Patent Publication Number: US-2022225937-A1

Title: Dry electrodes in a wearable garment

Description:
TECHNICAL FIELD 
     This disclosure relates to medical devices, and more particularly, to devices to monitor cardiac rhythm. 
     BACKGROUND 
     A number of wearable electronic devices exist that incorporate electrodes for monitoring bioelectric signals. In some examples, a patient may wear an ambulatory electrocardiography device, such as a Holter monitor, ambulatory cardiac monitoring patch, smart watch, or another external cardiac monitoring device, to collect data on the patient&#39;s cardiac activity. Such devices may record the patient&#39;s heart activity throughout the day and during events, such as sitting, standing, exercise, etc. Other examples of ambulatory measurement devices may monitor the patient for other sensed bioelectrical signals, such as biological impedance. 
     In other examples, a wearable automated external defibrillator (WAED), also referred to as a wearable cardiac defibrillator (WCD), is an option for patients having an identified risk of malignant tachyarrhythmia, but for whom an implantable cardioverter-defibrillator may not be indicated or desired. Malignant tachyarrhythmia, for example, ventricular fibrillation (VF), is an uncoordinated contraction of the cardiac muscle of the heart. If this arrhythmia continues for more than a few seconds, it may result in cardiogenic shock and stop effective blood circulation. Sudden cardiac death (SCD) may result in a matter of minutes. 
     In other examples, a wearable automated external defibrillator (WAED), also referred to as a wearable cardiac defibrillator (WCD), is an option for patients having an identified risk of malignant tachyarrhythmia, but for whom an implantable cardioverter-defibrillator may not be indicated or desired. WAEDs typically include straps or a garment carrying its components, such as sensing and defibrillation electrodes, processing circuitry, and shock generation and sensing circuitry, which may allow such components to be worn by a patient. Similar wearable devices may include sensing capability, but not necessarily defibrillation or other therapy delivery capability. 
     SUMMARY 
     In general, the disclosure describes a wearable garment and an arrangement of dry electrodes configured to measure signals from a patient. The dry electrodes are free from adhesives, such as conductive gels, to hold the electrodes in place on the patient&#39;s skin, which may provide improved comfort for the patient relative to electrodes including adhesives, and therefore may improve patient compliance in wearing the garment with the electrodes. However, dry electrodes may move on the skin and cause noise in the sensed bioelectrical signals. The arrangement of the electrodes of this disclosure may be configured to limit noise and thereby facilitate accurate signal sensing from the patient even with some amount of relative movement between the electrodes and the patient&#39;s skin. 
     In addition, the wearable garment may be controllable to change the amount of compression based on the sensed signals from the electrodes, and from other sensors. A wearable garment with a tight compression may provide improved electrode contact and reduce noise, but may reduce patient comfort, and therefore reduce patient compliance. A patient may decide not to wear a garment that is uncomfortable. The wearable garment of this disclosure may be controllable, such as by processing circuitry included in the garment, to maintain a comfortable level of compression until the processing circuitry detects a signal of interest, such as a cardiac arrhythmia, irregular respiration, or some other signal. The processing circuitry may cause the wearable garment to increase compression to improve the contact between the electrodes and the patient&#39;s skin and improve reception of the measured signals. In this disclosure, “in contact” requires the electrode to be touching on the patient&#39;s body tissue, such as the surface of the patient&#39;s skin. 
     In one example, this disclosure describes a medical device includes a first electrode, a second electrode, a third electrode, and a fourth electrode, each of the electrodes configured to be placed in contact with skin of a patient and held in position with a compressive member that is free of adhesives, wherein the first electrode and second electrode are configured to output an impedance measurement signal, and wherein the third electrode and the fourth electrode are configured to sense electrocardiogram (ECG) signals from the patient via the third electrode and the fourth electrode. 
     In another example, this disclosure describes a medical system includes first circuitry configured to measure a voltage, a second circuitry configured output a constant current signal, a medical device includes a first electrode, a second electrode, a third electrode, and a fourth electrode, each of the electrodes configured to be placed in contact with skin of a patient and held in position with a compressive member that is free of adhesives, and sensing circuitry configured to: output an impedance measurement signal via the first electrode and the second electrode; and sense electrocardiogram (ECG) signals from the patient via the third electrode and the fourth electrode. 
     In another example, this disclosure describes a method includes delivering, by a medical device, an impedance measurement signal via a first electrode and a second electrode; sensing, by the medical device, an electrocardiogram via a third electrode and a fourth electrode, wherein each of the first electrode, the second electrode, the third electrode, and the fourth electrode is configured to be placed in contact with skin of a patient and held in position with a compressive member that is free of adhesives. 
     In one example, this disclosure describes a method includes receiving, by processing circuitry, an indication from a mechanical sensor of a compression level for a garment, wherein a medical device comprises the garment, the processing circuitry, and the mechanical sensor; setting, by the processing circuitry, the compression level for the garment to a first compression level at a first time, wherein the garment comprises a plurality of electrodes, and wherein the first compression level locates the plurality of electrodes on a patient&#39;s skin such that the plurality of electrodes receives bioelectrical signals from a patient; and at a second time, changing, by the processing circuitry, the compression level from the first level to a second compression level based on one or more of: the indication of the compression level from the mechanical sensor and the received bioelectrical signals. 
     In another example, this disclosure describes a medical device includes a garment configured to be worn by a patient, the garment including: a mechanical sensor configured to output an indication of a compression level of the garment; an apparatus configured to adjust the compression level of the garment; and a plurality of electrodes located on the garment such that the plurality of electrodes receives bioelectrical signals from a patient&#39;s skin; processing circuitry attached to the garment and operatively coupled to the mechanical sensor, the plurality of electrodes and the apparatus, wherein the processing circuitry is configured to: receive the indication from the mechanical sensor of the compression level for the garment, set the compression level for the garment to a first compression level at a first time based on the indicated compression level; at a second time, change the compression level from the first compression level to a second compression level based on one or more of: the indication of the compression level from the mechanical sensor and the received bioelectrical signals. 
     In another example, this disclosure describes a computer-readable medium comprising instructions for causing a programmable processor to: receive an indication, from a mechanical sensor of a medical device, of a compression level for a garment, wherein the medical device comprises the garment, the programmable processor and the mechanical sensor; set the compression level for the garment to a first compression level at a first time, wherein the garment comprises a plurality of electrodes, and wherein the first compression level locates the plurality of electrodes on a patient&#39;s skin such that the plurality of electrodes receives bioelectrical signals from a patient; and at a second time different from the first time, change the compression level from the first compression level to a second compression level based on one or more of: the indication of the compression level from the mechanical sensor and the received bioelectrical signals. 
     The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example system that may be used to sense bioelectrical signals from a patient. 
         FIG. 2A  is a conceptual diagram illustrating a wearable garment configured to sense bioelectrical signals from a patient. 
         FIGS. 2B-2D  are conceptual diagrams illustrating examples of a controllable apparatus configured to adjust the compression level of a wearable garment according to one or more techniques of this disclosure. 
         FIG. 3  is a functional block diagram of an example wearable medical device configured to monitor cardiac function and deliver electrical therapy. 
         FIG. 4  is a functional block diagram of an external device configured to communicate with a wearable medical device. 
         FIG. 5  is a block diagram illustrating a system including an electrode device configured to sense bioelectrical signals from a patient according to one or more techniques of this disclosure. 
         FIGS. 6A-6D  are examples implementations of the electrode device according to one or more techniques of this disclosure. 
         FIG. 7  is a flow diagram illustrating an example operation of the electrode device according to one or more techniques of this disclosure. 
         FIG. 8  is a flow diagram illustrating an example operation of the wearable garment, which includes a medical device, according to one or more techniques of this disclosure. 
         FIG. 9  is a flow diagram illustrating an example operation of a medical device, such as a wearable automated external defibrillator, according to one or more techniques of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a conceptual diagram illustrating an example system  100  that may be used to monitor bioelectrical signals from a patient. In some examples, system  100  may deliver therapy to a heart of a patient  102 , such as to provide therapy for a cardiac arrhythmia. System  100  may include an apparatus  110 , one or more sensing devices  130 , a network  150 , and one or more external devices  170 . Apparatus  110  may be worn by patient  102 . Apparatus  110  may include a garment with electronics, devices to adjust the degree of compression of the garment, electrodes, and other components. Apparatus  110  may be configured to monitor bioelectrical signals from patient  102  and may configured to provide therapy. In some examples, apparatus  110  includes one or more sense electrodes configured to sense a phenomenon, e.g., the bioelectrical signal, based on the electrodes in contact with the skin of patient  102 . Some examples of bioelectrical, or physiological signals may include bioelectrical impedance, myoelectrical signals, cardiac signals such as electrocardiogram (ECG) or heart sounds and similar signals of patient  102 . In some examples, apparatus  110  is a WAED. In other examples, apparatus  110  may be any of a number of wearable electronic devices, including a smart watch, wearable patch, Holter monitor, and so on. In some examples, apparatus  110  may be configured to monitor patient  102  and communicate with external devices  170  either directly or through network  150 . 
     Sensing device(s)  130  may be configured to sense a phenomenon of patient  102  and/or the patient&#39;s environment. In some examples, sensing devices  130  may be included as part of apparatus  110 . For example, a sensing device including a movement sensor, such as an accelerometer, may be external to apparatus  110 , and/or included at some location in the garment that is part of apparatus  110 . In some examples, apparatus  110  may be configured to sense the same or different phenomena of patient  102  than sensing device(s)  130  that are external to apparatus  110 . As illustrated in  FIG. 1 , apparatus  110  and sensing device(s)  130  may communicate via one or more links  120 . In some examples, links  120  may be Bluetooth® links, such as Bluetooth® Low Energy (BLE) links, or other wired or wireless protocols. 
     In some examples, apparatus  110  will wake up a sensing device  130  using a specified magnetic, radio-frequency (RF), or electrical signal. In some examples, once a connection is established between apparatus  110  and sensing device  130 , periodic advertisements may maintain the connection. In some examples, organizational or globally unique identifiers may be used by apparatus  110  to distinguish among sensing devices  130 . In some examples, communication between apparatus  110  and sensing devices  130  may generally by according to the Bluetooth®, BLE or similar protocols. 
     Network  150  may represent any single network or combination of networks that facilitate communication between devices. As one example, network  150  may represent a combination of wireless and wired networks (e.g., the Internet) that facilitate communication between one or more external devices  170  and apparatus  110  (and/or sensing devices  130 ). External devices  170  and network  150  may comprise a remote patient monitoring system, such as the Carelink® network, available from Medtronic Inc., of. In some examples, external device(s)  170  may include one or more servers, and one or more personal computers, e.g., a computer that a healthcare provider may interact with via a user interface. In some examples, system  100  includes multiple external devices  170  (e.g., a remote patient monitoring system and one or more personal computers). In some examples, external device(s)  170  may comprise a cloud-based computing system. 
     External devices  170  may be configured to receive patient data from apparatus  110  and/or sensing devices  130  and store the data in memory. External device  170  may store data collected from populations of patients. In some examples the population data includes information about patient  102 , but in other examples the population data does not necessarily include information about patient  102 . 
       FIG. 2A  is a conceptual diagram illustrating an example wearable garment configured to sense bioelectrical signals from a patient. System  200  in the example of  FIG. 2A  is an example of system  100  described above in relation to  FIG. 1 . In the example of  FIG. 2A , system  200  includes wearable garment  202 , one or more other wearable devices  222  and  226  and a portable computing device  228 . 
     Wearable garment  202  is a medical device that, in the example of  FIG. 2A , includes a plurality of sensing electrodes  240 ,  242 ,  244 ,  246  located on the garment such that the plurality of sensing electrodes receives bioelectrical signals from the skin of patient  208 . Garment  202  also includes processing circuitry  210  and therapy delivery electrodes  230 ,  232  and  234 . In some examples, garment  202  may include a mechanical sensor configured to output an indication of a compression level of garment  202  and an apparatus configured to adjust the compression level of the garment (not shown in  FIG. 2A ). Power source  260  may provide power to the components of wearable garment  202 . In some examples power source  260  may be a replaceable or rechargeable battery. In some examples, power source  260  may be mounted on garment  202 . In other examples, power source  260  may be carried externally and connected to garment  202 , e.g., carried in a separate belt pack, a purse, and so on. 
     Processing circuitry  210  may be attached to garment  202  and operatively coupled to the mechanical sensor, the sensing electrodes  240 ,  242 ,  244  and  246 , and the apparatus for adjusting the compression level of garment  202 . Processing circuitry  210  may be configured to receive the indication from the mechanical sensor of the current compression level for garment  202  and control the apparatus for adjusting the compression level to maintain a target compression level. In some examples, each patient may have different body size and the target compression level of garment  202  may be adjusted to accommodate different sizes of patient. Processing circuitry  210  may set the compression level for the garment to a first compression level at a first time and a second compression level at a second time. In some examples, the first compression level may be set such that the sensing electrodes may be placed in contact with skin of patient  208  and held in position such that the sensing electrodes may measure bioelectrical signals from patient  208 . The first compression level may be such that the priority is patient comfort at the expense of a looser contact between the patient&#39;s skin and the sensing electrodes. At the first compression level, sensing electrodes  240 ,  242 ,  244  and  246  may move relative to the skin of patient  208  and cause some noise in the received bioelectrical signals but provide a higher level of comfort for patient  208 . Therefore, patient  208  may be more inclined to wear garment  202  more regularly. 
     When desired, e.g., when processing circuitry  210  receives bioelectrical signals indicating a cardiac arrhythmia, a lead off or disconnected condition, or some other predetermined condition, processing circuitry  210  may signal the apparatus to change the compression level to a second, tighter compression level when compared to the first compression level. For the tighter compression level, the priority may shift from patient comfort to improved contact between sensing electrodes  240 ,  242 ,  244  and  246  and the patient&#39;s skin. In some examples, for a lead off or noisy signal indication processing circuitry  210  may confirm whether there is an improved signal at the second compression level. In other examples, while at the second compression level, processing circuitry  210  may confirm that the cardiac arrhythmia is treatable by a defibrillation shock and, if necessary, alert patient  208  and deliver the shock. 
     In some examples, processing circuitry  210  may activate one or more conductive gel release mechanisms prior to delivering the defibrillation shock. In some examples the conductive gel may be contained in flattened, tubular capsules. In some examples, processing circuitry  210  may increase the compressive force such that the compression of garment  202  causes the capsule to release the gel. In other examples, processing circuitry  210  may trigger a mechanical, electrical, or chemical reaction to release gel, or trigger a primer which then releases the gel. In other examples, processing circuitry may activate a mechanism to move a gel capsule from a location that is not in contact with the therapy electrodes or the patient&#39;s skin, to a contacting location. For example, processing circuitry  210  may cause gel container to move from initial position, e.g., behind some protective mesh or film to a location in contact with the skin. In some examples the compressive force in one or more locations on garment  202  may cause the gel container, or the protective material, to move to allow the gel to release. 
     In other examples, therapy delivery electrodes  230 ,  232  or  234  may comprise non-liquid gel pads, or other similar material, that are positioned so the therapy delivery electrodes do not contact the patient&#39;s skin until triggered to do so by processing circuitry  210 . In other words, the therapy delivery electrodes may not remain in regular contact with skin but do so only when the garment is compressed, or otherwise triggered, by processing circuitry  210 . In some examples, increased garment compression may cause the therapy deliver pads to make mechanical contact with the patient&#39;s skin. In some examples, e.g., if processing circuitry fails to confirm a shockable arrhythmia, processing circuitry  210  may cause the therapy delivery pads to move off the patient&#39;s skin, e.g., back to a “ready position.” In other examples, compression of the garment may move the conductive material of the therapy pads through a porous barrier such that the conductive surface is then in contact with the patient&#39;s skin. 
     In some examples, the conductive gel may disperse under one or more of the therapy delivery electrodes  230 ,  232  or  234  to improve conduction between the electrode and the patient&#39;s skin. In other words, processing circuitry  210  may cause one or more conductive gel compartments associated with one or more therapy delivery electrodes to release conductive gel, prior to delivering electrical stimulation therapy via electrodes  230 ,  232  or  234 . By releasing conductive gel only when needed to deliver electrical stimulation therapy, patient  208  may avoid having to have gel in place during normal daily activity and therefore may improve comfort for patient  208 , when compared to having to apply conductive gel or other adhesive, when patient  208  dons garment  202 . 
     In other examples, the mechanical sensor may indicate that the compression level for garment  202  has changed, e.g., to a looser fit. In some examples, the received bioelectrical signals may also indicate a lead off, or increased noise or signal artifacts caused by the looser fit. A changed compression level may result from a relaxation in elastic material of garment  202  over time, patient  208  changes into different clothing, e.g., removes a coat, or for other reasons. Processing circuitry  210  may cause the apparatus of garment  202  to adjust the compression level to balance patient comfort with electrode-to-skin contact based on the indication of the compression level from the mechanical sensor and the received bioelectrical signals by executing an algorithm stored at a memory location operatively coupled to processing circuitry  210 . 
     In some examples, garment  202  may also include a motion sensor, such as an accelerometer or similar sensor (not shown in  FIG. 2 ). One or more motion sensors may be included in processing circuitry  210 , and/or located elsewhere in garment  202 . The motion sensor of this disclosure may be configured to determine one or more of movement or posture of the patient. For example, patient  208  may increase activity level, such as running, jumping and so on, which may cause increased movement between sensing electrodes  240 ,  242 ,  244  and  246  and the patient&#39;s skin. Also, patient  208  may change posture from an upright to a sitting or supine position. In some examples, external devices  222  and  226  as well as portable computing device  228  may include sensors that indicate movement, temperature, and so on. Processing circuitry  210  may receive the indication of movement and/or posture of patient  208 , and in some examples, may dynamically adjust the compression level of garment  202  based on one or more of the indication of the compression level from the mechanical sensor, the received bioelectrical signals or the indication from the motion sensor. 
     Processing circuitry  210  may be an example of a programmable processor, which may include any one or more of a microcontroller (MCU), e.g. a computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals, a microprocessor (μP), e.g. a central processing unit (CPU) on a single integrated circuit (IC), a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a system on chip (SoC) or equivalent discrete or integrated logic circuitry. A programmable processor may be integrated circuitry, i.e., integrated processing circuitry, and that the integrated processing circuitry may be realized as fixed hardware processing circuitry, programmable processing circuitry and/or a combination of both fixed and programmable processing circuitry. Accordingly, the terms “processing circuitry,” “processor” or “controller,” as used herein, may refer to any one or more of the foregoing structures or any other structure operable to perform techniques described herein. 
     Examples of a memory may include any type of computer-readable storage media, such as random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, and similar devices. In some examples the computer readable storage media may store instructions that cause the processing circuitry to execute the functions described herein. In some examples, the computer readable storage media may store data, such as configuration information, temporary values and other types of data used to perform the functions of this disclosure. 
     In the example of  FIG. 2A , therapy electrodes  230 ,  232  and  234  and sensing electrodes  240 ,  242 ,  244  and  246  are configured to be placed in contact with skin of a patient and held in position with a compressive member, such as garment  202 . Both therapy electrodes  230 ,  232  and  234  and sensing electrodes  240 ,  242 ,  244  and  246  may be free of adhesives. Electrodes held in position only with the compressive force from garment  202  may improve patient comfort when compared to electrodes held in place by adhesives. Though depicted as four sensing electrodes and three therapy electrodes in the example of  FIG. 2A , garment  202  may include any number of sensing, or therapy electrodes. The number and location of electrodes may depend on patient anatomy as well as a configuration to provide the best signal quality. 
     The example of  FIG. 2A  depicts sensing electrodes  242  and  246  as substantially circular disk shaped electrodes and sensing electrodes  244  and  240  as substantially circular ring shaped electrode surrounding electrodes  242  and  246 . For example,  FIG. 2A  depicts sensing electrode  242  as a disk surrounded by the ring of electrode  240 . In other examples, electrodes  240 ,  242 ,  244  and  246  may be any substantially circular shape, such as an oval, octagon, or similar shape. In other examples, electrodes  240 ,  242 ,  244  and  246  may be other geometric shapes such as square or rectangle. In contrast, other examples of multiple sensing electrodes may have arranged the electrodes that are approximately the same size with equal, or approximately equal spacing between the electrodes. 
     Electrodes  242  and  240  form a concentric arrangement and may appear to be a Laplacian bipolar type electrode. However, electrodes  242  and  240  connect to the circuitry of system  200  in a completely different manner than the high side and low side of a bipolar type electrode. In this disclosure, electrode  242 , as well as electrode  246  connects to circuitry that outputs an impedance measurement signal. In other words, the pair of electrodes  242  and  246  are configured to output the impedance measurement signal. In some examples the impedance measurement signal may be a constant current signal. In other examples, the impedance measurement signal may be a high frequency signal, e.g., approximately 8 kHz-16 kHz. 
     The third and fourth electrodes  244  and  240  may connect to a high side and low side inputs of an amplifier to measure voltage. In contrast, a bipolar Laplacian electrode would connect the inner disk and outer ring electrodes to the high side and low side of an amplifier to measure voltage. In some examples, the impedance measurement signal may be injected to electrode  242  and return from electrode  246 , or vice versa. Sensing circuitry connected to sensing electrodes  240  and  244  may measure the induced voltage in the patient&#39;s tissue caused by the impedance measurement signal. The measured induced voltage may provide an indication to processing circuitry  210  of a biological impedance of the patient&#39;s tissue. In some examples, the sensing circuitry may detect if an electrode is not connected to the patient&#39;s tissue. For example, if the induced voltage is outside a threshold range, the electrodes may be disconnected from the body. 
     As shown in  FIG. 2A , electrodes  244  and  240  may be separated and spaced at different locations relative to heart  212  and may sense electrocardiogram (ECG) signals as well as biological impedance signals from patient  208 . Sensing electrodes  240 ,  242 ,  244  and  246  may also measure bioelectrical signals related to biological impedance, fluid-status monitoring, heart failure, sleep apnea, ischemia detection, lead connectivity detection (also referred to as lead off detection), as well as cardiac arrythmia such as atrial fibrillation (AF), ventricular tachycardia (VT), ventricular fibrillation (VF) and so on. In some examples, electrodes  240 ,  242 ,  244  and  246  may be located at other positions different than shown in  FIG. 2A , e.g., lateral, or posterior relative to heart  212 . 
       FIGS. 2B -2D  are conceptual diagrams illustrating examples of a controllable apparatus configured to adjust the compression level of a wearable garment according to one or more techniques of this disclosure. Garments  252  and  254  are examples of garment  202  described above in relation to  FIG. 2A  and may have similar functions and characteristics. 
     In the example of  FIG. 2B , garment  252  includes a controllable apparatus  250  that may change the compression level of garment  252 . In some examples apparatus  250  may be implemented using a shape memory alloy (SMA), or similar material that when electrical current passes through conductors that include the shape memory alloy, the conductors may change shape, e.g., bend, expand or contract. One example shape memory alloy is nickel-titanium. In the example of  FIG. 2B , processing circuitry  210 , described above in relation to  FIG. 2A , may control driver circuitry that may control current through apparatus  250  to adjust the compression level of garment  252 . In some examples, apparatus  250  may include one or more mechanical sensors (not shown in  FIG. 2B ), such as a strain sensor, that may output an indication to processing circuitry  210  of the compression level of garment  252 . 
     Similarly, in the example of  FIG. 2C , garment  254  may include an apparatus, such as a controllable bladders  258  or similar mechanism, that may adjust the level of compression of garment  254 . Processing circuitry may control one or more pumps and/or valves, that may adjust the volume of air, or other fluid, within bladders  258  to control the compression level of garment  254 , similar to that described above in relation to  FIGS. 2B and 2A . Garment  254  may also include one or more mechanical sensors, e.g., near bladders  258 , (not shown in  FIG. 2C ) that may indicate to processing circuitry  210  the degree of compression for garment  254 . In other examples, garment  254  may include other sensors, such as a sensor to determine pulsatility, e.g., similar to sensors in an automated blood pressure device. 
       FIG. 2D  depicts a single shoulder version of wearable garment  255 . The single shoulder wearable garment  255  may include any of the variations described above for  FIGS. 2A-2C , including wearable bladders  258 , SMA, or other controllable apparatus  250 , mechanical sensors, and so on. 
     In some examples in which the garment comprises an WAED, processing circuitry  210  may cause the controllable apparatus to alert the patient that the WAED is preparing to deliver a defibrillation shock via the therapy electrodes described above in relation to  FIG. 2A . In some examples, processing circuitry  210  may cause the apparatus to change between two or more compression levels rapidly, e.g., over a period of ten seconds or less. For example, processing circuitry  210  may cause the apparatus to compress between a tight compression level and a loose compression level three to five times over a period of less than ten seconds. This may warn the patient that the WAED has nearly completed charging and is ready to confirm a shockable arrythmia and deliver a defibrillation shock. 
     In other examples, processing circuitry  210  may also alert the patient using an audible alert (e.g., tone or voice) or a tactile alert (e.g., vibration mechanism) or both. By using a compressive or vibration alert, the WAED of this disclosure may provide a warning in situations in which patient  208  is hearing impaired or is in a noisy environment where an audible alert may be less effective. 
       FIG. 3  is a functional block diagram of an example wearable medical device configured to monitor cardiac function and deliver electrical therapy. WAED  300  of  FIG. 3  is an example implementation of apparatus  110  described above in relation to  FIG. 1  and any of the garments described above in relation to  FIGS. 2A-2D . In the example illustrated by  FIG. 3 , WAED  300  includes sensing circuitry  302  connected to sensing device  330 , generation circuitry  320 , switching circuitry  322 , therapy generation circuitry  304  connected to therapy delivery electrodes  336 , processing circuitry  306 , memory  308 , and communication circuitry  318 . WAED  300  may receive power from power source  328  via connection  338 . 
     Processing circuitry  306  and memory  308  are respectively examples of processing circuitry  210  and the associated memory described above in relation to  FIG. 2A  and have similar functions and characteristics. For example, memory  308  may store program instructions, including one or more program modules, which are executable by processing circuitry  306 . When executed by processing circuitry  306 , such program instructions may cause processing circuitry  306  and WAED  300  to provide the functionality described herein. 
     As described above in relation to  FIG. 2A , sensing circuitry  302  may be configured to receive bioelectrical signals from sense electrodes  334 , such as biological impedance measurements and/or electrocardiogram (ECG) signals from a patient, such as patient  210  In some examples, sensing circuitry  302  is configured to sense cardiac events within the ECG signals based on the depolarization of myocardial tissue, e.g., P-waves and R-waves. Sensing circuitry  302  may include a switching circuitry  322  for selectively coupling output electrodes  332  to sensing circuitry  302  to monitor the bioelectrical signals. Switching circuitry  322  may include a switch array, switch matrix, multiplexer, or any other type of switching device suitable to selectively couple one or more of electrodes  332  to sensing circuitry  302 . In other examples, switching circuitry  322  may configure the electrodes to reverse the polarity of the signals, as well as to provide calibration functions. 
     Sensing circuitry  302  may include multiple sensing channels. Each sensing channel may be configured to amplify, filter, and rectify the bioelectrical signal received from selected electrodes coupled to the respective sensing channel to detect cardiac events, e.g., P-waves and/or R-waves, or ventricular fibrillation (VF) or ventricular tachycardiac (VT) signals, biological impedance, or other signals, as described above in relation to  FIG. 2A . Sensing circuitry  302  may include cardiac event detection circuitry that detects a cardiac event when the cardiac electrical signal crosses a sensing threshold. Sensing circuitry  302  may include a rectifier, filter and/or amplifier, a sense amplifier, comparator, and/or analog-to-digital converter. 
     Generation circuitry  320  may output the impedance measurement signal, for example via output electrodes  332 . Output electrodes  332  may be an example of electrodes  246  and  242  described above in relation to  FIG. 2A . In some examples, the impedance measurement signal may travel between output electrodes  332  through the patient&#39;s body tissue. For example, the impedance measurement signal may be a constant current signal that passes between output electrodes  332  (e.g., electrodes  246  and  242 ). The impedance measurement signal may measure an induced voltage during the delivery of the current signal via sense electrodes  334 . Sense electrodes  334  may be an example of sensing electrodes  240  and  244  described above in relation to  FIG. 2A . In other words, in some examples, sensing device  330  may provide a four-wire impedance measurement to processing circuitry  306 . In some examples, processing circuitry  306  may initiate an impedance measurement periodically and/or in response to an event. For example, processing circuitry  306  may initiate any of several types of biological impedance measurements one, two or more predetermined times per day. Processing circuitry may also initiate a measurement in response to a cardiac event, a predetermined movement or activity, a change in compression level, and so on. Some examples of types of biological impedance measurements may include body fluid level measurement (e.g., for kidney failure patient continuous monitoring or during dialysis), heart failure patient monitoring (as body fluid accumulated), sleep apnea (respiration monitor), body mass index (BMI) measurement, and so on. Processing circuitry  306  may cause generation circuitry  320  to adjust an output amplitude, frequency, pulse width or other characteristics of an output signal, for example, depending on the type of biological impedance measurement. In some examples, one or more electrodes may be used as part of common mode noise rejection circuitry. 
     In some examples, sensing circuitry  302  may include other types of sensors including accelerometers, temperature sensors, pressure sensors. In some examples, sensing circuitry  302  may communicate with other remote sensors implanted in, worn on or placed near patient  208 , described above in relation to  FIGS. 1 and 2A  (not shown in  FIG. 3 ) via communication circuitry  318 . In some examples, other remote sensors may communicate directly with processing circuitry  306  via communication circuitry  318 . In some examples the other types of sensors, may provide information such as patient posture, movement, activity, and other information. 
     Sensing circuitry  302  may output an indication to processing circuitry  306  in response to sensing of a cardiac event (e.g., detected P-waves or R-waves). In this manner, processing circuitry  306  may receive detected cardiac event signals corresponding to the occurrence of detected R-waves or P-waves. Indications of detected R-waves and P-waves may be used for detecting ventricular and/or atrial tachyarrhythmia episodes, e.g., ventricular, or atrial fibrillation episodes. Sensing circuitry  302  may also pass one or more digitized ECG signals to processing circuitry  306  for analysis, e.g., for use in cardiac rhythm discrimination. Processing circuitry  306  may use the indications of R-waves and/or the digitized ventricular ECG signals to detect diagnostic events and potential sensing issues, according to the techniques described herein. Indications of R-wave and P-wave timing, as well as digitized ECGs, may be stored in memory  308  as ECG data  310 . In some examples, analysis of the digitized ECG signals may include morphology (shape) analysis, such as ECG template matching and other types of ECG morphology analysis. 
     Memory  308  may also store an impedance analysis module  312 . Impedance analysis module  312  may be a software, firmware, or combination module executable by processing circuitry  306  to cause processing circuitry  306  to provide functionality related to identifying sensing issues, such as a lead off or other lead disconnection issues and sensing biological impedance. Functionality related to identifying sensing issues may include providing an alert, and/or modifying sensing or therapy provided by WAED  300 . As described above in relation to  FIG. 2A , biological impedance measurements may provide information regarding fluid-status monitoring, heart failure, sleep apnea, and ischemia detection, connectivity detection as examples. 
     Processing circuitry  306  may control therapy generation circuitry  304  to deliver electrical therapy, e.g., cardioversion or defibrillation shock pulses, to heart  212  (depicted in  FIG. 2A ) according to therapy parameters stored in memory  308 . In the example of  FIG. 3 , therapy generation circuitry  304  is electrically coupled to therapy electrodes  336 . Therapy delivery electrodes  336  is an example of therapy delivery electrodes  230 ,  232  and  234  described above in relation to  FIG. 2A . 
     Therapy delivery circuit  304  may include charging circuitry, one or more charge storage devices, such as one or more high voltage capacitors and/or one or more low voltage capacitors and switching circuitry (not shown in  FIG. 3 ) that controls when the capacitor(s) are discharged to selected combinations of therapy delivery electrodes  336 . Therapy delivery circuit  304  may control charging of capacitors to a programmed pulse amplitude and discharging of the capacitors for a programmed pulse width according to control signals received from processing circuitry  306 . 
     In some examples, communication circuitry  318  is used to communicate with an external device, for transmitting data accumulated by WAED  300  and for receiving interrogation and programming commands from external devices  130 , described above in relation to  FIG. 1 . Under the control of processing circuitry  306 , communication circuitry  318  may transmit an alert to notify a clinician and/or the patient that WAED  300  has detected a possible sensing issue with the atrial and/or ventricular leads. This alert enables the clinician to perform additional testing to confirm the issue and to intervene, if necessary, to replace the lead, fix the connection, reposition the lead, or to prevent unnecessary defibrillation therapy from being delivered to the patient. In other embodiments, WAED  300  may be equipped with alert circuitry configured to emit a sensory alert perceptible by the patient, e.g., a vibration, compressive or an audible tone, under the control of processing circuitry  306  to alert the patient to the possibility of a possible sensing issue, impeding defibrillation shock and so on, as described above in relation to  FIGS. 2B and 2C . 
     Processing circuitry  306  may adjust the compression level of the garment via compression control circuitry  319 . Compression control circuitry  319  may include driver circuits, logic circuitry or other processing circuitry, and other components to control the compression level of a wearable garment according to one or more techniques of this disclosure, such as garment 2   202 ,  252  and  254  described above in relation to  FIGS. 2A-2C . In some examples, compression control circuitry  319  may include circuitry configured to pass electrical current through the shape memory alloy of garment  252 . In other examples, compression control circuitry  319  may include circuitry to control the bladders, or other devices of garment  254 . 
     In some examples, compression control circuitry  319  may include feedback circuitry to receive signals from the one or more mechanical sensors (not shown in  FIG. 3 ) that provide an indication of the compression level of the garment. In other examples, processing circuitry  306  may receive the signals from the mechanical sensors and cause compression control circuitry  319  to adjust the compression level of the garment. 
     In some examples, processing circuitry  306  may send commands to compression control circuitry  319  to cause the compression level to rapidly change between two or more compression levels, for example, to alert the patient. In other examples, processing circuitry  306  may send an alert command to compression control circuitry  319 , and compression control circuitry  319  may manage the different compression levels as well as the timing between compression levels. In some examples, though shown as a separate block in the example of  FIG. 3 , compression control circuitry  319  may be embedded as part of processing circuitry  306 . 
       FIG. 4  is a functional block diagram of an external device configured to communicate with a wearable medical device. External device  400  depicted in  FIG. 4  is an example of external devices  170  described above in relation to  FIG. 1 . In the example of  FIG. 4 , external device  400  includes processing circuitry  440 , memory  442 , user interface (UI)  444 , and telemetry circuitry  446 . External device  400  may be a dedicated hardware device with dedicated software for the programming and/or interrogation of the wearable medical device. Alternatively, external device  400  may be an off-the-shelf computing device, which may execute an application that enables external device  400  to program and/or interrogate the wearable medical device. In some examples, external device  400  may be a portable computing device, such as portable device  228  as well as a wearable device, such as devices  222  and  226 , described above in relation to  FIG. 2A . In other examples, the functionality ascribed to external device  400  may also be implemented in medical devices worn by or implanted in patient  208  (not shown in  FIG. 3B ), such as an infusion pump for drug delivery, a leadless pacemaker, or other types of medical devices. 
     In some examples, a user may use external device  400  to select or program values for operational parameters of the wearable medical device, e.g., for cardiac sensing, therapy delivery, to configure the type of alert, e.g., audible, compressive, vibration, and other parameters. In some examples, a user may use external device  400  to receive data collected by the wearable medical device, such as cardiac ECG data  410  or other operational and performance data of the wearable medical device. The user may also receive one or more alerts provided by the wearable medical device. The user may interact with external device  400  via UI  444 , which may include a display to present a graphical user interface to a user, and a keypad or another mechanism for receiving input from a user. External device  400  may communicate wirelessly with the wearable medical device using telemetry circuitry  446 , which may be configured for RF communication, including inductive communication, with communication circuitry  318  of WAED  300  depicted in  FIG. 3 . In other examples, external device  400  may also communicate via a wired connection, such as via universal serial bus (USB), ethernet, or some other wired connection. 
     Similar to processing circuitry  306  described above in relation to  FIG. 3 , processing circuitry  440  may include any combination of integrated circuitry, discrete logic circuity, analog circuitry, such as one or more microprocessors, DSPs, ASICs, or FPGAs. In some examples, processing circuitry  440  may be configured with more processing power than the processing power available to processing circuitry  106 . Processing circuitry  440  may be configured to perform more complex calculations and analysis functions than performed by the processing circuitry in the wearable medical device. 
     As with memory  308 , described above in relation to  FIG. 3 , memory  442  may store program instructions, which may include one or more program modules and are executable by processing circuitry  440 . When executed by processing circuitry  440 , such program instructions may cause processing circuitry  440  and external device  400  to provide the functionality described herein. The program instructions may be embodied in software, firmware, or other volatile, non-volatile, magnetic, optical, or electrical media, such as a RAM, ROM, NVRAM, EEPROM, flash memory, or any other digital media. 
     In some examples, processing circuitry  440  of external device  400  may be configured to provide some or all of the functionality ascribed to processing circuitry  306  of the wearable medical device. For example, processing circuitry  440  may receive ECG data  410  from the wearable medical device via telemetry circuitry  446 , e.g., sensed via sensing device lead  330  depicted in  FIG. 3 , and may store the ECG data  410  in memory  442 . ECG data  410  may be current ECG data, or data previously collected and stored by the wearable medical device. Using ECG data  410 , processing circuitry  440  of external device  400  may identify characteristics of the atrial and ventricular EGM(s) indicative of possible sensing issues with WAED  300  described above in relation to  FIG. 3 , including sensed noise, biological impedance trends and so on. Based on the detection of possible sensing issues, or other diagnostic events, processing circuitry  440  may provide an alert to a user, e.g., via UI  444 . In some examples, impedance analysis module  412  may provide the functionality for the detection of possible sensing issues as well as biological impedance that may be used to monitor the patient. 
       FIG. 5  is a block diagram illustrating a system including an electrode device configured to sense bioelectrical signals from a patient according to one or more techniques of this disclosure. System  500  is an example of circuitry that may be included in the garments and WAED  300  described above in relation to  FIGS. 2A-2D and 3 . In the example of  FIG. 5 , system  500  includes sensing circuitry  503 , electrode device  514  and current source  502 . 
     Electrode device  514  includes output electrodes  504  and  506  arranged in an area between voltage sensing electrodes  508  and  510 . In other words, electrode  508  and electrode  510  define an area between electrodes  508  and  510 . Electrodes  504  and  506  are located in the area between electrodes  508  and  510 . In other examples, the arrangement of sensing electrodes  240 ,  242 ,  244  and  246  described above in relation to  FIG. 2A  is also an example of electrode device  514 . 
     Output electrodes  504  and  506  receive an impedance measurement signal, Iac  505 , e.g., from current source  502 . In the example of system  500 , current source  502  may be configured to output impedance measurement signal Iac  505  as a constant current magnitude signal, i.e., a fixed current amplitude. In some examples, Iac  505  may be an alternating current (AC) signal, e.g., with a constant current peak-to-peak amplitude. 
     In some examples, electrodes  508  and  510  may be configured to measure the biological impedance between electrodes  508  and  510  based on impedance measurement signal Iac  505 . In other words, electrode device  514  may be configured to provide a four-wire impedance measurement to processing circuitry  530 , via first stage amplifier  516 , as described above in relation to  FIG. 3 . Processing circuitry  530  may determine the biological impedance according to the following equation: 
     
       
         
           
             
               Z 
               biological 
             
             = 
             
               
                 
                   V 
                   
                     i 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     n 
                   
                 
                 
                   I 
                   
                     A 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     C 
                   
                 
               
               . 
             
           
         
       
     
     In other examples, processing circuitry  530  may be configured to determine an ECG signal based on V in    612 , as measured by electrodes  508  and  510 . 
     Sensing circuitry  503  may be an example of sensing circuitry  302  described above in relation to  FIG. 3  and may include the same functions and characteristics. Sensing circuity may include a first sensing path including low pass filter  518  and second stage amplifier  520 . Second stage amplifier  520  may also be referred to as ECG amplifier  520 . Second stage amplifier  520  may output the amplified and filtered voltage signal, Vin  612  to analog-to-digital converter ADC 1   522 . In some examples, system  500  may measure ECG and impedance simultaneously. For example, amplifier  516  may amplify both the ECG signal AND the impedance signals. The techniques of this disclosure may provide advantages over other types of measurement circuits. For example, a circuit arrangement like that depicted by system  500 , as well as system  300  described above in relation to  FIG. 3 , simplify the measurement circuitry, such by eliminating one or more multiplexer (MUX) circuits, and amplifier circuits. 
     Sensing circuitry  503  may include a second sensing path including high pass filter  524 , peak detector  526  and ADC 2   528 . In some examples, high pass filter  524  may be implemented by a capacitor. In some examples, peak detector  526  may include a diode and be configured to detect negative peaks from first state amplifier  516 . 
     In other examples, system  500  may include a second peak detector  527  coupled to ADC 3   529 . ADC 3   529  may receive the output of peak detector  527  and provide a digitized version to processing circuitry  530 . Including peak detector  527  may provide a different two-point measurement of the impedance, which may provide processing circuit  530  information to detect electrode-tissue interface issues, such as intermittent contact or an open circuit. In other words, the current is injected to electrode  504  and return from electrode  506 . Peak detector  527  may detect either electrode is not connected to body. The sensing electrodes  510  and  508 , which correlate to electrodes  240  and  244  depicted in  FIG. 2A , may measure the induced voltage in the patient&#39;s tissue. If the voltage is outside a threshold range, processing circuitry  530  may determine that the electrodes may be disconnected from the body. 
     As shown in the example of  FIG. 5 , ADC 1   522 , ADC 2   528  and ADC 3   529  may be part of processing circuitry  530 . In other examples, ADC 1   522 , ADC 2   528  and ADC 3   529  may be separate circuitry that is operatively connected to processing circuitry  530 . In some examples, the measured biological impedance may be recorded and reviewed for trends, e.g., trends in BMI, body fluid levels and so on. In other examples, the measured biological impedance may be compared to a threshold and used to provide patient or caregiver notification and/or cause processing circuitry  530  to perform some action. For example, breathing during sleep may be compared to a threshold to trigger a notification of potential sleep apnea. In other examples, when the induced voltage amplitude is higher than a threshold, processing circuitry  530  may determine that an electrode may be disconnected. 
       FIGS. 6A-6C  are examples implementations of the electrode device according to one or more techniques of this disclosure. Electrode devices  600 ,  610  and  630  are examples of sensing device  330  and electrode device  514  described above in relation to  FIGS. 3 and 5 , respectively. 
     Electrode device  600 , in the example of  FIG. 6A , is arranged similar to electrode device  514 , depicted in  FIG. 5 . Electrodes ECG 1   602  and ECG 2   604  define an area  605  between electrodes ECG  602  and ECG 2   604 . Electrodes Z 2   606  and Z 1   608  are located in area  605  between electrodes ECG 1   602  and ECG 2   604 . Also, a first distance  607  between electrode Z 2   606  and electrode and Z 1   608  is smaller than a second distance  603  between electrode ECG 1   602  and electrode ECG 2   604 . 
     The geometric shape of electrodes ECG 1   602  ECG 2   604  define a substantially circular disk with a diameter of a first dimension. In some examples the first dimension may be 20 millimeters (mm), 30 mm, 50 mm, or some other diameter. The geometric shape of electrodes Z 2   606  and Z 1   608  in the example of  FIG. 6A  also define a substantially circular disk with a diameter that is less than the diameter of electrodes ECG 1   602  and ECG 2   604 . In other examples, the geometric shape of the electrodes may be an oval, octagon, square, rectangle, or any other geometric shape. The dimensions for electrode ECG 1   602  may be approximately the same as for electrode ECG 2   604  in some examples. Similarly, in some examples, the dimensions for electrode Z 2   606  may be approximately the same as for electrode Z 1   608 . 
     Selecting the electrode size and shape may depend on a particular application, patient comfort and other factors. A larger size electrode may have better contact to the body but may take up more space. For ECG signals, larger electrode size may increase signal to noise ratio when compared to a smaller electrode. However, an electrode that is too large may make the electrode location less specific. Also, larger distance between the ECG electrodes may result higher signal amplitude. 
     Similar to electrode device  600 , in the example of  FIG. 6B , electrode device  610  may include electrodes ECG 1   612  and ECG 2   602  define an area  615  between electrodes ECG  1   612  and ECG 2   614 . Electrodes Z 2   616  and Z 1   618  are located in area  615  between electrodes ECG 1   612  and ECG 2   614 . Also, as described above in relation to  FIG. 6A , a first distance between electrode Z 2   616  electrode and Z 1   618  is smaller than a second distance between electrode ECG 1   612  and electrode ECG 2   614 . 
     The geometric shape of electrodes ECG 1   612  and ECG 2   614  define a substantially circular disk with a triangular or pie-shaped portion removed. Electrode Z 1   616 , in the example of  FIG. 6B , defines a triangular or pie-shape located in the removed portion of electrode ECG 1   612 . Similarly, electrode Z 1   616 , in the example of  FIG. 6B , defines a triangular or pie-shape located in the removed portion of electrode ECG 1   612 . 
     Also, as with the electrodes shown in any of  FIGS. 6A-6C , the geometric shape of the ECG 1  and ECG 2  electrodes form a first surface area in contact with the patient&#39;s skin and the geometric shape of the Z 1  and Z 2  electrodes form a second surface area in contact with the patient&#39;s skin. In the examples of  FIGS. 6A-6C , the first surface area may be the same as or larger than the second surface area. In some examples, the first surface area may be two to four times larger than the second surface area. 
     In the example of  FIG. 6C , electrode ECG 1   622  and Z 1   626  form a concentric arrangement. Electrode Z 1   626  defines a substantially circular disk-shaped electrode, while electrode ECG 1   622  defines a substantially circular ring shaped electrode surrounding electrode Z 1   626 . Similarly, electrode ECG 2   624  and Z 2   628  form a concentric arrangement. Electrode Z 2   628  defines a substantially circular disk shaped electrode, while electrode ECG 2   624  defines a substantially circular ring-shaped electrode surrounding electrode Z 2   628 . As described above in relation to  FIG. 6A , in other examples, the geometric shape of the electrodes may be implemented an oval, octagon, square, rectangle or any other geometric shape. The dimensions for electrode ECG 1   622  may be approximately the same as for electrode ECG 2   624  in some examples. Similarly, in some examples, the dimensions for electrode Z 2   626  may be approximately the same as for electrode Z 1   628 . As noted above, the distance  630  between the electrodes may impact the signal amplitude for some bioelectrical signals. In some examples, the distance  630  may be many times the diameter  632  of electrodes ECG 1   622  and ECG 2   624 .  FIG. 6C  is an example and may not be to scale. 
     Similar to  FIG. 6C , in the example of  FIG. 6D , each pair of electrode ECG 1   622  and Z 1   626  and electrode ECG 2   624  and Z 2   628  form a concentric arrangement. In  FIG. 6D , the locations of the output electrodes and the voltage sensing electrodes have been reversed, when compared to  FIG. 6C . Electrode ECG 1   646  defines a substantially circular disk-shaped electrode, while electrode Z 1   642  defines a substantially circular ring shaped electrode surrounding electrode ECG 1   646 . Electrode ECG 2   648  defines a substantially circular disk shaped electrode, while electrode Z 2   644  defines a substantially circular ring-shaped electrode surrounding electrode ECG 2   648 . As with  FIG. 6C , the distance  634  between the electrodes may be many times the diameter  636 . Also, the area of electrode ECG 1   622  in  FIG. 6C  may be approximately the same as for electrode Z 1   642  in  FIG. 6D . Similarly, in some examples, the dimensions for ECG 2  electrode  624  may be approximately the same as for electrode Z 2   646 . 
       FIG. 7  is a flowchart illustrating an example operation of the electrode device according to one or more techniques of this disclosure. The operation of  FIG. 7  will be described in terms of  FIG. 6C  for convenience. 
     A medical device, such as WAED  300 , arranged as shown by garment  202 , described above in relation to  FIGS. 3 and 2  respectively, may include circuitry configured to deliver an impedance measurement signal via a first electrode, e.g., electrode Z 1   626  and a second electrode, e.g., electrode Z 2   628  ( 700 ). The impedance measurement signal may travel through tissue of a patient, as described above in relation to  FIG. 5 . In some examples, electrode ECG 1   622  along with electrode ECG 2   624 , may measure a voltage which develops in the area between electrodes ECG 1   622  and electrode ECG 2   624  based on the impedance measurement signal. In other words, electrode device  620  may provide a four-wire impedance measurement to processing circuitry of the medical device. For example, signal generation circuitry  320  may be configured to output the impedance measurement signal, such as a constant current signal at a first time for a predetermined duration. Sensing circuitry  302  may be further configured to measure a biological impedance via sensing device  330  during the predetermined duration and based on the output constant current signal. 
     Also, the medical device, may sense an electrocardiogram from the patient via electrode ECG 1   622  and electrode ECG 2   624  based on a voltage measured between electrode ECG 1   622  and electrode ECG 2   624  ( 702 ). The measured voltage may be caused by cardiac activity, such as depolarization of heart tissue. As described above in relation to  FIG. 2A , each of ECG 1   622 , electrode ECG 2   624 , electrode Z 2   626  and electrode Z 1   628  may be configured to be placed in contact with skin of the patient and held in position with a compressive member, e.g., garment  202 . The electrodes may be free of adhesives, which may improve long term patient comfort, as described above in relation to  FIGS. 2A-2D . 
       FIG. 8  is a flow diagram illustrating an example operation of the wearable garment, which includes a medical device, according to one or more techniques of this disclosure. The flow diagram of  FIG. 8  may be described in terms of  FIGS. 2A and 3 , unless otherwise noted. 
     In the example of  FIG. 8 , processing circuitry  210  of garment  202  may start ( 800 ) by receiving an indication from a mechanical sensor, e.g., a strain sensor, or similar mechanical sensor that a patient has put on garment  202 . The processing circuitry may determine a compression level for garment  202 , based on the received indication, which may be affected by the size of garment  202  and height, weight and so on of patient  208 . 
     Processing circuitry  210  of the medical device may set the compression level for the garment to an initial compression level at a first time ( 802 ). In some examples, compression control circuitry  319  may set the initial compression level to prioritize patient comfort over receiving noise-free bioelectrical signals via the electrodes of sensing device  330 . In other examples, compression control circuitry  319  may set a tighter compression level to establish a baseline signal level for the bioelectrical signals from the skin of patient  208 , then relax the compression level to prioritize patient comfort. In any case, the compression level of garment  202  may locate the electrodes on the patient&#39;s skin such that the plurality of electrodes receives bioelectrical signals from a patient. 
     At a later time, the processing circuitry may cause compression control circuitry  319  to change the compression level from the initial level to a second compression level based on, for example, one or more of the indication of the compression level from the mechanical sensor and the received bioelectrical signals. In some examples processing circuitry  210  may sense and detect a cardiac arrhythmia using ECGs via the electrodes of sensing device  330  ( 804 ). Processing circuitry  210  determines whether the detected cardiac arrhythmia is a suspected shockable cardiac arrhythmia, e.g., ventricular tachycardia (VT) or ventricular fibrillation (VF) ( 806 ). When the processing circuitry  210  determines that the detected cardiac arrhythmia is not a suspected shockable cardiac arrythmia, such as a supraventricular tachycardia (SVT), atrial fibrillation, or other non-shockable rhythm (NO branch of  806 ), processing circuitry continues to monitor for a cardiac arrhythmia. 
     In other examples, processing circuitry  210  may detect that the cardiac arrythmia is a suspected shockable cardiac arrhythmia (YES branch of  806 ), and processing circuitry  210  may cause the compression level to change to a tighter compression level, which may press the electrodes more tightly against the skin of patient  208  ( 808 ). In some examples, pressing the electrodes more tightly to the skin may reduce skin-to-electrode movement, and increase the surface area of skin in contact with the electrode. In this manner the tighter compression level may provide a stronger (e.g., higher amplitude ECG) bioelectrical signal that may have less noise. In some examples, processing circuitry  210  may cause the compression level to change to a tighter level within a predetermined duration, e.g., within five seconds, three seconds, ten seconds, or some other duration ( 808 ). In some examples, processing circuitry  210  may retrieve from a memory one or more predetermined target compression levels, e.g., based on patient activity, posture, detected arrhythmia, patient comfort and so on. Processing circuitry may control the adjustment of the compression level of garment  202  to match a selected target compression level, e.g., within a threshold amount of compression as measured by the mechanical sensor. 
     While at the tighter compression level, processing circuitry  210  may confirm whether the cardiac arrhythmia is treatable by a defibrillation shock. In the example, of a short term VT episode, e.g., VT lasting for a few seconds, processing circuitry  210  may not be able to confirm the shockable rhythm (NO branch of  810 ). In the example of an arrhythmia such as VT or VF, the arrhythmia may be treatable by a defibrillation shock (YES branch of  810 ). 
     In response to confirming that the cardiac arrhythmia is treatable by the defibrillation shock, processing circuitry  210  may alert patient  208  that the medical device will deliver the defibrillation shock based on the confirmation ( 812 ). As described above in relation to  FIG. 2A , the medical device may output a vibration, audio signal, or may change compression levels in a manner that alerts the patient. In some examples, therapy generation circuitry  304  may begin charging an electrical energy storage device, e.g., one or more capacitors, to prepare for defibrillation shock delivery ( 812 ). 
     In some examples, garment  202  may include a button, switch, or some other input device e.g., a microphone for a voice command, configured to allow patient  208  to manually pause or disable delivery of the defibrillation shock. In other examples, the patient may provide the indication to disable the shock using any one of external devices  170 , wearable devices  222  and  226  and a portable computing device  228  described above in relation to  FIGS. 1 and 2 . For example, if the patient is driving, pausing the shock delivery may allow the patient to stop in a safe location. In response to receiving an input from patient  208  to disable the shock (YES branch of  814 ), processing circuitry may cause the medical device to stop charging and disable delivery of the defibrillation shock ( 816 ). 
     In other examples, patient  208  may not disable the shock (NO branch of  814 ). Once the charging is complete, processing circuitry  210  may reconfirm that processing circuitry  210  is still receiving an indication of a shockable arrythmia via sensing device  330  (YES branch of  818 ). In response, the processing circuitry may cause the therapy delivery circuitry, e.g., therapy generation circuitry  304  and therapy delivery electrodes  336  of the medical device to deliver the defibrillation shock ( 820 ). 
     In other examples, the arrhythmia may have ended on its own, or the indication of the arrhythmia may have been caused by noise or some other issue. Processing circuitry  210  may not be able to reconfirm the shockable rhythm (NO branch of  818 ) and stop charging ( 816 ). 
       FIG. 9  is a flow charge illustrating an example operation of a medical device, such as a wearable automated external defibrillator, according to one or more techniques of this disclosure. The flow diagram of  FIG. 9  may be described in terms of  FIGS. 2A-2D and 3 , unless otherwise noted. 
     Similar to the description of  FIG. 8  above, in the example of  FIG. 9 , a patient may start ( 900 ) by putting on garment  202 . Processing circuitry  210  of garment  202  may receive an indication from a mechanical sensor and may dynamically adjust the compression level for garment  202 , based on the received indication ( 902 ). For example, as described above in relation to  FIG. 2A , processing circuitry  210  may adjust the compression level of garment  202  to compensate for changes in activity, posture, donning or removing clothing, sensing noise and so on. In some examples, processing circuitry  210  may retrieve from a memory one or more predetermined target compression levels and adjust the compression level of garment  202  to match a selected target compression level, e.g., within a threshold amount of compression as measured by the mechanical sensor. In some examples, a patient may select one or more levels of comfort such as tight, less tight, least tight, and so on. In some examples, processing circuitry  210  may measure sensing performance and selects the least constricting compression that meets the sensing requirements. In some examples, the selected compression level may include a margin of compression to ensure sensing. In other examples, the selected target compression may be based on some combination, e.g., a selected comfort level, and further adjusted by processing circuity  210  based on the detected sensing. 
     As described above in relation to  FIG. 8 , in some examples processing circuitry  210  may sense and detect a cardiac arrhythmia using ECGs via the electrodes of sensing device  330  ( 904 ). In some examples, the arrhythmia may include arrhythmias that garment  202  may not be configured to treat or may be non-dangerous (NO branch of  906 ). In other examples, processing circuitry  210  may detect a dangerous arrhythmia, such as VT or VF (YES branch of  906 ). 
     In response to detecting the cardiac arrhythmia, processing circuitry  210  may cause the compression level to change to a tighter compression level ( 908 ). In some examples, processing circuitry  210  may cause the compression level to change to a tighter level within a predetermined duration, e.g., within a few seconds ( 908 ). 
     While at the tighter compression level, processing circuitry  210  may confirm whether the cardiac arrhythmia is treatable by a defibrillation shock. For a short term VT episode, or some other non-shockable rhythm, processing circuitry  210  may not be able to confirm the shockable rhythm (NO branch of  910 ). In some examples, when processing circuitry  210  is unable to confirm the shockable rhythm, processing circuity  210  may cause the compression level of the garment to reset to a more comfortable compression level for the patient. 
     In the example of an arrhythmia such as VT or VF, the arrhythmia may be treatable by a defibrillation shock (YES branch of  910 ). In response to confirming that the cardiac arrhythmia is treatable by the defibrillation shock, processing circuitry  210  may alert patient  208  that the medical device will deliver the defibrillation shock based on the confirmation and therapy generation circuitry  304  may begin charging an electrical energy storage device, to prepare for defibrillation shock delivery ( 912 ). 
     In some examples, processing circuitry  210  may compress the sleeve to verify pulsatility ( 914 ) for example, to compare with detected ECG or impedance signals via electrode device  514 . Mechanical or other sensors may be configured then determine if there is pulsatility by the sensor waveform. In some examples, processing circuitry  210  may use pulsatility measurements to confirm arrhythmia detection (e.g., a loss of pulsatility) and may avoid unnecessary therapy shocks. 
     In some examples, therapy delivery electrodes  336  may include associated conductive gel delivery systems, which may improve electrode-to-skin conduction when delivering a defibrillation shock. By including a gel delivery system, the medical system of this disclosure may provide advantages over other types of wearable medical devices. Examples of wearable medical devices that require gels, or other adhesives, as soon as the patient puts on the wearable medical device, may be less comfortable for a patient than the medical system of this disclosure which may include dry electrodes. Delivering a defibrillation shock without some sort of conductive medium, such as a conductive gel, may also be more uncomfortable for a patient when compared to therapy delivery electrodes in contact with the skin that have a conductive medium. Therefore, the medical system of this disclosure may include gel compartments near the therapy delivery electrodes, e.g., defibrillation pads. Processing circuitry  210  may cause the gel compartments to release the gel to improve defibrillation pad connectivity ( 916 ). As described above in relation to  FIG. 2A , processing circuitry  210  may activate one or more conductive gel release mechanisms prior to delivering the defibrillation shock, e.g., by causing an electrical signal to compress the gel compartments, which may cause the conductive gel to disperse under one or more of the therapy delivery electrodes  230 ,  232  or  234  to improve conduction between the electrode and the patient&#39;s skin. 
     In some examples, in response to receiving an input from patient  208  to disable the shock (YES branch of  918 ), processing circuitry may cause the medical device to stop charging and disable delivery of the defibrillation shock ( 920 ). In other examples, patient  208  may not disable the shock (NO branch of  918 ). 
     Once the charging is complete, processing circuitry  210  may reconfirm that processing circuitry  210  is still receiving an indication of a shockable arrythmia via sensing device  330  (YES branch of  922 ). In response, the processing circuitry may cause the therapy delivery circuitry, e.g., therapy generation circuitry  304  and therapy delivery electrodes  336  of the medical device to deliver the defibrillation shock ( 924 ). In other examples, processing circuitry  210  may not be able to reconfirm the shockable rhythm (NO branch of  922 ) and stop charging ( 920 ). 
     The techniques of this disclosure may also be described in the following examples. 
     Example 1: A medical device that includes a first electrode, a second electrode, a third electrode, and a fourth electrode, each of the electrodes configured to be placed in contact with skin of a patient and held in position with a compressive member that is free of adhesives, wherein the first electrode and second electrode are configured to output an impedance measurement signal, and wherein the third electrode and the fourth electrode are configured to sense electrocardiogram (ECG) signals from the patient via the third electrode and the fourth electrode. 
     Example 2: The medical device of example 1, wherein the first electrode and the third electrode form a concentric arrangement, wherein the third electrode surrounds the first electrode; and wherein the second electrode and the fourth electrode form a concentric arrangement, wherein the fourth electrode surrounds the second electrode. 
     Example 3: The medical device of examples 1 and 2, wherein the first electrode defines a substantially circular disk shaped electrode and, wherein the third electrode defines a substantially circular ring shaped electrode surrounding the first electrode. 
     Example 4: The medical device of any combination of examples 1-3, wherein the first electrode and the second electrode define a first dimension, wherein the third electrode and the fourth electrode define a second dimension, and wherein the second dimension is greater than the first dimension. 
     Example 5: The medical device of any combination of examples 1-4, wherein the first dimension and the second dimension is a diameter. 
     Example 6: The medical device of any combination of examples 1-5, wherein the first electrode and the third electrode define a geometric shape, wherein the first electrode defines a first portion of the geometric shape with a first surface area in contact with the patient&#39;s skin and the third electrode defines a second portion of the geometric shape with a second surface area in contact with the patient&#39;s skin, and wherein the first surface area is smaller than the second surface area. 
     Example 7: The medical device of any combination of examples 1-6, wherein the geometric shape comprises a substantially circular disk, and wherein the first portion comprises a substantially pie shaped portion of the circular disk. 
     Example 8: The medical device of any combination of examples 1-7, wherein a first distance between the first electrode and the second electrode is smaller than a second distance between the third electrode and the fourth electrode. 
     Example 9: The medical device of any combination of examples 1-8, wherein the third electrode and the fourth electrode define an area between the third electrode and the fourth electrode, and wherein the first electrode and the second electrode are located in the area. 
     Example 10: A medical system includes first circuitry configured to measure a voltage, a second circuitry configured output a constant current signal, a medical device includes a first electrode, a second electrode, a third electrode, and a fourth electrode, each of the electrodes configured to be placed in contact with skin of a patient and held in position with a compressive member that is free of adhesives; and sensing circuitry configured to: output an impedance measurement signal via the first electrode and the second electrode; and sense electrocardiogram (ECG) signals from the patient via the third electrode and the fourth electrode. 
     Example 11: The medical system of example 10, wherein the second circuitry is configured to output the constant current signal at a first time for a predetermined duration, and wherein the first circuitry is further configured to measure a biological impedance during the predetermined duration based on the output constant current signal. 
     Example 12: The medical system of examples 10 and 11, wherein the first circuitry comprises: a first stage amplifier circuit; a filter circuit; a second stage amplifier circuit; and a peak detector circuit. 
     Example 13: The medical system of any combination of examples 10-12, further comprising processing circuitry configured to: receive an output from the second stage amplifier circuit; receive an output from the peak detector circuit; determine an electrocardiogram based on the received outputs. 
     Example 14: The medical system of any combination of examples 10-13, wherein the first electrode and the third electrode form a concentric arrangement, wherein the third electrode surrounds the first electrode; and wherein the second electrode and the fourth electrode form a concentric arrangement, wherein the fourth electrode surrounds the second electrode. 
     Example 15: The medical system of any combination of examples 10-14, wherein the first electrode and the third electrode define a geometric shape, wherein the first electrode defines a first portion of the geometric shape with a first surface area in contact with the patient&#39;s skin and the third electrode defines a second portion of the geometric shape with a second surface area in contact with the patient&#39;s skin, and wherein the first surface area is smaller than the second surface area. 
     Example 16: The medical system of any combination of examples 10-15, wherein the geometric shape comprises a substantially circular disk, and wherein the first portion comprises a substantially pie shaped portion of the circular disk. 
     Example 17: The medical system of any combination of examples 10-16, wherein a first distance between the first electrode and the second electrode is smaller than a second distance between the third electrode and the fourth electrode. 
     Example 18: The medical system of any combination of examples 10-17, wherein the third electrode and the fourth electrode define an area between the third electrode and the fourth electrode, and wherein the first electrode and the second electrode are located in the area. 
     Example 19: A method includes delivering, by a medical device, an impedance measurement signal via a first electrode and a second electrode; sensing, by the medical device, an electrocardiogram via a third electrode and a fourth electrode, wherein each of the first electrode, the second electrode, the third electrode, and the fourth electrode is configured to be placed in contact with skin of a patient and held in position with a compressive member that is free of adhesives. 
     Example 20: The method of example 19, wherein the first electrode and the third electrode form a concentric arrangement, wherein the third electrode surrounds the first electrode; and wherein the second electrode and the fourth electrode form a concentric arrangement, wherein the fourth electrode surrounds the second electrode. 
     Example 21: The method of examples 19-20, wherein the first electrode defines a substantially circular disk shaped electrode and, wherein the third electrode defines a substantially circular ring shaped electrode surrounding the first electrode. 
     Example 22: A method includes receiving, by processing circuitry, an indication from a mechanical sensor of a compression level for a garment, wherein a medical device comprises the garment, the processing circuitry, and the mechanical sensor; setting, by the processing circuitry, the compression level for the garment to a first compression level at a first time, wherein the garment comprises a plurality of electrodes, and wherein the first compression level locates the plurality of electrodes on a patient&#39;s skin such that the plurality of electrodes receives bioelectrical signals from a patient; and at a second time, changing, by the processing circuitry, the compression level from the first level to a second compression level based on one or more of: the indication of the compression level from the mechanical sensor and the received bioelectrical signals from the plurality of electrodes. 
     Example 23: The method of example 22, further comprising a motion sensor, wherein the processing circuitry is further configured to dynamically adjust the compression level of the garment based on one or more of: the indication of the compression level from the mechanical sensor, the received bioelectrical signals or the indication from the motion sensor. 
     Example 24: The method of any combination of examples 22-23, wherein the one or more bioelectrical signals comprise: an electrocardiogram of the patient, and a biological impedance of the patient. 
     Example 25: The method of any combination of examples 22-24, wherein the received bioelectrical signal comprises a biological impedance of the patient. 
     Example 26: The method of any combination of examples 22-25, wherein the received bioelectrical signal comprises an electrocardiogram of the patient. 
     Example 27: The method of any combination of examples 22-26, further includes detecting, by the processing circuitry, a cardiac arrhythmia based on the received bioelectrical signals; in response to detecting the cardiac arrhythmia, changing the compression level to the second compression level, wherein the second compression level is tighter than the first compression level, while at the second compression level, confirming, by the processing circuitry, that the cardiac arrhythmia is treatable by a defibrillation shock; in response to confirming that the cardiac arrhythmia is treatable by the defibrillation shock, alerting the patient that the medical device will deliver the defibrillation shock based on the confirmation; and causing, by the processing circuitry, therapy delivery circuitry of the medical device to deliver the defibrillation shock. 
     Example 28: The method of any combination of examples 22-27, wherein the cardiac arrhythmia is one of ventricular fibrillation (VF) or ventricular tachycardia (VT). 
     Example 29: The method of any combination of examples 22-28, wherein alerting the patient comprises causing, by the processing circuitry the compression level to change between two or more compression levels. 
     Example 30: The method of any combination of examples 22-29, wherein changing the compression level to the second compression level causes the one or more conductive gel compartments to release the conductive gel. 
     Example 31: The method of any combination of examples 22-29, further comprising, before causing the therapy delivery circuitry to deliver the defibrillation shock, causing, by the processing circuitry, one or more conductive gel compartments associated with one or more therapy delivery electrodes to release conductive gel. 
     Example 32: The method of any combination of examples 22-30, further includes before causing the therapy delivery circuitry to deliver the defibrillation shock, receiving, by the processing circuitry, an indication from the patient to disable delivery of the defibrillation shock; and in response to receiving the indication from the patient, disabling the delivery of the defibrillation shock. 
     Example 33: A medical device includes a garment configured to be worn by a patient, the garment including: a mechanical sensor configured to output an indication of a compression level of the garment; an apparatus configured to adjust the compression level of the garment; and a plurality of electrodes located on the garment such that the plurality of electrodes receives bioelectrical signals from a patient&#39;s skin; processing circuitry attached to the garment and operatively coupled to the mechanical sensor, the plurality of electrodes and the apparatus, wherein the processing circuitry is configured to: receive the indication from the mechanical sensor of the compression level for the garment, set the compression level for the garment to a first compression level at a first time based on the indicated compression level; at a second time, change the compression level from the first compression level to a second compression level based on one or more of: the indication of the compression level from the mechanical sensor and the received bioelectrical signals from the plurality of electrodes. 
     Example 34: The medical device of example 32, further comprising a motion sensor, wherein the processing circuitry is further configured to dynamically adjust the compression level of the garment based on one or more of: the indication of the compression level from the mechanical sensor, the received bioelectrical signals or the indication from the motion sensor. 
     Example 35: The medical device of examples 32 and 33, wherein the processing circuitry is further configured to: detect a cardiac arrhythmia based on the received bioelectrical signals; change the compression level to the second compression level, wherein the second compression level is tighter than the first compression level, confirm that the cardiac arrhythmia is treatable by a defibrillation shock; alert the patient that the therapy delivery circuitry will deliver the defibrillation shock; cause therapy delivery circuitry to deliver the defibrillation shock. 
     Example 36: The medical system of any combination of examples 32-34, wherein alerting the patient comprises causing, by the processing circuitry the compression level to change between two or more compression levels. 
     Example 37: The medical device of any combination of examples 32-35, wherein the processing circuitry is further configured to: before causing the therapy delivery circuitry to deliver the defibrillation shock, receive an indication from the patient to disable delivery of the defibrillation shock; and in response to receiving the indication from the patient, disable the delivery of the defibrillation shock. 
     Example 38: The medical device of any combination of examples 32-36, wherein confirming that the cardiac arrhythmia is treatable by a defibrillation shock is based at least in part on bioelectrical signals sensed while at the second compression level. 
     Example 39: The medical device of any combination of examples 32-37, wherein the processing circuitry is further configured to, before causing the therapy delivery circuitry to deliver the defibrillation shock, cause one or more conductive gel compartments associated with one or more therapy delivery electrodes to release conductive gel. 
     Example 40: The medical device of any combination of examples 32-38, wherein changing the compression level to the second compression level causes the one or more conductive gel compartments to release the conductive gel. 
     Example 41: The medical device of any combination of examples 32-39, wherein the one or more bioelectrical signals comprise a biological impedance of the patient. 
     Example 42: The medical device of any combination of examples 32-40, wherein the one or more bioelectrical signals comprises an electrocardiogram of the patient. 
     Example 43: The medical device of any combination of examples 32-39, wherein the one or more bioelectrical signals comprise: an electrocardiogram of the patient, and a biological impedance of the patient. 
     Example 44: A computer-readable medium comprising instructions for causing a programmable processor to: receive an indication, from a mechanical sensor of a medical device, of a compression level for a garment, wherein the medical device comprises the garment, the programmable processor and the mechanical sensor; set the compression level for the garment to a first compression level at a first time, wherein the garment comprises a plurality of electrodes, and wherein the first compression level locates the plurality of electrodes on a patient&#39;s skin such that the plurality of electrodes receives bioelectrical signals from a patient; and at a second time different from the first time, change the compression level from the first compression level to a second compression level based on one or more of: the indication of the compression level from the mechanical sensor and the received bioelectrical signals. 
     Various examples of the disclosure have been described. These and other examples are within the scope of the following claims.