Analysis: For each SBW, the inspired and expired volumes, inspiratory and expiratory times, and the breath hold time were measured . RV was calculated by measuring the mass of helium inhaled and exhaled to determine the mass of helium remaining in the lung, as previously described . It was assumed that the mean helium concentration in the lung at RV was equal to the measured helium concentration at end expiration.
Because SBW manoeuvres were analyzed after storage of the flow, carbon monoxide and helium signals in digital form, DLcoSB-3EQ from any one or a number of simulated alveolar samples from the same manoeuvre could be analyzed . In this analysis the three-equation diffusing capacity using the simulated entire alveolar gas sample was computed . DLcosb-3EQ from the mean carbon monoxide concentration in each of four simulated sequential and equal alveolar gas samples was calculated, and each SB DLCO -3EQ value was normalized by expressing it as a per-SB centage of DLCO -3EQ calculated from the entire alveolar gas sample . For all DLcoSb-3EQ calculations, the mean carbon monoxide concentration in the exhaled alveolar gas of the deep breath of room air immediately preceding the single breath manoeuvre was used as the estimate of the carbon monoxide back pressure for the subsequent SBW manoeuvre .