Different effects of hypothermia on two syndromes of positive acceleration

¹ Aviation Medical Acceleration Laboratory, U. S. Naval Air Development Center, Johnsville, Pennsylvania

The tolerance to high positive (headward) acceleration at levels of 20–80 G was studied in normal and hypothermic rats. Normal rats have a mean survival time of 680 sec at 20 G, but this decreases to 105 sec at 30 G. Only slight decreases are noted at higher G, suggesting that 30 G is the point at which cerebral circulation is interrupted. Hypothermia at 22.5 C decreases acceleration tolerance at 20 G, but markedly increases acceleration tolerance at 30 G and above. At 40 G this effect is maximal, and the mean survival time for hypothermic rats is 252 sec, compared to 75 sec for controls, an increase of 236%. These different effects of hypothermia on acceleration tolerance suggest that there are two distinct syndromes of physiologic failure during positive acceleration. One is a "cardiac" syndrome, occurring at 20 G and below, characterized by partial maintenance of cerebral circulation until cardiac failure ensues, and the other is a "cerebral" syndrome, occurring at 30 G and above, characterized by immediate interruption of cerebral circulation and respiratory paralysis.