Postural variations of pulmonary diffusing capacity in insulin-dependent diabetes mellitus.

STUDY OBJECTIVE To assess whether posture-related changes of diffusing capacity could be considered as an early sensitive marker of pulmonary abnormalities in patients with insulin-dependent diabetes mellitus (IDDM) and whether the postural variations of pulmonary capillary blood volume (Vc) could reflect the lung capillary damage that characterizes the diabetic microangiopathy. DESIGN Carbon monoxide diffusing capacity (DCO) was measured by the single-breath method. Four DCO measurements, two in sitting and two in supine position, were performed in each subject using gas mixtures containing different oxygen concentrations. Membrane and capillary volume components of the diffusion capacity were calculated and both were expressed as absolute value and corrected by alveolar volume (VA). PATIENTS Twenty IDDM patients and 20 normal subjects matched for age and sex were studied. MEASUREMENTS AND RESULTS The IDDM patients showed normal pulmonary volumes and flows. No significant differences between the two groups were found for DCO, coefficient of diffusion, Vc, and pulmonary capillary blood volume corrected by alveolar volume in sitting position. All these indexes significantly increased in normal subjects but not in diabetics, by changing the posture of the subject from sitting to supine position. In a multivariate analysis, the presence of diabetes mellitus and the age of the subjects were the only significant predictors of Vc postural changes. CONCLUSIONS This postural test, adjusted for age, could be included in a screening diagnostic procedure for an early assessment of pulmonary abnormalities in diabetic patients. The lack of Vc postural increase in diabetics could reflect the presence of a microangiopathy involving the pulmonary small vessels.