Non-invasive intracranial pressure estimation during combined exposure to CO₂ and head-down tilt

BACKGROUND: Exposure to carbon dioxide (CO₂) and cephalad fluid shift are considered factors that affect intracranial pressure (ICP) during spaceflight. Increases in ICP were reported during cephalad fluid shift induced by head-down tilt (HDT), while little is known regarding the effect of additional CO₂ during HDT on ICP. Therefore, we tested the hypothesis that this combination increases ICP more than HDT alone. METHODS: There were 15 healthy male volunteers who underwent 4 types of 10-min interventions consisting of Placebo/Supine (air and supine), CO₂/Supine (3% CO₂ and supine, CO₂ alone), Placebo/HDT (air and 210° HDT, HDT alone), and CO₂/HDT (3% CO₂ and 210° HDT, combination). Using arterial blood pressure (ABP) and cerebral blood flow velocity waveforms, ICP was estimated noninvasively before and during the four interventions. Two calculation methods were employed. One is based on the signal transformation from ABP to ICP with the intracranial component as a "black box" system (nICP_BB), and the other is based on the equation ICP = ABP 2 cerebral perfusion pressure, reflecting critical closing pressure (nICP_CrCP). RESULTS: Both nICP_BB and nICP_CrCP significantly increased during Placebo/HDT and CO₂/HDT, although there was no statistically significant difference between the nICP indexes of these two interventions. DISCUSSION: Increases in ICP were observed during both Placebo/HDT and CO₂/HDT. Contrary to our hypothesis, the combination of 3% CO₂ and 210° HDT did not increase ICP remarkably compared to 210° HDT alone. Therefore, the addition of 3% CO₂ is considered to have little effect on increasing ICP during cephalad fluid shift.