Changes in transfer factor of the lung in response to bronchodilatation

Measurement of the transfer factor for carbon monoxide (T_LCO) is a widely used clinical lung function test. Although it is frequently applied in patients with bronchial obstruction, there is little information on the effects of bronchodilatation on the test. We therefore measured T_LCO in 40 patients before and after inhalation of terbutaline. T_LCO was measured with the single‐breath technique in 20 patients and with the intra‐breath technique in 20 patients. T_LCO was also measured in 20 healthy subjects with the single‐breath technique. Forced expiratory volume (FEV₁) increased from 2·9 ± 1·1 to 3·2 ± 1·2 l in patients with bronchial obstruction in response to terbutaline inhalation. T_LCO increased from 8·2 ± 2·6 to 8·6 ± 2·7 mmol min^–1 kPa^–1 (P< 0·001) and alveolar volume (V_A) from 5·74 ± 1·21 to 5·90 ± 1·21 l (P<0·001). There was no difference between the single‐breath and the intra‐breath techniques. There was little change in FEV₁ in the healthy subjects in response to terbutaline. T_LCO increased from 10·2 ± 2·1 to 10·5 ± 2·2 mmol min^–1 kPa^–1 (P< 0·01), but there was no change in V_A. The increase in T_LCO in patients may partly be explained by improved distribution of the inhaled gas. In healthy subjects, terbutaline may increase pulmonary capillary volume. We conclude that bronchodilatation results in a small increase in T_LCO in patients with light to moderate bronchoconstriction as well as in healthy subjects. The effect is small and should in most cases be simple to account for in the interpretation of pulmonary function tests, provided the patient’s treatment is known.