Influence of rate of induction of hypoxia on the ventilatory response

1. We have studied the influence of rate of induction of hypoxia on the duration of the phases of the respiratory cycle in seven conscious healthy subjects.2. Nine hypoxic procedures were performed on each subject, comprising three different rates of induction of hypoxia each at three different levels of P(CO2). Ventilation vs. arterial O(2) saturation plots were constructed and patterns of response were analysed in terms of ventilation vs. tidal volume and tidal volume vs. inspiratory duration (T(I)) and vs. expiratory duration (T(E)).3. The highest ventilatory responses were observed during steady-state hypoxia when P(CO2) was held at a level similar to the control mixed venous level; progressive hypoxia at this P(CO2) failed to stimulate ventilation to comparable levels. The ventilatory response was not influenced by rate of induction of hypoxia when P(CO2) was maintained at an end-tidal P(CO2) close to the control level.4. As tidal volume increased in response to hypoxia, T(I) and T(E) shortened. The progressive decrease in cycle duration occurred at all levels of P(CO2) and at all rates of induction of hypoxia, but was most marked under eucapnic conditions during steady state.5. It is concluded that steady-state hypoxia may produce a higher ventilatory response than progressive, non-steady-state hypoxia. Inspiratory time shortens as tidal volume increases irrespective of the rate of induction of hypoxia.