Implications of Different Types of Fatigue for Diagnosis

Although it is convenient to discuss the characteristics of central, peripheral high-frequency, and peripheral low-frequency fatigue separately, it is likely that these various phenomena do not occur in isolation during muscle activation. All three processes may be operating simultaneously when the respiratory muscles confront an excessive workload, with the relative importance of each depending on the duration of respiratory loading and other physiological variables (i.e., arterial pressure, arterial blood gas concentrations, nutritional state). Whereas all three processes may participate in the acute response to loading, both central and high-frequency fatigue resolve rapidly once fatiguing levels of muscle contraction cease, and only low-frequency fatigue is likely to persist over minutes to hours.

Because muscle fatigue is a complex phenomenon, a test that is well suited to detect one form of fatigue may be incapa ble of detecting another. Moreover, the necessity to make serial measurements of an index of muscle force generation over time to detect fatigue is a particularly difficult endeavor for the respiratory system, because a large number of variables (i.e., lung volume, thoracoabdominal configuration, muscle interaction) can vary over time. All these factors can influence the relationship between muscle force and pressure generation.

As an example, consider the utility of measuring maximum inspiratory pressure (Pi,max) serially to detect respiratory muscle fatigue. This parameter is highly effort dependent, and time-dependent reductions could represent lack of motivation, central fatigue, peripheral high-frequency fatigue, or simply an alteration in lung volume and a resultant mechanical change in the transduction of muscle force into pressure. In addition, failure of the PI,max to change does not exclude the development of fatigue, because this test would not be suitable to detect low-frequency fatigue. As a result, one must keep in mind the potential limitations of a given test for the detection of muscle fatigue. Most tests are suitable for detecting the presence of only one component of muscle fatigue, and complete characterization of fatigue requires a complex series of assessments.

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