Phase I and Phase II Oxygen Uptake Kinetics During Atrioventricular Dyssynchrony in Chronotropically Competent Pacemaker Patients

Objective To elucidate the effects of atrioventricular (AV) dyssynchrony on phase I and phase II oxygen uptake ( V ˙ o 2 ) 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 V ˙ o 2 amplitude response and gain (Δ V ˙ o 2 /WR ratio) were lower (p 2 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 V ˙ o 2 . A significant compensatory amplitude response in heart rate (HR) was observed in addition to a higher (p < 0.05) ΔHR/ V ˙ o 2 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 V ˙ o 2 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 O 2 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.