Effects of hemorrhagic hypotension on cerebral blood flow and perfused capillaries in newborn pigs.

We examined the effect of hemorrhagic hypotension on cerebral blood flow and perfused capillaries in newborn pigs, 2-10 days old. Cerebral blood flow was measured by using radioactive microspheres, perfused capillaries were determined by infusing a plasma marker, fluorescein isothiocyanate (FITC) - dextran (molecular mass 147,000 Da), and total capillaries were determined by staining with alkaline phosphatase. Rapid removal of 66 +/- 16 mL of blood resulted in a fall in mean blood pressure from 68 +/- 6 to 31 +/- 4 mmHg (1 mmHg = 133.3 Pa), an increase in heart rate from 137 +/- 18 to 240 +/- 34 beats/min, and a drop in arterial pH from 7.33 +/- 0.05 to 7.23 +/- 0.07. PCO2 was controlled by mechanical ventilation (36 +/- 4 mmHg before hemorrhage and 35 +/- 5 mmHg after hemorrhage) and PO2 remained stable (89 +/- 11 mmHg before hemorrhage and 94 +/- 10 mmHg after hemorrhage). Blood flow (n = 9) did not fall significantly in any brain region after hemorrhage. The percentage of perfused capillaries/mm2 (control, n = 7, hemorrhage, n = 6) was reduced in all brain regions during hypotension (cortex from 72 +/- 8 to 57 +/- 8%, cerebellum from 75 +/- 10 to 52 +/- 10%, and medulla from 76 +/- 8 to 51 +/- 9%). Similar results were seen for perfused capillary surface area We conclude that hemorrhagic hypotension resulted in a variable blood flow response with no significant reduction in cerebral blood flow but a reduction in perfused capillary number and surface area in all brain regions studied. We speculate that decreased capillary perfusion may be a contributing factor in diffuse neuronal injury after severe hemorrhagic hypotension.