Effects of partial anti-G suit inflation on thoracic volume and breathing pattern

The purpose of this study was to determine the changes in thoracic volume and pattern of breathing during partial anti-G suit (PAGS) inflation by respiratory inductive plethysmography (RIP). Nine normal subjects donned the PAGS, with bladders over legs and thighs, and rested for about 10 min in 60 degrees head-up tilt position. The subjects breathed with closed glottis at functional residual capacity while PAGS was suddenly inflated to 140 mmHg using a calves to thighs sequence. The increase in thoracic volume, as measured from deflection of RIP baseline was 252 ml (S.D. 43 ml), which reflected displacement of blood from the lower extremities into the thorax. On resuming normal breathing, thoracic volume returned to baseline level. Breathing pattern was then monitored for a 15 min baseline period, PAGS was inflated, expiratory reserve volume (ERV) was measured by spirometry, and breathing pattern was monitored another 15 min. ERV decreased 227 ml (+/- 60) after PAGS inflation, which did not differ from the change in thoracic volume expected from displacement of blood into the thorax. Breathing pattern was monitored for another 15 min after PAGS was deflated. No changes took place in minute ventilation, tidal volume, frequency, inspiratory time, fractional inspiratory time, and mean inspiratory flow from deflation to PAGS inflation. Thus, 1) increase in thoracic volume produced by displacement of blood from the calves and thighs is balanced by a decrease in gas volume and, 2) no changes in breathing pattern occur after partial anti-G suit inflation, probably because the pulmonary blood vessels and heart are sufficiently distensible to accept a 250 ml volume increment without leakage of fluid into pulmonary tissues.