Effects of high atmospheric pressure and oxygen on middle cerebral blood flow velocity in humans measured by transcranial Doppler.

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Abstract
Background and Purpose —There are several reports that have studied the effects of hyperbaric oxygen (HBO) on cerebral blood flow (CBF). However, most of the reports have been of animal experiments, and human studies are few so far. The aim of this study is to clarify the relationship between HBO and CBF in humans. Methods —Middle cerebral arterial blood flow velocity (MCV) was measured using transcranial Doppler (TCD) technique in a multiplace hyperbaric chamber. The Doppler probe was fixed on the temporal region by a head belt, and the transcutaneous gas measurement apparatus (tcP o 2 and tcP co 2 ) was fixed on the chest wall. MCV and transcutaneous gas were measured continuously in eight healthy volunteers under four various conditions: 1 atmosphere absolute (ATA) air, 1 ATA oxygen (O 2 ), 2 ATA air, and 2 ATA O 2 . On the next step, the effect of environmental pressure was studied in another eight healthy volunteers, in whom the tcPo 2 was kept at almost the same level under conditions of both 1 ATA and 4 ATA by inhaling oxygen at 1 ATA. Results —MCV of 1 ATA O 2 , 2 ATA air, and 2 ATA O 2 decreased, and tcP o 2 increased significantly in comparison with that of 1 ATA air. A significant difference in MCV was observed between the O 2 group and the air group under the same pressure circumstance. On the other hand, there were no differences in MCV or tcP o 2 between 4 ATA air and 1 ATA plus O 2 , and the influence for the MCV of the environmental pressure was not observed. Conclusions —We conclude that hyperoxemia caused by HBO reduces the CBF, but the high atmospheric pressure per se does not influence the CBF in humans.