Cardiorespiratory responses to prolonged normobaric inhalatory hypoxia in healthy men

Changes in the cardiorespiratory indicators due to the prolonged (25 min) inhalation of a respiratory mixture with an exponentially falling (from 20.9% to 10%) oxygen concentration were studied in healthy young men. The efficiency of oxygen uptake (as the ventilatory equivalent of O₂) in the lungs has been shown to fall particularly promptly (within the first 2–5 min). Individual manifestations of this fall were variable (ranging from 22 to 84% of the baseline data) and driven, to a considerable degree, by the subject’s current level of alveolar ventilation, mobilizing its urgent reserve to improve the ventilation-perfusion relations in the lungs. The subsequent response as a growth in the heart rate (HR) recorded in 100% of cases was delayed relative to the start of hypoxic exposure and combined with increased hypoxemia marked as a decrease in the blood oxygen saturation of hemoglobin. The individual HR growth in response to hypoxia ranged from 8 to 43% of the baseline level and was significantly related to the current level of diastolic arterial blood pressure. The hypoxic ventilatory response was expressed only in 71% of cases, including the 15% when it was reversed (decreased) against the background of a concomitant decrease in the respiratory frequency.