Impact of critical illness on expiratory and inspiratory muscle mass and function: a prospectively, observational study

Abstract

Introduction: Numerous studies have investigated the effect of critical illness on inspiratory muscle. Expiratory muscles, however, are neglected despite their importance in breathing and airway clearance. Objectives: To evaluate thickness changes of expiratory muscles and diaphragm in invasively ventilated ICU patients and determine their impact on the clinical outcome. Methods: Patients receiving invasive mechanical ventilation ≤ 48 hours at inclusion were eligible. Ultrasound of the lateral abdominal wall muscles and diaphragm was performed daily. An increase or decrease of muscle thickness was defined as >10% change on the last day of the first week versus initial thickness. Results: 77 patients (55 male, 62 ± 14 years) were analyzed. The thickness of the lateral abdominal wall muscles decreased in 31% of patients and increased in 17% of patients. The thickness of diaphragm muscle decreased in 21% of patients. No correlation was found between changes in thickness of the diaphragm and lateral abdominal muscles. The average thickening fraction of the transversus abdominis muscle was 17% (16.7 ± 14.9) during the partially supported modes. A dose-dependent correlation was found between diaphragm inspiratory effort and expiratory muscle activity. Low driving pressure or high tidal volume increased the contractile activity of expiratory muscles. Expiratory muscle atrophy increased the duration of mechanical ventilation, length of ICU stay, and rate of weaning failure. Conclusions: Expiratory muscle atrophy developed in a third of ventilated ICU patients. No correlation with diaphragm atrophy was found, suggesting a different pathophysiology.