Associations between changes in precerebral blood flow and cerebral oximetry in the lower body negative pressure model of hypovolemia in healthy volunteers

Reductions in cerebral oxygen saturation (ScO2) measured by near infra-red spectroscopy have been found during compensated hypovolemia in the lower body negative pressure (LBNP)-model, which may reflect reduced cerebral blood flow. However, ScO2 may also be contaminated from extracranial (scalp) tissues, mainly supplied by the external carotid artery (ECA), and it is possible that a ScO2 reduction during hypovolemia is caused by reduced scalp, and not cerebral, blood flow. The aim of the present study was to explore the associations between blood flow in precerebral arteries and ScO2 during LBNP-induced hypovolemia. Twenty healthy volunteers were exposed to LBNP 20, 40, 60 and 80 mmHg. Blood flow in the internal carotid artery (ICA), ECA and vertebral artery (VA) was measured by Doppler ultrasound. Stroke volume for calculating cardiac output was measured by suprasternal Doppler. Associations of changes within subjects were examined using linear mixed-effects regression models. LBNP reduced cardiac output, ScO2 and ICA and ECA blood flow. Changes in flow in both ICA and ECA were associated with changes in ScO2 and cardiac output. Flow in the VA did not change during LBNP and changes in VA flow were not associated with changes in ScO2 or cardiac output. During experimental compensated hypovolemia in healthy, conscious subjects, a reduced ScO2 may thus reflect a reduction in both cerebral and extracranial blood flow.