1. Nebulized inhaled morphine has been reported to increase exercise endurance in patients with chronic lung disease and to relieve dyspnoea in patients with malignant disease. Potential mechanisms include a central effect occurring after systemic drug absorption or a local action mediated by receptors in the lung.
2. The ventilatory effects of nebulized morphine (10 and 25 mg) were therefore compared with those of intravenous morphine (1.0 and 2.5 mg) and placebo in a double-blind study involving 12 young healthy males. Submaximal cycle ergometry with respiratory gas analysis was performed 15 min after drug administration and breathlessness assessed using a linear visual analogue scale.
3. Neither dose of inhaled morphine had statistically significant effects on spirometry, heart rate, ventilation, respiratory gases or breathlessness at any level of exercise. The slopes and intercepts of the lines relating ventilation to breathlessness were also unaffected.
4. Intravenous morphine 2.5 mg reduced breathlessness slightly at the highest equivalent workload [mean (least significant range) 33 mm (26–40 mm)] compared with placebo [41 mm (34–48 mm), P < 0.05] but had no other significant effects.
5. These results do not support the hypothesis that intrapulmonary opiate receptors modulate the sensation of breathlessness in healthy man. The possibility that inhaled morphine may affect breathlessness caused by other factors, such as disease, has not been excluded.