Pulmonary diffusing capacity for carbon monoxide and nitric oxide.

Abstract

Carbon monoxide (CO) is widely used to determine the diffusive conductance or diffusing capacity (D) of the lungs. Its advantage over O2 is the high affinity of hemoglobin for CO whereby PCO in pulmonary capillary blood is low and the veno-arterial PCO difference is minimal. Thus, the mean PCO difference effective for alveolar-capillary CO transfer is close to alveolar Pco. But since the reaction of CO with hemoglobin is relatively slow, the diffusing capacity for CO includes a reaction component which cannot be easily assessed. The affinity of hemoglobin for nitric oxide (NO) is even higher (about 2000 t imes) than for CO and, more importantly, the association velocity constant of hemoglobin with NO is about 75 times higher than that with CO (1). Thus the diffusing capacity for NO may represent a closer estimate of the true pulmonary diffusing capacity, i.e. not complicated by reaction velocity. Moreover, a comparison with DCO may provide information on the extent of reaction limitation in CO uptake.