Lack of effect of dextromethorphan on breathlessness and exercise performance in patients with chronic obstructive pulmonary disease (COPD)

We have previously shown that the exercise performance of patients with severe chronic obstructive pulmonary disease (COPD) can be increased with the administration of oral morphine (0.8 mg.kg-1). The purpose of this study was to determine whether the administration of dextromethorphan (DXT), an antitussive structurally similar to codeine, would result in increased exercise performance and decreased dyspnoea in patients with COPD, without the side-effects of opiates. Six eucapnic patients (mean age = 66 +/- 3.8 yrs) with COPD (mean forced expiratory volume in one second (FEV1) = 1.01 +/- 0.07 l) underwent two incremental cycle ergometer tests to exhaustion (Emax) and assessment of their hypercapnic and hypoxic ventilatory responses and mouth occlusion pressure responses following first the oral administration of placebo (P) and then dextromethorphan (60 mg) in a single-blind fashion. There was no statistically significant difference in the maximal exercise performance, perceived dyspnoea (modified Borg scale), breathing pattern or expired gases after the two different treatments. In addition, the ventilatory response to CO2 production during exercise (delta VE/VCO2) and the ventilatory and mouth occlusion pressure responses to hypoxia and hypercapnia did not differ significantly after DXT as compared with after P. Indeed the exercise performance was poorer and the ventilatory responses were brisker after DXT. We conclude from this study that the administration of this opiate analogue does not improve the exercise capacity or decrease the ventilatory response of patients with COPD.