EFFECTS OF HEMORRHAGIC HYPOTENSION ON CEREBRAL BLOOD-FLOW AND PERFUSED CAPILLARIES IN NEWBORN PIGS

We examined the effect of hemorrhagic hypotension on cerebral blood fl ow and perfused capillaries in newborn pigs, 2-10 days old. Cerebral b lood flow was measured by using radioactive microspheres, perfused cap illaries were determined by infusing a plasma marker, fluorescein isot hiocyanate (FITC) - dextran (molecular mass 147 000 Da), and total cap illaries were determined by staining with alkaline phosphatase. Rapid removal of 66 +/- 16 mL of blood resulted in a fall in mean blood pres sure from 68 +/- 6 to 31 +/- 4 mmHg (1 mmHg = 133.3 Pa), an increase i n heart rate from 137 +/- 18 to 240 +/- 34 beats/min, and a drop in ar terial pH from 7.33 +/- 0.05 to 7.23 +/- 0.07. Pco(2) was controlled b y mechanical ventilation (36 +/- 4 mmHg before hemorrhage and 35 +/- 5 mmHg after hemorrhage) and Po-2 remained stable (89 +/- 11 mmHg befor e hemorrhage and 94 +/- 10 mmHg after hemorrhage). Blood flow (n = 9) did not fall significantly in any brain region after hemorrhage. The p ercentage of perfused capillaries/mm(2) (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 m edulla from 76 +/- 8 to 51 +/- 9%). Similar results were seen for perf used capillary surface area. We conclude that hemorrhagic hypotension resulted in a variable blood flow response with no significant reducti on in cerebral blood flow but a reduction in perfused capillary number and surface area in all brain regions studied. We speculate that decr eased capillary perfusion may be a contributing factor in diffuse neur onal injury after severe hemorrhagic hypotension.