Terbutaline and diaphragm function in chronic obstructive pulmonary disease: A double-blind randomized clinical trial

We conducted a double-blind, randomized crossover trial to evaluate whether oral terbutaline (2.5 mg orally three times daily for a week) increased the force of diaphragmatic contraction in normocapnic patients with chronic obstructive pulmonary disease. Ten patients with moderate to severe airway obstruction completed the trial. Compared with placebo, terbutaline produced a mean increase of 5.8 cmH₂O in peak inspiratory mouth pressure and a mean increase of 5.0 cmH₂O in transdiaphragmatic pressure during a maximal inspiratory manoeuvre. These small changes with terbutaline failed to achieve statistical significance. Also, terbutaline failed to alter flow rates (FEV₁, $\text{V}\text{̇}{}_{\mathrm{max50}}$) or patients' dyspnoea ratings using two separate clinical scales (Pneumoconiosis Research Unit Score and the Modified Dyspnoea Index). Because all observed changes in respiratory muscle strength were small and because the trial had power to detect small changes in inspiratory mouth pressures, we suggest that oral terbutaline at the dose administered in this study has little noteworthy effect on respiratory muscle strength in normocapnic patients with chronic obstructive pulmonary disease.