Patent Publication Number: US-11648411-B2

Title: Defibrillation waveforms for a wearable cardiac defibrillator

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     The present patent application is a continuation of U.S. application Ser. No. 15/794,585 filed Oct. 26, 2017, which in turn claims benefit of U.S. Application No. 62/512,003 filed May 27, 2017 and the benefit of U.S. Application No. 62/550,518 filed Aug. 25, 2017, all of which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     When people suffer from some types of heart arrhythmias, the result may be that blood flow to various parts of the body is reduced. Some arrhythmias may even result in a Sudden Cardiac Arrest (SCA). SCA can lead to death very quickly, e.g. within 10 minutes, unless treated in the interim. 
     Some people have an increased risk of SCA. People at a higher risk include patients who have had a heart attack, or a prior SCA episode. A frequent recommendation is for these people to receive an Implantable Cardioverter Defibrillator (ICD). The ICD is surgically implanted in the chest, and continuously monitors the patient&#39;s electrocardiogram (ECG). If certain types of heart arrhythmias are detected, then the ICD delivers an electric shock through the heart. 
     After being identified as having an increased risk of an SCA, and before receiving an ICD, these people are sometimes given a Wearable Cardioverter Defibrillator (WCD) system (early versions of such systems were called wearable cardiac defibrillator systems). A WCD system typically includes a harness, vest, or other garment that the patient is to wear. The WCD system further includes electronic components, such as a defibrillator and electrodes, coupled to the harness, vest, or other garment. When the patient wears the WCD system, the external electrodes may then make electrical contact with the patient&#39;s skin, and therefore can help determine the patient&#39;s ECG. If a shockable heart arrhythmia is detected, then the WCD system may deliver the appropriate electric shock through the patient&#39;s body to try to defibrillate or cardiovert the patient&#39;s heart. 
     SUMMARY 
     In embodiments, a system includes an external defibrillator configured with at least two different algorithms for determining the duration of a shock administered to a patient being treated. In some embodiments, the defibrillator is configured with at least three different algorithms for determining the duration of the shock. In some embodiments, the system can include a WCD. 
     In some embodiments, a method of delivering a shock is configured to select from at least two different algorithms for determining the duration of the shock. In some embodiments, the method selects from at least three different algorithms for determining the duration of the shock. 
     In some embodiments, the at least two algorithms for determining the duration can be based on the Walcott algorithm and the Constant Energy algorithm (“CE algorithm”). In some embodiments, the at least two duration algorithms include a third duration algorithm such as, for example, an algorithm that sets the duration to preconfigured maximum duration without regard to the patient&#39;s TTI or durations of the waveform&#39;s periods or phases (referred to herein as a “Max Duration algorithm”). In some embodiments, the maximum duration is less than or equal to 25 ms. In some embodiments, other shock duration algorithms can be used in place of or in addition to the Max Duration, Walcott and CE algorithms, including modified versions of the Max Duration, Walcott and CE algorithms. 
     In some embodiments, the shock can be, for example, a defibrillation shock or a cardioversion shock. In some embodiments, the shock can be a multiphasic shock such as, for example, a biphasic truncated exponential shock. In some embodiments, the system or method is configured to select the duration algorithm based on one or more patient parameters such as, for example, the patient&#39;s transthoracic impedance (TTI) or a time for an energy storage module to discharge to a selected or set ratio. In some embodiments, the one or more patient parameter is measured before the shock is administered, and in some other embodiments while the shock is being administered to the patient. 
     In some embodiments, an energy storage device such as, for example, a capacitor is used to provide the energy for the shock that is administered to the patient. In some embodiments, in addition to using two or more algorithms to determine the duration of the shock, the charge voltage of the energy storage device can be varied depending on the patient&#39;s TTI. For example, in some embodiments the system may charge the energy storage device so that the charge voltage is higher or lower than a nominal charge voltage responsive to the patient&#39;s TTI being higher or lower compared to a predetermined average TTI, respectively. 
     The foregoing brief summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, which need not all be present in all embodiments of the inventions disclosed herein, further aspects, embodiments, and features are set forth in the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of components of a sample wearable cardioverter defibrillator (WCD) system, made according to embodiments. 
         FIG.  2    is a diagram showing sample components of an external defibrillator, such as the one illustrated in the system of  FIG.  1   , and which is made according to embodiments. 
         FIG.  3    is a diagram showing sample components of an external defibrillator similar to the external defibrillator of  FIG.  2   , in which some components are shown in more detail, according to embodiments. 
         FIGS.  4 A- 4 B  are diagrams showing energy delivery of an example biphasic truncated exponential (BTE) waveform, according to embodiments. 
         FIG.  5    is a flow diagram showing methods for determining shock duration, according to embodiments. 
         FIG.  6    is a flow diagram showing additional methods for determining shock duration, according to embodiments. 
         FIG.  7    is a flow diagram showing methods for determining charge voltage and shock duration, according to embodiments. 
         FIG.  8    is a diagram showing sample components of an external defibrillator similar to the external defibrillator of  FIGS.  2  and  3   , in which some components are shown in more detail, according to embodiments. 
         FIG.  9    is a flow diagram showing additional methods for determining shock duration and selecting a shock waveform, according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     A wearable cardioverter defibrillator (WCD) system made according to embodiments has several components. These components can be provided separately as modules that can be interconnected, or can be combined with other components, etc. 
       FIG.  1    depicts a patient  82 . Patient  82  may also be referred to as a person and/or wearer, since that patient wears components of the WCD system. 
       FIG.  1    also depicts components of a WCD system made according to embodiments. One such component is a support structure  170  that is wearable by patient  82 . It will be understood that support structure  170  is shown generically in  FIG.  1   , and partly conceptually.  FIG.  1    is provided to illustrate concepts about support structure  170 , and is not to be construed as limiting how support structure  170  is implemented, or how it is worn. 
     Support structure  170  can be implemented in many different ways in different embodiments. For example, in one embodiment support structure  170  is be implemented in a single component or a combination of multiple components. In some embodiments, support structure  170  includes a vest, or a half-vest, or shirt, or other type garment, etc. In such embodiments, such items can be worn similarly to parallel articles of clothing. In some embodiments, support structure  170  include a harness, one or more belts or straps, etc. In such embodiments, such items can be worn by the patient around the torso, hips, over the shoulder, etc. In some embodiments, support structure  170  includes a container or housing, which in some embodiments is waterproof. In such embodiments, the support structure can be worn by being attached to the patient by adhesive material, for example as shown in U.S. Pat. No. 8,024,037. In some embodiments, support structure  170  is implemented as described for the support structure of US Pat. App. No. US 2017/0056682A1, which is incorporated herein by reference. Of course, in such embodiments, in view of this disclosure a person skilled in the art will recognize that additional components of the WCD system can be in the housing of a support structure instead of attached externally to the support structure, for example as described in the document incorporated by reference. There can be other examples. 
     A WCD system according to embodiments is configured to defibrillate a patient who is wearing it, by delivering an electrical charge to the patient&#39;s body in the form of an electric shock delivered in one or more pulses.  FIG.  1    shows a sample external defibrillator  100 , and sample defibrillation electrodes  104 ,  108 , which are coupled to external defibrillator  100  via electrode leads  105 . Defibrillator  100  and defibrillation electrodes  104 ,  108  are coupled to support structure  170 . As such, many of the components of defibrillator  100  can be therefore coupled to support structure  170 . When defibrillation electrodes  104 ,  108  make good electrical contact with the body of patient  82 , defibrillator  100  can administer, via electrodes  104 ,  108 , a brief, strong electric pulse  111  through the body. Pulse  111 , also referred to herein as a shock, a defibrillation shock, a cardioversion shock, therapy or therapy shock, is intended to go through and restart heart  85 , in an effort to save the life of patient  82 . In some embodiments pulse  111  includes one or more pacing pulses, and so on. 
     A prior art defibrillator typically decides whether to defibrillate or not based on an ECG signal of the patient. However, external defibrillator  100  may initiate a shock (or hold-off a shock) based on a variety of inputs, with ECG merely being one of them. 
     Accordingly, in some embodiments of defibrillator  100 , signals such as physiological signals containing physiological data are obtained from patient  82 . While the patient may be a considered also a “user” of the WCD system, in some embodiments, for example, a user of the wearable cardioverter defibrillator (WCD) may be a clinician such as a doctor, nurse, emergency medical technician (EMT) or other similarly situated individual (or group of individuals). The particular context of these and other related terms within this description should be interpreted accordingly. 
     The WCD system may optionally include an outside monitoring device  180 . Device  180  is called an “outside” device because it could be provided as a standalone device, for example not within the housing of defibrillator  100 . Device  180  can be configured to sense or monitor at least one local parameter. A local parameter can be a parameter of patient  82 , or a parameter of the WCD system, or a parameter of the environment, as will be described later in this document. Device  180  may include one or more transducers or sensors that are configured to render one or more physiological inputs from one or more patient parameters that it senses. 
     Optionally, device  180  is physically coupled to support structure  170 . In addition, device  180  can be communicatively coupled with other components, which are coupled to support structure  170 . Such communication can be implemented by a communication module, as will be deemed applicable by a person skilled in the art in view of this description. 
       FIG.  2    is a diagram showing components of an external defibrillator  200 , made according to embodiments. These components can be, for example, included in external defibrillator  100  of  FIG.  1   . The components shown in  FIG.  2    can be provided in a housing  201 , which may also be referred to as casing  201 . 
     External defibrillator  200  is intended for a patient who would be wearing it, such as patient  82  of  FIG.  1   . Defibrillator  200  may further include a user interface  280  for a user  282 . User  282  can be patient  82 , also known as wearer  82 . Or user  282  can be a local rescuer at the scene, such as a bystander who might offer assistance, or a trained person. Or, user  282  might be a remotely located trained caregiver in communication with the WCD system. 
     User interface  280  can be made in many ways. User interface  280  may include output devices, which can be visual, audible or tactile, for communicating to a user by outputting images, sounds or vibrations. Images, sounds, vibrations, and anything that can be perceived by user  282  can also be called human perceptible indications. There are many examples of output devices. For example, an output device can be a light, or a screen to display what is sensed, detected and/or measured, and provide visual feedback to rescuer  282  for their resuscitation attempts, and so on. Another output device can be a speaker, which can be configured to issue voice prompts, beeps, loud alarm sounds to warn bystanders, etc. 
     User interface  280  may further include input devices for receiving inputs from users. Such input devices may additionally include various controls, such as pushbuttons, keyboards, touchscreens, one or more microphones, and so on. An input device can be a cancel switch, which is sometimes called an “I am alive” switch or “live man” switch. In some embodiments, actuating the cancel switch can prevent the impending delivery of a shock. 
     Defibrillator  200  may include an internal monitoring device  281 . Device  281  is called an “internal” device because it is incorporated within housing  201 . Monitoring device  281  can sense or monitor patient parameters such as patient physiological parameters, system parameters and/or environmental parameters, all of which can be called patient data. In other words, internal monitoring device  281  can be complementary or an alternative to outside monitoring device  180  of  FIG.  1   . Allocating which of the system parameters are to be monitored by which monitoring device can be done according to design considerations. Device  281  may include one or more transducers or sensors (not shown) that are configured to render one or more physiological inputs from one or more patient parameters that it senses. 
     Patient parameters may include patient physiological parameters. Patient physiological parameters may include, for example and without limitation, those physiological parameters that can be of any help in detecting by the wearable defibrillation system whether the patient is in need of a shock, plus optionally their medical history and/or event history. Examples of such parameters include the patient&#39;s ECG, blood oxygen level, blood flow, blood pressure, blood perfusion, pulsatile change in light transmission or reflection properties of perfused tissue, heart sounds, heart wall motion, breathing sounds and pulse. Accordingly, monitoring devices  180 ,  281  may include one or more sensors configured to acquire patient physiological signals. Examples of such sensors include electrodes to detect ECG data, a perfusion sensor, a pulse oximeter, a Doppler device for detecting blood flow, a cuff for detecting blood pressure, an optical sensor, illumination detectors and perhaps sources for detecting color change in tissue, a motion sensor, a device that can detect heart wall movement, a sound sensor, a device with a microphone, an SpO 2  sensor, and so on. It will be appreciated that such sensors can help detect the patient&#39;s pulse, and can therefore also be called pulse detection sensors, pulse sensors, and pulse rate sensors. Pulse detection is also taught at least in Physio-Control&#39;s U.S. Pat. No. 8,135,462, which is hereby incorporated by reference in its entirety. In addition, a person skilled in the art may implement other ways of performing pulse detection. In such cases, the transducer includes an appropriate sensor, and the physiological input is a measurement by the sensor of that patient parameter. For example, the appropriate sensor for a heart sound may include a microphone, etc. 
     In some embodiments, the local parameter is a trend that can be detected in a monitored physiological parameter of patient  282 . A trend can be detected by comparing values of parameters at different times. Parameters whose detected trends can particularly help a cardiac rehabilitation program include: a) cardiac function (e.g. ejection fraction, stroke volume, cardiac output, etc.); b) heart rate variability at rest or during exercise; c) heart rate profile during exercise and measurement of activity vigor, such as from the profile of an accelerometer signal and informed from adaptive rate pacemaker technology; d) heart rate trending; e) perfusion, such as from SpO 2  or CO 2 ; f) respiratory function, respiratory rate, etc.; g) motion, level of activity; and so on. Once a trend is detected, it can be stored and/or reported via a communication link, along perhaps with a warning. From the report, a physician monitoring the progress of patient  282  will know about a condition that is either not improving or deteriorating. 
     Patient state parameters include recorded aspects of patient  282 , such as motion, posture, whether they have spoken recently plus maybe also what they said, and so on, plus optionally the history of these parameters. Or, one of these monitoring devices could include a location sensor such as a Global Positioning System (GPS) location sensor. Such a sensor can detect the location, plus a speed can be detected as a rate of change of location over time. Many motion detectors output a motion signal that is indicative of the motion of the detector, and thus of the patient&#39;s body. Patient state parameters can be very helpful in narrowing down the determination of whether SCA is indeed taking place. 
     A WCD system made according to embodiments may include a motion detector. In embodiments, a motion detector can be implemented within monitoring device  180  or monitoring device  281 . Such a motion detector can be configured to detect a motion event. In response, the motion detector may render or generate from the detected motion event a motion detection input that can be received by a subsequent device or functionality. A motion event can be defined as is convenient, for example a change in motion from a baseline motion or rest, etc. Such a motion detector can be made in many ways as is known in the art, for example by using an accelerometer. In such cases, the patient parameter is a motion, one of the transducers may include a motion detector, and the physiological input is a motion measurement. 
     System parameters of a WCD system can include system identification, battery status, system date and time, reports of self-testing, records of data entered, records of episodes and intervention, and so on. 
     Environmental parameters can include ambient temperature and pressure. Moreover, a humidity sensor may provide information as to whether it is likely raining. Presumed patient location could also be considered an environmental parameter. The patient location could be presumed if monitoring device  180  or  281  includes a GPS location sensor as per the above. 
     Defibrillator  200  typically includes a defibrillation port  210 , such as a socket in housing  201 . Defibrillation port  210  includes electrical nodes  214 ,  218 . Leads of defibrillation electrodes  204 ,  208 , such as leads  105  of  FIG.  1   , can be plugged into defibrillation port  210 , so as to make electrical contact with nodes  214 ,  218 , respectively. It is also possible that defibrillation electrodes  204 ,  208  are connected continuously to defibrillation port  210 , instead. Either way, defibrillation port  210  can be used for guiding, via electrodes, to the wearer the electrical charge that has been stored in an energy storage module  250  that is described more fully later in this document. The electric charge will be the shock for defibrillation, pacing, and so on. 
     Defibrillator  200  may optionally also have an ECG port  219  in housing  201 , for plugging in sensing electrodes  209 , which are also known as ECG electrodes and ECG leads. It is also possible that sensing electrodes  209  can be connected continuously to ECG port  219 , instead. Sensing electrodes  209  are types of transducers that can help sense an ECG signal, e.g. a 12-lead signal, or a signal from a different number of leads, especially if they make good electrical contact with the body of the patient. Sensing electrodes  209  can be attached to the inside of support structure  170  for making good electrical contact with the patient, similarly as defibrillation electrodes  204 ,  208 . 
     Optionally a WCD system according to embodiments also includes a fluid that it can deploy automatically between the electrodes and the patient skin. The fluid can be conductive, such as by including an electrolyte, for making a better electrical contact between the electrode and the skin. Electrically speaking, when the fluid is deployed, the electrical impedance between the electrode and the skin is reduced. Mechanically speaking, the fluid may be in the form of a low-viscosity gel, so that it does not flow away, after it has been deployed. The fluid can be used for both defibrillation electrodes  204 ,  208 , and sensing electrodes  209 . 
     The fluid may be initially stored in a fluid reservoir, not shown in  FIG.  2   , which can be coupled to the support structure. In addition, a WCD system according to embodiments further includes a fluid deploying mechanism  274 . Fluid deploying mechanism  274  can be configured to cause at least some of the fluid to be released from the reservoir, and be deployed near one or both of the patient locations, to which the electrodes are configured to be attached to the patient. In some embodiments, fluid deploying mechanism  274  is activated prior to the electrical discharge responsive to receiving activation signal AS from processor  230  that is described more fully later in this document. 
     In some embodiments, defibrillator  200  also includes a transducer that includes a measurement circuit  220 . Measurement circuit  220  senses one or more electrical physiological signal of the patient from ECG port  219 , if provided. Even if defibrillator  200  lacks ECG port  219 , measurement circuit  220  can obtain physiological signals through nodes  214 ,  218  instead, when defibrillation electrodes  204 ,  208  are attached to the patient. In these cases, the physiological input reflects an ECG measurement. The parameter can be an ECG, which can be sensed as a voltage difference between electrodes  204 ,  208 . In addition, the parameter can be an impedance, which can be sensed between electrodes  204 ,  208  and/or the connections of ECG port  219 . Sensing the impedance can be useful for detecting, among other things, whether these electrodes  204 ,  208  and/or sensing electrodes  209  are not making good electrical contact with the patient&#39;s body. These patient physiological signals can be sensed, when available. Measurement circuit  220  can then render or generate information about them as physiological inputs, data, other signals, etc. More strictly speaking, the information rendered by measurement circuit  220  is output from it, but this information can be called an input because it is received by a subsequent device or functionality as an input. 
     Defibrillator  200  also includes a processor  230 . Processor  230  may be implemented in many ways. Such ways include, by way of example and not of limitation, digital and/or analog processors such as microprocessors and Digital Signal Processors (DSPs); controllers such as microcontrollers; software running in a machine; programmable circuits such as Field Programmable Gate Arrays (FPGAs), Field-Programmable Analog Arrays (FPAAs), Programmable Logic Devices (PLDs), Application Specific Integrated Circuits (ASICs), any combination of one or more of these, and so on. 
     The processor  230  may include, or have access to, a non-transitory storage medium, such as memory  238  that is described more fully later in this document. Such a memory can have a non-volatile component for storage of machine-readable and machine-executable instructions. A set of such instructions can also be called a program. The instructions, which may also be referred to as “software,” generally provide for functionality by performing methods as may be disclosed herein or understood by one skilled in the art in view of the disclosed embodiments. In some embodiments, and as a matter of convention used herein, instances of the software may be referred to as a “module” and by other similar terms. Generally, a module includes a set of the instructions so as to offer or fulfill a particular functionality. Embodiments of modules and the functionality delivered are not limited by the embodiments described in this document. 
     Processor  230  can be considered to have a number of modules. One such module can be a detection module  232 . Detection module  232  can include a Ventricular Fibrillation (VF) detector. The patient&#39;s sensed ECG from measurement circuit  220 , which can be available as physiological inputs, data, or other signals, may be used by the VF detector to determine whether the patient is experiencing VF. Detecting VF is useful, because VF results in SCA. Detection module  232  can also include a Ventricular Tachycardia (VT) detector, and so on. 
     Another such module in processor  230  can be an advice module  234 , which generates advice for what to do. The advice can be based on outputs of detection module  232 . There can be many types of advice according to embodiments. In some embodiments, the advice is a shock/no shock determination that processor  230  can make, for example via advice module  234 . The shock/no shock determination can be made by executing a stored Shock Advisory Algorithm. A Shock Advisory Algorithm can make a shock/no shock determination from one or more of ECG signals that are captured according to embodiments, and determining whether a shock criterion is met. The determination can be made from a rhythm analysis of the captured ECG signal or otherwise. 
     In some embodiments, when the determination is to shock, an electrical charge is delivered to the patient. Delivering the electrical charge is also known as discharging. Shocking can be for defibrillation, pacing, and so on. 
     Processor  230  can include additional modules, such as other module  236 , for other functions. In addition, if internal monitoring device  281  is indeed provided, it may be operated in part by processor  230 , etc. 
     Defibrillator  200  optionally further includes a memory  238 , which can work together with processor  230 . Memory  238  may be implemented in many ways. Such ways include, by way of example and not of limitation, volatile memories, Nonvolatile Memories (NVM), Read-Only Memories (ROM), Random Access Memories (RAM), magnetic disk storage media, optical storage media, smart cards, flash memory devices, any combination of these, and so on. Memory  238  is thus a non-transitory storage medium. Memory  238 , if provided, can include programs for processor  230 , which processor  230  may be able to read and execute. More particularly, the programs can include sets of instructions in the form of code, which processor  230  may be able to execute upon reading. Executing is performed by physical manipulations of physical quantities, and may result in functions, operations, processes, actions and/or methods to be performed, and/or the processor to cause other devices or components or blocks to perform such functions, operations, processes, actions and/or methods. The programs can be operational for the inherent needs of processor  230 , and can also include protocols and ways that decisions can be made by advice module  234 . In addition, memory  238  can store prompts for user  282 , if this user is a local rescuer. Moreover, memory  238  can store data. This data can include patient data, system data and environmental data, for example as learned by internal monitoring device  281  and outside monitoring device  180 . The data can be stored in memory  238  before it is transmitted out of defibrillator  200 , or stored there after it is received by defibrillator  200 . 
     Defibrillator  200  may also include a power source  240 . To enable portability of defibrillator  200 , power source  240  typically includes a battery. Such a battery is typically implemented as a battery pack, which can be rechargeable or not. Sometimes a combination is used of rechargeable and non-rechargeable battery packs. Other embodiments of power source  240  can include an AC power override, for where AC power will be available, an energy storage capacitor, and so on. In some embodiments, power source  240  is controlled by processor  230 . Appropriate components may be included to provide for charging or replacing power source  240 . 
     Defibrillator  200  may additionally include an energy storage module  250 . Energy storage module  250  can be coupled to the support structure of the WCD system, for example either directly or via the electrodes and their leads. Module  250  is where some electrical energy can be stored temporarily in the form of an electrical charge, when preparing it for discharge to administer a shock. In embodiments, module  250  can be charged from power source  240  to the right amount of energy, as controlled by processor  230 . In typical implementations, module  250  includes a capacitor  252 , which can be a single capacitor or a system of capacitors, and so on. In some embodiments, energy storage module  250  includes a device that exhibits high power density, such as an ultracapacitor. As described above, capacitor  252  can store the energy in the form of an electrical charge, for delivering to the patient. 
     Defibrillator  200  moreover includes a discharge circuit  255 . When the decision is to shock, processor  230  can be configured to control discharge circuit  255  to discharge through the patient the electrical charge stored in energy storage module  250 . When so controlled, circuit  255  can permit energy stored in module  250  to be discharged to nodes  214 ,  218 , and from there also to defibrillation electrodes  204 ,  208 , so as to cause a shock to be delivered to the patient. Circuit  255  can include one or more switches  257 . Switches  257  can be made in a number of ways, such as by an H-bridge, and so on. In some embodiments, discharge circuit  255  is configurable to output multiphasic shocks, such as for example, biphasic shocks. In some embodiments, energy storage module has multiple distinct energy submodules (e.g., capacitors) and discharge circuit  255  is configurable to discharge a different energy storage submodule for each phase of a multiphasic shock. In some embodiments, circuit  255  can also be controlled via user interface  280 . 
     Defibrillator  200  can optionally include a communication module  290 , for establishing one or more wired or wireless communication links with other devices of other entities, such as a remote assistance center, Emergency Medical Services (EMS), and so on. Module  290  may also include such sub-components as may be deemed necessary by a person skilled in the art, for example an antenna, portions of a processor, supporting electronics, outlet for a telephone or a network cable, etc. This way, data, commands, etc. can be communicated. The data can include patient data, event information, therapy attempted, CPR performance, system data, environmental data, and so on. Defibrillator  200  in some embodiments can optionally include other components. 
     Returning to  FIG.  1   , in embodiments, one or more of the components of the shown WCD system have been customized for patient  82 . This customization may include a number of aspects. For instance, support structure  170  can be fitted to the body of patient  82 . For another instance, baseline physiological parameters of patient  82  can be measured, such as the heart rate of patient  82  while resting, while walking, motion detector outputs while walking, etc. Such baseline physiological parameters can be used to customize the WCD system, in order to make its diagnoses more accurate, since the patients&#39; bodies differ from one another. Of course, such parameters can be stored in a memory of the WCD system, and so on. 
     A programming interface can be made according to embodiments, which receives such measured baseline physiological parameters. Such a programming interface may input automatically in the WCD system the baseline physiological parameters, along with other data. 
       FIG.  3    is a diagram showing sample components of an external defibrillator  301  similar to the external defibrillator  200  of  FIG.  2   , in which some components are shown in more detail, according to embodiments. External defibrillator  301  can be implemented in a WCD in some embodiments to deliver appropriate therapy (e.g., defibrillation shocks, cardioversion shocks, pacing, etc.) to a patient with an arrhythmia. 
     In this embodiment, external defibrillator  301  includes a measurement circuit  320  and memory  338 , both coupled to a processor  330 . External defibrillator  301  also includes other modules and components as shown in  FIG.  2   , but are omitted in  FIG.  3    as they have already been described in detail in conjunction with  FIG.  2   . 
     Memory  338 , in some embodiments, is similar to memory  238  ( FIG.  2   ) and in addition is configured with at least two algorithms for determining the duration of a shock as a function of TTI. In  FIG.  3   , memory  338  is configured with three duration algorithms  341 - 343 . In some embodiments, external defibrillator  301  is also configured so that no single shock can exceed a set duration. While some practitioners may not consider a fixed duration to be an “algorithm” in a technical sense, for purposes of this disclosure a maximum duration being used instead of a larger duration determined by another algorithm is a referred to herein as a Max Duration Waveform algorithm. In one embodiment, duration algorithms  341 - 343  are the Walcott Algorithm, the CE Algorithm, and the Max Duration algorithm, respectively. The Walcott Algorithm, CE Algorithm and the Max Duration Algorithms are described in more detail below. Further, in some embodiments, memory  338  is configured with at least two algorithms (not shown) for shaping the waveform of a shock to be delivered to a patient with the duration determined using duration algorithms such as described above. For example, the waveform algorithm can be a biphasic truncated exponential (BTE) waveform with equal duration phases, a biphasic truncated exponential waveform with the duration of the second phase being ⅔ of the first phase, rectilinear biphasic waveforms, biphasic square waveforms, pulsed waveforms, or waveforms with three or more phases. In some embodiments, the duration algorithm also defines the waveform such as, for example, the Walcott algorithm which defines the duration of each phase of a BTE waveform. Of course, other duration algorithms may also define the duration of each phase. In some embodiments, the external defibrillator is also configured with a minimum duration for the shock such as, for example, 4 ms. 
     In some embodiments, other algorithms can be used. For example, in some embodiments a modified version of the Walcott Algorithm can be used in which the shock duration is calculated in the same way as the original Walcott Algorithm but the periods of the waveform are modified (e.g., to be equal). Other algorithms include for example, monophasic truncated exponential algorithms, or algorithms based on a fixed duration or step-wise control of the duration for given range of TTI. 
     In addition, memory  338  can be configured with one or more voltage compensation algorithms (not shown) for determining a charge voltage for a capacitor energy storage module used in external defibrillator  301  to provide the energy for the shock. In some embodiments, the charge voltage of a capacitor used in external defibrillator  301  has a maximum voltage rating and known capacitance, which is accounted for in the voltage compensation algorithm. As will be described further below, in embodiments with the voltage compensation algorithm, external defibrillator  301  can deliver shocks with the desired energy to patients with higher TTIs than is typically delivered by external defibrillators with no voltage compensation. 
     In this embodiment, measurement circuit  320  is similar to measurement circuit  220  ( FIG.  2   ), and more specifically shows an impedance measurement circuit  305  coupled to sensors coupled to the patient which are used in this embodiment to measure a patient&#39;s TTI. In some embodiments, impedance measurement circuit  305  provides a low voltage constant current AC signal (e.g., a series of pulses) to the patient via the sensors, such as described in U.S. Pat. No. 5,999,852 entitled “Defibrillator Method and Apparatus”, which is incorporated herein by reference in its entirety. Impedance measurement circuit  305  can determine the TTI itself, or in other embodiments a TTI measurement module  336 A of processor  330  determines the TTI. 
     Processor  330 , in this embodiment, also includes a duration algorithm selection module  336 B and a shock duration module  336 C. Duration algorithm selection module  336 B is configured to select between the duration algorithms configured in memory  338  based on the patient&#39;s measured TTI. In some embodiments, duration algorithm selection module  336 B is configured to select between the duration algorithms  341 - 343  as a function of the measured patient TTI. For example, in some embodiments, one or more TTI thresholds are preset by an administrator, while in other embodiments the TTI thresholds are dynamically determined by processor  330  as a function of the measured patient TTI or the charge voltage in embodiments with voltage compensation and/or selectable shock energies. Duration algorithm selection module  336 B then compares the measured TTI to the one or more thresholds. In some embodiments, if the measured TTI is below a first threshold, module  336 B is configured to select Algorithm 1, and if the measured TTI is above the first threshold, module  336 B is configured to select Algorithm 2. In embodiments with a third duration algorithm, a second threshold (higher than the first threshold) is determined or preset. If the measured TTI is above the second threshold, then module  336 B is configured in this embodiment to select Algorithm 3. 
     In some other embodiments, processor  330  is configured to select the duration algorithm dynamically by determining the energies delivered by each of the algorithms and selecting the algorithm with the highest delivered energy (or the energy closest to the selected energy in some other embodiments). 
     Using the algorithm selected by module  336 B, shock duration module  336 C then determines the duration of the shock to be delivered to the patient, as a function of the measured TTI. In some embodiments, the shock is delivered in the form of a biphasic truncated exponential (BTE) waveform. In other embodiments, in addition to previously mentioned waveforms, other waveforms are used such as, for example, rectilinear biphasic waveforms, biphasic square waveforms, pulsed waveforms, or waveforms with three or more phases. 
     In some embodiments, in addition to controlling the duration of the shock, the charge voltage of the energy storage module (e.g., energy storage module  252  in  FIG.  2   ) is controlled by processor  330  (or by processor  230   FIG.  2   ) as a function of patient TTI. This is sometimes referred to as voltage compensation. For example, in patients with relatively high TTI, for a given charge voltage, the energy delivered to the patient may not reach the desired level within the maximum duration. In these embodiments, the processor causes the energy storage module to be charged to a higher charge voltage for patients with relatively high TTI. This in effect causes the values of the first and second thresholds to increase (or move to the right in  FIG.  4 A , described below). As a result, more patients are likely to receive shocks with at least the CE Algorithm energy level. That is, this voltage compensation provides constant energy for patients with greater TTI than is achievable by duration compensation along. In some embodiments, the voltage compensation is performed in a step-wise manner based on the measured patient TTI. In other embodiments, the voltage compensation is performed in a continuous manner as a function of the measure patient TTI. 
       FIG.  4 A  shows a chart illustrating an example of the thresholds and duration algorithms, according to embodiments.  FIG.  4 A  shows energy (J) as a function of patient TTI (ohms). Note, the values for energy and TTI are illustrative for an example external defibrillator, and could be different for other defibrillators. In this example, for relatively low TTI (i.e., below Threshold  1 ), the Walcott Algorithm is selected (e.g., by duration selection module  336 B in  FIG.  3   ) to determine the shock duration. The durations of the Walcott algorithm result in shocks having energies that is inversely proportional to TTI. As can be seen in  FIG.  4 A , this results in a relatively high energy shock for low TTI patients ranging from about 150 J to about 135 J. 
     For “moderate” TTI patients (e.g., patients having a TTI between Threshold  1  and Threshold  2 ), the CE Algorithm is selected in this embodiment. As the name of the algorithm implies, such patients all receive the same energy, which in this example is about 135 J. This energy can be preset (for example, in a WCD or simple AEDs), or selectable by a user or rescuer (for example, via a user interface in external defibrillators designed for trained operators). 
     For “high” TTI patients (e.g., patients with TTI greater than Threshold  2 ), the Max Duration algorithm is selected in this embodiment. In some embodiments, the maximum duration of the shock according to the Max Duration Algorithm is preset in the external defibrillator by an administrator, physician, or in the factory when the external defibrillator is manufactured. In some embodiments, the maximum duration of the shock is less than 25 ms. As shown in  FIG.  4 A , using the Max Duration Algorithm, the energy delivered to the patient drops off as a function of patient TTI, reflecting that the delivered energy for a fixed duration decreases as TTI increases. 
     In addition to controlling total waveform duration, in embodiments using multiphasic shock waveforms, processor  330  also controls the relative proportion of the entire duration dedicated to the phases.  FIG.  4 B  shows the phases of an example BTE waveform in terms of current (I) vs time (T). The duration of the first phase of the waveform is T 1 , and with an initial current of I 1  that decays exponentially based on the patient&#39;s TTI and other resistances in the system such as from components in the external defibrillator and between the electrodes and the patient&#39;s skin. The second phase of the BTE waveform has a duration of T 2 , and at the end of which the truncated waveform has a current of I 4 . 
     In some embodiments, processor  330  is configured to control a single phase of the BTE waveform (relative duration) to improve defibrillation efficacy. For example, as shown in  FIG.  4 B , the BTE waveform has a total duration of d=T 1 +T 2  where the relative duration of T 1  is d 1rel =T 1 /d, and the relative duration of T 2  is d 2rel =T 2 /d. In some embodiments, d 1rel  is a value between 0.3 and 0.7 and is fixed regardless of TTI. In other embodiments, the relative duration of each phase varies as TTI varies. In yet other embodiments d 1rel  is fixed in a given range or ranges of TTI while it varies in another range or ranges of TTI. These relative durations can be selected by an administrator, physician, etc., or preset when the external defibrillator is manufactured. These relative durations can be used to define waveform algorithms as previously described. 
     Walcott Algorithm 
     In the Walcott Algorithm, the total duration of the shock, d, is described by the equations below. 
     
       
         
           
             
               
                 
                   d 
                   = 
                   
                     
                       T 
                       1 
                     
                     + 
                     
                       
                         0.67 
                         · 
                         
                           T 
                           1 
                         
                       
                       ⁢ 
                       
                         ∀ 
                         
                           
                             R 
                             pt 
                           
                           &lt; 
                           x 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
       
         
           
             
               
                 
                   
                     where 
                     ⁢ 
                         
                     
                       T 
                       1 
                     
                   
                   = 
                   
                     
                       
                         - 
                         γ 
                       
                       · 
                       ln 
                     
                     ⁢ 
                        
                     
                       ( 
                       
                         
                           τ 
                           m 
                         
                         
                           τ 
                           s 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     and τ s  and γ are described by equations 3 and 4. 
                     τ   s     =       C   d     ·     (       R   pt     +     R   d       )               (   3   )                           γ   =         τ   s     ·     τ   m           τ   s     -     τ   m                 (   4   )               
C d  is the device capacitance, R d  is the device impedance, R pt  is the patient TTI, τ m  is the Walcott Model time constant and is between 4 and 8 ms, and x is a first threshold. In one embodiment, the first threshold is determined by the intercept of the energy delivery curves for the Walcott and CE Algorithms and the second threshold is determined by the TTI at which the device parameters (e.g., the capacitance of the energy storage module, the charge voltage, device impedances) can no longer deliver a shock with the energy set by the CE Algorithm In some embodiments, the Walcott Algorithm is modified so that the total duration d is the same (as in the original Walcott Algorithm), but the durations of the phases are slightly different. In one embodiment, the modified Walcott Algorithm has the phases being the same, as opposed to the second phase being 0.67 of the first phase as in the original Walcott Algorithm (see equation 1 above). In some embodiments, the modified Walcott Algorithm includes a minimum duration for the shock such as, for example, 4 ms.
 
     Constant Energy (CE) Algorithm 
     Given the device configuration (capacitance, resistance, charge voltage), desired defibrillation energy, and measured patient impedance the following equation is used to calculate the total duration of the defibrillation waveform. 
                       T   1     +     T   2       =           -     τ   s       2     ·     ln   ⁡   (     1   -         E   pt     ·     (       R   pt     +     R   d       )         0.5   ·     R   pt     ·     V   chg   2     ·     C   d           )       ⁢     ∀       R   pt     &gt;   x                 (   5   )               
where T 1 +T 2  is the total duration of the waveform and should not exceed approximately 25 ms or be less than 4 ms, τ s  is the time constant for the circuit consisting of the device and patient, R pt  is the patient&#39;s measured TTI, R d  is the device impedance, E pt  is the target energy to be delivered to the patient, V chg  is the charge voltage on the device capacitor, and C d  is the device capacitance. In some embodiments, the CE algorithm is used only when R pt  is greater than x, where x is the first threshold.
 
     In accordance with this disclosure, external defibrillators can advantageously deliver effective shocks with greater energy to a wider range of patients than with any one waveform compensation technique alone, thereby maximizing the defibrillation efficacy. This disclosure can be especially advantageous in a defibrillator of limited energy storage capacity, such as a WCD. 
     The various embodiments of the devices and/or systems disclosed in this document perform functions, processes and/or methods as described above. These functions, processes and/or methods may be implemented by one or more devices that include logic circuitry. Such a device can be alternately called a computer, and so on. It may be a standalone device or computer, such as a general-purpose computer, or part of a device that has one or more additional functions. In some embodiments, the computer is a specialized computer adapted to and optimally configured for a specific purpose such as for example, providing therapy shocks in emergency situations. The logic circuitry may include a processor and non-transitory computer-readable storage media, such as memories, of the type described in this document. Often, for the sake of convenience, it is preferred to implement and describe a program as various interconnected distinct software modules or features. These, along with data are individually and also collectively referred to herein as software. In some instances, software is combined with hardware, in a mix called firmware. 
     Various embodiments of methods and algorithms are described below. These methods and algorithms are not necessarily inherently associated with any particular logic device or other apparatus. Rather, they can be advantageously implemented by programs for use by a computing machine, such as a general-purpose computer, a special purpose computer, a microprocessor, a microcontroller, a processor and/or a combination of these devices such as described elsewhere in this document, and so on. 
     This detailed description includes flowcharts, display images, algorithms, and symbolic representations of program operations within at least one computer readable medium. An economy is achieved in that a single set of flowcharts is used to describe both programs, and also methods. So, while flowcharts describe methods in terms of boxes, they also concurrently describe programs. 
       FIG.  5    is a flow diagram illustrating a method  500  for determining shock duration, according to embodiments. In an operation  505 , a first threshold is stored. In some embodiments, the first threshold is stored in a memory of an external defibrillator such as for example, memory  238  ( FIG.  2   ) or memory  338  ( FIG.  3   ). 
     In an operation  507  and an operation  509 , first and second algorithms for determining a duration of a shock are stored, respectively. In some embodiments, the algorithms are stored in a memory of an external defibrillator such as for example, memory  238  ( FIG.  2   ) or memory  338  ( FIG.  3   ). In some embodiments, the first algorithm is the Walcott Algorithm. In other embodiments, the first algorithm is a modified Walcott Algorithm. In some embodiments, the second algorithm is the CE Algorithm. In yet other embodiments, other duration algorithms or other modifications of the Walcott or CE Algorithms can be used for the first and second algorithms such as, for example, the previously mentioned constant duration, step-wise duration, and monophasic truncated exponential algorithms. 
     In an operation  511 , the patient&#39;s TTI is measured. In some embodiments, the patient&#39;s TTI is measured via external sensors such as ECG or therapy electrodes (e.g., electrodes  204 ,  208 ,  209  in  FIG.  2   ) by a measurement circuit of an external defibrillator such as, for example, measurement circuit  220  ( FIG.  2   ) or impedance measurement circuit  305  in conjunction with TTI measurement module  336 A ( FIG.  3   ). 
     The duration of the shock is then determined based on the measured patient TTI. In some embodiments, the duration is determined by a processor of an external defibrillator such as, for example, processor  230  ( FIG.  2   ) or processor  330  ( FIG.  3   ). Embodiments of this process are described in more detail below. 
     In embodiments, in an operation  513 , the measured TTI is analyzed or compared to determine if it is greater than the first threshold. In some embodiments, this determination is performed by a processor such as, for example, duration selection module  336 B ( FIG.  3   ) of processor  330 . 
     If the measured TTI is not greater than (or not greater than or equal to in some embodiments) the first threshold, in an operation  515  the shock duration is determined using the first algorithm. In some embodiments, the first algorithm (per operation  507 ) is performed by a shock duration module such as, for example, shock duration module  336 C of processor  330  ( FIG.  3   ). 
     If the measured TTI is greater than (or greater than or equal to in some embodiments) the first threshold, in an operation  517  the shock duration is determined using the second algorithm. In some embodiments, the second algorithm (per operation  509 ) is performed by a shock duration module such as, for example, shock duration module  336 C of processor  330  ( FIG.  3   ). 
     In an operation  521 , the external defibrillator is caused to output a shock with the duration determined in operation  515  or  517 , depending on the measured patient TTI. In some embodiments, this operation is performed by a processor of an external defibrillator such as, for example, processor  230  ( FIG.  2   ) or processor  330  ( FIG.  3   ), controlling other components of the external defibrillator (e.g., as described above in conjunction with  FIGS.  2  and  3   ). 
       FIG.  6    is a flow diagram illustrating a method  600  for determining shock duration, according to embodiments. In an operation  601 , a first threshold and a second threshold are stored. In some embodiments, the first and second thresholds are stored in a memory of an external defibrillator such as for example, memory  238  ( FIG.  2   ) or memory  338  ( FIG.  3   ). 
     In an operation  602 , first and second algorithms for determining a duration of a shock are stored. In some embodiments, the algorithms are stored in a memory of an external defibrillator such as for example, memory  238  ( FIG.  2   ) or memory  338  ( FIG.  3   ). In some embodiments, the first algorithm is the Walcott Algorithm. In other embodiments, the first algorithm is a modified Walcott Algorithm. In some embodiments, the second algorithm is the CE Algorithm. In yet other embodiments, other duration algorithms or other modifications of the Walcott or CE Algorithms can be used for the first and second algorithms such as, for example, the previously mentioned constant duration, step-wise duration, and monophasic truncated exponential algorithms. 
     In an operation  603 , a third algorithm for determining a duration of a shock is stored. In some embodiments, the algorithm is stored in the same memory as described above in conjunction with operation  602 . In some embodiments, the third algorithm is the previously described Max Duration Algorithm. In other embodiments, other duration algorithms or modifications of the Walcott or CE or Max Duration Algorithms are used for the third algorithm. 
     In an operation  611 , the patient&#39;s TTI is measured. In some embodiments, the patient&#39;s TTI is measured via external sensors such as ECG or therapy electrodes (e.g., electrodes  204 ,  208 ,  209  in  FIG.  2   ) by a measurement circuit of an external defibrillator such as, for example, measurement circuit  220  ( FIG.  2   ) or impedance measurement circuit  305  in conjunction with TTI measurement module  336 A ( FIG.  3   ). 
     The duration of the shock is then determined based on the measured patient TTI, in embodiments. In some embodiments, the duration is determined by a processor of an external defibrillator such as, for example, processor  230  ( FIG.  2   ) or processor  330  ( FIG.  3   ). Embodiments of this process are described in more detail below. 
     In embodiments, in an operation  613 , the measured TTI is analyzed or compared to determine if it is greater than the first threshold. In some embodiments, this determination is performed by a processor such as, for example, duration selection module  336 B ( FIG.  3   ) of processor  330 . 
     If the measured TTI is not greater than (or not greater than or equal to in some embodiments) the first threshold, in an operation  615  the shock duration is determined using the first algorithm. In some embodiments, the first algorithm (per operation  602 ) is performed by a shock duration module such as, for example, shock duration module  336 C of processor  330  ( FIG.  3   ). 
     If the measured TTI is greater than (or greater than or equal to in some embodiments) the first threshold, in an operation  616  the measured TTI is analyzed or compared to determine if it is greater than the second threshold. In some embodiments, this determination is performed by the processor as described above in conjunction with operation  613 . 
     If the measured TTI is not greater than (or not greater than or equal to in some embodiments) the second threshold, in an operation  617  the shock duration is determined using the second algorithm. In some embodiments, the second algorithm (per operation  602 ) is performed by a shock duration module such as, for example, shock duration module  336 C of processor  330  ( FIG.  3   ). 
     If the measured TTI is greater than (or greater than or equal to in some embodiments) the second threshold, in an operation  619  the shock duration is determined using the third algorithm. In some embodiments, the third algorithm (per operation  603 ) is performed by a shock duration module such as, for example, shock duration module  336 C of processor  330  ( FIG.  3   ). 
     In an operation  621 , the external defibrillator is caused to output a shock with the duration determined in operation  615  or  617  or  619 , depending on the measured patient TTI. In some embodiments, this operation is performed by a processor of an external defibrillator such as, for example, processor  230  ( FIG.  2   ) or processor  330  ( FIG.  3   ), controlling other components of the external defibrillator (e.g., as described above in conjunction with  FIGS.  2  and  3   ). 
       FIG.  7    is a flow diagram illustrating a method  700  for determining charge voltage and shock duration, according to embodiments. In an operation  703 , first, second and third algorithms for determining a duration of a shock are stored. In some embodiments, the algorithms are stored in a memory of an external defibrillator such as for example, memory  238  ( FIG.  2   ) or memory  338  ( FIG.  3   ). In some embodiments, the first algorithm is the Walcott Algorithm. In other embodiments, the first algorithm is a modified Walcott Algorithm. In some embodiments, the second algorithm is the CE Algorithm. In some embodiments, the third algorithm is the Max Duration Algorithm In yet other embodiments, other duration algorithms or other modifications of the Walcott or CE or Max Duration Algorithms can be used for the first, second and third algorithms such as, for example, the previously mentioned constant duration, step-wise duration, and monophasic truncated exponential algorithms. 
     In an operation  705 , the patient&#39;s TTI is measured. In some embodiments, the patient&#39;s TTI is measured via external sensors such as ECG or therapy electrodes (e.g., electrodes  204 ,  208 ,  209  in  FIG.  2   ) by a measurement circuit of an external defibrillator such as, for example, measurement circuit  220  ( FIG.  2   ) or impedance measurement circuit  305  in conjunction with TTI measurement module  336 A ( FIG.  3   ). 
     In an operation  707 , a charge voltage is determined based on the measured patient TTI. In some embodiments, the charge voltage is determined by a processor of an external defibrillator such as, for example, processor  230  ( FIG.  2   ) or processor  330  ( FIG.  3   ) using a voltage compensation algorithm. In some embodiments, the voltage compensation algorithm increases the charge voltage of an energy storage module such as, for example, energy storage module  250  ( FIG.  2   ), and the processor then causes an energy storage module of the external defibrillator to be charged to the determined voltage. In some embodiments, the voltage compensation algorithm increases the charge voltage as a step-wise function of measured patient TTI, and in other embodiments as a continuous function of measured patient TTI. 
     In an operation  709 , a first threshold and a second threshold are determined. In some embodiments, the first and second thresholds are determined by a processor such as described above in conjunction with operation  707  as a function of the charge voltage and/or measured patient TTI. In some embodiments, the determined first and second threshold are then stored in a memory of an external defibrillator such as for example, memory  238  ( FIG.  2   ) or memory  338  ( FIG.  3   ). 
     The duration of the shock is then determined based on the measured patient TTI, in embodiments. In some embodiments, the duration is determined by a processor of an external defibrillator such as, for example, processor  230  ( FIG.  2   ) or processor  330  ( FIG.  3   ). Embodiments of this process are described in more detail below. 
     In embodiments, in an operation  713 , the measured TTI is analyzed or compared to determine if it is greater than the first threshold. In some embodiments, this determination is performed by a processor such as, for example, duration selection module  336 B ( FIG.  3   ) of processor  330 . 
     If the measured TTI is not greater than (or not greater than or equal to in some embodiments) the first threshold, in an operation  715  the shock duration is determined using the first algorithm. In some embodiments, the first algorithm (per operation  703 ) is performed by a shock duration module such as, for example, shock duration module  336 C of processor  330  ( FIG.  3   ). 
     If the measured TTI is greater than (or greater than or equal to in some embodiments) the first threshold, in an operation  716  the measured TTI is analyzed or compared to determine if it is greater than the second threshold. In some embodiments, this determination is performed by the processor as described above in conjunction with operation  713 . 
     If the measured TTI is not greater than (or not greater than or equal to in some embodiments) the second threshold, in an operation  717  the shock duration is determined using the second algorithm. In some embodiments, the second algorithm (per operation  703 ) is performed by a shock duration module such as, for example, shock duration module  336 C of processor  330  ( FIG.  3   ). 
     If the measured TTI is greater than (or greater than or equal to in some embodiments) the second threshold, in an operation  719  the shock duration is determined using the third algorithm. In some embodiments, the third algorithm (per operation  703 ) is performed by a shock duration module such as, for example, shock duration module  336 C of processor  330  ( FIG.  3   ). 
     In an operation  721 , the external defibrillator is caused to output a shock with the duration determined in operation  715  or  717  or  719 , depending on the measured patient TTI. In some embodiments, this operation is performed by a processor of an external defibrillator such as, for example, processor  230  ( FIG.  2   ) or processor  330  ( FIG.  3   ), controlling other components of the external defibrillator (e.g., as described above in conjunction with  FIGS.  2  and  3   ). 
     Time to Ratio (UR) Embodiments 
       FIG.  8    is a diagram showing sample components of an external defibrillator  801  similar to external defibrillator  200  ( FIG.  2   ) and external defibrillator  301  ( FIG.  3   ), in which some components are shown in more detail, according to embodiments. External defibrillator  801  can be implemented in a WCD in some embodiments to deliver appropriate therapy (e.g., defibrillation shocks, cardioversion shocks, pacing, etc.) to a patient with an arrhythmia. 
     In this embodiment, external defibrillator  801  includes measurement circuit  320 , memory  338 , and processor  330  as described above in conjunction with  FIG.  3   , and other modules and components as shown in  FIG.  2    (but omitted in  FIG.  8    as they have already been described in detail in conjunction with  FIG.  2   ). 
     Memory  338  of external defibrillator  801 , in some embodiments, is similar to memory  338  ( FIG.  3   ), being configured with at least two duration algorithms such as for example, the Walcott Algorithm, the Mod-cott Algorithm, the CE Algorithm or the previously described Max Duration Algorithm. In some embodiments, the duration algorithms include a minimum duration (e.g., 4 ms) for the shock. In addition, in some embodiments memory  338  is configured with at least one algorithm (not shown) for shaping (e.g., determining the durations of each phase of a multiphasic waveform) the waveform of a shock to be delivered to a patient. In some embodiments, the waveform is a BTE waveform with equal phases. In other embodiments, the waveform is a multiphasic waveform with one phase being an exponential, and another phase being a rectilinear. In some embodiments, the phase durations of a multiphasic exponential waveform are configured in a similar manner as described above in conjunction with  FIG.  4 B . In some embodiments, one or more of the duration algorithms also define the waveform (e.g., the duration of each phase of a BTE waveform as defined by the Walcott or equal phase Mod-cott duration algorithms). 
     In this embodiment, measurement circuit  320  is similar to measurement circuit  320  ( FIG.  3   ), and more specifically also shows a voltage measurement circuit  805  coupled to the energy storage module such as, for example, energy storage module  250  ( FIG.  2   ). In some embodiments, voltage measurement circuit  505  measures the charge voltage of a capacitor of the energy storage module while a shock is being administered to the patient. 
     Processor  330  of external defibrillator  801 , in some embodiments, is similar to processor  330  ( FIG.  3   ) and in addition includes a Time To Ratio (TTR) module  836 . In some embodiments, TTR module  836  is configured to determine the time it takes a fully charged energy storage module to discharge to a selected voltage ratio (also referred to as the decay time). For example, in some embodiments, the ratio is 82% of the peak voltage. The decay time needed to discharge to the set voltage ratio can then be used to calculate the durations according to the first and second duration algorithms  342  and  342  as described below. 
     In general, the decay time τ 1  for a ratio of 82% can be calculated according to equation (6) below.
 
τ 1   =C   d ·( R   pt   +R   d )·ln(0.82)  (6)
 
where the capacitance and resistances are defined as in equation (3) above. In this embodiment, the patient TTI is not measured and instead TTR module  836  measures τ 1  during delivery of the shock. But as can be seen in equation (6), patients with a relatively low TTI will have a lower decay time τ 1  than patients with a relatively high TTI.
 
     In one embodiment, duration algorithm  341  of  FIG.  8    is configured to determine the duration D MC  of a Mod-Cott (modified Walcott) waveform using the measured τ 1  according to equation (7) below. In one embodiment, the Mod-Cott waveform is the same as the Walcott waveform except that it has phases of equal duration. 
     
       
         
           
             
               
                 
                   
                     D 
                     MC 
                   
                   = 
                   
                     
                       5 
                       3 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           ( 
                           
                             
                               ( 
                               
                                 
                                   ( 
                                   
                                     
                                       - 
                                       
                                         τ 
                                         1 
                                       
                                     
                                     
                                       ln 
                                       ⁡ 
                                       ( 
                                       0.82 
                                       ) 
                                     
                                   
                                   ) 
                                 
                                 · 
                                 
                                   τ 
                                   m 
                                 
                               
                               ) 
                             
                             ÷ 
                             
                               ( 
                               
                                 
                                   ( 
                                   
                                     
                                       - 
                                       
                                         τ 
                                         1 
                                       
                                     
                                     
                                       ln 
                                       ⁡ 
                                       ( 
                                       0.82 
                                       ) 
                                     
                                   
                                   ) 
                                 
                                 - 
                                 
                                   τ 
                                   m 
                                 
                               
                               ) 
                             
                           
                           ) 
                         
                         · 
                         
                           ln 
                           ( 
                           
                             
                               τ 
                               m 
                             
                             
                               
                                 - 
                                 
                                   τ 
                                   1 
                                 
                               
                               
                                 ln 
                                 ⁡ 
                                 ( 
                                 0.82 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     where τ m  is the Walcott Model time constant and in in this embodiment is 0.0051. In other embodiments, other duration algorithms based on TTI can be mathematically recast as a function the decay time τ 1  instead of TTI by one skilled in the art after review of this disclosure. 
     In some embodiments, duration algorithm  342  of  FIG.  8    is configured to determine the duration D CE  of a CE algorithm waveform as a function of the measured decay time τ 1  instead of measured patient TTI. In some embodiments, equation (5) above is solved to be a function of a UR of 82%. For this UR, it can be shown that for a nominal 150 J Constant Energy waveform sourced from a 138 μF capacitor charged to 1600 volts, the duration D CE  can be determined as a function of the measured τ 1  according to equation (8) below.
 
 D   CE =6.13·τ 1   (8)
 
In other embodiments, different shock energies, capacitances, and/or charge voltage will result in the measured decay time τ 1  being multiplied by a different constant.
 
     In some embodiments, the capacitance of a nominally 138 μF energy storage module is calibrated. In such embodiments, the duration D CE  can be determined as a function of the measured τ 1  according to equation (9) below. 
                     D   CE     =       (     6.13   ·     τ   1       )     /     (     Ccal     138   ⁢         µ   ⁢   F       )               (   9   )               
where Ccal is the actual capacitance measured during calibration.
 
     In some embodiments, processor  330  of  FIG.  8    then compares the durations determined using the first and second duration algorithms, and selects the algorithm having the larger duration. In one embodiment, TTR module  836  performs this selection. Using the selected algorithm, processor  330  of  FIG.  8   , then controls the discharge circuit (e.g., discharge circuit  255  of  FIG.  2   ) so that the shock delivered to the patient has the waveform associated with the selected duration algorithm. In some embodiments where the first duration algorithm is the Walcott algorithm and the second waveform algorithm is the CE duration algorithm, the selection of the duration algorithm somewhat follows the graph of  FIG.  4 A  in that relatively low decay times will result in selection of the Walcott algorithm and a moderate decay time will result in the selection of the CE algorithm. In some embodiments, both duration algorithms are associated with a BTE waveform having equal duration phases. In other embodiments, the duration algorithm may each be associated with a waveform algorithm with different phase durations, different types of waveforms (e.g., exponential versus rectilinear), different number of phases, or even with specific phases having shapes according to different types of waveforms (e.g., first phase being rectilinear and a second phase being exponential). 
       FIG.  9    is a flow diagram illustrating a method  900  for selecting a shock duration based on TTR, according to embodiments. In an operation  905 , a first waveform algorithm and a first duration algorithm are stored. In some embodiments a single algorithm may define both a duration and waveform (e.g., the Walcott Algorithm). In some embodiments, the first waveform algorithm is for delivering a shock having Walcott waveform (i.e., a BTE waveform with the second phase having a duration that is ⅔ of the first phase) and first duration algorithm is for determining the duration the Walcott waveform as a function of measured decay time (i.e., the Walcott Algorithm as a function of TTR). In some other embodiments, the Mod-cott waveform algorithm is used instead of the Walcott waveform algorithm. In some embodiments, these waveforms are stored in a memory of an external defibrillator such as for example, memory  338  ( FIG.  8   ). 
     In an operation  907 , a second waveform algorithm and a second duration algorithm are stored. In some embodiments, the second waveform algorithm is for delivering a shock having an equal phase duration BTE waveform and second duration algorithm is for determining the duration the CE waveform as a function of measured decay time (i.e., the CE Algorithm as a function of TTR). In some embodiments, if the determined duration exceeds a predetermined or selected maximum, the duration is then set to this maximum. In some embodiments, these waveform algorithms are stored in a memory of an external defibrillator such as for example, memory  338  ( FIG.  8   ). As stated earlier in operation  905 , the waveform algorithm may be the same as the duration algorithm (e.g., the Mod-cott Algorithm can define both the total duration and the duration of each phase of a BTE waveform). In some embodiments, while durations algorithms 1 and 2 are different, the waveform algorithms may be the same (e.g., a BTE waveform with equal phases). In yet other embodiments, other waveform and duration algorithms or other modifications of the Walcott or CE Algorithms can be used for the first and second waveform and duration algorithms such as, for example, monophasic truncated exponential algorithms. 
     In an operation  909 , a delivery of a shock to the patient is initiated and the decay to reach a selected or set ratio measured. In some embodiments, the ratio is 82%. In other embodiments, the ratio can be different within the range from about 80% to about 99%, and so that there is still time to control the shock to have a waveform according to both first and second waveform algorithms. For example, if the ratio is too low (meaning more time is needed to discharge to that lower percentage) it might be that the discharge is past the time for ending the first phase of one of the waveform algorithms, especially as described below, some additional time may be needed to determine durations using both the first and second algorithms. If the ratio is too high, the decay time measurement might be less accurate. In some embodiments, this operation is performed by a processor of an external defibrillator such as, for example, TTR module  836  of processor  330  ( FIG.  8   ), controlling and/or receiving measurements from other components of the external defibrillator (e.g., as described above in conjunction with  FIGS.  2 ,  3  and  8   ). 
     In an operation  911 , the first and second duration algorithms are executed to determine durations of the waveforms according to the first and second waveform algorithms. In some embodiments, shock duration module  336 C of processor  330  ( FIG.  8   ) determines these durations. 
     In embodiments, in an operation  913 , the determined durations are analyzed or compared, including comparing to a maximum duration in some embodiments For example, the maximum duration is set to a value below 25 ms (e.g., 22 ms) in one embodiment. In some embodiments, the determined durations are also compared to a minimum duration (e.g., 4 ms). In some embodiments, these determinations are performed by a processor such as, for example, processor  330  ( FIG.  8   ). In some embodiments, this determination is in particular performed by TTR module  836  and/or duration selection module  336 B of processor  330  ( FIG.  8   ). 
     If the determined duration of the waveform according to waveform algorithm 1 (i.e., duration D 1 ) is greater than (or greater than or equal to in some embodiments) the determined duration of the waveform according to waveform algorithm 2 (i.e. duration D 2 ), in an operation  915  the external defibrillator is controlled to deliver energy according to waveform algorithm 1 with duration D 1 . In some embodiments, waveform algorithm 1 produces a BTE waveform with each phase having the same duration. In some embodiments, this operation is performed by a processor of the external defibrillator such as, for example, processor  330  ( FIG.  8   ), controlling other components of the external defibrillator (e.g., as described above in conjunction with  FIGS.  2 ,  3  and  8   ). 
     In other embodiments configured with a selected or predetermined minimum duration, if in addition duration D 1  is greater than the minimum duration, then operation  915  is performed as described above. However, if the determined duration is less than the minimum duration, then the algorithm sets the duration to this minimum duration and causes the external defibrillator to deliver a shock to the patient according to waveform algorithm 1 having the minimum duration. In other embodiments the minimum duration waveform is another waveform different from that produced by waveform algorithm 1. 
     However, if duration D 1  is not greater than (or not greater than or equal to in some embodiments) duration D 2 , in an operation  917  the external defibrillator is controlled to deliver energy according to waveform algorithm 2 with duration D 2 . In some embodiments, waveform algorithm 2 also produces a BTE waveform with each phase having the same duration. In other embodiments, waveform algorithm 2 produces a different waveform than waveform algorithm 1. In embodiments configured with a maximum duration, if in addition duration D 2  is less than the maximum duration, operation  917  is performed as previously described. In some embodiments, this determination is performed by the processor as described above in conjunction with operation  915 . 
     In embodiments configured with a maximum duration, responsive to both durations D 1  and D 2  exceeding the maximum duration, in an operation  919  the external defibrillator is controlled to deliver energy with the maximum duration (e.g., the previously described Max Duration Algorithm). In some embodiments, the waveform is a BTE waveform with equal duration phases. In some embodiments, the waveform algorithms 1-3 are all the same in that they cause the shock to have a BTE waveform with equal duration phases. In the methods described above, each operation can be performed as an affirmative step of doing, or causing to happen, what is written that can take place. Such doing or causing to happen can be by the whole system or device, or just one or more components of it. After review of this disclosure, a person of skill in the art will recognize that the methods and the operations may be implemented in many ways, including using systems, devices and implementations described above. In addition, the order of operations is not constrained to what is shown, and different orders may be possible according to different embodiments. Examples of such alternate orderings may include overlapping, interleaved, interrupted, reordered, incremental, preparatory, supplemental, simultaneous, reverse, or other variant orderings, unless context dictates otherwise. Moreover, in certain embodiments, new operations may be added, or individual operations may be modified or deleted. The added operations can be, for example, from what is mentioned while primarily describing a different system, apparatus, device or method. 
     A person skilled in the art will be able to practice the present invention in view of this description, which is to be taken as a whole. Details have been included to provide a thorough understanding. In other instances, well-known aspects have not been described, in order to not obscure unnecessarily this description. Plus, any reference to any prior art in this description is not, and should not be taken as, an acknowledgement or any form of suggestion that such prior art forms parts of the common general knowledge in any country or any art. 
     This description includes one or more examples, but this fact does not limit how the invention may be practiced. Indeed, examples, instances, versions or embodiments of the invention may be practiced according to what is described, or yet differently, and also in conjunction with other present or future technologies. Other such embodiments include combinations and sub-combinations of features described herein, including for example, embodiments that are equivalent to the following: providing or applying a feature in a different order than in a described embodiment; extracting an individual feature from one embodiment and inserting such feature into another embodiment; removing one or more features from an embodiment; or both removing a feature from an embodiment and adding a feature extracted from another embodiment, while providing the features incorporated in such combinations and sub-combinations. 
     In general, the present disclosure reflects preferred embodiments of the invention. The attentive reader will note, however, that some aspects of the disclosed embodiments extend beyond the scope of the claims. To the respect that the disclosed embodiments indeed extend beyond the scope of the claims, the disclosed embodiments are to be considered supplementary background information and do not constitute definitions of the claimed invention. 
     In this document, depending on the context, the phrases “constructed to” and/or “configured to” denote one or more actual states of construction and/or configuration that is fundamentally tied to physical characteristics of the element or feature preceding these phrases and, as such, reach well beyond merely describing an intended use. Any such elements or features can be implemented in many ways, by a person skilled in the art after reviewing the present disclosure, beyond any examples shown in this document. 
     All parent, grandparent, great-grandparent, etc. patent applications, whether mentioned in this document or in an Application Data Sheet (“ADS”) of this patent application, are hereby incorporated by reference herein as originally disclosed, including any priority claims made in those applications and any material incorporated by reference, to the extent such subject matter is not inconsistent herewith. 
     In this description, a single reference numeral may be used consistently to denote a single item, aspect, component, or process. Moreover, a further effort may have been made in the drafting of this description to use similar though not identical reference numerals to denote other versions or embodiments of an item, aspect, component or process that are identical or at least similar or related. Where made, such a further effort was not required, but was nevertheless made gratuitously to accelerate comprehension by the reader. Even where made in this document, such a further effort might not have been made completely consistently for all of the versions or embodiments that are made possible by this description. Accordingly, the description controls in defining an item, aspect, component or process, rather than its reference numeral. Any similarity in reference numerals may be used to infer a similarity in the text, but not to confuse aspects where the text or other context indicates otherwise. 
     This disclosure is meant to be illustrative and not limiting on the scope of the following claims. The claims of this document define certain combinations and subcombinations of elements, features and steps or operations, which are regarded as novel and non-obvious. Additional claims for other such combinations and subcombinations may be presented in this or a related document. These claims are intended to encompass within their scope all changes and modifications that are within the true spirit and scope of the subject matter described herein. The terms used herein, including in the claims, are generally intended as “open” terms. For example, the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” etc. If a specific number is ascribed to a claim recitation, this number is a minimum but not a maximum unless stated otherwise. For example, where a claim recites “a” component or “an” item, it means that it can have one or more of this component or item.