Hypobaric hypoxia: dual sympathetic control in the light of RR and QT spectra.

ECGs of 21 candidate-pilots and 19 pilots were recorded during: 1) exposure to 5500 m hypobaric hypoxia (HH) while sitting, in the sequence: 1a) initial 7 min of adaptation (A); 1b) later 7 min of recovery (R) after short but intense tread-mill effort; and 2) final 7 min baseline (B), while sitting and requested to relax, at "0 m altitude" in the hypobaric room. RR and QT short-term variability were studied using spectral powers within Traube-Hering-Mayer (THM: 0.05-0.15 Hz) and respiratory (RESP: 0.2-0.4 Hz) bands. Mean RR proved highest capability to aggregate individual response-profiles: 15 pilots and 9 candidates entered the main (normal) cluster, featured by a comparison "triangle" set as expected: A > R < B > A. QT-THM power closely followed: 10 subjects (ss) in very normal cluster, defined as: A < R > B < A, while secondary clusters in candidates and pilots were interpreted by not-successful relaxation and exaggerated start-effects, respectively. Subjects with QT-THM normal clusters (A < R > B < A has group averages, p < = 0.05), also showed a quasi-normal "triangle" for mean RR, (A = R < B > A). During adaptation to hypoxia, pilots' QT-THM was higher than candidates' one (p < 0.02, Wilcoxon test). Study supports the emerging capability of QT-THM spectral power to index ventricular sympathetic control. Exposure to hypobaric hypoxia proved to be in these subjects a psycho-physiologic rather than a purely physiologic test.