QUESTION: What are the physiologic effects of respiratory muscle fatigue?
ANSWER: These effects are revealed by the use of a special mechanical ventilator that is capable of unloading the respiratory muscles and substantially reducing the work of breathing during heavy exercise and thereby preventing diaphragm fatigue. Relief of a substantial amount of this high sustained level of respiratory muscle work reduces the rate of development of limb muscle fatigue during whole-body exercise and significantly improves endurance exercise performance. Preventing respiratory muscle fatigue does not result in an improvement in ventilation or pulmonary gas exchange, apparently because there is sufficient reserve in the accessory respiratory muscles, and even in the primary inspiratory and expiratory muscles, to provide sufficient alveolar ventilation in healthy subjects. Rather, the major effect of preventing exercise- induced respiratory muscle fatigue is more likely the prevention of the respiratory muscle metaboreflex effects on increasing sympathetically mediated vasoconstriction and an improved vasodilation and blood flow to limb locomotor muscles (see earlier). Please note that these effects of reducing respiratory muscle work on alleviating the development of limb muscle fatigue and increasing exercise performance are present in normal healthy subjects exercising at sea level. These cardiovascular effects of exercise-induced respiratory muscle work are greatly enhanced under conditions of in- creased ventilation during exercise, such as occurs in healthy subjects in the hypoxia of high altitude and to an even greater extent in patients with chronic heart failure exercising at relatively mild levels at sea level.