To identify conditions associated with severe carbon monoxide transfer coefficient reduction (Kco < 40% of predicted values).

We retrospectively reviewed pulmonary function tests, such as Kco measurement, in consecutive patients evaluated in the physiology unit of a University hospital over a 6-year period. Patients were included if they had at least 1 measure of severe Kco reduction, with further detailed pulmonary function tests, clinical data, and diagnostic procedures conducted within 6 months.

5576 patients underwent 9061 Kco measurements. 195 patients (3.5%) with Kco < 40% were investigated (156 males; mean age 62 ± 12 years). Mean Kco was 29 ± 9% of predicted values. The main conditions associated with severe Kco reduction were: emphysema (46%); interstitial lung disease (28%); combined pulmonary fibrosis and emphysema (16%); and pre-capillary pulmonary hypertension (8%). Systolic pulmonary artery pressure was ≥35 mmHg at echocardiography in 88% of patients. Right heart catheterization performed in 97 patients showed pre-capillary pulmonary hypertension in 86 of 195 patients (44%). Pulmonary hypertension was the most frequent condition associated with Kco severe reduction. Pulmonary hypertension was present in 29%, 53%, and 48% of patients with chronic obstructive pulmonary disease, interstitial lung disease, and combined pulmonary fibrosis and emphysema, respectively, and was disproportionate to the parenchymal lung disease (mean pulmonary artery pressure > 35 mmHg) in 63% of cases.

Severe Kco reduction is frequently related to pulmonary hypertension, especially when associated with emphysema and/or interstitial lung disease. Systematic echocardiography is thus warranted in any patient with severe Kco diminution.