Brief exposure to -Gz reduces cerebral perfusion pressure during subsequent +Gz stress in rats

Introduction: In humans, +Gz exposure immediately preceded by exposure to z ero or -Gz can result in unexpected incapacitation ("pushpull" effect). Our goals were to establish whether this phenomenon exists in rats and to eval uate the importance of varying the duration of -Gz exposure on magnitude of the push-pull effect on cerebral perfusion pressure. Methods: Eight consci ous male rats were studied in the transition from + 5 Gr to +10 Cr imposed by centrifugation. This was done with (push-pull or without (control) 2 s e xposure to -5 Gz applied using a counterbalanced design. Seven isoflurane a nesthetized rats were studied in the transition from 0Gz (+1Gy) to + 1Gz im posed by tilting. This was done with (push-pu[ii or without (control) 0.5, 1, 3, or 9 a exposure to -1Gz imposed immediately prior to the transition a pplied using a counterbalanced designed. Results: Exposure to 2 s of -5 Gz significantly (p < 0.01) reduced carotid artery pressure in the 4(th) throu gh a s of exposure to +10 Ct by an average of 15 mmHg compared with control . In the tilt experiments, a push-pull effect was found with mild Cr exposu re (+/-1Gz) with as little as 0.5 s -Gz exposure. Varying the head-down dwe ll time did not alter the magnitude of the exaggerated hypotension induced by "push-pull" (p = 0.90. Conclusions: We conclude that rats express a "pus h-pull" effect similar to that observed in humans but that altering the dur ation of exposure to -Gz does not influence the magnitude of the "push-puil " effect.