Effect of acute blood volume variations in man on the circulatory response to isometric handgrip

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Abstract
In normovolaemia (NV) a strong muscle activation performed as an isometric handgrip induced an initial transient decrease in vascular resistance in the resting forearm, probably mediated partly by β-adrenergic mechanisms. In hypovolaemia (HV) this reaction was less pronounced. To elucidate if the altered peripheral reaction to a contralateral isometric handgrip (CIH) could be explained by an altered response of the central circulation, the peripheral and central circulatory response to CIH was studied in eight healthy males after withdrawal and reinfusion of approximately 16% of the blood volume. The effects of blood loss at rest were the same as previously reported except for the fact that arterial pressures were not significantly lowered. During CIH in NV heart rate, cardiac output, arterial and pulmonary pressures and total systemic vascular resistance (TSR) increased and stroke volume decreased. The forearm vascular resistance fell. In HV, CIH induced the same changes in cardiac output, heart rate, stroke volume, arterial and pulmonary pressures and TSR. However, in accordance with previous findings the increase in blood flow and decrease in resistance in the resting forearm was significantly less pronounced. This altered response pattern might be caused by more pronounced α-adrenergic effects after bleeding both at rest and during CIH, superseding the suggested β-adrenergically mediated increase in blood flow on muscle activation. Since arterial pressures were not affected by bleeding at rest or during CIH, the arterial baroreceptors do not seem to be responsible, and thus receptors on the low pressure side may mediate the more pronounced sympathetic α-adrenergic effects. Furthermore, a dissociation of sympathetic effects on heart rate and peripheral vascular bed seems to be present in HV.

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