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.