Atrial Synchronous Ventricular Pacing with a Single Lead: Reliability of Atrial Sensing During Physical Activities, and Long-term Stability of Atrial Sensing

A VDD pacing system with bipolar single-pass leads, were implanted in 36 consecutive patients (average age 72 ± 2years) with high degree atrioventricular block and normal sinus node function. At implant the atrial signal amplitude was 2.6 ± 0.2mV measured by a pacing system analyser (PSA), 1.8 ± 0.1mV measured peak-to-peak from the telemetered calibrated electrogram, and 1.3 ± 0.1mV measured from the sensing threshold. At one month follow-up the peak-to-peak amplitudes (mV) of the telemetered atrial electrograms were not significantly different measured continuously during resting supine with quiet breathing (1.4 ± 0.1), sitting (1.6 ± 0.2). standing (1.5 ± 0.1), arm swinging (1.4 ± 0.2), hyperventilation (1.3 ± 0.1), Vaisalva manoeuvre (1.4 ± 0.1), and treadmill exercise (1.9 ± 0.6). The telemetered atrial electrogram amplitude and the atrial sensing threshold varied between 1.2 ± 0.09mV and 1.8 ± 0.1mV, and between 0.95 ± 0.07mV and 1.3 ± 0.01mV, respectively at 0.5, 1, 3, 6 and 12 months follow-up, but the changes were statistically nonsignificant. The Event Summary showed sensing of 98% to 99% of the atrial events at the different follow-up periods.     

Myopotential Inhibition of Unipolar QRS-inhibited (VVI) Pacemakers, Assessed by Ambulatory Holter Monitoring of the Electrocardiogram

Seventy-four patients with unipolar QRS-inhibited pacemakers (VVI) were Holter monitored to assess the occurrence of pacemaker inhibition caused by skeletal muscle potentials during daily activities. Fifty patients had high-grade atrioventricular block and 24 had sinoatrial disease. Chest wall stimulation prior to monitoring revealed asystole of > 4 seconds duration in 22 patients, and ventricular rates between 25 and 56 beats per minute in 52 patients. Fifty-one patients (69%) had one or more episodes of pacemaker inhibition from myopotentials. Inhibition occurred in all types of pacemakers studies, but was most frequent and of longest duration in patients with Siemens-Elema 207/70 (13/14 patients), Cordis Omni-Stanicor (6/7 patients), CPI Microlith (5/6 patients), and Medtronic 5945 (8/10 patients). This was not unexpected considering the filter characteristics of the pacemakers. Nine patients (12%) presented symptoms which might be ascribed to pacemaker inhibition. The longest asystole observed was 3.2 s. Seven patients had pacemakers spikes falling on or near T-waves of spontaneous heart beats because their pacemakers had been rendered refractory by myopotentials. No serious arrhythmias were seen during episodes of pacemaker inhibition or interference. More emphasis should be put on the improvement of filter characteristics of unipolar VVI-pacemakers. Pacemaker patients with symptoms of myopotential inhibition should be equipped with either a bipolar or ventricular triggered (VVT) pacemaker or with a sensitivity and/or pacing mode programmable pacemaker.     

Pulse Width Dependent Increase in the Pacemaker Automatic Interval

From a total of 51 patients equipped with rate and pulse width adjustable pulse gen erators (Microlith-P 0505, Microthin-PI 0522) implanted over the last 3 years, 10 (19.6%) showed an un expected drop in pacemaker pulse rate during pulse width programming. For one of the pulse gen erators used (Microthin-PI 0522), unexpected rate decrease occurred in 7/13 cases (53.8%). For all ex cept one patient, decrease in pacemaker pulse rate corresponded with the total refractory period of the pulse generator (320 ms), at a certain pulse width when rate drop first occurred. In seven of the patients the pulse generator automatic interval was extended from 13 ms to 171 ms beyond the re fractory period. In two patients it was necessary to replace the pulse generators. Our study strongly proves that this abnormal pacemaker functioning is a result of sensing of the polarization voltage at the pacemaker electrode/tissue interface and/or the T-wave. The polarization voltage is highly dependent on the total pacemaker electrode/tissue interface impedance. Using typical values for pulse genera tor output and input resistance and output capacitance, Faraday resistance, Helmholtz capacitance and tissue resistance at the electrode/tissue interface it was shown mathematically that in some cases the polarization voltage alone would be of sufficient amplitude and slew rate for pacemaker inhibi tion. The study demonstrates an urgent need for change in the filter characteristics by making the pulse generators less sensitive in the low frequency region and reducing the polarization voltage by reducing the output circuit capacitance. (PACE, Vol. 5, May-June, 1982)     

A Time-Related Study of the Hemodynamic Benefit of Atrioventricular Synchronous Pacing Evaluated by Doppler Echocardiography

We have used Doppler echocardiography to estimate the stroke volume (SV) in a study of 13 patients equipped with DDD pacemakers. SV was measured both during DDD and VVI pacing after observation times of 1,3,6, and 12 months of DDD pacing. SV was also measured at seven atrioventricular (AV) intervals (75-250 ms) in the search for optimal AV intervals. Mitral flow velocity was investigated to see if DDD pacing resulted in synchronous atrial contraction, and if mitral insufficiency existed at any of the pacing modes. Compared with the VVI mode, DDD pacing resulted in a mean increase in SV of 21 +/- 2% for the four observation periods. Two patients with severe left ventricular failure had no significant increase in SV during DDD vs VVI pacing. In each patient, an optimal AV interval ranging between 100-250 ms for the SV was found. Velocity profiles of mitral flow showed synchronous atrial contraction during DDD pacing, but not during VVI pacing. Mitral insufficiency was not seen in any pacing mode. DDD pacing resulted in a reduction in SV during the first 6 months, and was constant thereafter. Doppler echocardiography can be used repeatedly to evaluate the hemodynamic response of DDD pacing vs VVI pacing, and to find which AV interval gives the highest SV in the individual patient. Our study further shows that the hemodynamic benefit of DDD pacing is present after short-term as well as after long-term DDD pacing.     

Inhibition/Filter Characteristics and Input Impedances of QRS-Inhibited Demand Pacemakers Determined by in vitro Studies

There is still no standardized test procedure established for demand pacemakers. Much work has been done to reduce demand failures, but more knowledge is needed to arrive at better results. This study was initiated by in vivo observations of pacemaker malfunctions and unwanted pacemaker effects, the objective being to arrive at a better match between spontaneous cardiac activity and the pacemaker system. The study describes inhibition characteristics and input impedances in some modern temporary as well as permanent QRS-inhibited pulse generators, based on in vitro experiments with various signal waveforms. The different pulse generators tested showed a wide variety of inhibition characteristics. The interrelationship between signal amplitude and maximum derivative required to obtain pacemaker inhibition is pointed out. A better approach to describe the inhibition characteristics of demand pacemakers seems to be the introduction of the time integral (voltseconds) instead of the maximum derivative of a signal (Fig. 3). It is shown that this method nearly removed the discrepancies in inhibition characteristics between different pulse waveforms used. The input impedances were also widely dispersed and were in some instances of a magnitude so low that it would lead to marked reduction of the electrogram amplitude in case the electrode/tissue interface impedance was high. The characteristics of temporary pulse generators were in several respects different from those of the permanent ones. The results obtained with a temporary unit during a test procedure are therefore not the same as for a permanent pacemaker system.     

Long-Term Clinical Performance of a Central Venous Oxygen Saturation Sensor for Rate Adaptive Cardiac Pacing

Rate adaptive ventricular pacemakers using central venous oxygen saturation (O₂Sat) to control the pacing rate have been implanted in 14 patients (mean age 71 years), with a mean follow-up period of 44 months (range 2–63 months). In eight patients the pacemakers were replaced due to signs of battery depletion after an implant duration of 39–58 months. During bicycle exercise testing the O₂Sat decreased on average from 61%± 4% at rest to 36%± 4% (P < 0,0001) at peak exercise, and the maximum pacing rate was 122 ± 5 beats/min. The time delay until the O₂Sat bad dropped 10%, 65%, and 90% of the total reduction during exercise was 4.8 ± 0.9 seconds, 39.8 ± 3.8 seconds, and 71.3 ± 7.5 seconds, respectively. The O₂Sat decreased 9.4%± 2% (P <0.005) from resting supine to resting sitting. Oxygen breathing increased the telemetered O₂Sat from the pacemaker by 8.4 %± 1 % (P < 0.001). During follow-up the O₂Sats were relatively stable in 50% of the patients, but demonstrated significant fluctuations in the others. At 1-year invasive follow-up O₂Sat measured by the pacemaker decreased 22%± 2%, and in blood samples from the right ventricle 22%± 2% from rest to 3 minutes exercise at 25 watts. There was a significant correlation between O₂Sat measured by the pacemaker and in blood samples from right ventricle (n = 105; r = 0.73; P < 0.001). In two patients the O₂Sat dropped significantly during pneumonia. In another patient episodes of angina pectoris was associated with low O₂Sat and a concomitant fast pacing rate.     

The convergence angle in teeth prepared for artificial crowns

The bucco-lingual and mesio-distal convergence angles in vital and rootfilled teeth prepared for complete crowns were measured microscopically on 190 stone dies. In vital teeth the mean convergence angles varied between approximately 19 and 27 degrees. In rootfilled teeth the mean angles varied between 12 and 37.     

Temporary Pacemaker Treatment in Open Heart Surgery: Pre- to Postoperative Changes in the Electrogram Characteristics

Thirty-three patients undergoing cardiac surgery in general hypothermia were investigated during temporary pacemaker treatment for changes in right ventricular electrogram amplitudes (AMAX, UMAX) and maximum derivatives (DMAX, SMAX) from pre- to postoperative phase (AMAX = amplitude of the part of the electrogram with highest mean maximum derivative (SMAX), DMAX = maximum derivative, UMAX = maximum amplitude deflection). Standard commercially available electrodes were used in 28 of the patients. A paired comparison (n=29) showed a fall in AMAX from 8.64 ± 0.91 mV (mean ± SEM) preoperatively to 4.94 ± 0.43 mV (p < 0.001) between the 4th and 6th postoperative day; UMAX dropped from 11.09 ± 0.95 mV preoperatively to 5.44 ± 0.42 mV (p < 0.000001) from the fourth to the sixth postoperative day. In the same period DMAX fell from 1.57 ± 0.13 V/s to 0.67 ± 0.05 V/s (p < 0.000001), and SMAX from 0.76 ± 0.06 V/s to 0.32 ± 0.02 V/s (p < 0.000001). The most marked fall in amplitudes and maximum derivatives occurred during the first 24 hours. A slight, but nonsignificant increase occurred in amplitudes and maximum derivatives from the 4th to 6th postoperative day until the electrodes were removed the 10th to 19th postoperative day. Amplitudes and maximum derivatives were of the same value in patients with aortic valve compared with coronary heart diseases in spite of a more deteriorated mypcardial function in the former group. The changes in amplitudes and maximum derivatives followed the same pattern in the two groups from the pre-to postoperative phase. This indicates that the additional local hyperthermia applied to the patients undergoing valve surgery was of no importance in the electrogram changes. Despite the fact that the electrogram maximum derivative and maximum amplitude needed to inhibit a temporary pulse generator are of a low magnitude, the values found were so small that they might provoke demand failure. This actually occurred in four patients.