Changes in carbon monoxide transfer over 22 years in middle-aged men

We measured single breath CO transfer (T(LCO)), single breath alveolar volume (VA), CO transfer coefficient (K(CO)) and forced expiratory volume in 1 sec (FEV1) in 84 men, mean age 40.5 years at recruitment, in 1975 and in 1997. At recruitment, 42 men were cigarette smokers and 42 were not smoking. Mean annual decline in FEV1 was similar in never- (34.2 ml yr(-1)) and ex- (33.1 ml yr(-1)) smokers and faster (51.0 ml yr(-1)) in continuing smokers. In contrast to predictions from cross-sectional reference values, there was no fall in T(LCO) or K(CO) in men who did not smoke over the period of follow-up. In the 16 men who smoked throughout follow-up there was a 10% fall in T(LCO) (P = 0.043) but most of this was due to a significant fall in VA (P = 0.017), presumably reflecting uneven gas mixing. These results indicate the need for population-based longitudinal studies of T(LCO) and K(CO). If single breath estimates of VA are used in subjects with even mild airflow obstruction, K(CO) rather than T(LCO) should be used to assess alveolar function.