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The relationship between the maximum duration of a voluntary breath hold (BHT) and the alveolar Pco 2 at the start and finish of the period of apnoea has been investigated in six subjects over a wide range of CO 2 tensions, for breath holds made at VC, 60 per cent VC, and FRC. Alveolar Po 2 was maintained above the level at which hypoxia is a significant factor in stimulating pulmonary ventilation. The results suggest that the relationship between BHT and Pco 2 at breaking point may be approximated by two straight line segments: for short breath holds (with a high initial Pco 2 ), BHT and breaking point Pco 2 are linearly related; for long breath holds (with a low initial Pco 2 ) final Pco 2 is independent of the duration of the breath hold. These results suggest that the mechanical component of the drive to resume breathing saturates during long breath holds, the level of saturation depending on the lung volume at which the breath is held.
Kelman et al. (Wed,) studied this question.
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