G tolerance and pulmonary effects of removing chest counterpressure during pressure breathing.
BACKGROUND: In agile fighter aircraft positive pressure breathing is commonly used as part of the anti-G ensemble. To optimize G protection and prevent over-distention of the lungs, increased airway pressure is balanced by applying a counterpressure to the chest. The aim was to investigate the efficacy of chest counterpressure. METHODS: Three series of experiments were performed using the anti-G ensemble of the 39 Gripen aircraft (AGE-39) and exposing the subjects to 20-s G time profiles; in the first (n = 12) up to +8.0 Gz, in the second (n = 9) up to + 9.0 Gz, and also to simulated aerial combat maneuvers (SACM). Central and peripheral vision, arterial and airway pressures, pressure in the lower portion of the esophagus, and chest wall distension were measured. In the third series, six subjects were exposed to up to +7.0 Gz and esophageal pressure was measured in the upper thorax. In all series, two conditions were compared: with and without pressurized chest bladder. RESULTS: During the 20-s profiles arterial and esophageal pressures, chest wall distension, and visual impairment were similar with and without pressurized chest bladder. Upper esophageal pressure was slightly higher by 10-24% with than without chest bladder (p = 0.03). During SACM, time to exhaustion and the level of perceived exertion were similar with and without pressurized chest bladder. SUMMARY: The results suggest that the chest counterpressure can be removed from the AGE-39 without diminishing G tolerance or G endurance or significantly increasing the risk of lung parenchyma disruption.