Determination of lung capillary blood volume and membrane diffusing capacity in patients with COLD using the NO-CO method.

Lung capillary blood volume (Qc) and the membrane diffusing capacity (Dm) can both be determined from the combined measurement of nitric oxide (NO) and carbon monoxide (CO) transfers using the single-breath method. In ten healthy subjects, no differences was observed between the values of transfer factor of the lungs for carbon monoxide (TLCO) recovered after a 3 s or 9 s breath-holding time (tBH). The NO-CO method could thus be used with a short tBH and a low fraction of inspired nitric oxide (FINO) (8 ppm). However, in ten patients with chronic obstructive lung disease (COLD), the values of both transfer factor of the lungs for nitric oxide (TLNO) and TLCO were underestimated by around 20% at a short tBH (3 s). In COLD patients, the NO-CO method therefore requires a longer tBH and a higher inspired fraction of NO (30 ppm) than in healthy subjects. Similar values of Dm and Qc were obtained using the NO-CO method and the two-step conventional method, at two levels of the oxygenation. The former method gave less scatter. Furthermore, TLNO is independent of the fraction of inspired oxygen (FIO2) and directly proportional to carbon monoxide membrane diffusing capacity (DmCO).