Hyperinflation, trapped gas and theophylline in chronic obstructive pulmonary disease

Assessment of patients with chronic airflow limitation traditionally involves the measurement of airway obstruction by forced expiratory volume in one second (FEV1) and forced vital capacity (FVC), as well as hyperinflation by residual volume (RV). Patients often claim reduction of breathlessness during theophylline therapy, without change of spirometry. In severe airflow limitation there is often a difference in lung volumes measured by helium dilution and plethysmography. The latter is thought to measure the absolute amount of gas in the lungs at functional residual capacity (FRC), whereas helium dilution measures only air in rapid communication with the mouth. The difference between the two is considered to represent poorly-ventilated areas of the lung, and is termed trapped gas volume. Twenty two patients with "irreversible" airflow limitation, mean (SD) FEV1 1.15 (0.52) l, FVC 2.57 (0.88) l, were studied. Uniphyllin Continus tablets (Napp Laboratories) were given at a dose to provide a mean steady-state plasma level of 14 mg.l-1, in a placebo-controlled, double-blind, cross-over study. The measurements made were FEV1, FVC, static lung volumes, 6 min walking distance (6WD), breathlessness after exercise assessed by visual analogue scales (VAB), and arterial blood gases. No significant correlation was found between changes in VAB and changes in RV measured by the helium dilution technique (RV (He)), FEV1 or FVC, but there were good correlations with trapped gas volume (r = 0.61, p < 0.01). The plethysmographic element of the calculated trapped gas volume reflects change in the sensation of breathlessness, and is affected by treatment with theophylline.