Functional assessment of the diaphragm by speckle tracking ultrasound during inspiratory loading

Background: Assessment of diaphragmatic effort is challenging especially in critically ill patients in the phase of weaning. Fractional thickening during inspiration assessed by ultrasound has been used to estimate diaphragm effort. It is unknown whether more sophisticated ultrasound techniques such as speckle tracking are superior in the quantification of inspiratory effort. This study evaluates the validity of speckle tracking ultrasound to quantify diaphragm contractility. Methods: Thirteen healthy volunteers underwent a randomized stepwise threshold loading protocol of 0 to 50% of the maximal inspiratory pressure. Electric activity of the diaphragm and transdiaphragmatic pressures were recorded. Speckle tracking ultrasound was used to assess strain and strain rate as measures of diaphragm tissue deformation and deformation velocity, respectively. Fractional thickening was assessed by measurement of diaphragm thickness at end-inspiration and end-expiration. Results: Strain and strain rate increased with progressive loading of the diaphragm. Both strain and strain rate were highly correlated to transdiaphragmatic pressure (strain R²=0.72; strain rate R²=0.80) and diaphragm electric activity (strain R²=0.60; strain rate R²=0.66). No correlation between fractional thickening and strain and strain rate was found. Conclusion: Speckle tracking ultrasound is superior to conventional ultrasound techniques to estimate diaphragm contractility under inspiratory threshold loading.

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