Changes in venous return alter the plateau of stroke volume during dynamic exercise

Six average fit (26 ± 2 yrs; V̇O_2peak = 38 ± 1 ml·kg^-1·min^-1) subjects performed dynamic cycling to determine if increases or decreases in venous return affect the plateau that occurs in stroke volume during dynamic exercise. Each subject performed three bouts of exercise to volitional fatigue: control; exercise with Lower Body Negative Pressure (LBNP) of-40 mmHg [to reduce venous return (VR)]; and exercise with volume expansion (VE) to 10% of total blood volume with 6% dextran in saline [to increase VR]. Heart rate (HR, beats·min^-1), cardiac output (Q_c, L·min^-1) and oxygen uptake (V̇O₂) were measured and stroke volume (SV; ml·beat^-1) was calculated (mean ± SEM). In the average fit subjects, the mean plateau of SV began at 43 ± 5% of V̇O_2peak. However, the plateau of SV during LBNP (reduced VR) occurred at a greater percentage of V̇O_2peak (53 ±4), while the plateau of SV during VE 10% (increased VR) occurred at a much earlier percent of V̇O_2peak (7 ± 5), which was equivalent to rest. We conclude that the plateau of stroke volume that occurs during dynamic exercise to maximum was primarily determined by venous return and cardiac filling time.