Diaphragm ultrasound: A novel approach to assessing pulmonary function in patients with traumatic rib fractures

BACKGROUND Rib fractures following blunt trauma are a major cause of morbidity. Various factors have been used for risk stratification for complications. Ultrasound (US) measurements of diaphragm thickness (T di) and related measures such as thickening fraction (TF) have been verified for use in the evaluation of diaphragm function. In healthy individuals, T di by US is known to have a positive and direct relationship with lung volumes including inspiratory capacity (IC). However, TF has not been previously been described in, or used to assess, pulmonary function in rib fracture patients. We examined TF and IC to elucidate the association between acute rib fractures and respiratory function. We hypothesized that TF and IC were related. Secondarily, we examined the relationship of TF in rib fractures patients, in the context of values reported for healthy controls in the literature. METHODS We prospectively enrolled adults with acute blunt traumatic rib fractures within 48 hours of admission to a level 1 trauma center. Patients requiring a chest tube or mechanical ventilation at time of consent were excluded. Inspiratory capacity was determined via incentive spirometry. Thickening fraction was determined by bedside US measurements of minimum and maximum T di during tidal breathing (TF tidal) or deep breathing (TF DB) was calculated (TF = [T di max Tdi - T di min Tdi]/T di min Tdi). TF DB values were also compared with previously reported mean ± SD values of 2.04 ± 0.62 in healthy males and 1.70 ± 0.89 in females. Univariate and multivariate analyses were performed. RESULTS A total of 41 subjects (58.5% male) with a median age of 64 years (interquartile range [IQR], 53-77 years) were enrolled. Diaphragm US demonstrated a median TF tidal of 0.30 (IQR, 0.24-0.46). Median IC was 1,750 mL (IQR, 1,250-2,000 mL). As compared with previously reported controls, our mean ± SD TF DB in males 0.90 ± 0.51 and 0.88 ± 0.89 in females were significantly lower. Multivariate analysis revealed a significant inverse correlation (-0.439, p = 0.004) between TF tidal and IC, and no relationship between TF DB and IC. CONCLUSION To our knowledge, this is the first report of TF in rib fracture patients. The significant inverse association between TF tidal and IC, along with lower than normal TF DB ranges, suggests that, in the setting of rib fractures, there are alterations in the diaphragm-chest cage mechanics, whereby other muscles may play more prominent roles. LEVEL OF EVIDENCE Diagnostic tests or criteria, Level III.