QT versus RR spectrum shades into sympathetic control during hypobaric hypoxia

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 trend-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 to have the 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 every 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 as 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). The 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-physiological rather than a purely physiological test