Cardiovascular Effects of Sustained Norepinephrine Infusions in Dogs

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Abstract
Sustained infusions of norepinephrine (NE) are known to produce deleterious pathophysiologic effects on the circulatory system. The mechanisms responsible for these effects were investigated in 25 dogs; 17 were treated with selective block of alpha receptors by phenoxybenzamine or of beta receptors by propranolol, or by both agents, and then infused with NE (4 µg.kg-1.min-1) for 4 hours; 8 control animals were given only adrenergic blocking agents. Left ventricular and systemic pressure, cardiac output and arterial blood gases were measured at selected intervals throughout the infusion period, and histologic examinations of the heart and other vital organs were performed at the conclusion of each study. In animals with alpha-receptor blockade, NE infusions were associated with extensive subendocardial hemorrhage and focal myofiber fatty degeneration, yet blood pressure, cardiac output, and derived cardiac work parameters were well maintained. In animals with beta-receptor blockade, NE caused minimal pathologic changes in the heart despite significant reduction in cardiac output, minute and stroke work, and significant increase in left ventricular end-diastolic pressure (LVEDP) and peripheral vascular resistance. In animals receiving combined alpha- and beta-receptor blockade, NE infusions were not associated with significant hemodynamic or morphologic abnormality. These findings indicate: (1) the hemodynamic abnormalities are the result of the action of NE mediated primarily through alpha receptors; (2) the morphologic changes are produced by NE mediated mostly through beta receptors; and (3) the reductions in cardiac output and minute and stroke work and the increase in LVEDP produced by sustained NE infusions are not necessarily a consequence of the pathologic changes which develop in the heart muscle.