Altitude-induced periodic breathing optimises respiratory efficiency during sleep in young healthy males

Objective To assess the impact of acute hypoxia-induced periodic breathing on respiratory efficiency in young healthy males. Methods 20 healthy male (median (IQR) age: 24 (22 to 25) years; body mass index: 22.5 (20.9 to 24.9) kg/m 2 )—no sleep-disordered breathing in normoxia—underwent polysomnography in normobaric hypoxia simulating 3500 m altitude. Measurements included oesophageal pressure-time curve, airflow and exhaled fraction of carbon dioxide (CO 2 ). Inspiratory pressure-time product (iPTP, index of respiratory muscle effort) was calculated from oesophageal pressure. Physiological dead-space volume, ventilation ( <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mover accent="true"> <m:mrow> <m:mi>V</m:mi> </m:mrow> <m:mo>˙</m:mo> </m:mover> </m:math> D est ) and alveolar ventilation ( <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mover accent="true"> <m:mrow> <m:mi>V</m:mi> </m:mrow> <m:mo>˙</m:mo> </m:mover> </m:math> A est ) were calculated from exhaled fractions of CO 2 (Bohr equation). Within-subject comparisons of periodic (PB) and regular breathing (RB) periods were made using the Wilcoxon signed-rank test. Results In hypoxia, 17 participants had sufficient (>3 min) breathing periods of both periodic and non-periodic breathing for analysis. The median (IQR) Apnoea-Hypopnoea Index was 76.9 (62.8 to 122.7)/hour and participants spent 40.8% (27.3% to 64.6%) of sleep in periodic breathing (n=17). Tidal volume was greater in periodic breathing (n=17, median (IQR), PB vs RB; 0.815 (0.636 to 0.928) L vs 0.633 (0.517 to 0.673) L, p<0.001). Physiological dead space was similar in both breathing patterns (n=17, PB vs RB, 0.172 (0.160 to 0.196) L vs 0.184 (0.162 to 0.205) L, p=0.782). Respiratory rate was lower in periodic breathing (n=17; PB vs RB; 10.6 (9.8 to 13.6)/min vs 16.8 (15.4 to 17.7)/min, p<0.001). <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mover accent="true"> <m:mrow> <m:mi>V</m:mi> </m:mrow> <m:mo>˙</m:mo> </m:mover> </m:math> D est was lower in periodic breathing (n=17; PB vs RB; 1.9 (1.7 to 2.2) L/min vs 3.0 (2.6 to 3.4) L/min, p<0.001), while <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mover accent="true"> <m:mrow> <m:mi>V</m:mi> </m:mrow> <m:mo>˙</m:mo> </m:mover> </m:math> A est remained similar (n=17, PB vs RB; 6.9 (6.3 to 7.4) L/min vs 7.3 (6.8 to 7.7) L/min, p=0.102). iPTP/min was lower in PB (n=14, PB vs RB; 185.4 (154.3 to 203.8) cm H 2O×s/min vs 221.5 (189.2 to 291.9) cm H 2O×s/min, p=0.002). Conclusions Periodic breathing in acute normobaric hypoxia reduces inspiratory effort without impairing alveolar ventilation, suggesting an adaptive mechanism to optimise respiratory efficiency in young healthy males.