Wu B, Xue Y, Wu P, Gu Z, Wang Y, Jing X. Physiological responses of astronaut candidates to simulated +G_x orbital emergency re-entry. Aviat Space Environ Med 2012; 83:758–63.Introduction: We investigated astronaut candidates’ physiological and pathological responses to +G_x exposure during simulated emergency return from a running orbit to advance astronaut +G_x tolerance training and medical support in manned spaceflight. Methods: There were 13 male astronaut candidates who were exposed to a simulated high +G_x acceleration profile in a spacecraft during an emergency return lasting for 230 s. The peak value was 8.5 G. Subjective feelings and symptoms, cardiovascular and respiratory responses, and changes in urine component before, during, and after +G_x exposure were investigated. Results: Under high +G_x exposure, 15.4% of subjects exhibited arrhythmia. Heart rate (HR) increased significantly and four different types of HR response curves were distinguished. The ratio of QT to RR interval on the electrocardiograms was significantly increased. Arterial oxygen saturation (S_ao₂) declined with increasing G value and then returned gradually. S_ao₂ reached a minimum (87.7%) at 3 G during the decline phase of the +G_x curve. Respiratory rate increased significantly with increasing G value, while the amplitude and area of the respiratory waves were significantly reduced. The overshoot appeared immediately after +G_x exposure. A few subjects suffered from slight injuries, including positive urine protein (1/13), positive urinary occult blood (1/13), and a large area of petechiae on the back (1/13). Conclusions: Astronaut candidates have relatively good tolerance to the +G_x profile during a simulation of spacecraft emergent ballistic re-entry. However, a few subjects exhibited adverse physiological responses and slight reversible pathological injuries.