Appropriate body position and site for diaphragm ultrasound: Comparison with inspiratory mouth pressure

This study aimed to examine the appropriate body position and anatomical site for DUS through comparisons with maximal inspiratory pressure (MIP). Thirty healthy adults were included in the study. MIP was measured using a spirometer. DUS was performed in both the right and left intercostal spaces in both sitting and supine positions. The diaphragm thickness at functional residual volume (TDFRC) and total lung capacity (TDTLC) were measured, and the fractional change in diaphragm thickness (TF) was calculated under each condition. To assess interobserver reproducibility, DUS was performed by an experienced examiner and a novice examiner. Among the DUS parameters, the TDFRC measured at the right intercostal space in the sitting position showed the strongest correlation with MIP (r = 0.68, p < 0.001). The correlations of TDTLC with MIP tended to be inferior to those of TDFRC with MIP, and the TF parameters did not correlate significantly with MIP. DUS measurements in the supine position demonstrated greater reproducibility than those in the sitting position, and those at the right intercostal space tended to be better than those at the left intercostal space. When evaluating respiratory muscle function during inspiration using DUS, TDFRC at the right zone of the apposition in the sitting position was found to be appropriate. DUS measurements at the right zone of apposition in the supine position were considered adequate for clinical use because the influence of examiner experience was small.