Changes in Q-T and Q-aT Intervals at Rest and During Exercise With Different Modes of Cardiac Pacing

The influence of heart rate variation on the Q-T and Q-aT intervals (measured from the onset of the QRS to the end or the apex, respectively, of the T-wave) was studied both at rest and during exercise using different modes of pacing. The studies were made on 21 patients with high-degree atrioventricular block. In seven patients with programmable ventricular inhibited (VVI) pacemakers, an increase in pacing rate during rest produced significant shortening of both Q-T and Q-aT. During observations made at rest and during exercise in 14 patients with fixed rate VVI, atrial rate matched asynchronous (VVIm) or atrial triggered (VAT) pacing. Significant shortening of Q-T and Q-aT intervals occurred during exercise in all pacing modes, but was greatest with VVIm and VAT. The Q-T and Q-aT changes were almost parallel in all situations. For measurements made by two independent observers the coefficient of variation was lower for Q-aT than for Q-T (2.2 versus 2.5) and the correlation coefficient was higher (0.96 versus 0.93), indicating easier identification of Q-aT than of Q-T. This study indicated that changes in Q-T and in Q-aT are influenced by intrinsic factors in addition to the ventricular rate. Atrioventricular synchronization did not seem to influence these changes.     

Comparative Functional Effects of Chronic Ventricular Demand and Atrial Synchronous Ventricular Inhibited Pacing

We compared the effects of chronic ventricular inhibited (VVI) and atrial synchronous ventricular inhibited (VDD) pacing on functional capacity in 8 patients with complete atrioventricular heart block. Permanent VDD (Medtronic #2409, ASVIP) pacemakers were implanted in four men and four women (age range 27-76 years, mean 58.9 +/- 18.4 years), and randomly assigned to a three-month period of VDD or VVI pacing in this single blinded, crossover study. Functional capacity was assessed by questionnaire, graded treadmill exercise testing and radionuclide angiocardiography prior to pacemaker implant and following each pacing period. Following 3 months of pacing in each of VVI and VDD pacing modes, maximum heart rate (83.4 +/- 14 vs 134.9 +/- 16.4 beats/min, p less than 0.001) and double product (147.5 +/- 58.3 vs 218.9 +/- 52.7, p less than .001) were greater with VDD pacing. Although exercise duration on treadmill exercise testing (5.3 +/- 2.9 vs 6.9 +/- 3.1 minutes, p less than 0.1) was greater in the VDD mode, the difference was not significant. Similarly, there was no significant difference in functional capacity as measured by questionnaire scores (50.1 +/- 8.4 vs 46.9 +/- 8.9, p less than 0.1) or in left ventricular ejection fraction for the two pacing modes (.54 vs .55, p less than .5). Only one patient reported a subjective improvement with physiologic (VDD) pacing, whereas the remaining patients stated no preference. We conclude that VDD pacing offers improved maximal cardiac work during exercise compared to VVI pacing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Importance of Heart Rate Response During Exercise in Patients Using Atrioventricular Synchronous and Ventricular Pacemakers

Atrioventricular synchronous pacing offers advantages over fixed-rate ventricular (VVI) pacing both at rest and during exercise. This study compared the hemodynamic effects at rest and exercise of ventricular pacing at a rate of 70 beats/min, ventricular pacing where the rate was increased during exercise and dual chamber pacing. Ten patients, age 63 +/- 8 years, with multiprogrammable DDD pacemakers were studied using supine bicycle radionuclide ventriculography. Radionuclide data during dual chamber pacing was acquired at rest and during a submaximal workload of 200-400 kpm/min. The pacemakers were then programmed to VVI pacing at a rate of 70 beats/min, and 1 week later, studies were repeated in the VVI mode at rest, during exercise at a rate of 70 beats/min, and during exercise with the VVI pacemaker programmed to a rate adapted to the DDD pacing exercise rate. At rest, the cardiac output was lower in the VVI compared with the AV sequential mode (4.1 +/- 1.1 vs 5.7 +/- 1.1 1/min, P less than 0.01). During exercise, the cardiac output increased from resting values in the DDD and VVI pacing modes, however cardiac output in the rate-adapted VVI mode was higher than in the VVI mode with the rate maintained at 70 beats/min (8.1 +/- 1.5 vs 6.3 +/- 1.1 1/min, P = 0.02). Three patients completed lower workloads with VVI pacing at 70 beats/min compared with AV synchronous pacing. At rest, AV sequential pacing was superior to VVI pacing, suggesting the importance of the atrial contribution to ventricular filling. With VVI pacing during exercise, cardiac output was improved with an increased pacemaker rate, suggesting that the heart rate response during exercise was the major determinant of the higher cardiac output.     

Pacemakers of the 1980s. An overview

Methods and devices for permanent cardiac pacing remained relatively stable for over two decades with use of the single-chamber ventricular demand (VVI) pacemaker. However, changes have occurred in the 1980s and are expected to continue with the availability of more advanced technology and with increasing knowledge about cardiac pacing. The physiologic benefit of the newer dual-chamber atrial synchronous (VDD) and fully automatic, universal (DDD) pacemakers over the VVI pacemaker in patients with permanent complete heart block and normal sinus node function has been established. These newer units not only reestablish atrioventricular synchrony but also are physiologically rate-responsive. The VDD pacemaker is expected to be phased out in favor of the DDD pacemaker. When the atrial rate or interval is lower than the lower rate limit, the VDD pacemaker functions as a VVI, whereas the DDD pacemaker functions as an atrioventricular sequential (DVI) pacemaker to maintain continuous atrioventricular synchrony. Contrary to general belief, patients with complete heart block and normal sinus node function may gain very little physiologic benefit, if any, from DVI pacing. The sinus node will compete with the pacemaker's atrial stimulation when the sinus rate is faster than the DVI pacemaker rate (which usually occurs during activity). Also, the ventricular pacing rate will not vary with physiologic change. The DVI and atrial demand (AAI) pacemakers have been used in some patients with sinus node dysfunction. Increasing exercise tolerance should not be expected in the majority of patients because they are not pacemaker-dependent during activity, ie, their heart rate is higher than the pacemaker rate. However, these pacemakers appear to help in eliminating pacemaker syndrome, which does not infrequently occur with VVI pacemakers. Patients with sinus node dysfunction but without atrioventricular block do not gain more physiologic benefit with a DDD than with a DVI pacemaker. Whether these patients have severe sinus node dysfunction all the time or adequate sinus node function most of the time during follow-up, the DDD pacemaker will function as a noncommitted DVI with atrial sensing (DDI). The early report of DVI pacemaker-induced atrial fibrillation during follow-up has been refuted by more recent works. If the DDD pacemaker is significantly more expensive than the DVI pacemaker, the latter type may be a good alternative for this condition.     

Is DDD Pacing Superior to VVI,R? A Study on Cardiac Sympathetic Nerve Activity and Myocardial Oxygen Consumption at Rest and During Exercise

Rate responsive ventricular pacing (VVI,R) has been demonstrated to equal atrial synchronous ventricular pacing (DDD) with regard to hemodynamics and exercise tolerance. Whether the two modes are also comparable, with regard to cardiac metabolic effects, is not yet dear. We assessed central hemodynamics, cardiac sympathetic nerve activity fcardiac norepinephrine overflow), and myocardial oxygen consumption in 16 patients treated with rate responsive atrial synchronous ventricular pacemakers (DDD,R), due to high degree AV block. The study was performed at rest and during supine exercise at two workloads (30 ± 12 and 68 ± 24 watts, respectively) during VDD and rate matched VVI pacing (VVI_m). Ventricular rates at rest and during both workloads were almost identical. Cardiac output at rest tended to be higher in the VDD mode, due to a slightly higher stroke volume. Central pressures including right atrial pressure and pulmonary capillary wedge pressure were similar in the pacing modes. The coronary sinus blood flow, the coronary sinus arteriovenous oxygen difference, and the myocardial oxygen consumption did not differ between the two pacing modes. Cardiac norepinephrine overflow was similar in the two pacing modes, at rest or during exercise. Thus, we found no significant differences between VDD and VVI_m pacing with regard to central hemodynamics, cardiac sympathetic nerve activity (cardiac norepinephrine overflow), or myocardial oxygen consumption either at rest or during moderate exercise.     

Long-term complication rates in ventricular, single lead VDD, and dual chamber pacing

A higher incidence of pacemaker related complications has been reported in DDD systems as compared to VVI devices. The implantation of single lead VDD pacemakers might reduce the complication rate of physiological pacing in patients with AV block. In a retrospective study, the data records of 1,214 consecutive patients with pacemaker implantation for AV block between 1990 and 2001 (VVI 36.5%, DDD 32.9%, VDD 30.6%) were analyzed. Complications requiring surgical interventions were compared during a follow-up period of 64 +/- 31 months. Operation and fluoroscopic times were longer in DDD pacemaker implantation compared to VDD and VVI devices:58 +/- 23 versus 39 +/- 10 and 37 +/- 13 minutes (P<0.001), 9.2 +/- 5.2 versus 4.1 +/- 2.4 and 3.5 +/- 2.3 minutes, respectively. Differences remained significant after correction for covariates. In a multivariate Cox regression model, the corrected complication hazard of a DDD pacemaker implantation was increased by 3.9 (1.4-11.3) compared to VVI and increased by 2.3 (1.1-4.5) compared to VDD pacing. Higher complication rates in DDD pacing were mainly due to a higher incidence of early reoperation for atrial lead dysfunction, whereas the long-term complication rate was not different from VDD or VVI pacing. Early and long-term complication rates did not differ between VDD and VVI pacemaker systems. In conclusion, operation time and complication rates of physiological pacing are reduced by VDD pacemaker implantation achieving values comparable to VVI pacing. Thus, single lead VDD pacing can be recommended for patients with AV block.     

Is Atrial Sensing of Ventricular Far-Field Signals Important in Single-Lead VDD Pacing?

In single-lead VDD pacing the atrial sensitivity frequently is programmed to sensitive values. Atrial sensing of ventricular far-field signals should be reduced by differential atrial sensing. The aim of the study was to evaluate the effectiveness of this approach. Methods: The study included 10 patients with a single-lead VDD pacemaker (Thera 8948, Lead 5032). The atrial sensitivity was set to its most sensitive value of 0.18 mV and the telemetered intraatrial EGM was continuously recorded. After atrial tracked ventricular pacing, VVI pacing was performed with pacing rates from 100 to 160 beats/min in steps of 10 beats/min and up to 165 beats/min. The peak-to-peak amplitudes of P waves (P) and ventricular far-field signals (VFFS) were measured from the recordings. The ratio P/VFFS that defines the atrial signal-to-noise ratio was calculated, and the time from stimulus to maximum of the far-field signals amplitude (Tmax) was measured. Results: P measured 0.98 ± 0.76 mV. A VFFS was visible in the atrial channel in all patients with an amplitude of 0.45 ± 0.25 mV (range 0.01–1.0 mV), independent of the pacing rate. The ratio P/VFFS was 3.9 ± 4.2 (range 0.9–21.0). Tmax measured 99.4 ± 15.2 ms during sinus rhythm. A rate dependent shortening of Tmax to 92.7 ± 11.2 ms at 140 beats/min was observed (P = 0.001). At rates above 140 beats/min no further shortening occurred. Conclusion: Ventricular far-field signals are measurable in the atrial channel of VDD systems and may reach considerable amplitudes, which are not rate dependent. Although differential sensing provides favorable P waves to ventricular far-field signal ratios, refractory periods are needed to avoid far-field sensing. The rate dependent shortening of the ventricular signal can be detected in the atrial channel in VDD pacing.     

Improvement of exercise tolerance by single lead VDD pacemaker: evaluation using cardiopulmonary exercise test

We used a cardiopulmonary test to assess the physiological benefit of single lead VDD pacing in ten patients (six men, four women; aged 32-84 years, mean 69 years) with atrioventricular block. Maximal symptom-limited treadmill exercise test using a ramp protocol was performed under VDD and VVIR or VVI pacing (VVI) in random sequence. The pacemaker was then programmed to the VDD mode, and Holter ECG was recorded in nine patients. Compared with findings during the VVI, the VDD mode had a greater chronotropic response (mean maximal heart rate, VDD 106 +/- 17 beats/min vs VVI 79 +/- 19 beats/min, P = 0.03), and was associated with prolongation of exercise duration (VDD 11.2 +/- 2.9 minute vs VVI 10.5 +/- 3.1 minute; P = 0.01), and the onset of anaerobic threshold at a higher oxygen uptake (VDD 12.4 +/- 3.4 mL/min per kilogram vs VVI 10.0 +/- 2.1 mL/min per kilogram; P < 0.01). Atrial sensing was recognized in almost all normal sinus P waves for all cases examined using Holter ECG. Thus, chronotropic response during exercise by VDD pacemaker improved exercise tolerance, indicating that a VDD pacemaker might be useful for patients requiring physical activity.     

The Effect of VVI Pacing and Resultant Atrioventricular Dyssynchrony on Segmental Volumes

Left and right ventricular volumes were monitored simultaneously in four anesthetized dogs by intracardiac impedance ventriculography during normal sinus rhythm, spontaneous ventricular rhythm, and VVI and VDD pacing. Cardiac output was found to increase with VVI pacing rate but remained somewhat lower than normal sinus or VDD values. The dissynchronous atrial contraction was found to distort the volume waveforms but had little effect on right ventricular volumes. Left atrial contraction had the greatest impact on left ventricular filling at near normal AV delays. Cardiac output during VDD pacing was found to be a maximum at 170 msec and decreased in three of four dogs at 220 msec. Examination of volume waveforms during "filling" indicates that the right ventricle is dominated by passive filling while the left ventricle demonstrates a large active or "fast" filling phase.     

The Ventricular Paced QT Interval—The Effects of Rate and Exercise

Changes in the QT and QTc intervals in 19 patients were studied at a ventricular paced rate difference of 50 beats/min. In all patients the measured QT interval shortened as the pacing rate was increased, from a mean value of 441 ms to 380 ms (p < 0.001), but when correct ed for heart rate the QTc- lengthened from a mean value of 518 ms to 575 ms. In 11 patients the QT in terval was measured at rest and immediately following exercise sufficient to increase the atrial rate by approximately 50 beats/min at identical ventricular paced rates. In all patients exercise-induced QT interval shortening from a mean value of 433 ms to 399 ms (p < 0.001). These results show first that Bazett's formula is unsuitable for correction of QT interval changes induced by ventricular pacing, and second that heart rate and changes in sympathetic tone independently influence the duration of the QT interval. It is suggested that these resuits are relevant to the design of physiological pacemakers in which the duration of the QT interval influences the discharge frequency of the pacemaker and to the consideration of ventricular pacing for the treatment of abnormal repolarization syndromes. (PACE, Vol. 5, May-June, 1982)     

Pacemaker Syndrome Evaluated by Cardiopulmonary Exercise Testing

Two patients who presented with dyspnea on effort, persisting after insertion of a fixed rate ventricular demand pacemaker (VVI) for sick sinus syndrome, were evaluated by cardiopulmonary exercise testing. During VVI pacing a heightened ventilatory response to exercise and a fluctuation of ventilation occurred. The high ventilatory equivalent for CO2 throughout exercise with VVI pacing suggests that the patients had ventilation-perfusion mismatching due to an increase in the pulmonary capillary wedge pressure caused by 1:1 ventriculoatrial conduction. Rate responsive ventricular (VVIR) pacing associated with intact 1:1 ventriculoatrial conduction exaggerated the exertional dyspnea, while rate responsive atrial (AAIR) pacing improved the ventilatory response to exercise. We suggest that a heightened ventilatory response to exercise due to ventilation-perfusion mismatching may be an important factor causing the pacemaker syndrome, and that cardiopulmonary exercise testing is useful in identifying the exercise-induced symptoms with ventricular pacing.     

Reliability of the Evoked Response in Determining the Paced Ventricular Rate and Performance of the QT or Rate Responsive (TX) Pacemaker

The TX pacemaker uses a conventional transvenous electrode to sense T-waves of paced ventricular complexes and it adapts the pacing rate to varying physiological demands by responding to changes in the QT or, more correctly, the stimulus artifact-to-T-wave (stimulus-T) interval. This pacing system was assessed in 13 patients. The relation between heart rate and stimulus-T interval and the effect of programming on the performance of this pacemaker were studied on several occasions in each patient. Treadmill exercise performance during TX pacing mode was compared with atrial synchronized ventricular (VAT) and asynchronous ventricular demand (VOO and VVI--70 beats per minute) pacing modes. T-wave sensing problems arose in three patients. In one, this was overcome by reducing the pulse amplitude from 5.0 to 2.5 V. In another patient, spontaneous recovery of T-wave sensing occurred 5 months after pacemaker implantation. T-wave sensing deteriorated with the passage of time in most patients. Satisfactory rate response as assessed by treadmill exercise testing and Holter monitoring was achieved in 12 patients through adjustments of two programmable parameters: the slope that defines the alteration in heart rate in response to a millisecond change in stimulus-T interval and the "sensing window" that is the interval during which T-waves can be sensed and a rate response is possible. Exercise performance was significantly better during rate responsive pacing (TX) mode as compared with VVI pacing but was comparable to that during VAT pacing. The resting heart rate/stimulus-T interval can be described by the following linear regression equation: stimulus-T interval = 466 - 1.68 X paced-rate, r2 = -0.62. This relation, however, was subject to wide inter- and intra-patient variation. Consequently, given identical programmed parameters and exercise protocol, the chronotropic response differed significantly from patient to patient and in the same patient from one occasion to another. Our results show that a physiologically beneficial chronotropic response can be achieved in most patients. However, reprogramming, based on results of exercise tests and Holter monitoring, may be necessary to adjust for changes in T-wave sensing and the heart rate/stimulus-T interval relation and, thus to ensure that the pacemaker continues to function optimally.     

Changes in QT Interval during Exercise Testing in Patients with VVI Pacemakers

. The changes in QT intervals were studied in nine patients with normal sinus node function who had VVI pacemakers. Though PP intervals uniformly shortened during exercise, the change in QaT* intervals during exercise was variable. The correlation between PP and QaT intervals varied from case to case. A good correlation was found in only two cases (r =+0.816 or +0.897); a fair correlation was found in four cases (r =+0.672, +0.615, +0.615, or −0.669) and in the remaining three, the correlation was poor (r =+0.494, +0.467 or−0.424). In patients who are candidates for VTI pacemaker implantation, changes in QaT intervals should be assessed during exercise stress testing to determine if the intervals shorten during exercise or not.
(QaT*: Interval from the pacing spike to the apex of T-wave.)     

Holter-Monitored Heart Rhythm During Atrioventricular Synchronous and Fixed-Rate Ventricular Pacing

The purpose of this investigation was to study rate variability during normal everyday activities among physiologically paced patients. A comparison of the spontaneous occurrence of arrhythmias with ventricular inhibited pacing was also made. VDD pacemakers in 44 patients were randomly programmed to three-week long periods of VVI (70 bpm) or VDD (back-up rate 40-50 bpm, upper rate 125-150 bpm) pacing, respectively. A Holter ECG was recorded during the last 24 hours of each period. Although the total number of QRS complexes was similar in the two pacing modes (4.3 X 10(6)/24 hours), the rate span was extensively utilized during VDD pacing. During VDD pacing, only five patients of 39 with a programmed upper rate of 150 bpm reached this limit, while one of five with 125 bpm reached that rate. Occasional episodes of pacing at the back-up rate of 40 or 50 bpm were recorded in 33 patients. Episodes of asymptomatic rhythm disturbances were recorded in both pacing modes. No significant differences existed as regards ventricular tachyarrhythmias between the two modes of pacing and it did not seem that the incidence was higher than could be expected in a patient population of the present age. Since severe AV block does not permit conduction of atrial impulses during VVI pacing, no direct comparison between the two pacing modes can be made concerning atrial tachyarrhythmias. Only one episode of pacemaker-mediated SVT was found, and this was unsymptomatic.     

Single Lead VDD Pacing: Multicenter Study

Optimal treatment for patients with AV block and normal sinoatrial node (SA) function entails atrial sensing and ventricular pacing (VDD mode). Single-lead VDD pacing preserves AV synchrony, precludes the need to insert two leads, and makes the implanter's work simpler and quicker. Our objectives were to verify the performance of the Thera(tm) VDD pacing system (Medtronic, Inc., Minneapolis, MN, USA), and evaluate the effectiveness of its atrial sensing and its ventricular sensing and pacing. In 165 patients, 150 adults (mean age 62 ± 18 years) and 15 children (mean age 7 ± 5 years) with 1°–3° AV block and normal SA node function, a Thera VDD system (Models 8948 or 8968) was implanted. Intraoperative ventricular electrical measurements were not significantly different from those of VVI pacemakers. The mean amplitude of the atrial signal during implantation was 4.1 ± 1.9 mV. Optimal atrial signals during implantation were usually obtained in the mid or lower part of the right atrium by using a special technique. Adequate atrial measurements remained stable throughout 24 months. There was no difference between serial measurements of atrial signal amplitudes at predischarge and during follow-up visits. Reposition of the lead was done in 2 patients (1.4%), and reprogramming to VVI in 7 patients: due to atrial fibrillation in 3 (1.8%) and due to atrial undersensing in 4 patients (2.4%). Thera VDD pacing is reliable and easy to manage with dependable atrial sensing and ventricular pacing. The survival rate of VDD pacing at 2 years was 96%.     

A Multicenter Evaluation of a Single-Pass Lead VDD Pacing System

Since June 1985 until April 1989, 237 patients (130 males, 107 females, aged 22 to 95 years, mean 71) with symptomatic AV conduction disturbances and competent sinus node, were implanted with a single-pass lead VDD pacing system in 30 centers and followed-up for at least 6 months. The ventricular pacing lead incorporated two atrial ring 3-cm apart electrodes, positioned within the right atrial cavity without contact with the heart wall, in order to detect the atrial activity, which is differentially processed by the pacemaker. At implant, mean atrial electrogram amplitude, derived from a custom pacemaker system analyzer (PSA) with the same input filter of the pacemaker was 1.7 +/- 0.8 mV (n = 93). In all cases, atrial sensitivity at implant was the default value +/- 0.15 mV. The atrial tracking capability of the pacing systems was assessed within the month and every 6 months after implantation by means of clinical evaluation, resting ECG, 24-hour Holter monitoring and the following tests: exercise stress testing, mental stress, isometric exercise, and nifedipine test. These tests evoke an increase of atrial rate in consequence of metabolic needs or as a reflex response. The criterion used to evaluate the correct operation of the system was the percentage of atrial synchronization. This was defined as the ratio between atrial triggered ventricular paced complexes and all ventricular paced complexes. All monitorings showed a ratio higher than or equal to 98% in a percentage of patients not lower than 95%. Mean follow-up was 385 days (range 183-1,370 days).(ABSTRACT TRUNCATED AT 250 WORDS)     

QT Sensing Rate Responsive Pacing Compared to Fixed Rate Ventricular Inhibited Pacing: A Controlled Clinical Study

Eighteen patients, five women and 13 men, (mean age 70 +/- S.E.M. 2 years) treated with QT sensing rate responsive pacemakers due to symptomatic high degree AV block took part in a double-blind study, comparing the rate responsive (TX) mode with fixed rate ventricular inhibited (VVI) pacing. The pacemaker was blindly programmed to either mode in a cross-over design. During the 1 month period a daily diary of symptoms (chest pain, vertigo, dyspnea, and palpitations) was kept. At the end of each period, a mental stress test and an exercise test were performed. The patient rated the general well-being and stated a preference for one of the modes. In the TX mode the heart rate was significantly higher at the end of exercise compared with VVI (107 +/- 4 vs 73 +/- 3 bpm; P less than 0.001) and the exercise tolerance was improved by 9% (104 +/- 8 vs 96 +/- 7 W; P less than 0.01). The patients reported significantly less dyspnea and fatigue at comparable workloads with TX pacing. During the mental stress test the pacing rate increased by 10% in the TX mode (from 73 +/- 2 to 82 +/- 4 bpm; P less than 0.001). There was a physiological rate variability on 24-hour Holter monitoring. Ten patients reported a significant improvement in feeling of general well-being in the TX mode. Eleven patients preferred the TX mode, five patients could not distinguish between the modes and two patients preferred the VVI mode due to worsening of angina pectoris with TX pacing. This preference for the TX mode was significant (P less than 0.05). The results of this controlled study indicate that TX is preferable to VVI in most cases, but the worsening of angina pectoris in two of the patients and the occurrence of rapid rate oscillations in a third patient are factors that warrant some caution in selecting patients.     

A Double-Blind Study of Submaximal Exercise Tolerance and Variation in Paced Rate in Atrial Synchronous Compared to Activity Sensor Modulated Ventricular Pacing

To assess the variation in paced rate during everyday activity and the importance of atrioventricular synchronization (AV synchrony) for submaximal exercise tolerance, atrial synchronous (DDD) and activity rate modulated ventricular (VVI,R) pacing were compared in 17 patients with high degree AV block. The patients were randomly assigned to either mode and evaluated by treadmill exercise to moderate exertion and by 24-hour Holter monitoring after 2 months in the DDD and VVI,R modes, respectively. At the end of the study, the patients were programmed to the pacing mode corresponding to the preferred study period. During the treadmill test, the mean exercise time to submaximal exertion (Borg 5/10), exertion ratings and respiratory rate did not differ between pacing modes despite a significantly lower ventricular rate in the VVI,R mode. The atrial rate during VVI,R pacing was significantly higher than the ventricular rate, but did not differ from the ventricular rate during DDD pacing. There was a diurnal variation in paced rate in both pacing modes. Paced ventricular rate was, however, higher and variation in paced rate greater in DDD compared to VVI,R pacing. Nine patients preferred the DDD mode, three patients preferred the VVI,R mode, while five subjects did not express any preference. The results from this study indicate that the variation in paced rate during activity sensor-driven VVI,R pacing does not match that during DDD pacing neither during everyday activities nor during submaximal treadmill exercise. Nevertheless, no differences in exercise time, Borg ratings, and respiratory rate during submaximal exercise were found. Thus, for most patients with high degree AV block, DDD and VVI,R pacing seem equally satisfactory for submaximal exercise.     

Atrial Synchronized Ventricular Pacing: Contribution of the Chronotropic Response to Improved Exercise Performance

In contrast to asynchronous ventricular pacing (VOO, VVI), alrial synchronized ventricular pacing (VAT, VDD, DDD) maintains the normal sequence of cardiac chamber activation and permits a chronotropic response to exercise. thereby improving exercise performance. To assess the separate contributions of these two factors to improved work capacity. 14 patients with implanted programmable VAT pacemakers were exercised according to the Bruce protocol, in three different pacing modes, selected in a random orderand on a double blind basis: (a) VAT: (b) chest wall stimulation triggered ventricular (V-CWS-T) pacing, during which the pacemaker was programmed to VAT mode but driven externally using chest wall stimulation at rates fractionally above the patients'atrial rate, thereby providing a chronotropic response to exercise without atrioventricular synchronization; and (c) VOO mode at 70 beats per minute. There was a significant improvement in exercise performance in all patients during both VAT and V-CWS-T pacing as compared to VOO mode; the average increase in work capacity being similar: VAT: 44 ± 31, (range, 12 to 140) percent and V-CWS-T; 40 ± 24 (range, 5 to 85) percent. It is concluded that in patients with adaptive pacing systems, the chronotropic response is the major determinant of any improvement in exercise performance.     

Dynamic Relationship Between the Q-aT Interval and Heart Rate in Patients with Long QT Syndrome During 24-Hour Holter ECG Monitoring

The purpose of this study was to investigate the dynamic relationship between heart rate and the Q-aT interval (the interval from the Q wave to the T wave apex) in patients with long QT syndrome. The QT to heart rate relation is useful for evaluating abnormalities of the ventricular repolarization, but its clinical application to the long QT syndrome requires accurate computer aided measurement of the QT interval and the sampling of a large number of beats. Therefore, the Q-aT interval was used on the basis of some reports that the heart rate dependency of the QT interval was concentrated in the Q-aT interval. Recent advances in the computer technology have allowed analysis of the relationship between the Q-aT and RR intervals on Holter ECG recordings. However, in addition to a prolonged QT interval, most patients with long QT syndrome have bizarre and variable T waves and the influence of this T wave morphology on the Q-aT to heart rate relation has not been clarified. We investigated the dynamic relationship between the Q-aT interval and heart rate in 10 patients with long QT syndrome and 11 control subjects using our original computer algorithm for the analysis of 24-hour Holter ECG recordings. The patients showed morphological T wave changes associated with heart rate changes during Holter recordings and these affected the Q-aT interval. The patients showed the following characteristics in the relationship between the major T wave peak and the RR interval: (1) a modestly decreased correlation between Q-aT and RR than in the control subjects (a median r value of 0.87 vs 0.93; P = 0.001); and (2) a steeper Q-aT/RR slope than in controls (a median slope of 0.24 vs 0.16; P < 0.05). Abnormal and variable T wave morphology in the long QT patients was closely related to a modestly decreased correlation between Q-aT and RR than in the control subjects. The steep Q-aT/RR slope might reflect unstable repolarization of the ventricle, which could act as a substrate for ventricular tachyarrhythmias.