The gold standard in evaluating lung mechanics is the esophageal pressure. And although diaphragm ultrasound has been reported to be feasible in assessing diaphragm activity in sedated patients, no study has thus far looked at the relationship between the ultrasound parameters (i.e. thickening fraction) and the esophageal pressure. This is a single-centre physiological cohort study to evaluate the feasibility of using the thickening fraction as a parameter for diaphragm activity during spontaneous breathing in sedated children. Data will be collected during an elective surgical procedure with a patient under general anesthesia. If the patient's legal representative provides written informed consent, the patient is enrolled in the study before start of the anesthetic procedure and the trial ends on the moment of removal of the laryngeal mask.The registration of the physiological data will not interfere with the surgery.The thickening fraction (using ultrasound) and pressure measurements will be performed simultaneously and repetitive during different loads on the respiratory system.Esophageal pressures (Pes) will be measured with a single air-filled balloon catheter(AVEA™ smarthcath pediatric 7 Fr, Vyaire Medical Inc., IL, USA). The balloon catheter will be inserted nasally after the induction of anesthesia. The catheter will then be connected to a pressure transducer (FluxMed, MBMED, Argentina). The appropriate placement will be checked by commonly used methods. Firstly, the appropriate depth of insertion will be estimated by measuring the nose-ear-xyphoid distance, which is the distance from the nose to the diaphragm. Secondly, the balloon catheter will be inserted more deeply than this predicted distance, placing it in the stomach, and a positive pressure swing when mild abdominal pressure is exerted on a slightly inflated balloon will be noted. Thirdly, when retracted into the esophagus, cardiac reflections on the pressure monitor will appear. Fourth, when applying positive pressure ventilation, we will look for a positive deflection on the pressure curve. The output from the pressure transducer will be saved from the Fluxmed respiratory mechanics monitor. Pes will be measured over at least five consecutive breaths selected at the end of the pressure recordings. Ultrasound recordings will be done with the BK 3500 ultrasound machine, using a linear high-resolution probe. The parameter measured is the distance between the pleura and peritoneum as seen on a 2D image. The thickness using B-mode ultrasound imaging at end expiration on 10cm H2O of positive end-expiratory pressure (PEEP) and on zero PEEP (ZEEP) will be assessed. Furthermore, the thickness with Mmode imaging, both at end-inspiration (TEI) and end-expiration (TEE) will be measured. A transducer will be placed on the right side of the body in the midaxillary line at the level of the 8th and 9th intercostal space, the so-called zone of apposition. The probe will be positioned perpendicular to the skin, and temporary markings will be applied to the skin. Subjects are positioned in a supine position. TEE will be measured just before the thickening start and TEI measured at maximal thickening. Measurements will be averaged out of three or more consecutive breaths. The thickening fraction (TF) is calculated as (TEI - TEE)/TEE and expressed as a percentage. Digital images will be stored and re-interpreted by a separate, blinded interpreter.The thickening fraction recordings will be repeated during different loads on the respiratory system, with varying the depth of anesthesia (mean alveolar concentration of anesthetic vapor - MAC) and different added respiratory loads with a threshold valve and different levels of PEEP. An anesthesia circle circuit with standard pediatric tubing will consistently be used. The lowest dose of opioids will be used to the extent clinically possible.

Condition #1: during positive pressure ventilation, no spontaneous breaths Condition #2: spontaneous, MAC 1.5, PEEP, no added respiratory load Condition #3: spontaneous, MAC 1.5, PEEP 10, no added respiratory load (only Tdi, no TF) Condition #4: spontaneous, MAC 1.5, ZEEP, no extra added respiratory load Condition #5: spontaneous, MAC 1, ZEEP, no added respiratory load Condition #6: spontaneous, MAC 1, ZEEP, 7 cm H2O added respiratory load Condition #7: spontaneous, MAC 1, ZEEP, 15 cm H2O added respiratory load

These different conditions will alter the work of breathing the diaphragm has to perform and will allow us to better correlate thickening fraction with esophageal pressure.Preceding this study, a reproducibility study will assess intra-observer reproducibility for the researcher doing the ultrasound measurements.

A power analysis is not possible, as this is the first study comparing these parameters in children. Data will be analyzed using the SPSS statistical software package (SPSS Inc., IL, USA). Continuous variables will be reported as median (interquartile range). Friedman test and Wilcoxon paired tests (with post hoc Bonferonni correction) will be used to assess differences between related variables. Correlations between TF and Pes will be calculated using the Spearman method and performed for the entire data set (including all four respiratory conditions of each patient) because of the limited sample size. Two-tailed p values less than 0.05 will be considered significant.

All precautions will be taken to prevent complications due to placement of the balloon catheter but nevertheless all adverse events will be assessed.