Relief of distress of breathholding: separate effects of expiration and inspiration.

It is well known that rebreathing relieves the respiratory distress of maximal breathholding despite worsening blood gases, and it has been suggested that vagal input has a role in ameliorating this sensation via activation of pulmonary stretch receptors (PSR). However, it is believed by divers that expiration can lead to partial relief of distress of breathholding at total lung capacity (TLC) allowing a prolongation of breathholding. We studied the independent effects of an expiration and an inspiration on relief of respiratory distress of breathholding. Subjects held their breath at TLC until distress became intolerable, then exhaled to FRC and performed a second breathhold. When distress again became intolerable, subjects inspired to TLC a gas that resembled their exhaled gas and performed a third breathhold. Subjects noted partial relief with both an expiration and an inspiration. However, relief of distress was greater and the subsequent breathhold longer after an inspiration than after an expiration. We suggest that relief of distress after an inspiration is compatible with the inhibitory effect of PSR input; the mechanism of relief that occurs after an expiration is as yet uncertain.

The Fowler breathholding study revisited: continuous rating of respiratory sensation.

The respiratory distress of breathholding has been shown to be relieved by breathing, even without correction of worsening blood gases (Fowler, 1954). We repeated the study by having untrained normal subjects perform maximal breathholds which were followed by the rebreathing of a gas mixture containing 7.5% CO2 and 8.2% O2, and then by second breathholds. In addition, we had the subjects continuously rate their respiratory distress using a visual analog scale (VAS). The ratings were easy to perform and were highly reproducible on repeated trials in a given subject. Subjects experienced increasing distress during the breathhold, rapid and substantial relief upon rebreathing, and then were capable of performing second breathholds, all consistent with Fowler's results. The findings are consistent with animal studies in which a neural mechanism associated with stimulation of pulmonary stretch receptors inhibits the firing of midbrain neurons which may be involved in transmission to the cortex of sensory information about breathing.