Chest
Recent Advances in Chest MedicineUpdate in the Understanding of Respiratory Limitations to Exercise Performance in Fit, Active Adults
Section snippets
Physiologic Determinants of Exercise Performance
The capability for maximum O2 transport (or the product of arterial O2 content × blood flow) to the working locomotor muscles and, in turn, diffusion from muscle capillaries to mitochondria are major determinants of maximum oxygen uptake (o2max) in the muscle, of peripheral muscle fatigue, and, by implication, of exercise performance.1, 2 Exercise performance is also dictated by the perception of effort of the brain and its inhibitory effects on central motor drive to the periphery.3 We believe
Intrathoracic and Extrathoracic Airway Resistance
The ventilatory demands of heavy-intensity exercise require airway flow rates that often exceed 10 times resting levels and tidal volumes that approach 5 times resting levels. In order to avoid an increase in airway resistance causing excessive work of the respiratory muscles, adaptations must occur in the airways, including the following: (1) maximum relaxation of the bronchial smooth muscle due to withdrawal of parasympathetic tone; (2) increases in tidal end-inspiratory lung volumes, which
Expiratory Flow Limitation in the Nonasthmatic Intrathoracic Airway
There are many endurance athletes who show significant expiratory flow limitation (EFL) in heavy exercise resulting in hyperinflation of their end-expiratory lung volume25, 26, 27 (Fig 2). These athletes, unlike those discussed above with EIA or VCD, have “normal” airways and normal age-predicted maximal flow-volume envelopes. However, their high peak exercise capacities demand extreme ventilations and flow rates, resulting in EFL, hyperinflation, and reduced inspiratory capacity.
On the one
Inefficient Alveolar-to-Arterial O2 Exchange
During exercise in all healthy subjects, the P(A-a)O2 widens progressively with increasing exercise intensity. A significant portion of male and female young and older adults with high o2max show an exaggeration of this inefficiency in gas exchange during progressive and sustained heavy-intensity exercise,34 especially when running.35 When this excessive P(A-a)O2 is combined with a limited hyperventilatory response along with an acid pH-induced rightward shift of the hemoglobin-O2 dissociation
Diaphragm Fatigue
The diaphragm is certainly a very special skeletal muscle with many unique fatigue-resistant properties compared to limb muscle.49 However, during sustained heavy-intensity exercise, at > 80% of maximum in both untrained and highly trained subjects, objective measurements (ie, assessing changes in diaphragmatic force in response to supramaximal motor nerve stimulation) show a significant fatigue of both the diaphragm and the expiratory abdominal muscles.50, 51 This fatigue does not compromise
Extra Problems Presented Via the Hypoxia of High Altitudes
The hypoxic environment of high altitude has major effects on the cardiorespiratory responses to exercise, and causes decrements in exercise capacity and performance, which average about 5 to 10% per 1,000 feet of elevation. Some of the key respiratory maladaptations to exercise in hypoxic environments include the following: (1) arterial hypoxemia achieved via enhanced alveolar-capillary diffusion limitation36; (2) pulmonary hypertension and an enhanced propensity for pulmonary interstitial
Summary/Solutions
We have outlined the ways in which the compromised function of the intrathoracic and extrathoracic airways, gas exchange, and respiratory muscles present significant limitations to exercise performance in otherwise healthy endurance-trained individuals. Airway and gas-exchange limitations occur rarely, mostly at the extremes of exercise intensity and primarily in highly trained individuals of all ages and both sexes; women and the elderly are especially vulnerable. Exercise-induced diaphragm
Acknowledgment
This work was supported in part by the National Heart, Lung, and Blood Institute and the American Heart Association.
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