The Importance of Defining Respiratory Muscle Fatigue

I t is now more than 12 years since it was suggested that inspiratory muscle fatigue might be a cause of hypercapnic respiratory failure (1). The fact that we do not yet know for sure that this is so reflects a very serious lack of knowledge because skeletal muscle fatigue is eminently treatable. This issue of the REVIEW contains the summary of a National Heart, Lung, and Blood Institute (NHLBI) workshop on respiratory muscle fatigue (2). It is timely for a number of reasons. Perhaps the most important achievement of this workshop was a consensus on the definition of fatigue, which is a condition in which there is loss in the capacity for developing force and/or velocity of a muscle in response to a load and that is reversible by rest. This is a substantial departure from previous definitions, which usually went something like this: a condition in which a muscle is unable to continue to develop or maintain a predetermined force or work output. One of the purposes of this editorial is to explore the implications of this change in definition. Definitions are important. A great drawback in research on asthma is our inability to define what we are talking about. Thus, investigators in one laboratory may be studying a condition that they think is the same as a condition being investigated in another laboratory, whereas in reality the two may be quite different. An agreed upon definition of skeletal muscle fatigue allows investigators to be certain that they are all talking, studying, and experimenting with the same condition. It may seem presumptuous that a group of chest physicians and respiratory physiologists should attempt to define skeletal muscle fatigue, which some might Gustifiably) consider to be outside their area of expertise. However, two of the workshop participants (Drs. Ritchie and Faulkner) are skeletal muscle physiologists and it was they who pointed out the deficiencies of existing definitions and provided the framework in which the new

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definition was achieved. Here are some of the implications of this definition: (1) Fatigue may be present at a time considerably preceding the point in time at which a muscle is unable to continue to perform a particular task (task failure). In applying this concept to the inspiratory muscles, one concludes that they may be fatigued before there is hypercapnia due to failure of alveolar ventilation. Those of us who have used the power spectral shift ofthe diaphragmatic electromyogram to detect fatigue (a method not enthusiastically endorsed by the workshop) have generally stated that the shift, which occurs early when a muscle is performing a fatiguing task, predicts and precedes the development of fatigue. With the new definition it is possible that the power spectral shift tracks the development of fatigue temporally with considerable accuracy. (2) Precise criteria can now be established to determine if fatigue is present or not and to distinguish it from weakness (also defined by consensus as a condition in which the capacity for a rested muscle to generate force is decreased). To the extent that the new definition of fatigue is accepted, its detection implies the demonstration of a loss in the capacity of a muscle to develop force or velocity and that is recoverable by rest. This is useful if one wishes to establish clinically applicable diagnostic criteria because a therapeutic trial of respiratory muscle rest can be used as a diagnostic test. If rest improves the capacity of the inspiratory muscles to develop force or velocity (keeping other variables such as fiber length constant), it is reasonable to infer that before rest there was a loss in the capacity to develop force or velocity. (3) The establishment of precise diagnostic criteria should finally allow us to determine if respiratory muscle fatigue actually exists clinically, and, if so, how important it is, its prevalence, the risk factors for its development, and how to treat it in the most cost-effective way. (4) Determination of prevalence risk

factors and cost-effectiveness of treatment will assist health care authorities and granting agencies in establishing policies and priorities. All of this has been made possible by the new definition. It was not previously possible because detection of task failure (hypercapnia) is not specific. There are presumably several causes of this condition. The detection of the failure of the respiratory muscles to continue to develop the pressure swings or work output to overcome the mechanical load to breathing was not useful clinically because it required observing the patient in the transition period when pressure swings or work output diminished. This is not often possible because this phase is frequently over by the time the patient presents him or herself to a physician. It is evident that the new definition calls for much research. Important research priorities are given in the workshop summary. Throughout the history of science, there has traditionally been competition between the holistic and the reductionist approaches. In the present day with scientific emphasis on molecular mechanisms, cell biology, and subcellular organelles, reductionism is in the ascendency. However, the reductionistic approach is unlikely to answer the important question posed by the workshop. The pendulum continues to swing. PETER

T.

~ACKLEM

Meakins-Christie Laboratories McGill University Clinic Royal Victoria Hospital Montreal Chest Hospital Montrea/, Quebec Canada References 1. Roussos C, Macklem PT. Diaphragmatic fatigue in man. J Appl Physio) 1977; 43:189-97. 2. NHLBI Workshop. Respiratory muscle fatigue: report of the respiratory muscle fatigue workshop group. Am Rev Respir Dis 1990; 142:474-480.

AM REV RESPIR DIS 1990; 142:274

The importance of defining respiratory muscle fatigue.

The Importance of Defining Respiratory Muscle Fatigue I t is now more than 12 years since it was suggested that inspiratory muscle fatigue might be a...
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