A small dose of epinephrine causes a fall in mean and diastolic pressure with little or no effect on systolic pressure. This is due to the net decrease in total peripheral resistance that results from the predominance of vasodilation in the skeletal muscle vascular bed. The intravenous infusion or subcutaneous administration of epinephrine in the range of doses used in humans generally increases the systolic pressure, but the diastolic pressure is decreased. Therefore, the mean pressure may decrease, remain unchanged, or increase slightly, depending on the balance between the rise in systolic and fall in diastolic blood pressures (Fig. 10.4).
The cardiac effects of epinephrine are due to its action on p-adrenoceptors in the heart. The rate and contractile force of the heart are increased; consequently, cardiac output is markedly increased. Because total peripheral resistance is decreased, the increase in cardiac output is largely responsible for the increase in systolic pressure. Since epinephrine causes little change in the
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