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.     

Trends in Pacemakers Which Physiologically Increase Rate: DDD and Rate Responsive

Exercise (rate) responsive pacemakers benefit patients by providing increased cardiac output when needed and permitting lower rate during rest. This paper briefly reviews trends in reported studies on rate responsive pacemakers. For patients with reliable atrial rhythms, atrial-triggered pacemakers (DDD) provide physiological ventricular rates unless complications arise. At low rates, A-V synchrony benefits patients with refractory cardiac decompensation; however, in patients with healthy myocardiums, achieving higher pacing rates is more significant than maintaining synchrony. If atrial rhythms are unreliable, an alternative sensor for determining pacing rate is indicated. Pacemakers that respond to changes in right ventricular blood temperature, respiratory rate, QT interval, body vibration (motion), and pH have been implanted in humans. Clinical evaluations have shown that increased pacing rate leads to increased exercise tolerance and cardiac output when needed, independent of the sensor type (DDD, QT, respiratory rate, etc.). The effectiveness of any sensor type to increase pacing rate appropriately requires reliable sensors that respond specifically to the need for increased pacing rate. Sensors for stroke volume, venous oxygen saturation, right atrial or ventricular pressure and catecholamines are also under preclinical investigation. The availability of a reliable, long-term sensor is a key limitation to several techniques including pH. stroke volume, oxygen saturation, pressure, and catecholamines. Sensor technology and clinical effectiveness are the keys to rate responsive pacing.     

Enhanced sympathetic nervous activity after intravenous propranolol in ischaemic heart disease: plasma noradrenaline splanchnic blood flow and mixed venous oxygen saturation at rest and during exercise

To study the mechanisms by which acute beta-adrenergic blockade may change the activity of the sympathetic nervous system we have measured haemodynamic responses including splanchnic blood flow in twenty-three patients with ischaemic heart disease at rest and during supine exercise before and after i.v. injection of 0.039 mmol (10 mg) dl-propranolol. After propranolol both at rest and on exercise blood pressure, cardiac output and heart rate decreased, while splanchnic vascular resistance increased; mixed venous oxygen saturation decreased whilst arterial oxygen saturation and oxygen uptake were unchanged. Plasma noradrenaline increased after propranolol, values correlating with mixed venous oxygen saturation and splanchnic vascular resistance, both at rest and during exercise before and after propranolol, only at rest was there any correlation with arterial blood pressure. The increase in sympathetic nervous activity after propranolol may be due to a reduction in cardiac output and thereby alteration of the metabolic state (oxygen or related factors) in tissues. Afferent neural signals from the tissues may play a significant role in the regulation of sympathetic nervous activity.     

Myopotential Interference in Unipolar Rate Responsive Pacemakers

The effects of myopotential interference on unipolar rate responsive pacemakers were assessed in 22 patients. Six types of pacemakers (from four manufacturers) were studied: five TX2 (QT sensing), seven Biorate (five RDP3 and two MB-1, respiratory rate sensing), seven Activitrax (activity sensing), two Medtronic 2503 (dP/dt sensing), and one Sensolog P703 (activity sensing). Provocative tests using arm exercises were performed in both VVI and rate responsive modes. At nominal sensitivity settings (1.8-2.5 mV), 55% of these patients were myopotential positive for at least 1 provocative test. Pressing the palms together was found to be the most sensitive provocative test. Rate response was achieved with treadmill exercise (all patients), hyperventilation (RDP3 and MB-1) and tapping (Activitrax) or wobbling the pacemaker in its pocket (Sensolog). During continued rate acceleration, myopotential interference was induced by arm exercises. The duration of inhibition was shorter when the provocative tests were performed during rate response compared to that occurred at rest. Short periods of myopotential interference resulted in temporary inhibition of pacing but rate response continued immediately on removal of the interference. In one patient with a RDP3 pacemaker, a prolonged episode of myopotential interference during treadmill exercise resulted in reversion of the pacemaker to the interference mode. Appropriate adjustment of the sensitivity setting effectively controlled the symptoms in most patients. However, one patient with a QT sensing pacemaker and symptomatic myopotential interference required programming to the VVT pacing mode. Two out of five patients with RDP3 required pacemaker replacement because of uncontrolled myopotential interference.(ABSTRACT TRUNCATED AT 250 WORDS)     

The Effect of Maximum Heart Rate On Oxygen Kinetics and Exercise Performance at Low and High Workloads

The normal heart rate is lineurly related to oxygen consumption during exercise. The maximum heart rate of the normal sinus node is approximated by the formula: HR_max= (220-age) with a variance of approximately 15%. However, the nominal upper rate of most permanent pacemakers is 120 beats/min, a value that remains unchanged for many patients. As this nominal setting falls well below the maximum predicted heart rate for most patients, it is possible that the chronotropic response of rate adaptive pacemakers during moderate und maximal exercise workloads may be less than optimal. The purpose of this study was to determine the effect of the upper programmed rate on oxygen kinetics during submaximal exercise workloads and maximum exercise performance during symptom-limited treadmill exercise. Exercise performance with an upper rate programmed to 220-age was compared with an upper rate of 120 beats/min. Eleven patients (5 men and 6 women, mean age 54 ± 10 years) with complete heart block following catheter ablation of the atrioventricular junction for refractory atrial fibrillation who were implanted with permanent, rate-modulating VVIR pacemakers comprised the study population. The rate adaptive sensors were based on activity in 8 patients, minute ventilation in 2 patients, and mixed venous oxygen saturation in 1 patient. After performing a symptom-limited treadmill exercise test to determine maximum exercise capacity and to optimize programming of the rate adaptive sensor, each subject performed two treadmill exercise tests in random sequence with a rest period of at least 1 hour between tests. During one of the tests the upper rate was programmed to a value calculated by the formula: HR_max= (220-age). During the other exercise test the upper rate was programmed to 120 beats/min. Patients were blinded as to their programmed values and to the hypothesis of the study. A novel treadmill exercise protocol was used that consisted of a 6 minute, constant-workload phase at approximately 50% of maximum workload followed immedictely by incremental, symptom-limited exercise using a modified Chronotropic Assessment Exercise Protocol (CAEP) with 1 minute stages until peak exertion. Breath-by-breath analysis of expired gases was performed with subjective scoring of exertional difficulty at the end of the constant workload phase and during each stage of incremental exercise using the Borg Perceived Exertion Scale. Exercise duration was significantly longer (6.37 ± 47 vs 611 ±48 seconds. P < 0.005) with the higher programmed upper rate. Oxygen kinetics were also significantly improved with an age predicted upper rate with a lower O₂ deficit (258 ± 88 vs 395 ± 155 ml, P = 0.002) and higher VO₂ rate constant (3.6 ± 1.0 vs 2.4 ± 0.7. P < 0.001.). The V0_2maxduring peak exertion was higher with an age predicted upper rate than with an upper rate of 120 beats/min (1807 ± 751 vs 1716 ± 702 mL/min, P = 0.01). The mean Borg score was lower during the last common treadmill stage during maximum exercise with an age predicted upper rate than with an upper rate of 120 beats/min (15.7 ± 2.0 vs 16.5 ± 1.9. P = 0.04). The mean Borg score during submaximal. constant workload exercise was also lower with a higher upper rate (9.0 ±2.5 vs 9.6 ± 2.2, P = 0.10). Programming the upper rate of rate adaptive pacemakers based on the age of the patient improves exercise performance and exertional symptoms during both low and high exercise workloads as compared with a standard nominal value of 120 beats/min.     

Physiological Benefits of a Pacemaker with Dual Chamber Pacing at Low Heart Rates and Single Chamber Rate Responsive Pacing During Exercise

Dual chamber rate responsive pacing may be an ideal mode but may result in high current drain and premature battery depletion. To minimize battery drain during exercise, this study compared a combination pacing mode of IDDD and ventricular rate responsive pacing (WIR). Nine patients were studied who had complete heart block, sinus rhythm, DDD pacemakers, and a reduced mean left ventricular ejection fraction of 44%. Patients were exercised in DDD, WIR, and a combination of DDD at low heart rates and WIR at mean heart rates over 89 bpm. Blood pressure, heart rate, exercise duration, work rate, oxygen uptake, anaerobic threshold, and oxygen pulse were measured. There was no difference in symptoms or in mean cardiopulmonary function indices including exercise duration 10.7. 10.3. 10.3 minutes; heart rate 127. 133. 136 bpm; oxygen uptake 1.4. 1.5. 1.5 L/minute; or anaerobic threshold 5.6, 5.5, 5.7 minutes (p > 0.05) in any mode. A pacemaker that provides atrioventricular synchrony at low heart rates with ventricular rate responsiveness at high heart rates may be an alternative mode for some patients.     

The Effects of Rate Responsive Pacing on Exercise Performance in the Postoperative Univentricular Heart

Following the Fontan operation for definitive palliation of the univentricular heart, sinus node dysfunction, and/or atrioventricular block requiring pacemaker therapy is common. In previous studies ventricular rate responsive pacing (VVI, R) resulted in improved exercise performance over VVI pacing in anatomically normal hearts with either sinus node disease or atrioventricular block. In this study, the usefulness of both VVI, R and DDD, R pacing are evaluated in the postoperative univentricular heart following the Fontan operation. Eight postoperative Fontan patients with sinus node disease or atrioventricular block underwent exercise testing using a treadmill protocol. Six patients had single chamber ventricular pacemakers and two patients had dual chambered rate responsive pacemakers. Median age at exercise testing was 14 years. Patients were tested in the VVI, VVI, R, and DDD, R modes acting as their own controls. Heart rate, work rate, oxygen consumption, and respiratory exchange ratio were monitored continuously. Heart rate was significantly increased in the rate responsive modes compared to the VVI mode. In spite of the significant increase in heart rate, there was no change in maximal work rate or oxygen consumption. There was also no significant change in oxygen consumption at ventilatory anaerobic threshold. From these data we would conclude that VVI, R pacing in postoperative univentricular hearts does not result in improved exercise performance and that further study with DDD, R pacing is needed to determine its usefulness in this group of patients.     

Atrial Rate Adaptive Pacing: What Happens to AV Conduction?

To investigate if an nonphysiological prolongation of the AV inteii-al is common during activity sensor modulated atrial rate adaptive (AAIR) pacing, 21 patients witb sinus node disease treated with fixed rate atrial (AAI) or AAIR pacemakers were examined. Spike-Q intervals were compared at different beaii: rates obtained by overdrive pacing at rest and during exercise (Study I), measured during exercise at unresponsive (AAI), optimal (AAIR) and over responsive programming (AAIR +) of the activity sensor (Study II), and finally examined by 24-hour Holter recording in AAI and AAIR pacing modes (Study III). Study I: The spike-Q interval increased significantly with increasing heart rate at rest, but not during exercise. At rest the spike-Q interval was significantly higher at all heart rates compared to exercise. There was a significant positive correlation between the maximal spike-Q interval at rest and the maximal spike-Q inteival during exercise (r = 0.63). Study II: The spike-Q interval was shortest in the AAI and longest in the AAIR+ mode in all patients. Study III: During AAI or AAIR pacing the spike-Q interval was longest at night and shortest in the morning. The mean spike-Q interval was longer in AAIB than in AAI pacing. No statistical difference between the maximal spike-Q intervals observed during the two modes was, however, found. Variations in spike-Q interval are generally caused by changes in autonomic tone or medication with drugs with antiarrhythmic effect. Our results indicate that the risk for an nonphysiological prolongation of the AV interval during AAIR pacing is rather small and can be predicted by studying the spike-Q interval at rest during overdrive pacing.     

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.     

Atrial Inhibited Pacing in the Sick Sinus Node Syndrome: Clinical Value and the Demand for Rate Responsiveness

A consecutive series of 66 patients (males = 32; mean age +/- SD = 71 +/- 9 years) given atrial inhibited pacemakers for sick sinus nodes were followed to study the incidence of lead failures, chronic atrial tachyarrhythmias, and atrioventricular conduction disturbances. The need for rate responsive pacing was also assessed. Pre and postoperative investigation could include carotid sinus massage, Holter monitoring, exercise testing, and invasive electrophysiology. The mean follow-up time +/- SD was 32 +/- 29 months (median = 26 months). Three patients (5%) had their pacemakers replaced due to lead failures (loss of sensing = 2; exit block = 1). Two pacemakers (3%) were replaced after 5 and 22 months due to atrial fibrillation. Four patients (6%) received new pacemakers because of development of second-degree or complete atrioventricular block after 1, 6, 12, and 31 months, respectively. During exercise, most patients (76%) responded with an increase in sinus rate at least as marked as that achievable with the currently available rate responsive pacemakers. Assuming careful patient selection, atrial inhibited pacing is well suited for many patients with sinus node dysfunction and preserved atrioventricular conduction. There is a limited need for rate responsive pacemakers in these patients.     

The A-R Interval as Exercise Indicator: A New Option for Rate Adaptation in Single and Dual Chamber Pacing

RUITER, J.H., ET AL.: The A-R Interval as Exercise Indicator: A New Option for Rate Adaptation in Single and Dual Chamber Pacing. We investigated the possibility to use the interval from an atrial stimulus to the Ventricular R wave [A-R interval) as an indicator of physical stress, in 16 patients with pacemakers implanted for severe atrial bradycardia but with intact AV conduction. The A-R interval was studied during incremental atrial pacing at rest and during exercise with a constant workload. In addition, the atrial pacing rate was kept constant just above spontaneous sinus rate and the dynamics of the A-R interval were studied during exercise with a low constant workload and during a maximal exercise test with increasing workload. Incremental atrial pacing prolonged the A-R interval and this response was blunted during exercise [p < 0.003). Atrial pacing at a constant rate and during a constant workload resulted in an almost direct shortening of the A-R interval. When the workload was increased but the atrial rate kept constant, a pronounced shortening of the A-R interval was noted [p < 0.0001). It is concluded that changes of the A-R interval during different kinds of exercise were prompt and predictable in patients with sinus node dysfunction but intact AV conduction. In these patients the shortening of the A-R interval during exercise may be a suitable indicator for rate adaptive atrial pacing.     

Evaluation of a New Pacing Algorithm to Prevent Rapid Tracking of Atrial Tachyarrhythmias

The new SMARTracking (SMT) algorithm was evaluated in seven patients with the Intermedics Relay DDDE pacemakers and a history of atrial tachyarrhythmias. The SMT algorithm uses the sensor calculated rate to define a physiological band whose upper limit is defined by the SMT rate. Pacemakers were programmed to DDDR with SMT (DDDRSM), DDDR with Conditional Ventricular Tracking Limit (DDDRC), DDDR with standard upper rate behavior, and VVIR, for a period of one month each. Patients underwent a CAEP exercise test and 24-hour ECG Holter monitoring in each mode. They also had ambulatory ECG monitoring during daily activities including rest, slow and fast walk, stairs ascent and descent. Three patients were in atrial fibrillation during the daily activities protocol. Their ventricular rates were paced and highly irregular, in both DDDRSM and DDDRC modes. The heart rate was lower in DDDRSM than DDDRC at rest and low levels of exercise but not during more strenuous activity. Two patients in DDDRSM and 3 in DDDRC requested early change of their mode due to unacceptable symptoms. Two patients exhibited Wenckebach behavior at atrial rates below the upper rate limit in both DDDRSM and DDDRC modes. In conclusion, CVTL or SMARTracking are not adequate protection against atrial tachyarrythmias in patients with DDDR pacemakers.     

Atrial Sensing Performance of AV Universal Pacemakers During Exercise

To investigate the atrial sensing function of dual chamber pacemakers during exercise, we studied 57 patients aged 12 to 81 years (m = 65). They were paced for sinoatrial disorders (n = 15), second or third degree AV block (n = 37), or binodal disease (n = 11). The examination was performed 3-24 months (m = 11) after pacemaker implantation. Individual sensing thresholds were determined at rest in the supine position, and proper detection of atrial signals at the programmed sensitivity level was verified during a period of 5-9 minutes (m = 5.8). Without a change in the program of the unit, symptom-limited bicycle ergometry was performed at a maximum load of 25-200 Watts (m = 95) and 6-channel chest wall electrocardiograms were continuously recorded throughout the test, including recovery. During exercise, 25/57 patients (44%) exhibited poor atrial sensing of the pulse generator; after termination of exercise in 16 of the 25 patients, proper atrial sensing resumed within 1 to 7 minutes of recovery. In the remaining nine cases, ergometry was continued after lowering the sensitivity threshold to half the initial setting or, depending on the pacemaker model, by a value of 0.4 mV. This resulted in normal function of the pulse generator in all patients but one, who needed a sensitivity adjustment of another 0.4 mV. In a subgroup of 25 patients, telemetric atrial electrogram recordings were monitored during ergometry, 19 of which could be evaluated quantitatively. Besides random atrial signal variations, presumably due to ectopic beats or runs, a systematic decrease of the peak-to-peak amplitude by 0.1 to 1.6 mV (3-30%) was observed in 16/19 patients during exercise. Mean signal reduction amounted to 11.8% and was statistically significant at the 0.1% level. It is concluded from these findings that, besides testing of atrial sensing at rest, the follow-up of dual chamber pacemakers should include an exercise test.     

Clinical Experience with Sensolog 703: A New Activity Sensing Rate Responsive Pacemaker

Sensolog 703 is a new activity sensing rate responsive pacemaker which detects body vibration during physical exercise and uses the vibration as an indicator of the physiological need for a rate increase. This pacemaker was implanted in 11 patients with complete heart block and atrial arrhythmias. Their mean age was 58 (range 39-72) years. With appropriate rate response, exercise capacity, as assessed by the duration of graded treadmill exercise using the Bruce protocol, was significantly improved over the VVI pacing mode (mean +/- SEM, 462 +/- 52 s in the rate responsive mode and 368 +/- 34 s in the VVI mode, P less than 0.02). Cardiac output at peak exercise, as assessed by continuous wave Doppler sampling of aortic root blood flow, was also significantly increased compared to the resting value in both piecing modes. However, the increase was more marked when exercise was performed in the rate response mode (93 +/- 22% increase over resting cardiac output in the rate responsive mode and 57 +/- 13% increase in the VVI mode, P less than 0.05). The rate responses of this pacemaker were compared with those of a Medtronic Activitrax pacemaker. Although both pacemakers responded to an increase in walking speed, neither responded appropriately to walking up different gradients, In both cases, ascending and descending four flights of stairs resulted in similar pacing rates. There was no response to physiological activities with minimal body movements such as isometric exercise and the Valsalva maneuver. Technical problems were encountered in two implanted Sensolog pacemakers: one had spontaneous rate acceleration at rest immediately following implantation and one showed intermittent rate acceleration while the patient was at rest. Both units were programmed to the VVI mode. In conclusion, satisfactory rate response, improvement in exercise duration and increase in cardiac output were achieved with the Sensolog 703 pacemaker. However, as body vibration is not closely related to physiological needs, it has similar limitations in rate response as the Activitrax pacemaker.     

Comparative Evaluation of Chronotropic Responses of QT Sensing and Activity Sensing Rate Responsive Pacemakers

The rate responses of activity sensing (ATS) and QT sensing (QTS) rate responsive pacemakers to different forms and durations of exercises were compared. Nine patients with ATS and five with QTS were studied. All had complete heart block and atrial arrhythmias. At the onset, the pacemakers were programmed to achieve a pacing rate of 100-110 bpm by the end of stage 1 of the Bruce protocol, and to a pacing rate range of 70-150 bpm. With progressive exercise, using a treadmill (Bruce protocol), the maximum pacing rates in the two groups were not significantly different (mean +/- SD: 123 +/- 18 vs 129 +/- 23 bpm, ATS vs QTS). The time taken to return to the baseline pacing rate during recovery was significantly longer with QTS (178 +/- 70 vs 264 +/- 68 s, p less than 0.05). Brief exercise tests on a treadmill were performed for 3 min each with different combinations of treadmill speeds (1.2 and 2.5 mph) and gradients (0, 5, 10 and 15%). In both groups of patients, faster walking speed was associated with a faster pacing rate at each gradient. However, with increasing gradients, at each speed, there was a rise in the maximum pacing rate only in patients with QTS. During brief exercise tests, the maximum rate was achieved by the end of exercise in patients with ATS, but was delayed by 33 +/- 20 s after exercise in patients with QTS.(ABSTRACT TRUNCATED AT 250 WORDS)     

Efficacy and Safety of Ventricular Rate Responsive Pacing in Children with Complete Atrioventricular Block

Single chamber rate responsive pacing offers many potential advantages over the more complex dual chamber atrial tracking pacing mode in children, and the preservation of atrioventricular synchrony could be unnecessary in selected groups of pediatric patients. Twenty-two pediatric patients (age range 9 months to 12 years; mean 6.5 years) had implantation of ventricular rate responsive (VVIR) pacemakers over a 2-year period. All patients had chronic third-degree atrioventricular block, and a normal ventricular function at rest. During the follow-up each patient underwent a 24-hour Holter monitoring, and ten performed a graded treadmill test in both ventricular fixed rate (VVI) and rate responsive (VVIR) pacing mode. Paced ventricular rates were found to be normal for age in all 22 patients; maximum rate did not reach the higher programmed rate during daily activities in any patient. Comparing the mean paced ventricular rate to the mean rates of blocked P waves, six patients showed a difference of more than 20 beats/min, which induced the pacemaker parameters to be reprogrammed. In all patients a significant correlation was found between variations of paced ventricular rate and variations of spontaneous blocked atrial rhythm (P < 0.05); this correlation persisted in the subsequent Holter controls in the ten patients with longer follow-up. Exercise tolerance resulted normal in the ten patients who performed a treadmill test either in VVIR or VVI mode, with increased maximal heart rates and maximal systolic blood pressure in VVIR mode (P < 0.0013). Rate responsive ventricular pacemakers seem to adequately respond to the physiological needs of daily life of this selected group of children requiring permanent pacing.     

Failure of Plasma Atrial Natriuretic Peptide Levels to Increase During Exercise in Patients with Chronic Atrial Fibrillation

It is generally accepted that plasma atrial natriuretic peptide release occurs secondary to atrial stretch. The influence of coordinated atrial contraction (AC) upon this process is not fully appreciated. The aim of the study was to determine the importance of coordinated AC upon peripheral atrial natriuretic peptide levels (α-hANP) during exercise. Peripheral α-hANP levels were measured at rest and during exercise in 12 patients with complete heart block (CHB) and permanent rate responsive pacemakers. Seven patients had coordinated AC and five had chronic atrial fibrillation (AE). Each patient performed three treadmill exercise tests. Maximal inspired oxygen volume (VO₂ max) was determined during test 1. Tests 2 and 3 were performed to 70% VO₂ max, the pacemaker being programmed to either VVI or VVIR mode. Plasma α-hANP was measured using a two-site immunoradiometric assay. At rest there was a small but significant difference between the two patient groups: AF 60.2 pg/mL versus AC 97.6 pg/mL; P = 0.03. During exercise in the AC patients, there was a significant increase in α-hANP levels, in VVIR mode, to 238.4 pg/mL, and in VVI mode, to 207.9 pg/mL, P = 0.002 and 0.003, respectively. In those patients with chronic AF, there was no significant rise or fall in α-hANP levels in either pacing mode, VVIR 65.2 pg/mL, VVI 46.6 pg/mL. Previous workers have suggested that α-hANP release by nonfunctioning atria is normal. We have shown that the presence of coordinated AC is required for the release of α-hANP during exercise in patients with CHB, and that this appears to be independent of ventricular rate.     

Chronotropic Response to Exercise in Patients with Atrial Fibrillation

The exercise chronotropic response of patients with chronic atrial fibrillation requiring pacemaker therapy for bradycardia was assessed to characterize the chronotropic response and identify patients who would potentially benefit from a rate adaptive pacemaker. The population consisted of all patients who received a VVI pacemaker between January 1980 and November 1987 who underwent exercise tolerance testing (n = 130) and were in atrial fibrillation (n = 19). There were 11 males and 8 females with a mean age of 62 +/- 14 years. Left ventricular function was normal in eight patients, mildly impaired in three, and severely impaired in two. Long-term medications were continued until the morning of the test; digoxin 73%, beta blockers 21%, calcium channel blockers 26%, and nitrates 21%. Maximal exercise tolerance tests were performed. Parameters assessed were: (a) heart rate at rest, the end of each stage, and at peak exercise; (b) percent heart rate reserve per stage; and (c) percent metabolic reserve per stage. Results were compared to 100 normal subjects on no medications and without evidence of medical illness or cardiac disease. Heart rates were depressed in 58% of the study patients (21% early, 53% late) and elevated in 74% (74% early, 32% late). All 19 (100%) patients had abnormal heart rate responses at some point during the exercise test. Only four patients (21%) had a response within one standard deviation from the mean of the normal population during either the early or late segment of the exercise test. Patients in chronic atrial fibrillation requiring pacemakers for bradycardiac support at rest have an abnormal chronotropic response to exercise.(ABSTRACT TRUNCATED AT 400 WORDS)     

Evaluation of the Temperature Response to Exercise Testing in Patients with Single Chamher, Rate Adaptive Pacemakers: A Multicenter Study

Temperature responsive pacemakers were implanted in 45 patients (ages 44 to 90); 31 patients were evaluated by randomized, paired treadmill exercise tests 1 month postimplant. Of 28 males and 17 females, 19 had coronary artery disease; 8 had congestive heart failure. Pacing indications included sinus node disease (26), atrial fibrillation (15), AV block (10), and brady/tachy syndrome (10); some had multiple indications. Blood temperature (every 10 seconds, resolution = 0.004 degrees C) and pacing rate (every minute) were telemetered from the pacemaker. Average heart rate, exercise duration (5.7 min VVI; 6.7 min VVIR), VVIR response time (22 sec), initial temperature drop (0.23 degrees C) and maximum rate of drop (0.65 degrees C/min), temperature rise (0.31 degrees C VVI; 0.38 degrees C VVIR) and rate of rise (0.27 degrees C/min) were studied in a subset of patients. In pacer-dependent patients, average paired increases in exercise duration and heart rate was 56% and 34%, respectively. Including all (31) patients, some with intermittent sinus rhythm, increases were 28% and 9%, respectively. Because exercise duration increased, temperature rise was higher with rate adaptation. Rate adaptation was obtainable in all patients and patients averaged 99 +/- 48 increases above basic pacing rate per day at nominal temperature sensitivity. Conclusion: Beneficial rate adaptation is achievable using blood temperature to modify rate in a sensor based system.     

Tissue oxygenation and muscular substrate turnover in two subjects with high hemoglobin oxygen affinity.

Oxygen transport to and substrate turnover in leg muscle were studied at rest and during light and heavy upright bicycle exercise in two brothers with a hereditary hemoglobinopathy associated with high oxygen affinity (P50 = 13 mmHg). Femoral venous oxygen tension was below normal and femoral venous oxygen saturation above normal at rest and during exercise. Thus, the arterial-femoral venous oxygen saturation difference was decreased. Despite a compensatory increase in hemoglobin concentration, the arterial-femoral venous oxygen content difference tended to be below normal at heavy exercise. Approximately 25% of the oxygen was delivered via the abnormal hemoglobin at relative heavy exercise. Arterial lactate levels, lactate release, and muscle lactate concentration were not increased at any level of exercise. Glucose, alanine, pyruvate, and glycerol turnover were essentially normal, but the glycogen and creatine phosphate stores were abnormally depleted at the termination of heavy exercise. The exercise electrocardiogram (ECG) was normal, indicating that myocardial oxygenation was adequate. Muscle-surface oxygen pressure fields were normal at rest (not investigated during exercise). It is concluded that the high oxygen affinity of the hemoglobin in our two subjects did not lead to heart or skeletal muscle hypoxia during heavy exercise, as judged from the ECG and from the leg lactate turnover. Despite the lack of evidence for muscle hypoxia, the subjects experienced leg muscle fatigue and the creatine phosphate and glycogen stores were depleted more than normally.