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Reduced Inspiratory Muscle Endurance Following Successful Weaning From Prolonged Mechanical Ventilation^*

^* From the Division of Physiotherapy (Drs. Chang and Hodges), The University of Queensland, St. Lucia, QLD, Australia; Departments of Intensive Care Medicine (Dr. Boots) and Women’s Thoracic Medicine (Mr. Brown), Royal Brisbane Hospital, Brisbane, QLD, Australia; and Cardiopulmonary Research Centre (Dr. Paratz), Alfred Hospital/La Trobe University, Melbourne, VIC, Australia.

Correspondence to: Angela T. Chang, PhD, BPhty (Hons), Division of Physiotherapy, The University of Queensland, St. Lucia, QLD 4072, Australia; e-mail: a.chang{at}shrs.uq.edu.au

Study objectives: Respiratory muscle weakness and decreased endurance have been demonstrated following mechanical ventilation. However, its relationship to the duration of mechanical ventilation is not known. The aim of this study was to assess respiratory muscle endurance and its relationship to the duration of mechanical ventilation.

Design: Prospective study.

Setting: Tertiary teaching hospital ICU.

Patients: Twenty subjects were recruited for the study who had received mechanical ventilation for 48 h and had been discharged from the ICU.

Measurements: FEV₁, FVC, and maximal inspiratory pressure (PImax) at functional residual capacity were recorded. The PImax attained following resisted inspiration at 30% of the initial PImax for 2 min was recorded, and the fatigue resistance index (FRI) [PImax final/PImax initial] was calculated. The duration of ICU length of stay (ICULOS), duration of mechanical ventilation (MVD), duration of weaning (WD), and Charlson comorbidities score (CCS) were also recorded. Relationships between fatigue and other parameters were analyzed using the Spearman correlations ().

Results: Subjects were admitted to the ICU for a mean duration of 7.7 days (SD, 3.7 days) and required mechanical ventilation for a mean duration of 4.6 days (SD, 2.5 days). The mean FRI was 0.88 (SD, 0.13), indicating a 12% fall in PImax, and was negatively correlated with MVD (r = –0.65; p = 0.007). No correlations were found between the FRI and FEV₁, FVC, ICULOS, WD, or CCS.

Conclusions: Patients who had received mechanical ventilation for > 48 h have reduced inspiratory muscle endurance that worsens with the duration of mechanical ventilation and is present following successful weaning. These data suggest that patients needing prolonged mechanical ventilation are at risk of respiratory muscle fatigue and may benefit from respiratory muscle training.

Key Words: fatigue • inspiratory muscle • mechanical ventilation