Hemodynamics of hemorrhage in the conscious rat and chicken

Hemodynamic responses to hemorrhage in conscious chicks (n = 10, 233 g) and rats (n = 10, 309 g) were compared. The animals were fitted with miniature pulsed Doppler aortic flow probes 2 days (chickens) or 5 days (rats) before catheterization, and the experiment began 1 (chickens) or 2 (rats) days later. Mean arterial pressure (MAP) and cardiac output (CO) were recorded continuously and simultaneously digitized to compute total peripheral resistance (TPR). MAP, CO, and TPR values were graphed on-line by a microcomputer and stored for later analysis. A 4-ml hemorrhage reduced MAP and CO by 25 and 43% in the rat, and 15 and 4% in the chickens, respectively. The fall in CO in the rat was due to reduction of stroke volume (SV) unlike the birds where SV was well maintained. TPR was elevated 65% in the rats and fell 13% in the chickens. The minimal fall in CO and SV in these conscious birds suggests that anesthetic agents used previously (i.e., urethane, paraldehyde, phenobarbital, and pentobarbital sodium) suppressed cardiac function. However, they do not account for the lack of a peripheral vascular response during hemorrhage. The chicken apparently maintains MAP by a volume regulating mechanism operating independently of peripheral vascular tone inasmuch as circulating fluid volume restitution is rapid and occurs without vasoconstriction. The rat maintains MAP through reflex cardiac and peripheral vascular responses which eventually may contribute to transvascular fluid loss and the ultimate collapse after prolonged hemorrhagic hypotension.