The effects of roll vs. pitch rotation in humans under orthostatic stress

Background: It has been known since 1953 that pre-exposure to less than +1 Gz will reduce subsequent +Gz-tolerance. With few exceptions, during operat ional flying, the transition from hypogravity to hypergravity involves roll as well as pitch rotation. We examined the effect of roll vs. pitch rotati on while undergoing transition from hypogravity to +1 Ct on a tilt table. M ethods: Twelve subjects (28-47 yr old) were rotated at 45 degrees . s(-1) f rom head-up (HU) at 15 degrees relative to gravitational vertical to 135 de grees head-down (HD) and back to the HU position after different HD dwell t imes. HD dwell times were set at 7, 15, and 30 s. The subject was rotated a bout the interaural axis (pitch) and about the naso-occipital axis (roll). Both the HD dwell times and axes of rotation were randomized within and acr oss subjects. BP and heart rate were recorded during the HU-HD-HU maneuver. Results: Analysis of variance, repeated measure design revealed that the r ate and magnitude of BP decrease induced by the HD to HU maneuver is signif icantly higher (p < 0.01) in roll than in pitch during all HD dwell times. The decrease of BP at 7s is significantly (p < 0.01) higher than at 15s and 30s. Heart rate increases significantly higher (p < 0.01) in pitch than in roll at 7s-dwell time. Conclusion: Our results suggest that the compensato ry mechanism to orthostatic stress is more efficient in response to pitch t han roll rotation. This is reflected from the findings that the mean magnit ude of OH (orthostatic hypotension) and the rate of BP decrease induced by the HD-HU maneuver is significantly greater in roll rotation than pitch rot ation. The mean HR increase post HD-HU relation is significantly higher in the pitch than the roll rotation. The significant rate of BP decrease durin g HD-HU roll relation could have important implications for maintaining G-t olerance and spatial orientation during subsequent exposure to hypergravity .