Treadmill assessment of an activity-modulated pacemaker: the importance of individual programming.

Maximum benefit from a rate-modulated pacemaker requires individualized programming of rate response settings. We tested an externally strapped activity-sensing pacemaker (Activitrax-Medtronic 8400) in eight healthy volunteers, to assess the pacing responses of the different rate response and activity threshold settings. Five males and three females, aged 20 to 70 years (mean 40), performed a total of 67 treadmill exercise tests, using a specific protocol designed to assess the activity-sensing unit. The external unit was compared to implanted units in four patients, to validate its accuracy. A reproducible sinus response to the treadmill protocol was observed, against which pacing responses were compared. The activity threshold determines the degree of activity required to elicit a pacing rate response, whereas the rate response setting determines the rate attained. Rates of 140 bpm were rarely achieved, despite vigorous exercise. The sensor responds rapidly to activity, not to physiologic demand; to increase in speed, not grade. Four patients performed repeated limited treadmill tests to determine their optimum program setting, with symptomatic status and the healthy volunteer sinus response as guides. These results, and those from the external Activitrax unit, suggest that LOW 6 and MEDIUM 6-10 settings will prove optimum for most patients.

Lyme carditis: an important cause of reversible heart block.

Lyme disease is a tick-borne spirochetal infection, characterized by erythema chronicum migrans and an acute systemic illness. The disease is endemic in many parts of the north-eastern United States. Without treatment, late rheumatic, neurologic, and cardiac complications frequently occur. We report four serologically confirmed cases of Lyme carditis in previously healthy young men (mean age, 45 years) from endemic areas. Each presented with severe symptomatic atrioventricular block, three with episodes of prolonged ventricular asystole. Two had permanent pacemakers implanted (one was later removed), and another, very nearly did, before diagnosis. All four patients were treated with antibiotics, and in each case their rhythm returned to sinus, though one patient has Wenckebach second degree block with atrial pacing at 120 beats/min 16 months later. Carditis occurs in 4% to 10% of cases of Lyme disease and usually begins 3 to 6 weeks after the initial illness. It manifests as a transient myocarditis with varying degrees of atrioventricular block. The diagnosis is made primarily on clinical grounds and confirmed by serologic testing. Temporary cardiac pacing is frequently needed by patients who have severe heart block with hemodynamic instability. The evidence suggests that, in most cases, the block is at the level of the atrioventricular node. The block generally resolves completely with antibiotic treatment. Complete heart block rarely persists more than 1 week and the long-term prognosis appears to be excellent. Consideration and prompt recognition of this potentially lethal, but reversible, cause of heart block is crucial in order to avoid inappropriate permanent pacemaker implantation.

Atrial electrogram analysis: antegrade versus retrograde.

Automatic discrimination between antegrade and retrograde atrial electrograms would prevent endless loop tachycardia and contribute to tachycardia detection algorithms. We tested its feasibility by comparing antegrade and retrograde atrial electrograms in 129 patients at the time of atrial lead implantation. Only unipolar, passive-fixation, endocardial, right atrial appendage leads were included. The mean antegrade amplitude was 4.2 +/- 2.2 mV, and retrograde 2.4 +/- 1.5 mV (P less than 0.001); the mean antegrade slew rate was 2.6 +/- 2.1 mV/ms, and retrograde 1.3 +/- 1.1 mV/ms (P less than 0.001). Morphology was similar in 84 patients (65%). The antegrade amplitude exceeded the retrograde by 1.0 mV in 67%, and by 0.5 mV in 81% of patients. Morphology and slew rate contributed little to the discriminating power of amplitude alone. Thus, amplitude criteria reliably distinguish antegrade from retrograde atrial activity.

Retained pacemaker leads.

Increasingly, functionless pacemaker leads are being abandoned in place because they cannot be safely removed. One hundred eighty-nine intact or partially removed pacemaker leads were abandoned in situ in 152 patients between Jan. 1, 1965, and Dec. 31, 1985. The leads, sometimes several leads in a single patient, were deemed uninfected at the time of abandonment in 137 patients and contaminated with Staphylococcus epidermidis in 15 patients. All of the contaminated leads have remained clinically uninfected during follow-up. One clean lead became infected early after implantation and the patient died after an open cardiac operation to remove that lead and an adjacent abandoned lead that was adherent to the subclavian vein. No other patient has had a late complication during follow-up to 256 months (mean 47.6). Properly managed abandonment of an uninfected lead can carry a very low complication rate.

Comparison of myopotential interference in unipolar-bipolar programmable DDD pacemakers.

Myopotential interference (MPI) can inhibit or trigger single and dual chamber unipolar pacemakers while bipolar pacemakers are resistant. Twenty units of two different models of dual chamber pacemaker, each capable of being programmed to single chamber or dual chamber and unipolar or bipolar function were tested to provoke myopotential interference. No patient had evidence of myopotential interference at any sensitivity setting in the bipolar configuration either in atrium or in ventricle. All patients (20/20) interfered with pacemaker function at the highest atrial or ventricular sensitivity settings in the unipolar configuration. T wave sensing occurred at the 0.25 mV sensitivity setting in four patients in pacemaker model 925, in both bipolar and unipolar configurations. Twenty-five percent of patients had myopotential interference at the unipolar atrial sensing threshold and did not allow a setting which would reject myopotential interference while providing satisfactory atrial sensing. Twenty percent (2/10) had myopotential caused ventricular inhibition at the least sensitive ventricular channel setting in model 240G so that myopotential interference could not be avoided in that unit no matter how large the electrogram.

Interatrial conduction during cardiac pacing.

DDD pacemakers sense and pace right-sided cardiac chambers. The relationship of atrial to ventricular systole on the left side of the heart is of importance for systemic hemodynamics. Effective atrioventricular synchrony is partially determined by interatrial conduction time (IACT). At the time of DDD pacemaker implantation, interatrial conduction was measured using an intraesophageal pill electrode in 25 patients who were on no cardiac medications. Mean interatrial conduction time for all patients prolonged from 95 +/- 18 ms during sinus rhythm to 122 +/- 30 ms during right atrial pacing (p less than 0.001). In 16 patients with P wave duration less than 110 ms interatrial conduction prolonged from 85 +/- 10 ms during sinus rhythm to 111 +/- 9 ms during right atrial pacing (p less than 0.01) compared to 114 +/- 20 ms prolonging to 111 +/- 19 ms (p less than 0.01) in 9 patients with P wave duration greater than 110 ms. In each patient, while atrioventricular conduction prolonged with incremental right atrial pacing, interatrial conduction times did not vary. Interatrial conduction prolongs from baseline during atrial pacing and remains constant at all paced rates from 60-160 beats per minute. In addition to longer interatrial conduction times during sinus rhythm, patients with electrocardiographic P wave prolongation have longer interatrial conduction times during right atrial pacing than do normals (p less than 0.001). Based on interatrial conduction times alone, the AV interval during DDD cardiac pacing should be approximately 25 ms longer during AV pacing as compared to atrial tracking.

The dynamic nature of ventriculoatrial conduction.

An endless loop tachycardia starts when the atrial sensory amplifier of a dual chamber pacemaker identifies an early atrial signal originating from a ventricular or atrial premature depolarization or from myopotential noise. The tachycardia will continue as long as ventriculoatrial conduction is sustained. By selecting the appropriate atrial sensitivity setting, postventricular atrial refractory period, or upper rate limit, it is possible to eliminate sustained endless loop tachycardia. Electrophysiological data obtained at the time of dual chamber pacemaker implantation can assist the physician when selecting these settings. This report summarizes our intraoperative data on ventriculoatrial conduction obtained from 432 consecutive patients. One hundred sixty-two patients had evidence of ventriculoatrial conduction including 14% of patients with antegrade complete heart block and 32% with 2:1 AVB. The majority of patients with preserved antegrade conduction had sustained retrograde conduction. During incremental ventricular pacing, ventriculoatrial conduction prolonged in the majority of patients, and with faster ventricular pacing rates, ventriculoatrial block developed. Ventriculoatrial block developed in half of the patients at a ventricular pacing rate exceeding 120 bpm. Analysis of these data suggests that by selecting an upper rate limit of 140 bpm, a postventricular atrial refractory period of 300 msec, and an atrioventricular interval of 125 msec, approximately 90% of patients will not have sustained endless loop tachycardia.

Spontaneous endless loop tachycardia.

Pacemaker-mediated endless loop tachycardia is usually caused by a P wave displaced from the physiologic position preceding a QRS complex to a time of atrial channel sensitivity after the QRS. Five cases are described of endless loop tachycardia starting after a normally-timed P wave, either spontaneous and preceding a ventricular stimulus or a P wave produced by an atrial channel stimulus followed by a ventricular stimulus and QRS complex. In each instance, the atrial refractory interval (ARI) was shorter than the retrograde conduction time. In four of the cases, prolongation of the atrial refractory interval after the ventricular event ended the tachycardias. In the fifth, in which the pulse generator could not be so programmed, the ventricular inhibited mode was required.

Selective atrial sensing in dual chamber pacemakers eliminates endless loop tachycardia.

With the introduction of dual chamber pacemakers that have multiple atrial amplitude sensing values, selective P wave sensing is possible. Five consecutive patients were studied who had 1) retrograde atrioventricular conduction, 2) anterograde atrial signals that were at least 1.4 times larger than their corresponding retrograde atrial signals, and 3) dual chamber pulse generators that are capable of discriminating this difference in atrial amplitude. In each patient the pacemaker was programmed in the DDD mode and the postventricular atrial refractory interval was at least 100 ms shorter than the individual's minimal retrograde conduction time. Two atrial sensitivity settings were evaluated in each patient: a high setting to ensure sensing of both anterograde and retrograde P waves, and a lower setting to allow sensing of anterograde P waves only. Ambulatory electrocardiographic monitoring demonstrated that with a high sensitivity setting, each patient sustained endless loop tachycardia (mean number of episodes 41, range 6 to 143) and that a low atrial sensitivity setting eliminated the tachycardia. With the lower atrial sensitivity setting, there was only sporadic atrial undersensing (1.5 episodes for each 1,000 P waves). This study demonstrates that atrial signals having different amplitudes can be selectively sensed. Additionally, dual chamber pulse generators with multiple atrial amplitude sensitivity values can discriminate anterograde from retrograde P waves, ensure anterograde sensing, reject retrograde P waves and eliminate endless loop tachycardia.

Stability of atrial sensing and pacing after dual chamber pulse generator implantation.

The continued efficacy of dual chamber pacing is predicated on the stability of both atrial and ventricular electrodes. The introduction of the tined atrial J lead has decreased the incidence of atrial lead dislodgment, allowing for continued effective sensing and pacing. To study the evolution of atrial pacing and sensing threshold, 54 patients with identical pulse generators and atrial electrodes were evaluated for 58 +/- 29 weeks (mean +/- SD). Immediately after pacemaker implantation in 39 patients, the amplitude of the atrial signal was measured by programming the pulse generator to the lowest sensitivity that assured pacing in the atrial synchronous mode. Three levels of atrial sensing were possible: high (0.5 mV), intermediate (1.3 mV) and low (2.5 mV) sensitivity. Three patients had a high, 16 patients had a medium and 20 patients had a low atrial sensitivity. The P wave amplitude and slew rate measured on a physiologic recorder did not differ significantly between the latter two groups. The atrial charge threshold increased from 1.8 +/- 1.3 microcoulombs (microC) to a maximal value of 2.5 +/- 1.3 microC, 3 days to 1 week after implantation (p = 0.02). This remained elevated for 1 to 3 months (p = 0.05) and then decreased, remaining stable over the ensuing year. The atrial sensitivity for the group with noninvasive measurement did not change significantly, although there was considerable patient variation. For 54% of the patients, atrial sensing remained stable or improved. In 26% of the patients, further programming to higher sensitivity settings ws required. In the remaining 20% of the patients, the atrial sensitivity setting fluctuated.(ABSTRACT TRUNCATED AT 250 WORDS)

Maintenance of exercise stroke volume during ventricular versus atrial synchronous pacing: role of contractility.

Although atrial synchronous and rate-responsive ventricular pacing have been compared, the importance of maintaining synchronized atrial systole in addition to rate responsiveness has been incompletely defined. That is, the effects of these two pacing modes on cardiac volumes and contractility have not been studied. Accordingly, 16 patients with normal ventricular function were studied while in the upright position and at rest with gated radionuclide ventriculography during both atrial synchronous and ventricular pacing. Twelve of these patients were also studied during low-level upright exercise (300 kilopond-meters). Rest and exercise ventricular pacing heart rates were matched to those recorded with synchronous pacing. Ventricular volumes were determined with a counts-based method. The ejection fraction and peak systolic pressure/end-systolic volumes or contractility between the two pacing modes. However, during exercise to identical heart rates, blood pressures, and workloads, although stroke volume was the same during exercise with atrial synchronous and ventricular pacing (78 +/- 13 vs 75 +/- 12 ml), end-diastolic and end-systolic volumes were lower with ventricular pacing than with atrial synchronous pacing (end-diastolic volume 101 +/- 13 vs 113 +/- 16 ml, p less than .001; end-systolic volume 26 +/- 4 vs 35 +/- 7 ml, p less than .001). Stroke volume during ventricular paced exercise was maintained at atrial synchronous pacing levels by means of increased contractility (ejection fraction of 74 +/- 4% during ventricular pacing vs 69 +/- 5% during atrial synchronous pacing, p = .002; peak systolic pressure/end-systolic volume ratio of 6.51 +/- 1 during ventricular pacing vs 4.85 +/- 1 during atrial synchronous pacing, p less than .001).(ABSTRACT TRUNCATED AT 250 WORDS)