CEREBRAL VASCULAR-RESPONSE TO HEMORRHAGIC HYPOTENSION IN NEWBORN LAMBS - THE INFLUENCE OF DEVELOPING ANEMIA

The ability of newborn animals to autoregulate cerebral blood flow (CB F) has been documented. Most studies of the cerebral vascular response to hypotension utilize hemorrhage, generally confounded with anemia. We studied the cerebral blood flow and metabolic response of chloralos e and urethane anesthetized newborn lambs to regulated hypotension. La mbs (less than or equal to 7 days old) were catheterized for radioacti ve microsphere determinations of CBF. The dorsal sagittal sinus was ca theterized to obtain cerebral blood samples for the calculation of oxy gen uptake. Cerebral perfusion pressure was reduced in a step-wise fas hion with hemorrhagic hypotension. Animals spontaneously became anemic with hypotension (AH; n = 8). In a group of animals (NH; n = 6), anem ia was prevented by infusion of autologous red blood cells. Arterial p ressure was reduced from control to 50, 40, and 30 mm Hg. In the AH gr oup hematocrit fell 37% but was not different from control in the NH g roup. Total CBF was maintained in all groups. The lowest perfusion pre ssures studied were 25 +/- 1 and 22 +/- 1 mm Hg in AH and NH groups re spectively. Oxygen delivery decreased (37%) only in the AH group, seco ndary to anemia. Calculated oxygen consumption was maintained in the A H group but increased (approximate to 50%) in the NH group at 50 and 4 0 mm Hg. The ratio of oxygen uptake to oxygen delivery (fractional oxy gen extraction) increased linearly in both groups as arterial pressure decreased. The major findings of these experiments are (i) The anesth etized newborn lamb can maintain CBF when perfusion pressure falls to 25 mm Hg and this autoregulatory capacity (classically defined) is not dependent on a change in hematocrit and, presumably, viscosity; (ii) Cerebral hypotension, anemic or not, appears to be accompanied by an i ncrease in fractional extraction of oxygen.