MEASUREMENT OF SINGLE-BREATH CO DIFFUSING-CAPACITY BY CONTINUOUS RAPID CO ANALYSIS IN MAN

This study attempts to evaluate factors affecting the single-breath diffusing capacity for carbon monoxide (DLCO(SB)) measured from gas sampled at progressively lower lung volumes during exhalation. Helium and carbon monoxide 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 six 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 to 0.51/s as well as by decreasing the time of breath holding from 10 to 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 also calculated, DLCO(SB) no longer increased when measured from gas sampled later in exhalation. Our findings suggest that 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 depend to a large degree on incorrect values for lung volume and time rather than inhomogeneity of diffusing capacity throughout the lungs. Further, our results suggest that the diffusion properties of lung regions contributing to alveolar gas during the upper 90% of the exhaled volume are similar in healthy subjects; only over the lower 10% of the exhaled volume does DLCO(SB) decrease.