Respiratory Muscle Function in the Obese

Abstract

Obese patients perform poorly at task requiring increased ventilatory levels such as during maximum voluntary ventilation (MVV) manoeuvres and exercise.¹,² Their increased chest wall load with its attendant reduction in chest wall compliance causes increased work of breathing at all levels of ventilation. In addition, it has been reported that obese patients have reduced inspiratory muscle strength as assessed by peak inspiratory mouth pressures and transdiaphragmatic pressures.³,⁴ Normal subjects have been shown⁵,⁶ capable of sustaining minute ventilation levels ranging from 55 to 80% of their predicted maximum voluntary ventilation (MVV). More recent data⁷ suggest that normal subjects will demonstrate electromyographic and mechanical diaphragmatic fatigue at ventilatory levels above 76% MVV with diaphragmatic tension-time indices (TtDi) greater than 0.15. Given the reduced chest wall compliance and inspiratory muscle strength typical of the obese, we hypothesized that their poor mechanical ventilatory coupling should cause fatigue of their diaphragms at levels of ventilation well below that reported in normals, i.e., at levels of less than 76% of MVV.