Background: Oxygen uptake during constant workload exercise increases exponentially from its resting value before reaching a steady state. The difference between the actual rate of oxygen consumption at the onset of exercise and the steady state is an oxygen deficit. Similarly, the normal sinus node increases its rate at the onset of exercise before achieving a steady state, thereby producing a heart rate deficit. The purpose of this study was to test the hypothesis that elimination of the heart rate deficit by an :instantaneous increase in heart rate at the onset of constant workload exercise to the steady-state level would reduce the oxygen deficit and improve the perceived difficulty of exertion as compared with the chronotropic response of the normal sinus node. Methods and Results: Ten subjects with normal sinus node function who had DDD pacemakers implanted for A V block completed a symptom-limited maximal treadmill exercise test using the Chronotropic Assessment Exercise Protocol (CAEP) to assess sinus node function, maximal heart rate, and VO2max. The subjects then performed constant workload exercise tests (6-min duration) at a workload equal to approximately 50% of metabolic reserve with the pacemaker randomly programmed to each of three patterns of chronotropic response: (1) DDD (lower rate 60 beats/min); (2) Fast (lower rate abruptly programmed to the expected value at 50% metabolic reserve); and (3) Overpaced (lower rate at least 80% of the age predicted maximum). The oxygen deficit was lower with the fast chronotropic response (434 +/- 238 ml O-2) than with either the DDD (512 +/- 233; P = 0.02), or overpaced chronotropic patterns (488 +/- 238; P = 0.02 vs fast). The rate constant for change in VO2 was highest with the fast chronotropic pattern (2.85 +/- 1.38) compared with either the DDD (2.25 +/- 0.64; P = 0.01) or overpaced (2.38 +/- 0.43; P = 0.02) patterns. The Borg perceived exertion rating was lowest with the fast chronotropic response (P = 0.02 vs DDD and P = 0.02 vs overpaced). Conclusions: The results of this study suggest that oxygen kinetics and exertional symptoms are improved by an abrupt increase in pacing rate at the onset of exercise to a value that is appropriate for metabolic demand as compared with the DDD pacing mode in patients with normal sinus node function. In contrast, an overly aggressive chronotropic response was not associated with improved oxygen kinetics or exertional symptoms.

The normal heart rate (HR) is linearly related to oxygen consumption (VO2) during aerobic exercise and has been used as the gold standard for rate-adaptive pacing. However, at the onset of constant-workload exercise both VO2and the sinus HR increase exponentially before reaching their steady-state levels, producing oxygen and HR deficits. Whether the rate constants (k) for VO2and HR are related has not been proven. We hypothesized that an instantaneous increase in HR at the onset of exercise to the steady state value (k = ∞) would reduce oxygen deficit and increase the VO2rate constant as compared with normal sinus node function during treadmill exercise. 10 pts with rigorously-proven normal sinus node function and AV block with DDD pacemakers underwent maximal treadmill exercise to determine VCO2max. Oxygen kinetics were then measured during constant-workload exercise tests (6-min) at 50% VO2maxwith pacemakers programmed to 3 modes in random sequence: 1) DDD, with the sinus node controlling pacing rate; 2) Fastan instantaneous increase in HR at onset of exercise to the expected steady-state level for 50% VO2max; and 3) Max, an instantaneous increase to the age-predicted maximum at exercise onset. The O2deficit was lower in the Fast (434 ± 76 ml) than either the DDD (512 ± 74) (p = 0.01) or Max (503 ± 84) (p = 0.02) pacing modes. The VO2rate constant was highest in the Fast mode (2.85 ± 1.38 Fast vs 2.25 ± 0.63 DDD vs 2.38 ± 0.43 Max, p = 0.03). The median Borg perceived exertion score was also lowest in the Fast mode (7.5 Fast vs 8.5 DDD vs 9 Max). Thus, oxygen deficit is improved by an instantaneous increase in HR at the onset of exercise to the steady state level as compared with either the normal sinus node or an overly-aggressive pacing rate. The rate constant for VO2is directly related to that for HR.