Cerebral blood flow and metabolism in relation to electrocortical activity with severe umbilical cord occlusion in the near-term ovine fetus.

Objective: The purpose of this study was to determine the change in cerebral blood flow and substrate metabolism in relation to electrocortical activity in the near-term ovine fetus with repeated umbilical cord occlusion of a severe degree.
Study Design: Eight near-term fetal sheep were studied through a 2-hour control period, a 6-hour experimental period with repeated cord occlusion of 4 minutes' duration every 90 minutes, and a 16-hour recovery period. Regional cerebral blood flow was measured with the microsphere technique before, during, and after the first cord occlusion; blood flow in the superior sagittal sinus, the cerebral perfusion pressure, and the electrocortical activity were monitored continuously. Brachiocephalic arterial and sagittal venous blood were sampled at selected time points for blood gas and pH, oxygen content, and glucose and lactate levels.
Results: Severe umbilical cord occlusion as studied resulted in profound hypoxemia with modest hypercapnia and acidemia, to a similar degree with each insult, but with a return to preocclusion values after occluder release. Glucose values also fell acutely with each cord occlusion by approximately 30% but showed an overall increase through the experimental period, from 0.80 to 1.44 mmol/L; lactate values showed an increase, from 1.21 to 6.10 mmol/L (both P 1 <.01). Fetal electrocortical activity was disrupted markedly, with an abrupt flattening of the electrocortical amplitude by 1.5 minutes of each cord occlusion on average and with an overall increase in indeterminate state activity during the experimental and through the recovery periods. Cerebral blood flow increased approximately 2.5- and 2.8-fold, as measured at 2 and 3.5 minutes during the first cord occlusion (both P <.01) and with the regional flow increase greater in the subcortex and brainstem. Cerebral extraction of oxygen fell toward zero, as measured at 2 minutes during the second and fourth occlusions (P <.05) with oxygen uptake no longer measurable; glucose extraction was now increased approximately 2-fold (P <.05), which indicates that anaerobic metabolism of glucose must be the predominant source of energy at this time. Superior sagittal sinus blood flow also increased in all animals, approximately 1.4- and 1.6-fold at 2 and 3.5 minutes of the first cord occlusion, but much less than the corresponding increase in arterial inflow; the increase was in response to subsequent occlusions was further reduced.
Conclusion: Severe umbilical cord occlusion in the near-term ovine fetus results in a rapid decrease in the availability of oxygen to the brain. The low PO (2) gradient from blood to tissue rate limits for oxygen consumption by 2 minutes of insult (despite the marked increase in blood flow) and signals the shift to anaerobic metabolism, the suppression in electrocortical activity, and the probable shutdown of other energy-using processes.