Cardiovascular Effects of Hypercapnia During Lower Body Negative Pressure and Head-Up Tilt

Exposure to head-to-toe acceleration (Gz) may lead to G-induced loss of consciousness due to reduced head-level blood pressure and flow. Despite various countermeasures, G-induced loss of consciousness remains a safety concern, causing accidents and fatalities. Inhalation of 5% carbon dioxide (CO2) and higher concentrations can improve G-tolerance but lead to adverse symptoms. This study investigated the impact of 5% and lower concentrations of CO2 on the cardiovascular system during orthostatic stress.

To simulate +Gz cardiovascular effects, 20 healthy subjects (11 males, 9 females) were exposed to 5 gas mixtures (room air, 2%, 3%, 4%, and 5% CO2) while undergoing lower body negative pressure of -60 mmHg combined with an 80° head-up tilt. Cardiovascular and respiratory parameters were continuously monitored and subjective symptoms were assessed.

Compared to room air, breathing 5% CO2 significantly increased systolic (+18.4 ± 16.4 mmHg), diastolic (+30.0 ± 9.5 mmHg), and mean arterial (+26.1 ± 11.0 mmHg) blood pressure during orthostatic stress. However, this was also associated with a perceived increase in breathlessness (median 2.5, interquartile range 1.25-3) and difficulty breathing (median 2, interquartile range 1-3). Lower concentrations of CO2 were better tolerated but did not significantly affect physiological response during orthostatic challenge.

These findings suggest breathing higher concentrations of CO2 (5%) can elicit significant cardiovascular changes which potentially could increase G-tolerance, although they are also associated with respiratory discomfort in some individuals. However, as this is an exploratory study, further research that more accurately replicates the complex physiological changes that occur during Gz exposure is still required. Twardowska K, Bortnowschi M, Skert R, Lee Y-H, Rafferty G, Pollock R. Cardiovascular effects of hypercapnia during lower body negative pressure and head-up tilt. Aerosp Med Hum Perform. 2026; 97(1):3-10.