Tests Of Respiratory Muscle Fatigue Breathing Pattern Tidal Volume and Breathing Frequency

Rationale. Rapid shallow breathing, characterized by high breathing frequency and low tidal volume, commonly develops in progressive respiratory failure or in unsuccessful attempts to wean from mechanical ventilation. These conditions are associated with an increased ventilatory load and/or a reduced respiratory muscle capacity and may therefore potentially lead to respiratory muscle fatigue (see Prediction of Weaning in Section 10 of this Statement).

Methodology and equipment. Breathing frequency can be easily counted at the bedside and should be included in standard monitoring. Tidal volume measurements of intubated patients are commonly accomplished with the flow sensors built into modern ventilatory equipment and can be displayed on a breath-by-breath basis by these machines. Volume measurements can also be made with Wright respirometers and other spirometric devices via a mouthpiece in nonintubated patients, albeit mouthpiece placement can artifactually alter tidal volumes and respiratory patterns. To avoid such artifacts, it is possible to noninvasively monitor tidal volume by respiratory inductance plethysmography. Use of this and similar methods is described in Devices Used to Monitor Breathing: Pneumograph, Magnetometer, and Respiratory inductive Pleth-ysmograph in Section 6 of this Statement.

Advantages. Monitoring changes in breathing frequency and tidal volume is simple and noninvasive.

Disadvantages. The relationship between fatigue and breathing pattern is complex. Moreover, rapid shallow breathing is most likely a reflex response to an increase in the respiratory workload (24) and not the consequence of respiratory muscle fatigue per se (25). Thus, although rapid shallow breathing may accompany respiratory muscle fatigue (2), it cannot be considered a specific marker of fatigue.

Applications. Monitoring breathing frequency and tidal volume represents a part of the routine respiratory surveillance of patients, but these parameters should not be used as specific indicators of the development of respiratory muscle fatigue (see Breathing Pattern in Section 10 of this Statement).

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