Measurement of single-breath CO diffusing capacity by continuous rapid CO analysis in man

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Abstract
Factors affecting the single-breath diffusing capacity for CO (DLCO(SB)) measured from gas sampled at progressively lower lung volumes during exhalation were evaluated. He and CO concentrations as well as lung volumes were measured continuously during single-breath maneuvers. The DLCO(SB) was calculated from gas concentrations measured at each 10% decrement in lung volume in 6 healthy male subjects. When conventional methods were employed to determine breath-holding time and alveolar volume, DLCO(SB) appeared to increase when measured from gas concentrations sampled later in exhalation. This trend was exaggerated by decreasing the exhaled flow from 2-0.5 l/s and by decreasing the time of breath-holding from 10-2 s (P < 0.001). However, when the times of breath-holding were calculated according to the postulate that gas entering the lung first exits last and when a separate time-weighted mean alveolar volume was calculated, DLCO(SB) no longer increased when measured from gas sampled later in exhalation. Apparently the major differences found in DLCO(SB) with changes in inhalation time, breath-holding time, exhalation time and the size and portion of the alveolar gas sampled depended on incorrect values for lung volume and time rather than inhomogeneity of diffusing capacity throughout the lungs. Apparently, the diffusion properties of lung regions contributing to alveolar gas during the upper 90% of the exhaled volume were similar in healthy subjects; only over the lower 10% of the exhaled volume did DLCO(SB) decrease.