Digital Signal Processing Chip Implementation for Detection and Analysis of Intracardiac Electrograms

The adoption of digital signal processing (DSP) microchips for detection and analysis of electrocardiographic signals offers a means for increased computational speed and the opportunity for design of customized architecture to address real-time requirements. A system using the Motorola 56001 DSP chip has been designed to realize cycle-by-cycle detection (triggering) and waveform analysis using a time-domain template matching technique, correlation waveform analysis (CWA). The system digitally samples an electrocardiographic signal at 1000 Hz, incorporates an adaptive trigger for detection of cardiac events, and classifies each waveform as normal or abnormal. Ten paired sets of single-chamber bipolar intracardiac electrograms (1–500 Hz) were processed with each pair containing a sinus rhythm (SR) passage and a corresponding arrhythmia segment from the same patient. Four of ten paired sets contained intraatrial electrograms that exhibited retrograde atrial conduction during ventricular pacing; the remaining six paired sets of intraventricular electrograms consisted of either ventricular tachycardia (4) or paced ventricular rhythm (2). Of 2,978 depolarizations in the test set, the adaptive trigger failed to detect 6 (99.8% detection sensitivity) and had 11 false triggers (99.6% specificity). Using patient dependent thresholds for CWA to classify waveforms, the program correctly identified 1,175 of 1,197 (98.2% specificity) sinus rhythm depolarizations and 1,771 of 1.781 (99.4% sensitivity) abnormal depolarizations. From the results, the algorithm appears to hold potential for applications such as realtime monitoring of electrophysiology studies or detection and classification of tachycardias in implantable antitachycardia devices.     

Intracardiac Electrograms and Sensing Test Signals: Electrophysiological, Physical, and Technical Considerations

Until now, the characterization of intracardiac electrograms was carried out mainly in the frequency domain by which filtering and sensitivity testing was decidingly influenced. The typical structure of the intracardiac electrogram derived with unipolar electrodes that is normally responsible for synchronization can be explained theoretically by assuming two dipoles perpendicular to each other moving along with an excitation wave. An abstraction of this typical structure yields, as a sensitivity test signal, an asymmetric triangle with a 4-ms decay time and a 16-ms rise time. A correlation of different test signals and real heart signals applied to 16 different pacemakers demonstrates that the proposed test signal is superior to all others known so far.     

Characteristics of Intracardiac Electrograms II: Atrial Endocardial Electrograms

The intra-alriai electrograms (P-wave and QRS complex) of newly implanted electrodes were recorded, and the time and frequency domain characteristics compared. The sensing impedance properties of the atrial electrode were studied in relation to the characteristics of both atrial and ventricular electrograms, and specific differences between the P and R-waves were identified. These values, particularly the amplitude and frequency components, were sufficiently distinct to make differentiation by pacemaker sensing circuits selective and reliable.     

Clinical Experience with Sensolog 703: A New Activity Sensing Rate Responsive Pacemaker

Sensolog 703 is a new activity sensing rate responsive pacemaker which detects body vibration during physical exercise and uses the vibration as an indicator of the physiological need for a rate increase. This pacemaker was implanted in 11 patients with complete heart block and atrial arrhythmias. Their mean age was 58 (range 39-72) years. With appropriate rate response, exercise capacity, as assessed by the duration of graded treadmill exercise using the Bruce protocol, was significantly improved over the VVI pacing mode (mean +/- SEM, 462 +/- 52 s in the rate responsive mode and 368 +/- 34 s in the VVI mode, P less than 0.02). Cardiac output at peak exercise, as assessed by continuous wave Doppler sampling of aortic root blood flow, was also significantly increased compared to the resting value in both piecing modes. However, the increase was more marked when exercise was performed in the rate response mode (93 +/- 22% increase over resting cardiac output in the rate responsive mode and 57 +/- 13% increase in the VVI mode, P less than 0.05). The rate responses of this pacemaker were compared with those of a Medtronic Activitrax pacemaker. Although both pacemakers responded to an increase in walking speed, neither responded appropriately to walking up different gradients, In both cases, ascending and descending four flights of stairs resulted in similar pacing rates. There was no response to physiological activities with minimal body movements such as isometric exercise and the Valsalva maneuver. Technical problems were encountered in two implanted Sensolog pacemakers: one had spontaneous rate acceleration at rest immediately following implantation and one showed intermittent rate acceleration while the patient was at rest. Both units were programmed to the VVI mode. In conclusion, satisfactory rate response, improvement in exercise duration and increase in cardiac output were achieved with the Sensolog 703 pacemaker. However, as body vibration is not closely related to physiological needs, it has similar limitations in rate response as the Activitrax pacemaker.     

Benefits of Intracardiac Electrograms and Programmable Sensing Polarity in Preventing Pacemaker Inhibition Due to Spurious Screw-In Lead Signals

False inhibition of ventricular output due to spurious signals from a bipolar Oscor Medical lead (PY58BV) is reported following implantation of a Pacesetter Synchrony DDDR 2020T. Telemetered bipolar and distal unipolar intracardiac electrograms revealed 20 mV spurious signals coming from the distal screw-in electrode. Reprogramming the sensing channel to unipolar ring prevented inhibition of pacemaker output.     

Filter Characteristics of the Atrial Sensing Circuit of a Rate Responsive Pacemaker. To See or Not to See

The intra-atrial electrograms (P waves) from floating orthogonal atrial electrodes of acutely implanted pacemaker leads (SRT lead) were recorded and the frequency characteristics were determined. The atrial sensing properties of the rate responsive pacemaker (RS4) used in conjunction with these leads, were studied in relation to the frequency spectra of atrial electrograms. Whereas the P waves showed a bandwidth to 65 Hz, the filter had an upper cutoff frequency of 35 Hz. We conclude that unreliable atrial sensing with the RS4-SRT pacing system is primarily due to an inappropriate filter match and therefore no satisfactory rate responsiveness is achieved.     

Clinical Utility of Intraatrial Pacemaker Stored Electrograms to Diagnose Atrial Fibrillation and Flutter

POLLAK, W.M., et al. : Clinical Utility of Intraatrial Pacemaker Stored Electrograms to Diagnose Atrial Fibrillation and Flutter. The purpose of this study was to determine if intraatrial electrograms (EGMs) are required to diagnose specific types of atrial tachyarrhythmias detected by pacemaker diagnostics. DDD pacemakers in 56 patients were programmed to store episodes of atrial tachyarrhythmias. Some episodes had a stored atrial EGM snapshot of the atrial tachyarrhythmia. The EGMs were analyzed to confirm whether the stored episodes were true atrial tachyarrhythmias or other pacemaker-sensed events. EGM confirmation of atrial tachyarrhythmias correlated with increasing duration and rate of episodes. In particular, using EGMs, 8 (18%) of 44 episodes < 10 seconds in duration confirmed atrial tachyarrhythmias compared to 16 (89%) of 18 episodes > 5 minutes in duration (  P < 0.001  ). Only 10 (18%) of 56 detected atrial arrhythmia episodes at rates < 250 complexes per minute were confirmed by the atrial EGM as true arrhythmias compared to 33 (57%) of 58 detected episodes at rates > 250/min (  P < 0.001  ) Twenty-nine (91%) of 32 EGM confirmed episodes of atrial fibrillation/flutter had an atrial rate > 250 complexes per minute and were a minimum of 10 seconds in duration. Fifteen (88%) of 17 episodes meeting the combined stored data criteria of > 250 complexes per minute and duration > 5 minutes were confirmed as atrial fibrillation or flutter by stored EGMs. Atrial EGMs identified that 71 (62%) of 114 stored high atrial rate (HAR) episodes were events other than true atrial tachyarrhythmias. Pacemaker diagnostic data with intraatrial EGMs can diagnose specific atrial tachyarrhythmias and identify other pacemaker-sensed events. Stored episodes > 250 complexes per minute and > 5 minutes in duration had a high correlation with atrial fibrillation and flutter.     

Reliability of an Automatic Sensing Test with Beat-to-Beat Display of the Signal Amplitude

In contrast to testing of the capture threshold, the reliability of sensing tests has been little studied. This study was performed to test the automatic sensing algorithm included in the Biotronik pacing systems. The automatic measurements made by the devices were compared with the direct manual measurements made of 271 atrial (72%) or ventricular (28%) electrograms recorded in ten patients. A high correlation(r = 0.995, P < 0.0001) was found between the two types of measurements. The use of this function should facilitate the verification of accurate sensing during the long-term follow-up of pacemaker patients, and offers a tool to analyze the variations in amplitude of intracardiac signals.     

Rate Modulation by Arm Movements of the Respiratory Dependent Rate Responsive Pacemaker

The rate response to arm movements of the respiratory dependent rate responsive pacemaker (RDP3, Biotec) was assessed in four patients implanted with this pacemaker. The pacemaker was implanted in the left prepectoral region and the auxiliary impedance measuring electrode positioned subcutaneously over the right second intercostal space with its tip lateral to the mid-clavicular line. The lower rate of the pacemaker was programmed to 75 bpm. While holding the breath, swinging arm movements (30 times) resulted in rate acceleration. The peak rate was faster when the arm on the side of the auxiliary electrode was swung (mean ± SEM, 117 ±8 compared to 130 ± 5 bpm, P < 0.5). The mean rate response of the subjects to brie/ treadmill exercise (Bruce stage 1) performed with both hands holding the support rails, swinging the right arm only, swinging left arm only and swinging both arms were 108, 140, 135 and 128 bpm respectively. Impedance measurement confirmed the significant influence of arm movements on thoracic "impedance" changes, which was mainly caused by electrode motion artifacts affecting the two electrode measuring system. This effect was dependent on the relative positions of the impedance measuring electrodes (i.e., between the pacemaker casing the auxiliary lead). Subsequently the auxiliary lead of the respiratory pacemaker (MB-1, and Biorate) was implanted in the lower part of the chest on the right sternal edge in another patient. Rate acceleration was only observed when the arm on the side of the pacemaker was swung. As arm movements often accompany physical activities, pacing rate can be affected and should be considered when programming this pacemaker.     

A Randomized Double-Blind, Cross-Over Study of the Linear and Nonlinear Algorithms for the QT Sensing Rate Adaptive Pacemaker

BAIG, M.W., ET AL.: A Randomized Double-Blind, Cross-Over Study of the Linear and Nonlinear Algorithms for the QT Sensing Rate Adaptive Pacemaker. We have compared the pacing rate responses during cardiopulmonary exercise testing in 11 patients (mean 59 years, six female) with implanted QT sensing rate adaptive pacemakers who were randomly programmed to 1-month periods in the linear and nonlinear algorithms using a double-blind, cross-over design. Exercise testing was performed at the end of each month block and symptoms were scored with the MacMaster questionnaire. With exercise, the time to a 10 beats/min increment in rate was significantly less with the nonlinear compared to the linear algorithm (126 sec vs 255 sec, P = 0.02) but there were no significant differences in exercise duration, the peak pacing rate, the peak VO₂, the VO₂ at the anaerobic threshold or the mean correlation coefficients of the pacing rate VO₂ relationship. Rate oscillation occurred in seven patients in the linear algorithm and in two patients in the nonlinear setting. Initial deceleration of the pacing rate at the onset of exercise occurred in seven patients in the linear algorithm and in four patients in the nonlinear setting. The nonlinear algorithm is associated with a faster response time during exercise and fewer instances of rate instability. However, it has not overcome the problem of a dip in the pacing rate at the beginning of exercise. The major difference in the function of the two algorithms is faster initial acceleration with the nonlinear algorithm. This is explained by the significantly higher values of the slope setting at the lower rate limit for the nonlinear versus the linear algorithm (6.3 ms/ms vs 5.1 ms/ms).     

Improved Pattern of Rate Responsiveness with Dynamic Slope Setting for the QT Sensing Pacemaker

We have recently described the electrophysiological basis of a new algorithm for the QT (TX) sensing rate responsive pacemaker. By using the new software program running on the standard programmer it has been possible to simulate the new algorithm in ten patients with complete heart block (seven patients had implanted TX units and three were paced with an external TX pacemaker) during routine exercise testing. In this way a single-blind, intra-patient comparison of the pattern of pacing rate change using both the existing and new algorithms was possible. In nine out of the ten cases the time taken to increase the pacing rate from 70 to 80 bpm was reduced significantly when the new algorithm was used (P = 0.037). Additionally, the correlation between the atrial and ventricular rates in those patients with normal sinus node function (seven patients) was determined. In all cases we have observed a significantly improved correlation between the atrial and ventricular paced rates during exercise with the new algorithm (P less than 0.001).     

Decrease in Canine Endocardial and Epicardial Electrogram Voltages with Exercise: Implications for Pacemaker Sensing

Individuals with permanently implanted pacemakers who have normal sensing and pacing at rest may demonstrate abnormalities, particularly of atrial sensing, with exercise. Exercise is known to cause changes in the surface electrocardiographic voltages. The purpose of this study was to evaluate changes in endocardial and epicardial voltages during exercise in canines with permanently implanted pacing leads similar to those permanently implanted in humans with pacemakers. Six adult mongrel dogs were equipped with transvenous atrial and ventricular endocardial bipolar screw-in leads from a jugular venosection, and with atrial and ventricular bipolar stab-on leads from a left thoracotomy. Exercise was performed at one week following implantation and at weekly intervals for a total of 4 weeks. The animals were studied at rest and at a single workload of 3 miles per hour at a 30% elevation. Unfiltered electrograms were recorded with a VR-12 Electronics-for-Medicine photographic recorder. The mean decreases of electrograms with exercise from resting voltage were: 15% for atrial endocardial unipolar leads; 11% for atrial endocardial bipolar leads; 4% for atrial epicardial unipolar leads; 15% for atrial epicardial bipolar leads; 8% for ventricular endocardial unipolar leads; 18% for ventricular endocardial bipolar leads; 0.1% for ventricular epicardial unipolar leads and 5% for ventricular epicardial bipolar leads.     

Variation in P Wave Amplitude Immediately After Pacemaker Implantation: Possible Mechanism and Implications for Early Programming

The P wave amplitude (PWA) plays an important role in determining atrial sensing capabilities. To assess early PWA change, we compared the unipolar PWA in 43 patients at the time of atrial lead placement, measured by a pacing systems analyzer, to the unipolar PWA recorded at the end of pacemaker surgery, from telemetered atrial endocardial electrograms. Individual PWA varied from a decrease of 5.2 mV to an increase of 2 mV (-63% to 267%). In 33 patients with active fixation leads, the implant PWA was 1.96 +/- 0.99 mV versus 2.4 +/- 1.4 mV after surgery. In 11 patients with passive fixation leads, the implant PWA was 2.8 +/- 1.9 mV versus 1.9 +/- 0.8 mV after surgery. The PWA change, measured as the difference between the postsurgical and implant PWA was 0.43 +/- 0.8 mV in active versus -0.86 +/- 1.6 mV in the passive fixation lead groups (P less than 0.05). Considerable change in individual P wave amplitude can therefore occur very early after pacemaker implantation. The direction differs significantly between active (predominantly positive) and passive fixation groups (predominantly negative). These data suggest that an adequate margin of safety is important when initially programming atrial sensitivity, particularly when using passive fixation leads.     

In Vitro Evaluation of a Sensor Sensitive to Acceleration Forces Included in a New Rate Modulated Pacemaker

The characteristics of the sensor and rate adaptive algorithm included in a new dual chamber rate responsive pacemaker (Relay 294–03, Intermedics, Inc.) were studied by submitting the device to calibrated to-and-fro movements of specific frequencies and peak accelerations by means of a mechanical arm connected to a speed adapter. Atrial pacing rate was continuously recorded on a Holter monitor. The influences of the frequency, the magnitude, and the axis of the acceleration forces as well as the reproducibility of the rate response were evaluated. Results: (1) the sensor was sensitive to frequencies ranging from 0.5 to 7 Hz with a peak sensitivity at 3 Hz; (2) the pacing rate was not affected during movements at frequencies > 6 Hz, commonly presented as nonactivity related signals fear, bus transportation, drilling); (3) the pacing rate increased as a function of the acceleration magnitude up to 0.5 G (0.3 G for 3 Hz), then remained constant. This level of acceleration usually corresponds to high degrees of activity; (4) rate response was maximum when acceleration was directed in the anteroposterior direction; (5) reproducibility of the rate response was excellent (R²: 0.999; slope of the regression line: 0.999). Conclusion: Relay 294-03 is a low frequency signal sensing rate modulated pacemaker using an accelerometer sensitive to motion, mainly in the anteroposterior direction.     

Circumvention of Maximum Tracking Limitations with a Rate Modulated Dual Chamber Pacemaker

A patient having high grade AV block with intact sinus node function is presented in whom DDDR pacing provided the benefit of preventing 2:1 pacemaker block in response to exercise-induced sinus tachycardia. In paired treadmill tests with the patient blinded as to pacing mode, she was able to exercise longer (7.5 vs 6.6 METS) when programmed in DDDR than in DDDO. This is attributable to circumvention of 2:1 pacemaker block which had resulted in abrupt onset of fatigue and SOB (shortness of breath) when the sinus rate exceeded the maximum tracking rate of 130/min. Outpatient ambulatory electrocardiographic monitoring confirmed this phenomenon during relatively strenuous activity. The theoretic advantages of dual chamber rate modulated pacing compared to the DDDO and VVIR modes are discussed.     

Cardiac Pacemaker Regulated by Respiratory Rate and Blood Temperature

A new method using respiratory rate and temperature as the guides for optimal pacing is proposed. A pacemaker was fabricated which senses these two parameters simultaneously. The pacemaker functions by calculating the cardiac rate, which would be derived from the respiratory rate and the blood temperature. The higher of the two rates is adopted as the cardiac pacing rate, i.e., at which stimuli will be delivered. The operation was tested in a mongrel dog with complete atrioventricular block. After the induction of anesthesia, a thermistor temperature probe was inserted into right atrium and a respiratory rate sensor was attached around the chest. After administration of a pyrogenic drug, both respiratory rate and blood temperature increased. The pacing rate was increased from 178 beats/minute(bpm) at 36.4 degrees C, blood temperature, and 26.5 acts/minute(apm), respiratory rate, to 233 bpm at 40.1 degrees C and 40.0 apm. Cardiac output was increased from 2.15 liters/minute(l/pm) at the beginning to 2.50 l/pm at maximum. The transition of the guide from respiratory rate to temperature was observed at about 38 degrees C.     

Diagnosis of Early Cardiac Transplant Rejection by Fall in Evoked T Wave Amplitude Measured Using an Externalized QT Driven Rate Responsive Pacemaker

Reliable diagnosis of cardiac ailograft rejection is at present only possible using endomyocardial biopsy. We have serially measured epicardial evoked T wave amplitude during ventricular pacing with an externalized QT driven rate responsive pacemaker telemetered to a TP2 analyzer in 13 patients (12 males) followed for 19 (14–26) days after transplantation. A total of 228 records were analyzed. Rejection was defined on endom-yocardial biopsy. On 17 of the 31 occasions on which biopsy was performed during the study, specimens showed significant (moderate) rejection. In 11 patients the initial biopsy proven rejection episode was associated with a significant fall in the evoked T wave amplitude from 1.3 (0.7–2.3) mV to 0.6 (0.5–1.8) mV (P < 0.005), which began 2 (1–4) days earlier. One patient with uncontrolled diabetes mellitus had no change in evoked T wave amplitude during rejection. The evoked T wave amplitude did not fall in the absence of histologic rejection. These results suggest a noninvasive method for detecting cardiac rejection, which appears both sensitive (92%) and specific (100%) in the first rejection episodes.     

Identification of Ventricular Tachycardia Using Intracardiac Electrograms: A Comparison of Unipolar Versus Bipolar Waveform Analysis

Implantable antitachycardia devices suffer a high false-positive rate of delivery of therapy because current detection schemes based upon ventricular rate and rate variations are excessively sensitive at the cost of specificity. Several methods have been proposed for providing complementary information derived from morphologic analysis of intraventricular electrograms in order to increase specificity. The majority of these techniques have utilized bipolar electrogram analysis to detect changes in ventricular activation indicative of ventricular tachycardia. Whether bipolar or unipolar intracardiac electrogram analysis might be preferred for discriminating ventricular tachycardia from sinus rhythm has not been determined. In this study, a previously demonstrated method for identification of ventricular tachycardia using intracardiac electrograms, correlation waveform analysis, was used to analyze both unipolar and bipolar signals during sinus rhythm and ventricular tachycardia recorded during electrophysiology studies of 15 patients with inducible sustained monomorphic ventricular tachycardia. Correlation waveform analysis consistently discriminated between all depolarizations during ventricular tachycardia in 14/15 patients (93%) using either electrogram configuration; 13 of the 14 patients were common to both groups. Of these patients, 8/15 (53%) had greater separation between sinus rhythm and ventricular waveforms with bipolar electrogram analysis while 7/15 (47%) had greater separation with unipolar electrogram analysis. We conclude that morphologic analysis of unipolar and bipolar electrograms may be equally effective in distinguishing ventricular tachycardia from sinus rhythm. For individual patients, either a unipolar or bipolar ventricular configuration may be preferable, and should be chosen on a patient-specific basis during electrophysiology study prior to antitachycardia device implantation.     

Impaired Activity Rate Responsiveness of an Atrial Activity-Triggered Pacemaker: The Role of Differential Atrial Sensing in its Prevention

The physiological benefit of rate responsive, single-chamber cardiac pacing is well documented. We studied the activity response of nine atrially placed Activitrax II pacemakers. Seven patients were noted to have an inadequate activity-rate response with maximal pacing rates of 85 to 101 beats/min. Marker Channel analysis revealed that the upper rate timeout was reset by far-field R wave sensing, even when sensing occurred in the atrial refractory period. These 9 pacemakers were tested by atrial sensitivity adjustment for ability to exclude far-field R wave sensing, while preserving P wave sensing. Unipolar implantation data were then examined for predictors of this differential far-R and P-wave sensing. Differential atrial sensing occurred in 4/9 pacemakers (2/2 bipolar in the right atrial appendage; 0/1 bipolar in the coronary sinus; and 4/9 unipolar). An empirically developed index utilizing unipolar implant parameters discriminated outcomes for 8/9 unipolar pacemakers. We conclude that: (1) the rate responsiveness of the atrial Activitrax II pacemaker is limited by far-field R wave sensing even when this occurs during atrial channel refractoriness; (2) reprogramming atrial sensitivity to differentially sensed P and far-field R waves may restore appropriate rate responsiveness; and (3) although a unipolar implant discriminant index may correctly identify adequacy of future rate responsiveness, the atrial application of the Activitrax II pacemaker is cautioned until further validation is forthcoming, particularly when used in unipolar and coronary sinus applications.     

The Use of Implantable Sensors for the Control of Pacemaker Mediated Tachycardias: A Comparative Evaluation Between Minute Ventilation Sensing and Acceleration Sensing Dual Chamber Rate Adaptive Pacemakers

The role of implantable sensors to control pacemaker mediated tachycardias was investigated in 16 patients with two different dual chamber rate adaptive (DDDR) pacemakers, which sensed eiter minute ventilation (DDDR-Meta, nine patients) or body acceleration (Relay, seven patients). Successive atrial sensed events beyond a programmable rate occurring in the absence of detection of exercise by the sensors were considered to represent retrograde conduction or atrial arrhythmias, and the pacemakers responded by either a mode shift from DDDR to ventricular rate adaptive (VVIR) pacing (DDDR-Meta) or by tracking at an interim rate, the so-called conditional ventricular tracking limit (CVTL, Relay). In the unipolar atrial sensing mode, myopotential sensing (MPI) and external chest wall stimulations (CWS) at 250 beats/min were induced to be preferentially sensed by the atrial channel to simulate the conditions of atrial arrhythmias. In the DDD mode, these maneuvers resulted in ventricular responses of 88 +/- 3 beats/min and 110 +/- 3 beats/min for MPI and CWS, respectively. The pacing rate was significantly reduced in the DDDR mode with the sensors correctly detecting and responding to the sensed abnormal atrial signals (68 +/- 5 beats/min during MPI and 71 +/- 5 beats/min during CWS, P less than 0.005 compared with the corresponding DDD rate). One patient with a Relay pacemaker developed spontaneous atrial flutter and the ventricular tracking responses were 140 and 85 beats/min in the DDD and DDDR pacing modes, respectively. Thus MPI and CWS are useful bedside testing methods to assess pacemaker response during atrial arrhythmias. The use of implantable sensors to judge the appropriateness of atrial rate is a new approach to the management of pacemaker mediated tachycardias.