Clinical applications of the threshold tracking pacemaker

Do daily threshold trend fluctuations of epicardial leads correlate with pacing and sensing characteristics in paediatric patients?

AIMS

To evaluate whether the magnitude of daily ventricular pacing threshold fluctuations (Deltafluctuation) in trend graphs of stored diagrams correlate with ventricular threshold and sensing changes over time.

METHODS AND RESULTS

A total of 56 children received AutoCapture devices (St. Jude Medical, Sylmar, CA, USA) connected to steroid-eluting epicardial leads. Maximum lead age at study closure was 12.2 years (median 4.0). Telemetry data and daily Deltafluctuation were obtained every 6 months. Regression slope coefficients and mean values of repeated measurements were calculated for each patient's course. High daily Deltafluctuation correlated with higher pacing thresholds (rho = 0.68, P < 0.001), lower impedances (rho = -0.38, P = 0.004), and a Deltafluctuation-incline (rho = 0.34, P = 0.01) over time. Furthermore, a Deltafluctuation-incline correlated with a pacing threshold-incline (rho = 0.34, P = 0.01). No correlation was observed for ventricular sensing. Higher daily Deltafluctuation were observed if lead age was > 5 years compared with <or= 5 years (0.75 vs. 0.55 V@0.5 ms, P = 0.028).

CONCLUSION

High amplitudes of daily Deltafluctuation correlate with higher and increasing pacing thresholds and lower impedances. Theoretically, this results from electrode microinstability on the epicardial surface. A decrease of the steroid-eluting potency of the electrode can be hypothesized to cause higher daily Deltafluctuation beyond a lead age of 5 years. Potential implications of marked daily Deltafluctuation are short-term follow-up and lead replacement in the presence of high pacing thresholds.

Acute performance evaluation of a new ventricular automatic capture algorithm

AIMS

This study evaluated the acute clinical performance of a new ventricular automatic capture algorithm developed to work with all lead types and pacing vectors.

METHODS AND RESULTS

During regular pacemaker implant or replacement, AutoThreshold and manual threshold tests were performed in ventricular unipolar (UP) and bipolar (BP, if applicable) pacing using a customized external prototype INSIGNIA pacemaker. The success rate and accuracy of two different modes (commanded and ambulatory) of the automatic capture algorithm were used to evaluate the performance. Loss-of-capture events (two consecutive non-captured beats without backup pacing) were used to assess safety. Data of 53 patients (33 DDD/20 VVI) from four medical centres were analysed. Tested leads included 43 BP and 10 UP from nine manufacturers, and seven had electrodes with low polarization. The rate of successful commanded and ambulatory AutoThreshold tests was 96 and 94%, respectively, with an average absolute threshold difference compared with manual threshold of < 0.1 V at 0.4 ms (commanded 0.07 +/- 0.07 V and ambulatory 0.08 +/- 0.07 V). There was no significant difference in performance between UP/BP pacing, polarization, and lead type. No loss-of-capture event was observed.

CONCLUSION

When successful, the ventricular automatic capture algorithm accurately determined pacing thresholds in either a UP or BP pacing configuration among all leads tested.

Use of the AutoCapture Pacing System with implantable defibrillator leads

INTRODUCTION

Previous studies using various bipolar pacemaker leads have shown that the AutoCapture (AC) Pacing System is able to verify ventricular capture and regulate pacing output, increasing patient safety with respect to unexpected threshold changes and potentially prolonging device longevity. An increasing number of patients with implantable cardioverter defibrillators (ICDs) require ventricular pacing that contributes to a shortening of longevity of these systems. This prospective study tested the compatibility of the AC system with bipolar ICD leads.

METHODS

The AC algorithm was evaluated prior to ICD testing in 30 ICD recipients. A single coil, active fixation, true bipolar ventricular lead was implanted in 21 patients, and a dual coil, passive fixation, integrated bipolar ventricular lead was implanted in 9 patients. A ventricular evoked response sensitivity test and an AC threshold test were performed using a pacemaker with the ventricular AC algorithm.

RESULTS

AC was recommended in 22/30 (73.3%) of implants, including 20/21 (95.2%) with the single coil and 2/9 (22.2%) with the dual coil lead. Mean polarization was lower (1.23 +/- 0.95 mV vs 3.70 +/- 2.33 mV, P = 0.013) while the mean evoked response was higher (18.04 +/- 8.29 mV vs 10.13 +/- 4.22 mV, P = 0.002) with the single coil leads.

CONCLUSION

Automatic threshold tracking using the AC is compatible with ICD leads. Leads with lower polarization and greater evoked response are more likely to result in recommendation of AC use. Use of this system offers the potential for increasing ICD generator longevity and improving patient safety in response to late unexpected threshold increases.

The impact of automatic threshold tracking on pulse generator longevity in patients with different chronic stimulation thresholds

Automatic adjustment of the stimulation output of pacemakers to changing stimulation thresholds using the Autocapture feature increases patient safety and decreases energy consumption. This study examined the impact of Autocapture on pulse generator longevity in patients with different chronic stimulation thresholds. Eighty patients (mean age 79 +/- 9 years; 37 men, 43 women) with Pacesetter Regency SR+ pacemakers were included in the study. Data were collected before discharge of the patients from the hospital, 6-12 weeks postimplant, and then every 6-12 months. Pulse generator longevity was calculated theoretically, assuming 100% stimulation with a stable threshold, at a pacing rate of 65 +/- 6 beats/min and 1% backup pulses. Theoretical pulse generator longevity was calculated for low (< 1 V), intermediate (> or = 1 V and < 2 V), and high (> or = 2 V) stimulation thresholds. Pulse generator longevity was compared among three groups: (A) Autocapture programmed On, (B) Autocapture programmed Off, (C) theoretical calculations using thresholds of patients in group A with the stimulation voltage programmed at twice pacing threshold, or at a minimum of 2.4 V. The mean follow-up time since implantation was 19 +/- 8 months. The calculated longevity benefits for patients in group A were 36%, 59%, and 30% compared to group B, and 19%, 32%, and 49% compared to group C in patients with low, intermediate, and high chronic stimulation thresholds, respectively. Theoretical calculations based on chronic stimulation thresholds in our patient population with Regency SR+ pacemakers suggest that Autocapture may markedly prolong pulse generator longevity in patients with a broad range of long-term pacing thresholds.

Threshold tracking pacing based on beat by beat evoked response detection: clinical benefits and potential problems

Continuous monitoring of pacemaker stimulation thresholds and automatic adjustment of pacemaker outputs were among the longstanding goals of the pacing community. The first clinically successful implementation of threshold tracking pacing was the Autocapture feature which has accomplished automatic ventricular capture verification for every single stimulus by monitoring the Evoked Response (ER) signal resulting from myocardial depolarization. The Autocapture feature not only decreases energy consumption by keeping the stimulation output slightly above the actual threshold, but also increases patient safety by access to high-output back-up pulses if there is loss of capture. Furthermore, it provides valuable documentation of stimulation thresholds over time and serves as a valuable research tool. Current limitations for its widespread use include the requirements for implantation of bipolar low polarization leads and unipolar pacing in the ventricle. Fusion/pseudofusion beats with resultant insufficient or even non-existent ER signal amplitudes followed by unnecessary delivery of back-up pulses and a possible increase in pacemaker output is not an uncommon observation unique to the Autocapture feature. The recent incorporation of the Autocapture algorithm in dual chamber pacemakers has been challenging because of more frequent occurrence of fusion/pseudofusion beats in the presence of normal AV conduction. Along with a review of the previously published studies and our clinical experience, this article discusses the clinical advantages and potential problems of Autocapture.

A cardiac evoked response algorithm providing threshold tracking: a North American multicenter study. Clinical Investigators of the Microny-Regency Clinical Evaluation Study

The purpose of this study was to evaluate a pacing system using the recognition of cardiac evoked response for the automatic adjustment of pacing output. Patients were prospectively followed after primary implantation of VVIR pacemakers using AutoCapture (St. Jude Medical CRMD). Sensing and pacing thresholds, polarization signal, evoked response, and AutoCapture performance were evaluated with serial visits and 24-hour Holter monitoring. Three hundred ninety-eight patients (mean age 71 +/- 15 years) were followed for an average duration of 1 year (3 days-1.75 years) with the algorithm functional in > 90% of patients. Backup pacing in the event of exit block was confirmed in all patients. Pacing thresholds remained stable at 0.89 +/- 0.34 V with a pulse width of 0.31 ms (with chronic output autoset at 0.3 V above the actual threshold). Evoked response exhibited a small but statistically significant increase with time (8.92 mV at implant, 9.60 mV at 12 months), however, this finding did not result in any change in AutoCapture function during our follow-up period. The polarization signal remained stable with minimal variation (1.12 mV at implant, 1.18 at 12 months). No clinical adverse events were observed using the AutoCapture algorithm. In this initial experience with the AutoCapture algorithm the evoked response and polarization measurements remained adequate, allowing the system to function in the majority of patients with safe, low output pacing. High energy backup pacing provided an added safety feature over fixed output devices in cases of unexpected threshold rises. Longer follow-up is required for continued long-term validation of the algorithm.

The next 5 years in cardiac pacemakers: a preview

Advances in cardiac pacing continue at an astounding rate, and, occasionally, technologic capabilities are developed almost faster than they can be implemented clinically. The development and implementation of single- and dual-chamber rate-adaptive pacemakers have been the major thrusts in cardiac pacing in recent years. Rate-adaptive pacing will continue to be of primary interest in the future as investigators search for the perfect "sensor" and attempt to develop rate-adaptive pacemakers with multiple sensors. The "smart" pacemaker--that is, an autoprogramming, autodiagnostic device--will also be refined. The ultimate "smart" pacemaker would be capable of automatically adjusting output and sensing factors as well as altering the rate-adaptive variables and even changing the pacing mode in response to variations in the underlying rhythm. Other aspects of cardiac pacing that will be actively investigated include new low-threshold pacing lead designs, refinements of the single-lead pacing system capable of P-synchronous pacing, and diagnostic information that can be derived from sensors used for rate-adaptive pacing.

Advances in pacing therapy for bradycardia

Advances have been made rapidly in the field of cardiac pacing. The most significant technologic advance is that of pacemakers capable of rate-adaptive pacing. Multiple types of sensors are now used for rate-adaptive pacing; some are commercially available and many are undergoing clinical investigation. In the near future, clinical investigation will begin on pacemakers that incorporate dual simultaneous sensors for rate-adaptive pacing. Significant improvement has been made in electrode design. Electrodes with low thresholds allow improved battery longevity. Steroid-eluting leads have proven reliable and capable of avoiding the early threshold rise seen with other electrodes. Standardization of pacemaker connector dimensions is now under way. The International Standards Organization has established the guidelines for connector standardization, and the guidelines have been adopted by the major manufacturers. The ultimate "smart" pacemaker would be capable of autoprogramming most or all of its programmable features. Many autoprogramming features have already been incorporated, and several others such as automatic programming of output and sensitivity are under investigation.

The autodiagnostic pacemaker

Loss of normal pacemaker stimulation and/or sensing functions requires prompt detection, automatic correction, and automatic and continuous "marking" of the intermittent failure. The autodiagnostic pacemaker (ADP) detects "failure to capture" (FC) by distinguishing, at its single stimulating and sensing electrode, between the normal biphasic cardiac response evoked by an adequate stimulus (corresponding to the QRS and T waves on the surface cardiogram) and the monophasic pseudo-response generated by electrotonic spread of a subthreshold stimulating current. Detection of "failure to sense" (FS) spontaneous cardiac activity requires two amplifiers: a "timing control" amplifier of standard fidelity and standard (approximately 250 ms) refractory period, and a second amplifier which has negligible refractoriness and provides high fidelity amplification of all evoked and spontaneous activity. Failure to sense (FS) is defined as a specified number of consecutive failures to recycle correctly the pacemaker's timing circuits. Similarly, a specified number of consecutive failures of the stimulus to evoke an active cardiac response is defined as a failure to capture (FC). When FC is detected, the ADP doubles the applied stimulus voltage and generates marker pulses which follow every subsequent stimulus by 40 ms. The marker pulses appear on the surface electrocardiogram, serving as an externally detectable "memory" of the earlier, possible corrected, failure. When FS is detected, non-stimulating marker pulses, of a different time relation (80 ms delay) to each stimulus, are generated continually and can also be detected externally. The ADP has been tested in 14 anesthetized, open-chest dogs. Unipolar rather than bipolar electrodes were used as they rpovided more reliable stimulation and more satisfactory electrograms for detection.

The automatic rate adjustment pacemaker. The possibilities of rate hysteresis

Since the first use of pacemakers there have been attempts to regulate the fixed, or basic rate of implanted pulse generators. Earlier models employed the use of magnets or percutaneous needles to change the pacemaker rate after implantation. A recent development is the programmable pacemaker, which utilizes external electromagnetic signals to alter the basic rate. A series of engineering advances have resulted in automatic pacemaker rate changes, as first embodied in the hysteresis pacemaker. Notable modifications of the basic hysteresis concept include gradual pacemaker rate changes, and variable hysteresis or rate changes dependent on electrophysiologic events. Many of these technical advances are unknown to physicians, but are disclosed in patents. In general, negative hysteresis favors emergence of underlying non-pacer rhythms, whereas positive hysteresis suppresses underlying rhythms of any type. The automatic rate adjustment pacemaker represents an attempt to derive the advantages of negative hysteresis while eliminating the disadvantages of abrupt rate changes. The unit automatically searches for a sinus rhythm slower than the basic pacing rate, by periodically gradually slowing its rate to a lower level.

Influence of output capacitor, electrode and pulse width on power consumption in cardiac pacing

Three different types of unipolar endocardial electrodes--47 in all--were compared in regard to power consumption at stimulation threshold with six different output capacitors and seven pulse widths. Fifteen were conventional large surface electrodes (area 47 mm2); 18 were conventional small surface electrodes (area 6 mm2), and 14 had a specially designed tip with a large area but small active surface of 8 mm2. Pulse widths ranged from 0.15 to 2.0 msec and output capacitors from 1.0 to 22.0 microFarads. All in all about 2,000 measurements were performed. The average current drain to the pacemaker output stage was measured and power consumption was calculated for each electrode--pulse width--output capacitor combination. In all combinations, the two small surface electrodes consumed approximately the same amount of power and, in both cases, significantly less than the larger one. With regard to power economy at stimulation threshold, the pulse width of choice was about 0.5 msec and, furthermore, power consumption decreased with increasing capacitor size. The optimal combination was a small surface electrode, an output capacitor of 22 microF and a pulse width of 0.5 msec.