TOTAL AND REGIONAL CEREBRAL BLOOD-FLOW DURING RECOVERY FROM G-LOC

Introduction: This study measured total and regional cerebral blood fl ow (BF) in baboons during +Gz-induced loss of consciousness (G-LOC) an d during recovery from G-LOC. Methods: Flowprobes (Transonic Inc., T20 1, Ithaca, NY) were placed on the common carotid and internal carotid arteries of five male baboons for continuous measurement of total ceph alic and cerebral BF, respectively. Radiolabeled microspheres were use d to measure regional central nervous system BF at discrete timepoints . G-LOC was determined from visual observations of the animals and fro m EEG recordings. Results: Cerebral blood flow was maintained and anim als remained conscious during 60 s exposure to +4 Gz. In contrast, G-L OC was observed during the first 16-25 s (mean = 20.3 +/- 3.7 s) of ex posure to +8 Gz in all five animals. Internal and common carotid arter y BF decreased rapidly to zero during the first few seconds of +8 Gz. BF always appeared to cease prior to the occurrence of CLOG. During ea rly recovery from G-LOC there was no hyperemic response recorded with flowprobes, whereas a hyperemic response was recorded following 60 s e xposures to +4 Gz in which the animals did not experience G-LOC. Micro sphere measurements of the regional distribution of BF are consistent with the hypothesis of a +Gz-induced differential perfusion deficit th roughout the brain and central nervous system during G-LOC. Conclusion s: We conclude that G-LOC is preceded by cessation of cerebral BF. The fact that the hyperemic response following +Gz exposure is less when G-LOC occurs than when G-LOC does not occur suggests CNS energy conser vation during G-LOC.