1. Atropine is known to diminish broncho-motor 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 broncho-motor tone.