The Hemodynamic Importance of Atrial Systole: A Function of the Kinetic Energy of Blood Flow?

The relative importance of atrial systole on left ventricular filling was investigated at rest and during exercise in 25 patients with dual chamber pacemakers. The mean blood flow velocity over the atrial valve, the velocities of the rapid filling phase (E), the active filling phase (A), and the E/ A ratio were determined for pulsed Doppler-echocardiography. The patients were first examined at rest during AV sequential pacing (DVI) at 70 and 104 beats/mm. The investigation was subsequently repeated during atrial synchronous pacing (VDD) at rest and during supine submaximal exercise at workloads adjusted to achieve heart rates corresponding to those during DVI pacing. The mean blood flow velocity at rest did not differ between DVI and VDD pacing at 70 beats/mm (0.46 vs 0.49 m/sec). When (he resting heart rate was increased to 104 beats/min (DVI) the mean blood flow velocity increased to 0.56 msec (P < 0.001). At a corresponding heart rate during exercise (VDD) the velocity increased to 0.70 msec (P < 0.001). At a resting heart rate of 70 beats/min the E/A ratio (n = 14) did not differ significantly between DVI and VDD pacing. With an increased resting heart rate (DVI) the E/A ratio decreased from 0,94 ± 0.45 to 0.78 ± 0.18; NS. When the heart rate increased during exercise (VDD) the E/A ratio increased from 0.75 ± 0.14 to 0.97 ± 0.16; P < 0.001. There was a positive correlation between the increase of the mean blood flow velocity and the increase of the E/A ratio during exercise (r = 0.69, P < 0,01). No such correlation was found when the heart rate was changed at rest. Thus, the importance of atrial systole on ventricular filling diminishes during exercise in accordance with increasing blood flow velocity, which by physical principles is related to the kinetic energy. The relative importance of atrial systole is hence inversely correlated to the kinetic energy of the blood flow.     

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.     

Right Ventricular Blood Temperature Profiles for Rate Responsive Pacing

To establish the efficacy of a temperature-based pacemaker control algorithm, right ventricular temperature and heart rate were measured for 12–70 hours in eight patients (51 ± 17 years) and in one normal volunteer (28 years) during a variety of activities including exercise, rest, sleeping, eating, drinking, and bathing. A diurnal variation in heart rate and temperature was observed. Drinking caused transient temperature changes (less than one minute); during eating, increases of 0.07–0.36°C over 3–12 minutes were observed. An increase of 0.24°C over 8.5 minutes was observed in one patient during bathing. An abrupt drop in temperature was typically observed at the onset of exercise, followed by a steady temperature rise. During treadmill exercise, after a drop (0.13–0.48°C, Bruce n - 4; 0A6–0.34°C, Naughton, n = 3) during the first 1–2 minutes, temperature rose steadily through the end of peak exercise (0.45–1.01°C, Bruce; 0.28–0.47°C, Naughton). A temperature dip was also observed when a patient was told exercise would start but the treadmill failed to turn on. The dip is probably secondary to changes in blood flow from the peripheral circulation to the central system at the onset of exercise. Repeated exercise separated by short rests caused progressive blunting of the initial dip. Right ventricular temperature changes in a predictable manner with daily activity, allowing a temperature algorithm to detect rest and exercise.     

Central Body Temperature as a Guide to Optimal Heart Rate

Les stimulateurs cardiaques à fréquence fixe ne restaurent pas la fonction hémodynamique normale. Puisque un pourceniage élevé de patients porteurs ďun stimuJateur cardiaque a des anomaJies auriculaires, un outre moyen de "piloter" le pacemaker est présentér celui de la témperature du sang dans ľoreillette droite. Les sondes à enregistrement thermique mesurent la variation de température pendant ľexercice et peuvent servir de guide pour déterminer la fréquence cardiaque optimale.
Studies in man suggest that fixed-rote artificial pacemakers do not return hemodynamic function to normal, since the principal mechanism for the increase in cardiac output with exercise, increased heart rate, is not restored. Special pacemakers are available that can detect atrial activity and pace the ventricles in coordination, but nearly half of the patients receiving artificial pacemakers have abnormal atrial function fatrial fibrillation, sick sinus syndrome). This study examined the effects of exercise on the temperature of blood returning to the right atrium. Precision thermistors, placed in the right hearts of conscious dogs, recorded temperature increases of 1°C (range 0.4–1.5°C) during submaximal treadmill exercise. Temperature change correlated well with work load and changes in heart rate.     

Relationship Between Right Atrial and Mixed Venous Oxygen Saturation and Heart Rate During Exercise in Normal Subjects and Patients with Cardiac Disease

FRENCH, W.J., ET AL.: Relationship Between Right Atrial and Mixed Venous Oxygen Saturation and Heart Rate During Exercise in Normal Subjects and Patients with Cardiac Disease. An ideal sensing variable for use in rate responsive pacemakers should measure a physiological parameter that closely correlates with heart rate during various activities in a diverse group of subjects. Nineteen patients, 12 normal and 7 patients with heart disease, were studied to assess the relationship between mixed venous oxygen saturation and heart rate. In patients with heart disease right atrial oxygen saturation and heart rate were also compared. Each subject underwent pulmonary artery catheterization and performed seated cycle ergometer exercise. Gas exchange and heart rate were measured continuously and blood sampled at frequent intervals. Normal patients were studied at rest and during steady-state exercise (mean work rate 149 watts). Patients were studied at rest, steady-state exercise [mean work rate 37 watts), and during incremental exercise (5–10 wattsimin) to tolerance. There were 248 paired right atrial or mixed venous oxygen saturation/heart rate observations obtained. Changes in mixed venous oxygen saturation and heart rate were not substantially altered by fitness or cardiac disease. Rate responsive pacemakers sensing changes in oxygen saturation may be a superior sensing variable for both normal and patients with heart disease.     

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.     

The Optimal Pacing Rate: An Unpredictable Parameter

A three phase relation has been demonstrated between increasing heart rate and cardiac output at rest. Phase I with cardiac output increasing with increasing heart rate, phase II a plateau, and phase III decreasing cardiac output with any further increase in heart rate. The “optimal rate” can be defined as the heart rate at the onset of phase II. Twenty patients were studied, 13 male, mean age 60 years (range 31–71 years). All had chronic complete heart block and established DDD pacing. A maximal exercise test was performed to determine peak sinus rate. Exercise hemodynamics were measured using an ambulatory monitor (Capintec Vest), which permits measurement of relative cardiac output and relative ejection fraction. The patients were programmed to VVI pacing at a rate of 60 beats/min and performed three exercise tests at different workloads. The order of workloads was randomized and selected from a range (0, 25, 50, or 75 W) depending on fitness. After 3-minute stabilization, the VVI pacing rate was increased at 1-minute intervals until higher than peak sinus rate giving a total exercise time of 12 minutes. The “optimal rate band” was determined at each workload. The mean of this “optimal rate band” for each workload varied in a nonlinear manner. There was no correlation between “mean optimal rate” and age or the peak rate predicted by the Astrand formula. Current definitions of chronotropic incompetence are inaccurate. Are some of these people at their “optimal rate” already? The arbitrary selection of rate response curves on age related criteria may lead to an impaired hemodynamic response.     

Varying the heart rate response to dynamic exercise in pacemaker-dependent subjects: effects on cardiac output and cerebral blood velocity

Cerebral blood flow increases upon the transition from rest to moderate exercise, but becomes affected when the ability to raise CO (cardiac output) is limited. HR (heart rate) is considered to contribute significantly to the increase in CO in the early stages of dynamic exercise. The aim of the present study was to test whether manipulation of the HR response in patients dependent on permanent rate-responsive ventricular pacing contributes to the increase in CO, MCA V(mean) [mean MCA (middle cerebral artery) velocity] and work capacity during exercise. The effect of setting the pacemaker to DSS ('default' sensor setting) compared with OSS ('optimized' sensor setting) on blood pressure, CO, SV (stroke volume) and MCA V(mean) was evaluated during ergometry cycling. From rest to exercise at 75 W, the rise in HR in OSS [from 73 (65-87) to 116 (73-152) beats/min; P<0.05] compared with DSS [70 (60-76) to 97 (67-117) beats/min; P<0.05] was larger. There was an increase in SV during exercise with DSS, but not with OSS, such that, at all workloads, SVs were greater during DSS than OSS. The slope of the HR-CO relationship was larger with DSS than OSS (P<0.05). From rest to exercise, MCA V(sys) (systolic MCA velocity) increased in OSS and DSS, and MCA V(dias) (diastolic MCA velocity) was reduced with DSS. No changes were observed in MCA V(mean). Manipulation of the pacemaker setting had no effect on the maximal workload [133 (100-225) W in OSS compared with 129 (75-200) W in DSS]. The results indicate that, in pacemaker-dependent subjects with complete heart block and preserved myocardial function, enhancing the HR response to exercise neither augments CO by a proportional offset of the exercise-induced increase in SV nor improves cerebral perfusion.     

A New Rate-Modulated Pacemaker System Optimized by Combination of Two Sensors

A new rate-modulated pacemaker system optimized by combination of two sensors is described. The parameter body activity and central venous blood temperature control the pacemaker rate. The specific characteristic of each parameter determines its role within the algorithm. While the motion sensor yields a fast reaction following the onset or a change of stress intensity, central venous blood temperature corresponds better to body metabolism. An indication of increased exercise from the motion sensor results in an accordingly rapid increase in the pacing rate. Unless this increased exercise is confirmed by an increase in central venous blood temperature within 2 or 3 minutes, the new motion level will be assumed to be the new baseline motion value and the pace rate will return to a basic pacing rate. Prolonged inappropriate responses are therefore avoided. Longer lasting exercise, fever and nonphysiological signals are recognized and handled safely. Exercise tests with five volunteers under various conditions showed pacing rate behavior that was close to normal.     

Static exercise-induced increase in blood pressure in individuals with cervical spinal cord injury

OBJECTIVE: To compare the pressor response to static exercise in subjects with cervical spinal cord injury (SCI) at the C6 to C8 level with that in able-bodied control subjects. In these SCI subjects, the descending supraspinal sympathetic neurons and afferent pathways from the contracting muscles to peripheral vessels via the medullary cardiovascular center are damaged. DESIGN: Mean arterial blood pressure, heart rate, and plasma concentrations of norepinephrine, epinephrine, renin activity, vasopressin, aldosterone, and human atrial natriuretic peptide were measured during a 2-minute period of sustained contraction of elbow flexor group muscle in 7 SCI subjects and 7 age-matched able-bodied control subjects. RESULTS: Static exercise resulted in a significant increase in mean blood pressure (p<.05) in both SCI subjects (pre-exercise. 74.7+/-2.2 mm Hg; static exercise, 81.9+/-4.1 mm Hg) and control subjects (pre-exercise, 101.0+/-4.2 mm Hg; static exercise, 117.0+/-4.9 mm Hg). In SCI subjects, there was no change in heart rate during exercise, whereas in control subjects heart rate increased during exercise (p<.05) (pre-exercise, 8.7+/-3.8 beats/min: static exercise, 76.0+/-3.1 beats/min). There were no significant changes in the hormone levels in the SCI subjects throughout the experiment. CONCLUSION: The significant increase in mean blood pressure observed in the present study indicates the presence of peripheral control from muscle receptors and evoked pressor response during static exercise in SCI subjects.     

Assessment of the Chronotropic Response at the Anaerobic Threshold: An Objective Measure of Chronotropic Function

MEINE, M., et al. : Assessment of the Chronotropic Response at the Anaerobic Threshold: An Objective Measure of Chronotropic Function. The evaluation of the heart rate response to exercise is important for the diagnosis of chronotropic incompetence and the assessment of a rate responsive algorithm of sensorcontrolled pacemakers. The aim of the present study was to examine a classification of the chronotropic response at an individually moderate exercise level. Sixteen pacemaker patients (patient group, age 62.9 ± 7.6 years ) with sick sinus syndrome and 15 age‐matched healthy subjects (control group, age 57.6 ± 9.4 years ) underwent a maximum cardiopulmonary exercise test on a treadmill after a protocol with individually selected incremental steps. To analyze the patients' intrinsic heart rate response, the rate responsive mode of the pacemaker was switched off. Chronotropic incompetence was diagnosed in eight patients whose maximal heart rate was < 80% of the age‐predicted heart rate. The heart rate at the anaerobic threshold was significantly lower in the chronotropically incompetent subgroup than in the chronotropically competent patients and the healthy subjects (85.9 ± 6.6 beats/min vs 100.3 ± 9.9 beats/min and 112.9 ± 11.7 beats/min , respectively). The chronotropic slope of the heart rate reserve as a function of the metabolic reserve was significantly higher in the control group than in the patient groups with either mild or severe chronotropic incompetence (0.94 ± 0.17 vs 0.64 ± 0.08 and 0.43 ± 0.14 , respectively). Furthermore, the chronotropically incompetent response could be divided into a linear type with and without a threshold, an exponential, and a logarithmic type. The anaerobic threshold was an objectively detectable breakpoint at an individually moderate exercise level that could be used for characterization of chronotropic function. At the anaerobic threshold, a physiological heart rate response was about 220 ‐ age – 50 beats/min. A deviation of more than 10 beats/min below this physiological value characterized chronotropic incompetence.     

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)     

Comparison of Continuously Recorded Sensor and Sinus Rates During Daily Life Activities and Standardized Exercise Testing: Efficacy of Automatically Optimized Rate Adaptive Dual Sensor Pacing to Simulate Sinus Rhythm

The normal sinus rhythm remains the gold standard to compare the rate response of a rate adaptive pacemaker. The aim of this study was to assess an automatically optimized dual sensor system by continuous comparison of the normal sinus (SR) and sensor indicated rates (SIR). Twelve patients with complete heart block (mean age 60 ± 9 years) with normal sinus rhythm received a dual sensor pacemaker driven by combined, automatically adaptive activity and QT sensors. After I month of automatic adaptation, patients performed a treadmill exercise in the VDD mode with simultaneous collection of SR and combined SIR. Thereafter the difference between SR and SIR was recorded over a 1-month period using a software downloaded into the pacemakers, with the patients ambulatory during this period. During exercise testing, the SR and SIR were significantly correlated (r =0.96 ± 0.02, P < 0.001), and the mean difference between SR and SIR was 4.01 ± 4.47 beats/mm. The percentages of paced beats, over the 1 month ambulatory period, that exhibited a difference between SR and SIR of 8 beats/mm were 98%± 2%, 90%± 4% and 67%± 8% for low, medium, and high workloads, respectively (P < 0.05, ANOVA). whereas > 95% of SIR were within 15 beats/min of SR independent of the level of activities. Thus, an automatically programmed dual sensor gives an accurate reflection of SR during exercise. SIR was less accurate for more vigorous daily life activities, but most of the SIR were within the normal SR variation of 15 beats/min.     

The physiologic sequelae of chronic dynamic exercise

The physiologic results of acute dynamic exercise include complex neurologic, hormonal, pulmonary, and cardiovascular adjustments that provide an integrated response perfectly matching oxygen supply with oxygen demands. Long-term repeated bouts of dynamic exercise of sufficient intensity and duration yield predictable changes in anatomy and physiology. These changes affect active skeletal muscle and the heart. Changes in skeletal muscle include an increased capillary blood volume, increased mitochondrial density, increased oxidative pathway enzymes, and more efficient regulation of blood flow. These adaptations result in an increased oxidative capacity and more favorable fuel utilization. Oxygen extraction increases, accounting for up to 50 per cent of the increased maximal oxygen consumption, and endurance improves. Following chronic dynamic exercise the heart beats slower and has a larger stroke volume at rest and throughout a broad range of work intensities. The maximal cardiac output increases substantially, accounting for up to 50 per cent of the increased maximal oxygen consumption. The metabolic and biochemical changes found in skeletal muscle are not found in cardiac muscle. Changes found in isolated cardiac muscle do not always correlate with heart performance. The separation of central and peripheral factors in assessing heart performance is difficult because preload and afterload are major determinants of heart function and are altered by chronic dynamic exercise. Ischemia is a major stimulus for the development of coronary collateral vessel development in animals. Because dynamic exercise does not induce ischemia in normal humans, collateral vessel development may only occur in those with coronary heart disease. However, there is no convincing evidence that chronic dynamic exercise results in physiologically important coronary collateral vasculature in patients with angina. Improved work capacity is predictable following chronic dynamic exercise in patients with coronary heart disease. Although the rate pressure product that produces angina does not change following training, heart rates are lower at matched absolute workloads and the maximal consumption of oxygen increases. Changes in heart function are largely secondary to peripheral changes in these patients.     

Plasma concentrations of adrenaline,noradrenaline and dopamine during forearm dynamic exercise

Summary. In six healthy volunteers plasma concentrations of adrenaline, noradrenaline and dopamine were measured at rest and during dynamic forearm exercise at submaximal and maximal intensities. Arterial and venous concentrations of adrenaline and noradrenaline increased with forearm exercise at all workloads. Dopamine concentrations did not change. The increases in adrenaline and noradrenaline were almost linearly related to the increase in heart rate with no levelling off at maximal exercise intensities. It is concluded that dynamic exercise with the forearm muscle group causes a small but significant activation of the sympatho-adrenal system as reflected by increases in plasma concentrations of adrenaline and noradrenaline.     

Rate-Adaptive Cardiac Pacing in Children Using a Minute Ventilation Biosensor

YABEK, S.M., ET AL.: Rate-Adaptive Cardiac Pacing in Children Using a Minute Ventilation Biosensor. Chronotropic integrity is required for a normal cardiac output response to exercise. We evaluated a rate-adaptive ventricular demand pacemaker (Telectronics, META-MV) which uses minute ventilation as the sensed physiological variable for adjusting pacing rate, in seven young patients with a mean age of 11.4 years. All patients had clinically significant bradycardia related to complete heart block (n = 4) or sinus node dysfunction (n = 3). For the entire group, paced heart rates increased from 70 ± 10 beats/min to 151 ± 19 beats/min with exercise testing. The onset of rate adaptation took < 30 seconds. Changes in paced rate were linearly related to workload, VO₂ (5.9 to 20.7 mL/min/kg) and minute ventilation (8–65 L/min). The decline in pacing rate after exercise was related directly to the gradual decrease in minute ventilation and VO₂. Our data show that minute ventilation closely and accurately reflects the metabolic demands of varying workloads in children and can be used to achieve physiological, rate-adaptive pacing.     

Comparison between arm and leg exercise in women and men.

Arm and leg work was performed on bicycle ergometers in sitting position by fourteen women and sixteen men. Heart rate, minute volume of ventilation (VE), and oxygen consumption (VO2) were measured. Arm exercise was performed until (muscular) exhaustion, leg exercise up to a heart rate of circa 170 beats/min. At comparable work loads arm exercise evoked higher VO2, VE, and heart rate than leg exercise irrespective of sex. At comparable VO2, the heart rate and VE were higher during arm work in both sexes, VE more so among the men. With the same limbs working, the mechanical efficiency was equal in both sexes. The regression coefficients of heart rate on load or VO2 was higher for the women irrespective of work type. A close correlation was obtained between working capacity at a heart rate of 170 beats/min (W170) during leg and arm exercise and between W170 of leg exercise and W150, similarly calculated during arm work. Thus W170 of leg exercise could be calculated from either a maximal or submaximal arm work.     

Long-Term Pacing in Heart Transplant Recipients is Usually Unnecessary

The indications for and timing of permanent pacing were reviewed in all 17 of 154 adult heart transplant recipients at this center who have had permanent pacemakers implanted. Resting 12-lead ECGs recorded during routine follow-up were examined. A prospective study of pacing requirement was then undertaken. Holter monitoring was performed before and after reprogramming the pacemakers to VVI mode at 50 beats/min. Exercise responses in various pacing modes were then assessed in seven patients with rate responsive pacemakers using a standard Bruce protocol treadmill test. The indication for pacing was sinus node dysfunction in 59% (10/17) and atrioventricular (AV) block in 41% (7/17). The majority of pacemakers were implanted between seven and 21 days after transplantation. There was a progressive reduction in the frequency of pacing on 12-Jead ECGs with time after transplantation. Eight of 14 patients with empirically selected programming paced during Holter monitoring. After reprogramming to 50 beats/ min VVI mode only three of 14 patients, all with sinus node dysfunction, paced. Rate responsive pacing made no difference to exercise time. The requirement for long-term pacing in cardiac transplant recipients is small (3/154) and is limited lo patients with sinus node dysfunction. Rate responsive pacing did not increase exercise tolerance.     

Rate Adaptive Atrial Pacing in the Bradycardia Tachycardia Syndrome

In 42 patients (26 men, 16 women; mean age 69 ± 10 years), who were paced and medicated with antiarrhythmic drugs for the bradycardia tachycardia syndrome, chronotropic response and AV conduction with rapid atrial pacing during exercise were studied. Patients were included if they had no second- or third-degree AV block, no complete bundle branch or bifascicular block, and a PQ interval ≤ 240 ms during sinus rhythm at rest. The interval between the atrial spike and the following Q wave (SQ) was measured in the supine position at rest with an AAI pacing rate of 5 beats/min above the sinus rate (SQ-R+5), and at the end of exercise with 110 beats/min (SQ-E110). Bicycle ergometry was performed using the Chronotropic Assessment Exercise Protocol with the pacemakers being programmed to AAI with a fixed rate of 60 beats/min. Chronotropic incompetence was defined as peak exercise heart rate: (1) < 100 beats/min; (2) < 75% of the maximum predicted heart rate; or (3) the heart rate at half the maximum workload < 60 + 2 beats/min per mL O₂/kg per minute (calculated O₂ consumption). During exercise, one patient developed atrial fibrillation. Chronotropic incompetence was present in 71 % (29/41) of the patients according to definition 2, and in 76% (31/41) according to definition 1 or 3. Ten out of 41 patients (24%) exhibited a second-degree AV block with atrial pacing at 110 beats/min at the end of exercise. Only 9 out of the remaining 31 patients (29%) showed a physiological adaptation of the SQ-E110, and 21 patients (68%) exhibited a paradoxical increase of the SQ interval with rapid atrial pacing at the end of exercise as compared to the SQ-R+5. These observations indicate that the pacing system to be used in most patients paced and medicated for the bradycardia tachycardia syndrome should be dual chamber, and the option of rate adaptation should be considered.     

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.