Influence of lung volume history on closing volume measurement during anaesthesia.

Airway closure measurements were made with the bolus technique on eight healthy subjects, who were in a supine position prior to and during anaesthesia. Measurements were made on an expiration following vital capacity (VC) and 30% VC. Closing volume (CV) was calculated prior to anaesthesia, and closing capacity (CC)--functional residual capacity (FRC) was estimated during anaesthesia. When measured from VC, CV was 703 +/- 20 ml (s.e. mean) and from 30% VC it was 440 +/- 51 ml (s.e. mean) (P less than 0.005) prior to anaesthesia. When measured from VC, CC--FRC was 370 +/- 34 ml (s.e. mean), and from 30% VC it was 343 +/- 37 (s.e. mean) (P greater than 0.05) during anaesthesia. It is concluded that volume history has little effect on CC measurement during anaesthesia and artificial ventilation, but a major influence on CC measurement in the conscious patient. Hence, it is suggested that CC, within the tidal range, is increased during anaesthesia.