Theophylline Improves Measurements of Respiratory Muscle Efficiency

¹ From the Division of Pulmonary and Critical Care Medicine, Department of Medicine, MCP—Hahnemann School of Medicine, Allegheny University Hospitals-Hahnemann Division, Philadelphia
² From the Division of Allergy and Immunology, Department of Medicine, MCP—Hahnemann School of Medicine, Allegheny University Hospitals-Hahnemann Division, Philadelphia

To determine the effect of theophylline on respiratory muscle efficiency (RME), 12 normal subjects were given theophylline vs placebo in a double-blind, randomized crossover protocol. Spirometry, resting energy expenditure, minute ventilation, RME, and oxygen cost of breathing were measured at baseline, after taking theophylline, and after placebo. RME was calculated by dividing the added work required to breathe through a threshold load by the added energy consumed during loaded breathing. Oxygen cost of breathing was calculated by dividing the increase in oxygen consumption induced by breathing an air/carbon dioxide mixture by the associated increase in minute ventilation. RME increased from 3.3±1.6% at baseline to 7.9±3.2% after theophylline (p<0.01) but did not change significantly after placebo (4.8±2.4%). Oxygen cost of breathing decreased from 3.9±2.4 mL O₂ per liter at baseline to 1.7±0.7 mL O₂ per liter after theophylline (p<0.05) but did not change significantly after placebo (2.8±1.3 mL O₂ per liter). Theophylline use was also associated with an 18% increase in minute ventilation (p<0.01) and a 15.7% increase in resting energy expenditure (p<0.01). Theophylline improves measured RME and reduces oxygen cost of breathing in normal subjects. These effects are offset by increases in resting energy expenditure and minute ventilation.

Key Words: oxygen consumption • oxygen cost of breathing • respiratory muscles • resting energy expenditure • theophylline • work of breathing

Submitted on January 16, 1996
Accepted on July 24, 2007