Mechanical constraints on exercise hyperpnea in endurance athletes

We determined how close highly trained athletes [n = 8; maximal oxygen consumption (V̇O(2 max)) = 73 ± 1 ml · kg^-1 · min^-1] came to their mechanical limits for generating expiratory airflow and inspiratory pleural pressure during maximal short-term exercise. Mechanical limits to expiratory flow were assessed at rest by measuring, over a range of lung volumes, the pleural pressures beyond which no further increases in flow rate are observed (Pmax(e)). The capacity to generate inspiratory pressure (Pcap(i)) was also measured at rest over a range of lung volumes and flow rates. During progressive exercise, tidal pleural pressure-volume loops were measured and plotted relative to Pmax(e) and Pcap(i) at the measured end-expiratory lung volume. During maximal exercise, expiratory flow limitation was reached over 27-76% of tidal volume, peak tidal inspiratory pressure reached an average of 89% of Pcap(i), and end-inspiratory lung volume averaged 86% of total lung capacity. Mechanical limits to ventilation (V̇E) were generally reached coincident with the achievement of V̇O(2 max); the greater the ventilatory response, the greater was the degree of mechanical limitation. Mean arterial blood gases measured during maximal exercise showed a moderate hyperventilation (arterial PCO₂ = 35.8 Torr, alveolar PO₂ = 110 Torr), a widened alveolar-to-arterial gas pressure difference (32 Torr), and variable degrees of hypoxemia (arterial PO₂ = 78 Torr, range 65-83 Torr). Increasing the stimulus to breathe during maximal exercise by inducing either hypercapnia (end-tidal PCO₂ = 65 Torr) or hypoxemia (saturation = 75%) failed to increase V̇E, inspiratory pressure, or expiratory pressure. We conclude that during maximal exercise, highly trained individuals often reach the mechanical limits of the lung and respiratory muscle for producing alveolar ventilation. This level of ventilation is achieved at a considerable metabolic cost but with a mechanically optimal pattern of breathing and respiratory muscle recruitment and without sacrifice of a significant alveolar hyperventilation.