Ultrasound Evaluation of Diaphragm Force Reserve in Patients with Chronic Obstructive Pulmonary Disease.

RATIONALE Diaphragm function is a key determinant of dyspnea in COPD; however, it is rarely assessed in clinical practice. Lung hyperinflation can also impair diaphragm function. Ultrasound can assess the activity, function and the force reserve of the diaphragm. OBJECTIVES To compare diaphragm activity, function, and force reserve among patients with COPD and in healthy controls. METHODS 80 patients with stable COPD and 20 healthy controls were enrolled (97% of them were male). Ultrasound was used to measure the thickening fraction of the diaphragm during tidal breathing and maximum volitional effort. Outcome measures were: 1) the difference in diaphragm force reserve, activity, and function between COPD patients and controls, 2) the correlation between lung volumes and diaphragm force reserve, activity, and function, and 3) the relationship between diaphragm force reserve and the rate of moderate to severe exacerbation of COPD. RESULTS The tidal thickening fraction of the diaphragm during resting breathing (TFdi-tidal) was higher in COPD patients than in controls (P = 0.002); it was approximately twice as high in patients with severe COPD than in controls. Patients with COPD had poorer diaphragm function than controls as assessed by the maximal thickening fraction of the diaphragm (TFdi-max) during Muller maneuver (P < 0.01). Diaphragm force reserve ratio assessed by 1 - (TFdi-tidal/TFdi-max) was lower in COPD patients than in controls and it fell with increasing GOLD stages (P < 0.001); it correlated with inspiratory capacity (r = 0.46) and the BODE index, a multidimensional scoring system (r = -0.49). Patients who developed exacerbation during the following two years had less force reserve than patients without exacerbation (P = 0.024). CONCLUSIONS Male patients with COPD have increased diaphragm workload, impaired diaphragm function, and reduced force reserve compared to healthy subjects. Ultrasound assessment of the diaphragm in COPD provides important functional information. Clinical trial registered with Thai Clinical Trials Registry (#TCTR20160411001).