Stroke volume response during prolonged exercise depends on left ventricular filling: Evidence from a beta-blockade study.

Abstract

Prolonged moderate intensity exercise leads to a progressive upward drift in heart rate (HR) that may compromise stroke volume (SV). Alternatively, the HR drift may be related to abated SV due to impaired ventricular function. The aim of this study was to examine the effects of cardiovascular drift on left ventricular volumes and in turn SV. Thirteen healthy young males completed two 60-minute cycling bouts on a semi-recumbent cycle ergometer at 57%VO2max either under placebo condition (CON) or after ingesting a small dose of β1-blockers (BB). Measurements of HR, end-diastolic volume (EDV), and end-systolic volume were obtained by echocardiography and used to calculate SV. Other variables such as ear temperature, skin temperature, blood pressure, and blood volume were measured to assess potential changes in thermoregulatory needs and loading conditions. HR drift was successfully prevented when using BB from min10 to min60 (128±9 to 126±8bpm, p=0.29) but not in CON (134±10 to 148±10bpm, p<0.01). Conversely, during the same time, SV increased by 13% when using BB (103±9 to 116±7mL, p<0.01) while it was unchanged in CON (99±7 to 101±9mL, p=0.37). The SV behavior was mediated by a 4% increase in EDV in the BB condition (164±18 to 170±18mL, p<0.01), while no change was observed in the CON condition (162±18 to 160±18mL, p=0.23). In conclusion, blocking HR drift enhances EDV and SV during prolonged exercise. These findings suggest that SV behavior is tightly related to filling time and loading conditions of the left ventricle.