Abstract:
An implantable medical device (IMD) is provided which is capable of sensing and determining its orientation, and of determining whether the IMD has been displaced over time away from its original or optimal position. Electronic components of the IMD, including a processor, digital memory, signal conditioning components, and a power supply, are preferably hermetically sealed within a biocompatible housing. At least three subcutaneous electrodes have fixed relative spacing for sensing electrical cardiac activity for various combinations of two electrodes, forming sensing vectors. Amplitude ratios and sign indicators associated with the sensing vectors are compared with a reference to determine an orientation of the device. In one embodiment, a telemetry unit transmits orientation data as a function of time to a remote device, and the remote device compares different stored orientations to detect displacement over time.

Description:
RELATED APPLICATION 
     This patent application claims the benefit of U.S. Provisional Patent Application No. 61/251,328 filed on Oct. 14, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to implantable medical devices and, in particular, to subcutaneous implantable medical devices capable of detecting electrical signals from the heart for monitoring heart activity. 
     BACKGROUND 
     An electrocardiogram (ECG) is well known in the art as a transthoracic interpretation of electrical activity of the heart as detected by electrodes. Electrical impulses in the heart originate in the sinoatrial node and travel through an intrinsic conducting system to the heart muscle. The impulses stimulate myocardial muscle fibers to contract and thus induce systole. These electrical impulses may be detected by a set of three electrodes (electrical contacts) selectively placed on the skin. An ECG represents a voltage measured between pairs of these electrodes and the muscle activity that the electrodes detect from different directions, or vectors. A typical shape of the systolic portion of an ECG output signal, plotted as a time-varying voltage, is known to those skilled in the art as the “QRS complex” in which Q, R, and S designate typical feature shapes that correspond to events in a cardiac cycle. 
     Present day implantable cardiac devices such as pacemakers, implantable defibrillators, and the like, include a monitoring function that may use either intracardiac or subcutaneous electrodes to sense electrical signals from the heart, in a similar fashion to a conventional, external ECG. Intracardiac electrodes are implanted directly into the heart tissue; whereas subcutaneous electrodes are fixedly attached to the housing of the implantable device. If subcutaneous electrodes are used, the resultant recording is called a subcutaneous electrocardiogram (SECG). Existing subcutaneous monitoring devices typically use only two electrodes (one pair), thus providing only one recording channel. Because the separation between the electrodes is small (due to the device size), the distance to the heart is relatively large, and skeletal muscle is in close proximity to the device, signals detected by subcutaneous electrodes are more susceptible to noise than are signals detected by implanted leads placed directly on the heart tissue. Signals detected by subcutaneous electrodes are also highly dependent upon orientation of the monitoring device with respect to the heart. Proper placement of the monitoring device is required during implantation to ensure optimal signal amplitude. The implanted device is preferably positioned so as to maximize the QRS signal amplitudes detected. 
     After implantation, it is generally difficult to identify movement of the device away from its optimal position. Such a displacement may cause a decrease in electrocardiographic signal amplitudes, resulting in a poor quality SECG. If such a decrease in signal strength occurs, existing devices generally cannot distinguish whether or not the reason is due to a shift in the device position. To confirm the orientation of the device, inspection by a physician currently requires the patient to travel to the physician and possibly to receive an X-ray. For at least these reasons, an improved method of determining the orientation of implantable heart monitoring devices is needed. 
     SUMMARY 
     An implantable medical device (IMD) is provided which is capable of a) sensing and self-determining its orientation, and b) determining whether the IMD has been displaced over time away from its original position. Electronic components within the IMD, including a processor, a digital memory, a power supply, signal conditioning components, and a telemetry unit, are preferably hermetically sealed within a biocompatible housing. A preferred embodiment of the device applies to monitoring for the purpose of assessing cardiac function. At least three subcutaneous electrodes are deployed for sensing electrical cardiac activity between pairs of electrodes, forming a set of at least three sensing vectors, thus providing three recording channels, each channel having one electrode in common with at least one other channel. Because subcutaneous electrodes are attached to a rigid device housing, their relative spacing is fixed as opposed to intracardiac electrodes which move as the heart expands and contracts, and skin electrodes, which also may be independently affected by different bodily motions. Amplitude ratios and sign indicators characterizing the sensing vectors are compared against a reference to determine an orientation of the IMD. In one embodiment, a telemetry unit transmits orientation data as a function of time to a remote device, and the remote device compares the orientation data against one or more stored orientations to detect displacement of the implanted device over time. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The present invention will now be explained in greater detail on the basis of exemplary embodiments with reference to the figures. In the Figures 
         FIG. 1  is a block diagram of an exemplary prior art implantable medical device (IMD) system that includes a telemetry unit for communicating with a remote device. 
         FIG. 2  is a diagram of Einthoven&#39;s triangle, showing a typical prior art configuration for placing external electrodes on the skin to sense electrocardiographic (ECG) signals. 
         FIG. 3  is a diagram of a preferred configuration for subcutaneous electrode placement in which three sensing vectors form an isosceles triangle. 
         FIG. 4  is a diagram of an alternative configuration for subcutaneous electrode placement in which three sensing vectors form a right triangle. 
         FIG. 5  is a vector diagram of a prior art coordinate system having three co-planar vector coordinate axes for representing bipolar surface ECG recordings. 
         FIG. 6  is a vector diagram of three sensing vectors, three normal lines (shown as dotted lines), and a resultant mean cardiac vector (shown as a dashed line) superimposed on the coordinate system shown in  FIG. 5 . 
         FIG. 7  is a graph of signal amplitudes recorded using the preferred configuration for subcutaneous electrode placement shown in  FIG. 3 . 
         FIG. 8  is a graph of signal amplitudes and their vector sum, recorded using the preferred configuration for subcutaneous electrode placement shown in  FIG. 3 . 
         FIG. 9  is a graph of signal amplitudes recorded using the alternate configuration for subcutaneous electrode placement shown in  FIG. 4 . 
         FIG. 10  is a graph of signal amplitudes and their vector sum, recorded using the alternate configuration for subcutaneous electrode placement shown in  FIG. 4 . 
         FIG. 11  is a plot of amplitude ratios and a sign indicator for signals recorded using the preferred configuration for subcutaneous electrode placement shown in  FIG. 3 . 
         FIG. 12  is a diagram showing an orientation of the IMD rotated 90 degrees relative to the mean cardiac vector. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , an implantable medical device (IMD) system  90  includes an IMD  100  in communication with a remote external device  105 . IMD  100 , having the disclosed features, may include existing cardiac monitoring device hardware such as, for example, an implantable loop recorder, a leadless pacemaker, or a leadless defibrillator. Such devices are well known in the art. The following embodiments describe features required in addition to features known in the art to carry out the present invention. As shown in  FIG. 1 , electronic components of IMD  100  are preferably surgically deployed within a hermetically sealed, biocompatible housing  110  that protects the components and minimizes reactions between the device and the surrounding living tissue. Electronic components within housing  110  include a processor  120  coupled to a digital memory  130  via a bus  140 ; a telemetry unit  150  coupled to processor  120 ; and a power supply  170 , typically in the form of a battery, for energizing processor  120 , digital memory  130 , and telemetry unit  150 . A suitable processor  120  may take the form of a Motorola 68000 series, Intel 8086, or similar 8-bit microprocessor, a programmable microcontroller, or another similar logic device typically implemented in IMDs. A suitable low power digital memory has the capacity to store SECG data. 
     Processor  120  is further coupled to a set of signal conditioning components  180 . Signal conditioning components  180  may comprise, for example, one or more of a sense amplifier, a filter, and an analog-to-digital converter that samples input signals using a predetermined sampling rate. Suitable signal conditioning components known in the art, include signal conditioning components implemented in implantable loop recorders, pacemakers or defibrillators. 
     Telemetry unit  150  enables wireless communication with remote device  105  of data recorded by signal conditioning components  180  or stored in the digital memory  130 . Telemetry unit  150  may be further wired, or wirelessly connected, to a remote server, expert center, or database. A suitable telemetry unit  150  known in the art may be inductive or radio frequency (RF) based. 
     Referring now to  FIG. 2 , electrodes used for obtaining conventional, surface ECG measurements are typically arranged in a triangle  200 , known to those skilled in the art as Einthoven&#39;s triangle, such that a first electrode  210  is placed on, or in the direction of the right arm, a second electrode  220  is placed on, or in the direction of the left arm, and a third electrode  230  is placed on or in the direction of the left leg. Signals measured by leads I, II, and III, extending between pairs of electrodes, are understood as vectors, wherein the amplitude of the vector is the voltage between the pair of electrodes and the direction (theta) of the vector is determined by the arrangement of the electrode pairs. By convention, when electrode  220  is at a positive electric potential with respect to electrode  210 , lead I is positive; when electrode  230  is at a positive electric potential with respect to electrode  220 , lead II is positive; and when electrode  230  is at a positive electric potential with respect to electrode  210 , lead III is positive; 
     Referring to  FIGS. 3 and 4 , IMD system  90  further includes three subcutaneous electrodes  310 - 330  that may be placed at, on, or in one end of housing  110 , in a triangular configuration similar to Einthoven&#39;s triangle  200 , for detecting electrical cardiac signals in a similar manner to skin electrodes  210 - 230  used in the exemplary external electrocardiogram (ECG) shown in  FIG. 2 . Signal conditioning components  180  coupled to electrodes  310 - 330  produce signals A, B, and C, for which three pairs of electrodes provide three SECG recording channels: a signal detected between electrodes  310  and  320  is measured as vector A; a signal detected between electrodes  330  and  310  is measured as vector B; and a signal detected between electrodes  330  and  320  is measured as vector C. Processor  120  is programmed to detect the QRS complex in sensing vectors A, B, or C and may provide the amplitude of the sensed vector signals at the point in time at which the QRS complex occurs. Or, processor  120  may combine sensing vectors A, B, or C, and may detect the QRS complex in the combined signal. 
     It is understood that the processing of sensing vectors A, B, and C is performed in a similar fashion as is known for some existing IMDs, in which signals are detected by bipolar electrodes implanted directly into the heart tissue. In particular, it is understood that signals A, B, and C are processed simultaneously, and that the sensed signal amplitudes may be stored in digital memory  130 .  FIGS. 3 and 4  show representations of two possible electrode placements: a preferred embodiment uses an isosceles triangular electrode configuration  350  for which vector orientations are shown in  FIG. 3 ; an alternative embodiment uses a right triangular electrode configuration  400 , for which two (A and B) of the three sensing vectors are orthogonal to each other as shown in  FIG. 4 . Thus, IMD  100  records an SECG of cardiac electrical activity in essentially the same fashion as described for a conventional ECG. 
     Referring to  FIGS. 5 and 6 , a mean cardiac vector  610  may be determined by finding the resultant electrical conduction vector (i.e., the vector sum) of at least two of the surface ECG lead signals I, II, and III at an instant in time at which at least one of the ECG signals is at an absolute maximum. In  FIG. 5 , a coordinate system  500  is presented in which three co-planar axes  510 ,  520 , and  530  are used to represent bipolar surface ECG recordings in accordance with established conventions that place lead I at 0 degrees, lead II at 120 degrees, and lead III at 60 degrees. The center of coordinate system  500  is represented by a crossing point  550 . In  FIG. 6  the three signal vectors I, II, and III are plotted on coordinate system  500  at the same time for example, at a time coinciding with the peak of the QRS complex, so as to maximize the signal amplitudes. Signal amplitudes I, II, and III are represented by the lengths of the arrows from the crossing point  550  along axes  510 ,  520 , and  530 , respectively If a normal line is extended from the tip of each signal vector, the normal crossing point  600  specifies the tip of the resultant mean cardiac vector  610 , the length and direction of which is represented by a dashed arrow in  FIG. 6 . Mean cardiac vector  610  then represents the direction of electrical activity in the heart at its maximum amplitude. 
     It can be seen that the maximum amplitude of each signal vector A, B, and C is partly dependent upon its angular orientation with respect to the mean cardiac vector  610 , and partly upon the distance from the IMD  100  to the heart. Electrode configuration  350  may be rotated relative to the heart, through a rotation angle between 0 and 360 degrees.  FIG. 7  shows an amplitude relationship  700  for electrode configuration  350  in which, for this example, the orientation of the device is rotated through 360 degrees relative to mean cardiac vector  610 . The amplitudes of signal vectors A, B, and C are recorded every 15 degrees to show the amplitude relationships. A plot  710  (solid line) of the signal amplitude of vector A crosses the abscissa at an intersection point  715  at 90 degrees. Plots  720  (dashed line) and  730  (dotted line) of the signal amplitudes of vectors B and C, respectively are shown relative to plot  710 .  FIG. 8  shows a similar amplitude relationship  800  between signal amplitudes  810 ,  820 , and  830  of sensing vectors A, B, and C, respectively, for electrode configuration  400 . The abscissa in  FIGS. 7 and 8  represents the orientation of sensing vectors relative to cardiac vector  610  in degrees. 
     An exemplary optimal orientation of IMD  100  relative to the mean cardiac vector  610  may be determined relative to mean cardiac vector  610  for which the sum of the absolute values exceeds a predetermined threshold. This is shown in  FIG. 9  for electrode configuration  350  and in  FIG. 10  for electrode configuration  400 . Plots  910 ,  920 , and  930  of normalized signal amplitudes, equal to the absolute values of signal amplitudes  710 ,  720 , and  730  of vectors A, B, and C are shown against a plot  940  showing the sum (dashed-dotted line) of normalized amplitudes  910 ,  920 , and  930 . Similarly, in  FIG. 10 , plots  950 ,  960 , and  970  of normalized signal amplitudes, equal to the absolute values of signal amplitudes  810 ,  820 , and  830  of sensing vectors A, B, and C are shown against a plot  980  showing the sum (dashed-dotted line) of normalized amplitudes  950 ,  960 , and  970 . 
     After implantation, the orientation of IMD  100  relative to the mean cardiac vector  610  may be determined by evaluating amplitude ratios of the sensing vectors. To determine the orientation of the device, a clean QRS waveform from each channel is required. This may be obtained by averaging QRS signals over a time interval, for example, ten seconds, to reduce inherent noise.  FIG. 11  shows a plot of amplitude ratios for electrode configuration  350 , in which the abscissa represents the orientation in degrees of vector A relative to the cardiac vector  610 . Plotted are the ratios A/B as triangles, the ratio B/C as quadrates and the ratio B/C as circles in steps of 10 degrees. Whereas the signal amplitudes of sensing vectors A, B, and C vary with both the orientation relative to, and the distance away from, the heart, the signal amplitude ratios of the vectors depend only on the orientation of the device relative to the mean cardiac vector  610 . 
     Alternatively, an orientation data set may be derived from the ratio of A:B:C as a unique identifier of the orientation. In a preferred embodiment, processor  120  calculates amplitude ratios for the sensed vector signals, and may store the calculated amplitude ratios in the digital memory  130 . Processor  120  may be programmed to verify whether the maximum amplitudes of sensing vectors A, B, and C occur at the same point in time and it may provide a corresponding indicator. It can be seen that a combination of the three amplitude ratios shown [A/B, A/C, B/C] is unique for each orientation of IMD  100  relative to the mean cardiac vector  610  over a range of 0 to 180 degrees. 
     If, in addition to the amplitude ratios, the signs of the sensing vectors are evaluated, a unique combination may be found for each orientation of IMD  100  relative to the mean cardiac vector  610 . Processor  120  may be programmed to compute a sign indicator by converting the signs of the amplitudes of sensing vectors A, B, and C into integer values (positive sign equals 1, negative sign equals −1 and 0 otherwise) and then summing these integer values. Exemplary sign indicators are denoted as SignI in  FIG. 11  and plotted as rhombuses. Sign indicator SignI may be stored in digital memory  130  as part of an orientation data set representing a particular orientation of IMD  100  relative to the mean cardiac vector  610 . Elements of the orientation data set include the corresponding three amplitude ratios [A/B, A/C, B/C] and the sign indicator SignI, all of which are preferably recorded at substantially the same time. 
     It is also possible to detect changes in the orientation of IMD  100 , for example, to determine the cause of sensing amplitude degradation. In this case, a known orientation data set recorded at a first point in time is stored as a reference. For example, in the event of gross changes in an SECG, a new comparison may be performed to determine if IMD  100  changed position relative to the mean cardiac vector  610  (see  FIG. 12 ). This comparison does not rely on any external devices and thus it can be performed remotely. For example, as the orientations of the vectors A, B and C relative to each other are determined by electrode orientation, expected vector ratios and sign indicators may be calculated for different orientations and stored as reference orientation data sets. Alternatively, IMD  100  may be placed in a fluid that simulates body fluid, (e.g., in a water bath), and vector ratios and a sign indicator for different orientations relative to a known signal vector representing the peak mean cardiac vector  610  may be recorded and stored as reference orientation data sets. A determination of the orientation of IMD  100  relative to the mean cardiac vector  610  may then be performed by comparing actual orientation data sets with the stored reference orientation data sets. Such a comparison may be accomplished in various ways, e.g., by storing reference orientation data sets as a look-up table and seeking in a preferred embodiment, a one-to-one identical match, or, in an alternative embodiment, a closest match of the orientation data set elements within a prescribed range. 
     It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teaching. The disclosed examples and embodiments are presented for purposes of illustration only. Therefore, it is the intent to cover all such modifications and alternate embodiments as may be evident within the true scope of this invention.