Cost of breathing during exercise

In healthy subjects, the metabolic cost of ventilation is small during rest or moderate exercise, but increases during heavy exercise, reaching ~10% of the total energy expenditure at peak exercise. Moreover, activation of metaboreceptors in the respiratory muscles during heavy exercise may trigger an increase of the sympathetic outflow to the locomotor muscles, blunting the increase of the bloodflow and limiting the performance. The cost of ventilation may become substantial in the presence of a pulmonary pathology, as chronic obstructive pulmonary disease. Despite its importance in physiological and pathological conditions, the measurement of the cost of breathing remains a challenge, as suggested by the wide range of values reported in the literature. To measure the cost of breathing via indirect calorimetry, ventilation should be increased while maintaining non-respiratory oxygen consumption constant. As the fraction of expired oxygen increases, measurement of oxygen consumption becomes problematic, because the relation between alveolar ventilation and alveolar partial pressure of oxygen becomes almost flat, and small errors in measuring expired gas composition produce big errors in the calculation of oxygen consumption. Additionally, the modality of ventilation may be strikingly different according to the stimulus which elicited the increase of ventilation, namely exercise or the addition of a dead space or of hypercapnic mixtures. Mimicking at rest the breathing pattern recorded at various levels of exercise may improve the accuracy of the estimation of the cost of breathing, but this method is technically difficult and requires a high level of cooperation from the experimental subject.