The effect of head and neck suction on G tolerance.

A device was constructed which allowed subatomospheric pressures of up to -50 mm Hg to be applied to the neck, or to the head plus neck, of volunteer subjects riding on the USAFSAM centrifuge. Breathing pressures were always atmospheric. The G tolerance was measured during gradual (0.1 G.s-1) and rapid onset (1.0 G.s-1) runs and heart rate was monitored. Neck suction decreased tolerance to an extent explicable by carotid sinus activation (about -0.8 G at -50 mm Hg), while combined head and neck suction of -25 mm Hg increased tolerance to an extent greater than predicted from pressure effects on the eye and carotid sinus. Neck suction of -50 mm Hg induced less bradycardia when applied at +3 Gz than under 1-G conditions. These results were discussed in relation to intraocular tension, baroreceptor responsiveness, and jugular venous siphon effect.