Chronology of the push pull effect on nonhuman primates-shift of the arterial pressure threshold.

Fighter pilots are frequently exposed to high Gz acceleration which may induce in-flight loss of consciousness (G-LOC). One factor reducing tolerance to accelerations is a previous exposure to negative accelerations. This phenomenon, which happens during the first few seconds after the onset of the positive plateau, is called the push pull effect. Our goal was to validate a non human primate model in order to study push pull physiological mechanisms and possible changes in arterial pressure, which may occur after the first ten seconds of the positive acceleration plateau. Eight rhesus monkeys were centrifuged in profile runs, including positive Gz accelerations (+1.4, +2 and +3 Gz) with or without previous negative Gz acceleration (-2 and -3 Gz vs. +1.4 Gz). Heart rate, blood pressure and esophageal pressure were recorded during the entire centrifugation run. Results showed that the push pull effect was observed in the non human primate model. Moreover, the reduced tolerance to acceleration lingered longer than that during the first ten seconds after exposure to +Gz acceleration. It was found that, after the fourteenth second, mean blood arterial pressure stabilizes at a lower value, when the positive acceleration is preceded by a negative acceleration (15.8 kPa for -1 Gz and 15.5 for -2 Gz vs. 16.9 for 1.4 Gz). The chronology of the push pull effect seems to involve two periods. One has a short time span. The other one has a longer time span and could be induced by shift of pressure threshold, coming from exposure to previous negative acceleration.