Detection of respiratory muscle fatigue

Diaphragmatic fatigue can be detected in various ways (DMrey!I..and,...Aybi§I..1988). Assessment of fatigue for the diaphragm is more difficult than for other skeletal muscles because the tension generated cannot be measured directly; only the mechanical transformation of that tension into pressure can be measured. The transdiaphragmatic pressure (Pdi), defined as the difference between the gastric pressure and the esophageal pressure, closely reflects the tension produced by the contracting muscle. The clinical manifestations of inspiratory muscle fatigue, which are a paradoxical inward abdominal motion during inspiration (abdominal paradox) and an alteration between abdominal and rib cage breathing (respiratory alternans), have been described by Cohen ef a/ (1982).

These investigators studied 12 patients intubated and ventilated for acute respiratory failure who exhibited difficulties during discontinuation of artificial ventilation. Ventilatory support was discontinued and the investigators searched for clinical signs of respiratory muscle fatigue that could be correlated with electromyographic signs of fatigue. Electromyographic evidence of inspiratory muscle fatigue was described in six patients with the following sequence of events leading to respiratory acidemia. Electromyographic fatigue followed or was accompanied by an increased respiratory rate, which was, in turn, followed by abnormalities of thoracoabdominal mechanics (respiratory alternans, abdominal paradox). These clinical manifestations were followed by an increase in arterial carbon dioxide levels ( PaCO2). It was concluded that these abnormal respiratory movements may be a reliable index of inspiratory muscle fatigue. Apart from diaphragmatic fatigue, abdominal paradox has been described only in diaphragmatic paralysis. Tobin eL&L (1987) recently studied normal subjects breathing against severe resistive loads and concluded that rib cage-abdominal paradox is due predominantly to an increase in respiratory load rather than to respiratory muscle fatigue, because the paradox was observed in this study for a level of exercise that can be sustained indefinitely.

Diaphragmatic function can he assessed by recording Pdi and analyzing two indices: peak Pdi and the rate of relaxation of Pdi. Impairment in diaphragmatic contractility reduces peak Pdi during maximum static inspiratory effort (Pdi(max)) or during maximum sniff (Pdi(sniff)). However, the values of peak Pdi depend not only on diaphragmatic contractility, but also on the subject's ability and motivation to co-operate. Furthermore, comparison of peak Pdi obtained at two different times is possible only if the abdominal compliance and muscle length and geometry (lung volumes) remain unchanged.

Changes in diaphragmatic contractility are also accompanied by changes in muscle relaxation time (fatigued muscles relax more slowly than fresh muscles). This change in diaphragmatic relaxation time, which appears before the decrease in muscular tension, can be quantified by measuring the maximum Pdi relaxation rate or time constant (T) of the monoexponential phase of relaxation. The latter index is interesting because it is not influenced by the amplitude of the peak Pdi, but only by lung volume (T decreases when lung volume increases). When T exceeds 75 ms, diaphragmatic dysfunction may be suspected.

Changes in the frequency content of the diaphragmatic electromyogram during spontaneous breathing also provide information concerning diaphragmatic fatigue. Diaphragmatic muscle fatigue is associated with electromyographic redistribution from high to low frequencies. The technique is to ascertain the ratio of power in a high-frequency band to that in a low-frequency band (high-to-low ratio) of the diaphragmatic electromyogram. The high-to-low ratio of the diaphragm decreases with fatigue with a time course and extent that closely matches the prolongation of relaxation. As T becomes longer, modification of the high-to-low ratio precedes fatigue. It is then possible to detect mechanical impairment of the diaphragm before the muscle fails as a pressure generator. The other ways of assessing diaphragmatic function, i.e. the force-frequency curve and bilateral phrenic stimulation with single pulses, are difficult to use routinely in the intensive care unit ( AubieL§i,Mi 1981).

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