The effect of various straining maneuvers on cardiac volumes at 1 G and during +Gz acceleration

The effects of Valsalva (Val), isometric contraction (Iso), and the L-1 maneuvers (L-1) on cardiac volumes at 1 G and during 4 G were evaluated. Two-dimensional echocardiography was used to measure volumes. In the 1 G phase, eight seated male subjects performed the three maneuvers. The end-diastolic volume (EDV) decreased during VAL and L-1 (p 0.05). EDV decreased initially, but then returned to baseline with Iso. In the 4 G phase, seven male subjects experienced three 30 second 4 G epochs while performing the three maneuvers. During 4 G, 3 of 7 performing Val, 2 of 7 performing Iso, and none performing L-1 experienced peripheral light loss. EDV decreased and stabilized below baseline during the 4 Gz epoch (p 0.05). Heart rate and cardiac output increased for all maneuvers. Differences between the effects of Val, Iso, and L-1 on G-tolerance do not appear to be related to a differential ability to maintain cardiac volumes. L-1 must provide more G protection due to the isometric contraction component of the maneuver and its ability to increase blood pressure