Cerebral Vascular Response to Hemorrhagic Hypotension in Newborn Lambs: The Influence of Developing Anemia

Abstract

The ability of newborn animals to autoregulate cerebral blood flow (CBF) 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 chloralose and urethane anesthetized newborn lambs to regulated hypotension. Lambs (< or = 7 days old) were catheterized for radioactive microsphere determinations of CBF. The dorsal sagittal sinus was catheterized to obtain cerebral blood samples for the calculation of oxygen uptake. Cerebral perfusion pressure was reduced in a step-wise fashion with hemorrhagic hypotension. Animals spontaneously became anemic with hypotension (AH; n = 8). In a group of animals (NH; n = 6), anemia was prevented by infusion of autologous red blood cells. Arterial pressure was reduced from control to 50, 40, and 30 mm Hg. In the AH group hematocrit fell 37% but was not different from control in the NH group. Total CBF was maintained in all groups. The lowest perfusion pressures studied were 25 +/- 1 and 22 +/- 1 mm Hg in AH and NH groups respectively. Oxygen delivery decreased (37%) only in the AH group, secondary to anemia. Calculated oxygen consumption was maintained in the AH group but increased (approximately 50%) in the NH group at 50 and 40 mm Hg. The ratio of oxygen uptake to oxygen delivery (fractional oxygen extraction) increased linearly in both groups as arterial pressure decreased. The major findings of these experiments are (i) The anesthetized 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 increase in fractional extraction of oxygen.

Cerebral tissue oxygen saturation and extraction in preterm infants before and after blood transfusion

“…It is possible for preterm infants to compensate for a low haemoglobin level by increasing cardiac output 5 23. Along with the higher cardiac output, cerebral blood flow will also increase,24–26 but this increase might be absent or limited in cases of mild anaemia 27 28. In fact, our data suggest that the increase in cerebral blood flow is insufficient to compensate for the decreased oxygen transport capacity.…”

Cerebral O2 Supply Thresholds for the Preservation of Electrocortical Brain Activity During Hypotension in Near-Term-Born Lambs

“…A low Hb concentration is accompanied by a low CaO 2 and low blood viscosity, which both reduce cerebral vascular resistance and therefore increase CBF (19,44,45). However, O'Neill et al (46) showed that the CBF response to hemorrhagic hypotension was not dependent on a change in hematocrit and, presumably, viscosity. Therefore, we assume that anemia has not disturbed cerebral regulation of CBF in our study.…”