Effect of atropine on alveolar gas mixing in man

1. Atropine is known to diminish bronchomotor tone. In order to investigate the acute effect of atropine on respiration and alveolar gas mixing, a dose of 2.4 mg was given intravenously. 2. Ten normal male volunteers were each studied three times with a nitrogen washout method, once before administration of atropine and then 20 min and 60 min thereafter. 3. After the administration of atropine there was a reduction in tidal volume, a slight increase in frequency of respiration and an increase in series dead space. The tidal mixing volume showed a fall of 25%. In spite of the reduced alveolar dead space the effective mixing volume fell by 29%. Multi-breath alveolar mixing efficiency fell by 3.5%. 4. Multi-breath alveolar mixing efficiency was found to be less with smaller tidal mixing volumes, a fall of 518 ml in the latter causing a reduction of 17.2% in mixing efficiency. 5. A reduction of 100 ml in tidal volume in normal subjects was associated with a decrease of 6.9% in alveolar mixing efficiency. In the subjects receiving atropine tidal volume reduced by 96 ml, but the observed fall in alveolar mixing efficiency was only 3.5%, This suggests an improvement in alveolar mixing of 3.4% due to the administration of atropine. Despite this small improvement, the mixing efficiency is still only 66%. The residual inefficiency of 34% cannot therefore be explained on the basis of bronchomotor tone.