CARDIOVASCULAR AND BLOOD VOLUME RESPONSES TO THREE LEVELS OF G-PROTECTION.

BACKGROUND: Anti-G Suits provide protection against +Gz acceleration through several physiological mechanisms. The contribution of each of these mechanisms to improving G-tolerance, and how each relate to the degree of protection provided, is not fully understood. This study investigates the cardiovascular response to three levels of G-protection: No (NP), Moderate (MP; 5-Bladder Anti-G Trousers) and Improved (IP; Full Coverage Anti-G Trousers) protection. METHODS: 11 subjects undertook a series of rapid onset (3 G/sec; 15 sec plateau) runs in a human centrifuge with target +Gz increased until Peripheral Light Loss (PLL) was reached. Impedance plethysmography (IPG) assessed blood volume changes in the whole lower body (Total), Abdomen, Thigh and Calf. Non-invasive pressure waveform monitoring was used to calculate Blood Pressure (BP), Cardiac Output (CO), Stroke Volume (SV) and Total Peripheral Resistance (TPR). Heart Rate (HR) was calculated from 3-lead ECG. For each variable, changes between preceding baseline and the +Gz plateau at PLL were analysed using repeated measures analysis and post-hoc pairwise comparisons. A comparison between MP and IP at +3.2 Gz was also performed. RESULTS: +Gz at PLL increased with increasing protection (mean ± SD: NP; +3.4 ± 0.5 Gz, MP; +4.5 ± 0.6, IP; +5.0 ± 1.0 Gz). At PLL, there was no difference in the change in eye-level BP, SV or CO between conditions. However, the increase in TPR was greater for IP than MP. At +3.2 Gz, eye-level ΔBP was the same for MP and IP, but ΔHR was higher in MP, whilst ΔTPR was higher in IP. At PLL, Total ΔIPG decreased (i.e blood volume increased) for NP, but changes in individual segments were not significant. Total and Abdominal ΔIPG increased equally with both MP and IP. Thigh ΔIPG increased significantly with IP but was unchanged in the other conditions. Calf ΔIPG was unchanged from baseline in all conditions. At +3.2 Gz, Abdominal ΔIPG was increased more by IP than MP. DISCUSSION: At PLL, regardless of the level of protection, key measures of cardiovascular physiology were similar. Greater protection allowed these physiological changes to occur at higher +Gz. Both Anti-G suits reduced blood volume re-distribution to the lower body. The main difference with improved protection was a greater blood volume shift from the thigh. Thigh and abdominal compression appear key to the level of G-protection, whilst calf compression appears less critical. Learning Objectives 1. The audience will learn about the differences in cardiovascular response between exposures to +Gz with and without G-protection. 2. The audience will learn about how differences in cardiovascular response reflect changes in G-tolerance. [ABSTRACT FROM AUTHOR]