Relationships between the extent of apnea-induced bradycardia and the vascular response in the arm and leg during dynamic two-legged knee extension exercise.

Our aim was to test the hypothesis that apnea-induced hemodynamic responses during dynamic exercise in humans differ between those who show strong bradycardia and those who show only mild bradycardia. After apnea-induced changes in heart rate (HR) were evaluated during dynamic exercise, 23 healthy subjects were selected and divided into a large response group (L group; n = 11) and a small response group (S group; n = 12). While subjects performed a two-legged dynamic knee extension exercise at a work load that increased HR by 30 beats/min, apnea-induced changes in HR, cardiac output (CO), mean arterial pressure (MAP), arterial O(2) saturation (Sa(O(2))), forearm blood flow (FBF), and leg blood flow (LBF) were measured. During apnea, HR in the L group (54 ± 2 beats/min) was lower than in the S group (92 ± 3 beats/min, P < 0.05). CO, Sa(O(2)), FBF, LBF, forearm vascular conductance (FVC), leg vascular conductance (LVC), and total vascular conductance (TVC) were all reduced, and MAP was increased in both groups, although the changes in CO, TVC, LBF, LVC, and MAP were larger in the L group than in the S group (P < 0.05). Moreover, there were significant positive linear relationships between the reduction in HR and the reductions in TVC, LVC, and FVC. We conclude that individuals who show greater apnea-induced bradycardia during exercise also show greater vasoconstriction in both active and inactive muscle regions.