Effect of Chronotropic Response Pattern on Oxygen Kinetics

Background: The sinus node is considered to be the model of chronotropic response for pacemakers that use artificial rate modulating sensors. Maximal metabolic exercise testing with measurement of oxygen consumption (VO₂) is frequently used to evaluate chronotropic response. Since activities of daily living are generally transient and involve submaximal effort, maximal exercise testing may not provide the most clinically relevant method of assessing rate modulation. The purpose of this study was to determine if an abrupt increase in heart rate (HR) at the onset of submaximal exercise provides improved oxygen kinetics compared with a linear response. Methods and Results: Thirteen patients with complete heart block and permanent rate modulating pacemakers implanted following catheter ablation of the atrioventricular junction for refractory atrial fibrillation were chosen for study. The patients first completed a maximal treadmill exercise test using the chronotropic assessment exercise protocol with breath-by-breath analysis of expired gases. The expected HR at 50% of metabolic reserve was calculated for each patient. Three submaximal constant workload exercise tests were then performed at 50% of each patient's metabolic reserve, with the pacemaker randomly programmed to provide three different patterns of chronotropic response: linear (in which HR increased from 70 beats/min to the expected HR at 50% of metabolic reserve), fast(in which HR was abruptly increased to the expected HR at 50% of metabolic reserve), and slow (VVI at 70 beats/ min). Oxygen kinetics were compared for the three patterns of chronotropic response. Cumulative oxygen (O₂) consumption was significantly greater for the fast pattern (3610 mL) as compared with the linear (3487 mL, P = 0.004) or slow pattern (3277 mL). The O₂ deficit was lower for the fast (361 ± 139 mL) than for the linear (539 ± 225 ml, P = 0.003) or slow chronotropic pattern (559 ± 194). Similar improvements in the rate constant of O₂ uptake and Borg perceived exertion scores were observed with the fast chronotropic response pattern. Conclusion: A rapid increase in pacing rate at the onset of exercise improves oxygen kinetics and results in less perceived exertion as compared to a more gradual rate increase that is more characteristic of sinus node behavior.