Diaphragmatic ultrasound measurement
All the ultrasonographic measurements were made by the same pediatric intensivist (who was specially trained) using a portable Esaote MyLab Omega® system equipped with a 4–15 Mhz linear transducer for diaphragmatic thickness (DT), diaphragmatic thickening fraction (DTF), and a 6–10 Mhz convex transducer for diaphragmatic excursion (DE), inspiratory slope (IS), and expiratory slope (ES). We evaluated right diaphragm since previous studies demonstrated that there was no difference in sonographic measurements between the two sides and right diaphragm was easier to assess19,20. All patients were supine with the head of the bed elevated 30. Measurements were obtained at the end of the two-hour ERT. First, as in previous studies21, the linear transducer in B-mode was placed between the anterior axillary and midaxillary lines, in the 9th and 10th intercostal spaces. The diaphragm is depicted as a three-layer structure in this image, with two parallel echoic lines (pleural and peritoneal lines) and a hypoechoic structure between them (the muscle itself). DT was measured three times, both at the end of inspiration (DTi) and expiration (DTe). The percent change in DT between end-expiration and end-inspiration (DTF) was calculated as (DTi-DTe/Dte) x 100 (Figure 1-A, B).
Second, using the right hepatic lobe as an acoustic window, the 2-5 Mhz transducer was placed immediately below the right costal margin along the mid-clavicular line and directed medially, cephalad, and dorsally. The posterior third of the diaphragmatic muscle was visualized using this technique. Then, M-mode is used to visualize the diaphragm’s motion during the respiratory cycle. The diaphragm moves caudally toward the probe during inspiration; it moves cranially away from the probe during expiration. Three respiratory cycles were recorded, and the average was calculated. DE is measured on the vertical axis of the trace from the baseline to the point of maximum height of inspiration22 (Figure 1-C, D). Three serial respiration cycles were recorded for each patient, and mean values for each diaphragmatic parameter was calculated. Intraobserver variability of ultrasonic measurements were less than 10%.