Sternomastoid Muscle Function and Fatigue in Breathless Patients with Severe Respiratory Disease

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Abstract
In patients with severe respiratory disease, the work of breathing is increased and the respiratory muscles, particularly those of inspiration, may become fatigued. Hitherto, there has been little information on the on the incidence of respiratory muscle fatigue in acutely breastless patients.We studied 34 patients with severe respiratory disease on admission to hospital when they were most breathless, and then, if possible, 7 to 14 days later after recovery for evidence of sternomastoid muscle fatigue or increased fatigability. Frequency force curves, numerically expressed as the 20:50 ratio, were carried out in all patients on admission. Three of the 34 patients had evidence of low frequency fatigue (i.e., > 15% reduction in 20:50 ratio) in the sternomastoid muscle on admission when first studied (mean .+-. SEM 20:50 ratio, 56.3 .+-. 1.2%; n = 3). The mean 20:50 ratio in the remaining 31 patients on admission was 75.7 .+-. 1.6% (n = 31) compared with 77.8 .+-. 1.4% (n = 25) when symptomatically better (p < 0.05). The mean 20:50 ratio on admission was also significantly lower than the mean 20:50 ratio in a group of age- and sex-matched normal control subjects (i.e., 78.5 .+-. 1.4%; n = 25; p < 0.05). Twenty-five patients were studied completely both on admission and recovery, including a fatigability test that involved the performance of 50 fatiguing head lifts with measurements of the 20:50 ratio 10 and 60 min later. Sternomastoid muscle fatigability was significantly increased on admission when the patients were most breathless, compared with recovery when they were less breathless (p < 0.001 at both 10 and 60 min). These results support the concept that respiratory muscle fatigue and increased fatigability may develop in breathless patients with severe respiratory disease, and that this may have important consequences in the development of ventilatory failure.