Body position and cardiac dynamic and chronotropic responses to steady-state isocapnic hypoxaemia in humans

Neural mediation of the human cardiac response to isocapnic (IC) steady-sta te hypoxaemia was investigated using coarse-graining spectral analysis of h eart rate variability (HRV). Six young adults were exposed in random order to a hypoxia or control protocol, in supine and sitting postures, while end -tidal PCO2 (P-ET,P-CO2) was clamped at resting eucapnic levels. An initial 11 min period of euoxia (P-ET,P-O2 100 mmHg; 13.3 kPa) was followed by a 2 2 min exposure to hypoxia (P-ET,P-O2 55 mmHg; 7.3 kPa), or continued euoxia (control). Harmonic and fractal powers of HRV were determined for the term inal 100 heart beats in each time period. Ventilation was stimulated (P < 0 .05) and cardiac dynamics altered only lay exposure to hypoxia. The cardiac interpulse interval was shortened (P < 0.001) similarly during hypoxia in both body positions. Vagally mediated high frequency harmonic power (P-h) o f HRV was decreased by hypoxia only in the supine position, while the fract al dimension, also linked to cardiac vagal control, was decreased in the si tting position (P < 0.05). However, low-frequency harmonic power (P-l) and the HRV indicator of sympathetic activity (P-l/P-h) were not altered by hyp oxia in either position. These results suggest that, in humans, tachycardia induced by moderate IC hypoxaemia (arterial O-2 saturation S-a,S-O2 approx imate to 85%) was mediated by vagal withdrawal, irrespective of body positi on and resting autonomic balance, while associated changes in HRV were posi tionally dependent.