Original ContributionDiaphragmatic Ultrasound Correlates with Inspiratory Muscle Strength and Pulmonary Function in Healthy Subjects
Introduction
Diaphragm ultrasound (DUS) is a simple, reproducible method for evaluating diaphragmatic mobility and thickness (Boon et al, 2013, Boussuges et al, 2009, Cohn et al, 1997, Kantarci et al, 2004, Testa et al, 2011, Ueki et al, 1995). DUS has been increasingly used to evaluate diaphragmatic function in many clinical situations because its advantages (absence of radiation, portability, real-time imaging and non-invasiveness) eliminate many of the limitations of previous standard imaging techniques (Matamis et al, 2013, Sarwal et al, 2013).
Despite these advantages, the correlation between DUS variables and inspiratory muscle strength is controversial in healthy patients (Brown et al, 2013, McCool et al, 1997, Summerhill et al, 2007, Ueki et al, 1995). Some authors (McCool et al, 1997, Summerhill et al, 2007) found a correlation between maximal inspiratory pressure (MIP) and diaphragm thickness at functional residual capacity (FRC) (ThFRC). However, Ueki et al. (1995) conversely found a correlation between MIP and diaphragm thickness at total lung capacity (TLC) (ThTLC) and between MIP and thickening ratio, but not between MIP and ThFRC. Methodologic issues such as a small number of patients, different ages and non-uniform distribution of sexes may explain the controversial findings among these studies. Considering that the strength of a muscle depends in part on its effective cross-sectional area (Freilich et al, 1995, Verdijk et al, 2010) and that muscle strength is greater in males than in females (Chen, Kuo, 1989, Wilson et al, 1984), sex in particular may represent a key factor in examining the relationship between diaphragm thickness and inspiratory strength.
Describing the characteristics of DUS in healthy individuals is clinically relevant as a reference value so that diaphragm dysfunction can be diagnosed using a non-invasive method. Furthermore, whether or not the mobility and thickness of the diaphragm have a correlation with inspiratory strength, DUS might represent a tool for the evaluation of patients unable to perform the manoeuvres of maximal ventilatory strength and even lung function tests.
This study aimed (i) to investigate the influence of sex on the mobility and thickness of the diaphragm; (ii) to characterize the relationships between mobility and thickness of the diaphragm evaluated by ultrasound and inspiratory muscle strength in healthy males and females; (iii) to obtain the mean values and the lower limit of normality (LLN) in both sexes.
We hypothesized that inspiratory muscle strength may be related to diaphragm mobility and thickness displayed on ultrasound, mainly during deep breathing (DB) and at TLC, respectively, with possible differences between sexes.
Section snippets
Methods
We performed a cross-sectional study that consecutively evaluated 64 healthy volunteers. The study was approved by the Research Ethics Committee of the University of São Paulo School of Medicine Hospital das Clínicas (protocol number 0835/11).
Results
Healthy male and female volunteers were well matched for age, body mass index and smoking status. As expected, lung volumes were lower in females than in males in absolute values but not when corrected to the predicted value (Table 1). Age range in females and males was also matched (20–80 y and 20–82 y, respectively).
Regarding non-invasive inspiratory muscle strength, females had lower absolute values than males, but not in percent of predicted value. For invasive ventilatory strength, during
Discussion
This is the first study involving a large sample of healthy individuals with all measurements aimed at investigating the relationship of several ventilatory muscle-strength measurements (invasive and conventional) and the influence of sex mainly on mobility and thickness of the diaphragm. We observed that females had significantly lower DB mobility, ThTLC and TF than males. In addition, we found significant correlations among DB mobility, ThTLC and TF of the diaphragm with inspiratory muscle
Conclusions and Clinical Implications
Ultrasound is feasible for evaluating the diaphragm in healthy patients, with a good correlation between lung function and inspiratory strength. Sex has an influence, with females having lower DB diaphragmatic mobility, TLC thickness and TF than males.
The main clinical implications are (i) DUS evaluations during DB and at TLC are more relevant as indexes of diaphragm weakness; (ii) there are lower limits not only of diaphragm mobility but also thickness in both sexes; (iii) particularly
Acknowledgments
The authors thank Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP). The authors have no conflicts of interest to disclose.
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