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 (Tdi) and related measures such as thickening fraction (TF) have been verified for use in the evaluation of diaphragm function. In healthy individuals, Tdi 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.
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. IC was determined via incentive spirometry. TF was determined by bedside US measurements of minimum and maximum diaphragm thickness (Tdi) during tidal breathing (TFtidal ) or deep breathing (TFDB) was calculated (TF= [Tdimax-Tdimin]/Tdimin). TFDB values were also compared to previously reported mean values of 2.04±0.62 in healthy males and 1.70±0.89 in females. Univariate and multivariate analyses were performed.
A total of 41 subjects (58.5% male) with a median age of 64 [IQR 53-77] years were enrolled. Diaphragm US demonstrated a median TFtidal of 0.30 [IQR 0.24-0.46]. Median IC was 1750 [IQR 1250-2000] mL. As compared to previously reported controls, our mean TFDB 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 TFtidal and IC, and no relationship between TFDB and IC.
To our knowledge, this is the first report of TF in rib fracture patients. The significant inverse association between TFtidal and IC, along with lower than normal TFDB ranges suggest 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: III Diagnostic tests or criteria.

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