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Body Position and Cardiac Dynamic and Chronotropic Responses to Steady‐State Isocapnic Hypoxaemia in Humans
Author(s) -
Lucy S. Deborah,
Hughson Richard L.,
Kowalchuk John M.,
Paterson Donald H.,
Cunningham David A.
Publication year - 2000
Publication title -
experimental physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.925
H-Index - 101
eISSN - 1469-445X
pISSN - 0958-0670
DOI - 10.1111/j.1469-445x.2000.01932.x
Subject(s) - supine position , heart rate , heart rate variability , medicine , cardiology , hypoxia (environmental) , anesthesia , chemistry , blood pressure , organic chemistry , oxygen
Neural mediation of the human cardiac response to isocapnic (IC) steady‐state hypoxaemia was investigated using coarse‐graining spectral analysis of heart 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 P CO2 ( P ET,CO2 ) was clamped at resting eucapnic levels. An initial 11 min period of euoxia ( P ET,O2 100 mmHg; 13.3 kPa) was followed by a 22 min exposure to hypoxia ( P ET,O2 55 mmHg; 7.3 kPa), or continued euoxia (control). Harmonic and fractal powers of HRV were determined for the terminal 400 heart beats in each time period. Ventilation was stimulated (P < 0.05) and cardiac dynamics altered only by 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 (Ph) of HRV was decreased by hypoxia only in the supine position, while the fractal dimension, also linked to cardiac vagal control, was decreased in the sitting position (P < 0.05). However, low‐frequency harmonic power (Pl) and the HRV indicator of sympathetic activity (Pl/Ph) were not altered by hypoxia in either position. These results suggest that, in humans, tachycardia induced by moderate IC hypoxaemia (arterial O 2 saturation S a,O2 ≈ 85%) was mediated by vagal withdrawal, irrespective of body position and resting autonomic balance, while associated changes in HRV were positionally dependent.