Diaphragm and peripheral muscle thickness on ultrasound: Intra-rater reliability and variability of a methodology using non-standard recumbent positions

Background and objective: Reliable measurement of diaphragm and peripheral muscle thickness, using diagnostic ultrasound, has only been validated in the erect posture. However, in many clinical populations, including critically ill patients, the erect posture presents logistic difficulties. This study aimed to validate ultrasound measurement of diaphragm and peripheral muscle thickness in the recumbent position. Methods: An observational methodology of repeated but blind ultrasound and anthropometric measurements was applied, to assess inta-rater reliability. Thirteen healthy volunteers (aged 20-73 years) participated. A pneumotachograph was used to target lung volume, as diaphragm thickness was measured from ultrasound at end-expiration, and both 25% and 50% of inspiratory capacity, while semi-recumbent. The thicknesses of the mid-upper arm, mid-forearm and mid-thigh musculature were also measured bilaterally while supine. Results: Diaphragm thickness could be reliably measured at end-expiration (intra-class correlation coefficient (ICC) = 0.990, 95% confidence interval: 0.918-0.998), 25% of inspiratory capacity (ICC = 0.959 (0.870-0.988)) and 50% of inspiratory capacity (ICC = 0.994 (0.980-0.998)). Peripheral muscle thickness measurements were also reliable (ICC = 0.998-1.0). Supine anthropometric measurements of limb segment lengths and girths were highly reproducible. Conclusions: This ultrasound technique has good reliability in recumbent positions, making it useful for application to clinical populations when the erect posture is not practical. Recumbent postures may affect the reliability of diaphragm and peripheral muscle thickness measured with ultrasound. While usually performed erect, patients may not tolerate this posture. We demonstrated that recumbent positions can be used to reliably measure diaphragm and peripheral muscle thickness, and that imaging sites can be accurately located supine.