Efficiency of the normal human diaphragm with hyperinflation

Abstract

We evaluated an index of diaphragm efficiency (Effdi), diaphragm power output (Ẇdi) relative to electrical activation, in five healthy adults during tidal breathing at usual end-expiratory lung volume (EELV) and diaphragm length ( Ldi ee) and at shorter Ldi eeduring hyperinflation with expiratory positive airway pressure (EPAP). Measurements were repeated with an inspiratory threshold (7.5 cmH2O) plus resistive (6.5 cmH2O·l−1·s) load. Ẇdi was the product of mean inspiratory transdiaphragmatic pressure (ΔPdimean), diaphragm volume displacement measured fluoroscopically, and 1/inspiratory duration (Ti−1). Diaphragm activation, measured with esophageal electrodes, was quantified by computing root-mean-square values (RMSdi). With EPAP, 1) EELV increased [mean r2= 0.91 (SD 0.01)]; 2) in four subjects, Ldi eedecreased [mean r2= 0.85 (SD 0.07)] and mean Effdidecreased 34% per 10% decrease in Ldi ee( P < 0.001); and 3) in one subject, gastric pressure at EELV increased two- to threefold, Ldi eewas unchanged or increased, and Effdiincreased at two of four levels of EPAP ( P ≤ 0.006, ANOVA). Inspiratory loading increased Ẇdi ( P = 0.003) and RMSdi( P = 0.004) with no change in Effdi( P = 0.63) or its relationship with Ldi ee. Effdiwas more accurate in defining changes in Ldi ee[(true positives + true negatives)/total = 0.78 (SD 0.13)] than ΔPdimean·RMSdi−1, RMSdi, or ΔPdimean·Ti (all <0.7, P ≤ 0.05, without load). Thus Effdiwas principally a function of Ldi eeindependent of inspiratory loading, behavior consistent with muscle force-length-velocity properties. We conclude that Effdi, measured during tidal breathing and in the absence of expiratory muscle activity at EELV, is a valid and accurate measure of diaphragm contractile function.