Effects of Prolonged Hypotension on Myocardial Function in Dogs

Left myocardial blood flow (MF), oxygen consumption (MVO2) and the first derivative of left ventricular pressure (max dP/dt) were determined in anesthetized dogs subjected either to hemorrhagic hypotension or to normovolemic hypotension by means of a ganglion blocker (trimethaphan camphor sulfonate).In dogs bled to a mean arterial pressure of 55 mm Hg, MF, MVO2, and max dP/dt were initially reduced to about half of control values, but showed a progressive increase towards control during a 3-hour hypotensive period. End-diastolic volumes decreased shortly after bleeding, and were further reduced in the late hypotensive period. In contrast, during normovolemic hypotension MF, MVO2, and max dP/dt, after an initial decrease to half of control, showed no change in the further course. Similar results were obtained when adrenalectomized dogs, or dogs pretreated with a ß-adrenergic receptor blocker (propranolol) were bled to a mean arterial pressure of 55 mm Hg, and when untreated dogs were bled to 40 mm Hg. End-diastolic volumes were reduced to variable degrees, and remained constant or increased during the further course. These findings suggest that the inotropic effects observed during moderate hemorrhagic hypotension (55 mm Hg) are due to slow increase of the sympathoadrenal activity, and that this inotropic effect is almost eliminated at more severe degrees of hemorrhagic hypotension.