Heart status following high G exposure in rats and the effect of brief preconditioning

Acceleration on the +Gz axis increases pressure in the cardiac chambers. Our research was designed to study whether: 1) such acceleration would impair cardiac function; and 2) a "preconditioning" exposure to +Gz would protect the heart from any such effects.

There were 45 male Sprague-Dawley rats that were randomly divided into 3 groups of 15: 1) control without acceleration (CTRL); 2) exposure to 5 min of +15 Gz (EXP); and 3) pretreatment whereby the 5-min exposure was preceded by two exposures of 30 s at +15 Gz (PRE). Within each group of 15, subsets of 7 animals were used to study: 1) echocardiographic heart function or 2) myocardial injury [lactic dehydrogenase (LDH) and malondialdehyde (MDA)], and 1 animal was used to examine the histology of cardiac tissue.

Acceleration (EXP) was found to have a significant effect on cardiac function; specifically the early diastolic myocardial velocity (Em) and systolic myocardial velocity (Sm) in both ventricles decreased in EXP compared to CTRL, while PRE significantly reduced this elect. The systolic mitral annular velocity in the left ventricular lateral wall was significantly less affected in PRE than in EXP (1.99 +/- 0.65 vs. 1.2 +/- 0.39 cm x s(-1), P = 0.017). Enzyme levels showed only minor changes. In histology, no neutrophil infiltration was found in three groups.

These results suggest that short bouts of acceleration might offer cardiac protection. Confirmation using larger animal models may allow application of the concept to pilots preparing to undertake sustained high G maneuvers and may explain some aspects of the "G layoff" effect.