Fetal O2 consumption in sheep during controlled long-term reductions in umbilical blood flow.

Electromagnetic flow sensors and inflatable occluders were placed on the distal aortas of nine fetal lambs of 111-124 days gestation; indwelling vascular catheters were placed in a lower body artery and vein and the common umbilical vein. After a control period of 7.6 +/- 2.1 (SD) days, distal aortic flow was reduced to about 60% of control flow; placental flow fractions of distal aortic flow were 77 +/- 5%, control; 69 +/- 11% during flow reduction. Duration of the flow reductions was 9.6 +/- 6.5 days. Fetal O2 consumption fell from 6.4 +/- 1.5 to 5.3 +/- 1.1 ml X min-1 X kg-1 (P less than 0.02) as umbilical blood flow was reduced from 202 +/- 47 to 116 +/- 36 ml X min-1 X kg-1 (P less than 0.001). Least-squares polynomial regression analysis showed that O2 consumption was a linear function of umbilical blood flow (P less than 0.01). With regard to long-term fetal O2 consumption, there was no evidence for a margin of safety in fetal umbilical blood flow.