Potential hazards of high anti-Gz suit protection

Although balanced positive pressure breathing increases protection by G suits and reduces fatigue during sustained high Gz exposures, it does not prevent dependent pulmonary right-to-left shunts (arterial hypoxemia) and potentially dangerous distention of nondependent alveoli. An incident of acute incapacitating mediastinal emphysema in a healthy young man during a sustained exposure to 5.5 Gx documents this possibility. This, plus neurogenic bradycardia during sustained high Gz exposures without pressure breathing, suggests that testing of full counterpressure to neck level suits at sustained > 9 Gz exposures may be hazardous. Fully instrumented studies of animal surrogates with comparable thoracic dimensions are indicated.