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BACKGROUND: Pilots in high-performance aircraft are exposed to acceleration in the +Gz (Head-to-foot) axis which can result in G-induced Loss of consciousness (G-LOC). Breathing elevated levels of CO2 have been reported to increase Systolic Blood Pressure (SBP) and Middle Cerebral Artery Velocity (MCAv) and therefore may potentially be used to improve G tolerance. Lower body negative pressure (LBNP) has been used as a tool to investigate responses to central hypovolemia and as a surrogate of hypergravity. The present study aimed to determine whether breathing 5% CO2 increased cerebral blood flow and blood pressure during LBNP. METHODS: 15 subjects (9 M/6 F, 31±8 yr) undertook the experiment which involved breathing either room air or a mix of 5% CO2, 21% oxygen, balance nitrogen for 3 mins followed by a 2 min period of -60mmHg LBNP. There was a 2-minute washout period between trials. We measured middle cerebral artery velocity (MCAv) with transcranial Doppler ultrasound. Continuous, non-invasive blood pressure was measured using finger plethysmograph with mean arterial pressure (MAP), stroke volume (SV), cardiac output (CO) and total peripheral resistance (TPR) determined. We corrected MCAv for blood pressure (MAP) and present it as cerebrovascular conductance index (CVCi). RESULTS: LBNP caused an increase in heart rate and TPR while SV and CO significantly decreased (p < 0.05, main effect of LBNP) with each of these changes being similar between breathing gas conditions. MCAv, MAP, and CVCi were all greater during the 5% inspired CO2 trials (p < 0.05, main effect of gas), irrespective of exposure to LBNP. DISCUSSION: Breathing a mixture of 5% CO2 increased cerebral blood flow and blood pressure with these increases remaining during an analogue of hypergravity. While technical challenges exist in integrating CO2 breathing inflight, increasing inspired CO2 could contribute to better G-tolerance. Learning Objectives 1. The audience will note that breathing a mixture of 5% CO2 increased cerebral blood flow and blood pressure, and these increases were demonstrated in an analogue of hypergravity. 2. The audience will learn that increasing inspired CO2 could contribute to better G tolerance. [ABSTRACT FROM AUTHOR]