Closing capacity measurement during general anesthesia.

A modification of the single-breath nitrogen closing volume (CV) test allows measurement of closing capacity (CC) during general anesthesia. In the modification, inspiration and expiration are mechanically produced by a hydraulically powered cylinder. For 14 awake, normal subjects results of the CV test performed using this mechanical method differed that those obtained following spontaneous inspiration and expiration. Mean (+/-SE) CC's were 2.25 (+/-0.15) and 2.42 1 (+/-0.14) (P less than 0.01) using spontaneous and mechanical methods, respectively. The slopes of Phase III of the CV traces were 2.24 (+/-0.27) and 2.66 per cent N2/L (+/-0.32) (P less than 0.01), respectively. To eliminate differences due to measurement technique, the modified CV test was used both before and during anesthesia with halothane in 70 per cent N2 in 11 normal, supine, spontaneously breathing subjects. CC's were 1.89 l (+/-0.16) before and 1.84 l (+/-0.15) during anesthesia (P greater than .5). Mean functional residual capacities (FRC) by the closed-circuit helium method were 1.77 l (+/-0.15) before and 1.45 l (+/-0.17) during anesthesia (P less than .001). With CC unchanged and FRC decreased following induction, CC/FRC increased from 1.07 (+/-0.08) to 1.37 (+/-0.11) (P less than .005), suggesting increased small-airway closure during anesthesia.