Chest
Volume 134, Issue 3, September 2008, Pages 613-622
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Recent Advances in Chest Medicine
Update in the Understanding of Respiratory Limitations to Exercise Performance in Fit, Active Adults

https://doi.org/10.1378/chest.07-2730Get rights and content

This review addresses three types of causes of respiratory system limitations to O2 transport and exercise performance that are experienced by significant numbers of active, highly fit younger and older adults. First, flow limitation in intrathoracic airways may occur during exercise because of narrowed, hyperactive airways or secondary to excessive ventilatory demands superimposed on a normal maximum flow-volume envelope. Narrowing of the extrathoracic, upper airway also occurs in some athletes at very high flow rates during heavy exercise. Examination of the breath-by-breath tidal flow-volume loop during exercise is key to a noninvasive diagnosis of flow limitation and to differentiation between intrathoracic and extrathoracic airway narrowing. Second exercise-induced arterial hypoxemia occurs secondary to an excessively widened alveolar-arterial oxygen pressure difference. This inefficient gas exchange may be attributable in part to small intracardiac or intrapulmonary shunts of deoxygenated mixed venous blood during exercise. The existence of these shunts at rest and during exercise may be determined by using saline solution contrast echocardiography. Finally, fatigue of the respiratory muscles resulting from sustained, high-intensity exercise and the resultant vasoconstrictor effects on limb muscle vasculature will also compromise O2 transport and performance. Exercise in the hypoxic environments of even moderately high alitudes will greatly exacerbate the negative influences of these respiratory system limitations to exercise performance, especially in highly fit individuals.

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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    The authors have reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

    Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestjournal.org/misc/reprints.shtml).

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