Bruce M. Steinhaus

OPTIMIZED STANDBY RATE REDUCES THE VENTRICULAR RATE VARIABILITY IN PACEMAKER PATIENTS WITH ATRIAL FIBRILLATION

Patients with chronic atria! fibrillation (AF) and symptomatic bradycardia often receive ventricular‐based pacemakers. However, many of these patients continue to have symptoms of palpitations, which may be due to ventricular rate variability. It has previously been shown that contin uous ventricular pacing during AF has a stabilizing effect on the ventricular rate. Hence, a study was initiated to determine whether a patient‐specific optimal ventricular standby rate that reduces the ventricular rate variability, without over‐pacing, could be predicted. A ventricular rate stabilization (VRS) pacing algorithm that increases the pacing rate until instability is reduced below a threshold was developed. The VRS algorithm was utilized to determine a patient‐specific standby rate in 15 patients with chronic AF, intact AV nodal conduction, and implanted pacemakers. The computer algorithm controlled a pacemaker programmer to automatically change the pacemakers ventricular pacing rate via telemetry. Patients were studied for 15 minutes with VRS and for 15 minutes with 50 ppm fixed rate pacing (control). The results were as follows: (1) VRS versus control = P < 0.05; (2) mean ventricular pacing rate (ppm): 77 ± 13 versus 50 ± 0; (3) mean ventricular rate (beats/mm); 82 ± 13 versus 79 ± 12; (4) ventricular rate coefficient of variation (%): 11 ± 1 versus 22 ± 5; (5) percent pacing: 75 ± 8 versus 6 ± 8; (6) percent of RR intervals less than minimum pacing interval eliminated: 58 ± 12; (8) regression analysis: mean VRS pacing rate (beats/min) = 0.96 X mean control ventricular rate + 2.3, r2= 0.85. We concluded that: (1) a moderate increase in the ventricular pacing rate was required to substantially stabilize the ventricular rate; (2) the resulting mean ventricular rate increased marginally: (3) a majority of RR cycles less than each patients minimum pacing interval were eliminated; and (4) there was a linear relationship between the mean ventricular rate during control and the optimal ventricular pacing rate. Thus, a ventricular pacing rate close to the mean ventricular rate during control consistently reduced the ventricular variability. Although pacing at an increased ventricular standby rate reduces variability at rest, the optimal solution would likely be an adaptive rate algorithm that changes the ventricular standby rate as the mean intrinsic rate varies.

The Information Content Of The Cardiac Electrogram At The Stimulus Site

A cardiac action potential propagation model was used to determine the electrophysiologic properties which influence the electrogram measured at the stimulus site. The electrogram at the stimulus site was anionophasic negative QS wave during depolarization and the area was computed to obtain the paced depolarization integral (PDI). The study showed that the PDI was proportional to the product of action potential amplitude and tissue cross sectional area divided by the product of cellular coupling resistance and conduction velocity (r=0.93, n=34). The results support the clinical use of the PDI as: (1) a robust parameter to differentiate subthreshold from suprathreshold stimuli and (2) an indicator of increased metabolic demand in a rate responsive pacemaker application. The results also suggest experimental use of the PDI to estimate changes in myocardial conduction velocity or ventricular wall thickness.

Detection of Ventricular Tachycardia Using Scanning Correlation Analysis

STEINHAUS, B.M., ET AL.: Detection of Ventricular Tachycardia Using Scanning Correlation Analysis. Cross correlation is an accurate method for distinguishing normal sinus rhythm (NSR) from ventricular arrhythmias. The computational demands of the method, however, have prohibited development of an implantable device using correlation. In this study, temporal data compression prior to correlation analysis was used to reduce the total number of computations. Unipolar and bipolar intracardiac electrograms of NSR and 23 episodes of ventricular tachycardia (VT) from 23 patients were obtained from a right ventricular apex electrode catheter during routine electrophysiology studies. The data were filtered (111 Hz), digitized (250 samples/sec) and temporally compressed to 50 samples/sec. Data compression removed four out of every five samples by only saving the sample with the maximum excursion from the last saved sample. The average squared correlation coefficient (r2) was computed for the NSR and VT episodes using each patients NSR waveform as a template. In all 23 patients, the r2 values showed large separation between NSR versus VT in both unipolar (0.93 ± 0.05 vs 0.20 ± 0.16, p < 0.005) and bipolar (0.91 ± 0.07 vs 0.17 ± 0.11, p < 0.005) electrode configurations using template lengths of 80% the intrinsic interval (avg ± SD). Narrow templates 40% intrinsic interval or less) often resulted in multiple r2 peaks during each heart cycle and degraded the r2 separation (n = 10, p < 0.005). High pass filtering at 3 Hz also degraded the r2 separation (n = 10, p < 0.05). Standard noncompressed correlations indicated that data compression had negligible effects on the results. Thus, a computationally efficient cross correlation method was found to be a reliable detector of VT. The results suggest that the T wave, as well as the QRS complex, is useful in VT recognition.

Separation of Ventricular Tachycardia From Sinus Rhythm Using a Practical, Real-Time Template Matching Computer System

Template matching morphology analysis of the infra‐ventricular electrogram (IVEG) has been proposed for inclusion in implantable cardioverter defibrillators (ICDs) to reduce the number of false ventricular tachyarrhythmia detections caused by rate overlap between ventricular tachycardia (VT) and sinus tachycardia and for supraventricular tachycardia. Template matching techniques have been developed that reduce the computational complexity while preserving the perceived important aspects of electrogram amplitude and baseline independence found in such computationally unsolved methods as correlation waveform analysis (CWA). These methods have been shown to work as well as CWA for separation of VT, however, they have not been proven in real‐time on a system that incorporates many of the constraints of present day ICDs. The present study was undertaken with two purposes: (1) to determine if real‐time IVEG template matching analysis on an ICD sensing emulator was accurate in separating VT from sinus rhythm (SR) electrograms; and (2) to compare amplitude normalized area of difference (NAD) with signature analysis (SIG), a new, computationally less expensive technique that normalizes for amplitude variation within the expected physiological level of variability. In this study, JVEGs, obtained from 16 patients who underwent electrophysiological study (EPS) for evaluation of sustained ventricular arrhythmia, were digitized to 250 Hz with 6‐bit quantization after filtering (16‐44 Hz) and differentiation. After an SR template was selected and periodically updated, it was compared to subsequent IVEGs using NAD and SIG. In general, SIG calculates the fraction of samples occurring outside template window boundaries. Eleven‐beat running medians from beat‐by‐beat NAD and SIG results were determined. The maximum median during VT was subtracted from the minimum median during SR with the result equal to the separation margin. With the minimum separation threshold set to 0 (i.e., no overlap), 0.1, and 0.2, NAD separated 16/16, 14/16, and 9/16 VTs, while SIG separated 15/16, 14/16, and 13/16 VTs, respectively. While NAD separated more VT episodes on the strict basis of no overlap, SIG separated more than NAD as the safety margin was further increased. Conclusions: (1) template matching morphology techniques can potentially be implemented in ICDs; (2) using a patient specific threshold, NAD and SIG appear capable of separating VTfrom SR in most patients; and (3) SIG and NAD appear to be similar in accuracy. Thus, SIG may be preferable since it significantly reduces the computational load.

Effects of Stress and Beta1 Blockade on the Ventricular Depolarization Gradient of the Rate Modulating Pacemaker

Prism‐CLR is a dosed loop, rate modulating pacemaker that uses ventricular depolarization gradient (Gd) to continuously adjust heart rate. Heart rate response to a formal mental stress protocol, esmolol (500meg/kg bolus, 75–125 mcg/kg/min infusion), and mental stress during esmolol infusion were studied in six patients to investigate if Gd and paced heart rate response are under direct beta‐adrenergic control. Paced heart rates increased in response to mental stress in a physiological manner (P < 0.001). Response to esmolol infusion was paradoxical, with increased paced heart rates during esmolol bolus and infusion (P < 0.05). There was no significant alteration in either systolic or diastolic blood pressure during mental stress or esmolol infusion (P > 0.05). Paradoxical increase in paced heart rates during esmolol administration suggests a primary or secondary effect of esmolol to decrease the ventricular depolarization gradient, This hypothesis was supported in four dog studies in which direct Gd measurements were made during esmolol infusion. Mental stress during esmolol infusion resulted in significantly increased paced heart rates (esmolol effect) with blunted changes in heart rate in response to the mental stress. The results of this study suggest that the physiological rate response during mental stress is attributable to sympathetic autonomic response.

Computer simulation of ventricular rate stabilization during atrial fibrillation

Ventricular rate instability in patients with atrial fibrillation (AF) and intact AV nodal conduction may cause patient symptoms. Previous studies have shown that ventricular pacing at rates less than the maximum intrinsic rate during AF resulted in ventricular rate stabilization. The results of this study show that a simplified AV nodal conduction model was able to simulate this finding. The mechanism was determined to be the protective nature of retrograde propagation of the ventricular paced beats which allowed a greater time to collide with and extinguish the short cycles due to the AF.

Distinguishing Ventricular Fibrillation From Noise Using The Paced Depolarization Integral

The discrimination of ventricular fibrillation from noise interference can be problematic in an implantable arrhythmia control device. This study investigated the electrogram following a pacing stimulus as a metric to distinguish ventricular fibrillation from normal sinus rhythm during the injection of electrical noise. The electrogram at the pacing site was low pass filtered and integrated to obtain the paced depolarization integral. Results from six animal studies indicated that when compared with normal sinus rhythm, the paced depolarization integral markedly decreased during ventricular fibrillation and so could distinguish ventricular fibrillation from normal sinus rhythm during injected noise interference.

Clinical significance of a new P wave lead vector for pacemaker follow-up of atrial functions.

Patient welfare requires routine follow‐up procedures of implantable pacemakers. However, the assessment of atrial sensing and pacing functions in implantable pacemakers is often a challenge due to difficult identification of low amplitude P waves on surface electrocardiograms (ECGs). A previous body surface mapping study suggested that a novel P wave lead vector (P lead) had larger root mean square values than other standard leads. However, for pacemaker follow‐up procedures, peak‐to‐peak amplitudes are more relevant than root mean square values. In this study, the peak‐to‐peak amplitudes of intrinsic and paced P waves recorded from surface ECG standard lead II and the P lead were compared. In addition, intrinsic and paced R waves were also compared. Data recorded from 15 patients undergoing electrophysiological studies indicated that peak‐to‐peak amplitudes of the P lead were significantly larger than standard lead II: 24% for intrinsic P waves, 30% for paced P waves, and 72% for intrinsic R waves. In addition, the P lead amplitude of paced R waves showed a nonsignificant increase of 24% compared with standard lead II. Therefore, the use of this new lead vector may improve the clinical ease‐of‐use and reduce the time required for follow‐up procedures of implantable pacemakers for atrial sensing and pacing assessments.

High frequency impedance measurements from implanted unipolar pacing leads

Impedance measurements using frequencies up to 500 MHz were used along with time domain reflectometry techniques to investigate respiration sensing using standard unipolar pacemaker leads. The results have demonstrated the unique nature of these impedance sensing methods versus traditional lower frequency impedance methods. Experimental evidence includes: separation in frequency (or time and thus distance along the lead) of the cardiac stroke volume signal from the respiration signal and respiration sensing using a fully insulated tip unipolar pacing lead.

A pacing parameter for controlling ventricular rate during atrial fibrillation: potential correlation to hemodynamic function

The irregular ventricular (V) rate of atrial fibrillation (AF) may contribute to adverse hemodynamics and symptoms. An algorithm that adjusts pacing rate based on RR interval mean absolute difference (MADIFF) to reduce AF V rate variability was tested. Previous algorithms used a fixed % of V pacing to change pacing rate. Patients with chronic AF, intact AV nodal conduction, and a pacemaker were studied during V rate stabilization (VRS) pacing and control. Previous work showed that cardiac output was inversely related to the % of RRs which were short relative to the preceding RR. VRS eliminated 74% and 81% of cycles more than 10% and 20% shorter than the preceding RR interval. MADIFF was better correlated to the % of cycles more than 10% and 20% less than the preceding RR than was % pacing. Hence, a MADIFF-based VRS algorithm reduced the % of short RRs and may be a better indicator/controller of hemodynamic performance in AF than % pacing.