Objective: To elucidate the effects of atrioventricular (AV) dyssynchrony on phase I and phase II oxygen uptake (V̇o2) kinetics in chronotropically competent pacemaker patients during exercise of an intensity comparable to activities of daily living.
Design: Blinded patients completed sub-ventilatory threshold (VT) work rate (WR) cycle ergometry exercise in random order during asynchronous AV pacing (AV OFF) and synchronous AV pacing.
Setting: Tertiary care hospital in a major city.
Subjects: Six chronotropically competent male pacemaker patients (mean [± SD] age, 68 ± 10 years) with high-degree AV block and varying cardiac histories.
Results: The phase I and phase II V̇o2 amplitude response and gain (ΔV̇o2/WR ratio) were lower (p < 0.05) and the time course of phase II was slower (p < 0.05) during AV OFF; however, the O2 deficit was similar (p > 0.05) across pacing modes. The stroke volume index (SVI) was consistently lower (p < 0.05) during AV OFF pacing and was significantly correlated with the time course of phase II V̇o2. A significant compensatory amplitude response in heart rate (HR) was observed in addition to a higher (p < 0.05) ΔHR/V̇o2 ratio during AV OFF. Ventilatory responses were consistent with ventilatory-perfusion mismatching and perceived exertion was higher during asynchronous pacing.
Conclusion: This study demonstrated that the contribution of SVI affects V̇o2 kinetics and underscores the importance of the atrial contribution to ventricular filling and, consequently, to metabolic and hemodynamic responses. This study supports the theory of an O2 transport limitation and further implicates SV as a potential limiting factor during sub-VT exercise intensities that are comparable to those encountered in activities of daily living.