This study was conducted in an academic, 57-bed PICU of First Hospital of Jilin University in China. Study subjects including 70 consecutive patients younger than 18 years old who required invasive MV for more than 24 hours were enrolled between January 2019 and January 2020. The institutional ethics committee of the hospital approved the study protocol (ChiCTR1800020196). The parents or guardians of the eligible children provided written informed consent. An information sheet was provided for the parents or guardians of the participants.
All children met the standard criteria for weaning readiness(improve-ment in the cause of primary disease, PaO2/FiO2 > 200, positive end-expiratory pressure (PEEP) ≤ 5-10cm H2O, FiO2 ≤ 50%, and hemodynamically stable in the absence of vasopressors) were included in the study. Exclusion criteria include known neuromuscular disease (such as amyotrophic lateral sclerosis, Guillain-Barre, or myasthenia gravis), cervical spinal cord injury, pneumothorax, unwillingness of the parents or guardians to participate in the study.
Enrolled subjects underwent a diaphragm assessment by using ultrasonography during the spontaneous breathing trial (SBT), which was performed using pressure support trials with a pressure support (8 cm H2O) and 5 cm H2O PEEP using a Drager Evita 4 ventilator for 30 min. Ultrasound measurements were taken at the fifth minute after the beginning SBT. All enrolled subjects were categorized into the DD group and the non-DD group according to the result of diaphragmatic echo. DD was defined as a diaphragmatic thickening fraction (DTF) of < 20% during tidal breathing.
Diaphragm ultrasound measurement
The diaphragm ultrasound was performed using a portable ultrasound machine (Mindray, M7 series, China) with a 10HMz linear probe by two experienced sonographers. Only the right hemidiaphragm was measured because the right hemidiaphragm was more feasible and repeatable compared with the left hemidiaphragm. All subjects were placed in a semi-recumbent position with the head of the bed at a 30-degree angle. The probe was placed between the mid-axillary or antero-axillary line, in the 8th to 11th intercostal space, and positioned in a cranio-caudal direction and perpendicular to the skin to achieve the best view of the right hemidiaphragm. At this position, the diaphragmatic ultrasound image was a hypoechoic structure between two echoic lines (the pleural and the peritoneal membrane)(Fig 1). In the B-mode image, diaphragm thickness (Tdi) was measured from the inner edge of the pleural line to the inner edge of the peritoneal line at both end inspiration and end expiration. The calculation formula of DTF was (Tdi-inspiration – Tdi-expiration) / Tdi-expiration. Tdi and body weight (BW) have significant positive correlations in children. Therefore, Tdi was standardised by BW (DE/BW).
Patient Characteristics and Clinical Outcomes
We collected the basic demographic of all subjects, beside, primary diagnosis, medications, inflammatory factor levels at discharge and duration of elevated inflammatory factors were also collected, as the previous study demonstrated that systemic inflammation is associated with muscle atrophy in critically ill adult patients. The clinical outcomes we observed include delay or difficulty in weaning, extubation failure, total length of time on mechanical ventilation, length of PICU stay and mortality.
For comparisons of demographic, clinical characteristics and outcomes between DD and non-DD patient group, continuous variables were compared with Student t-test or Mann-Whitney U test. Categorical variables were compared with Chi-squared test or Fisher’s exact test. Data are presented as mean ± standard deviation for continuous variables with a normal distribution and as median with interquartile range for variables with a non-normal distribution. Categorical variables were described as n (%). To determine which factors are significantly associated with DD, logistic regression analysis was then performed. All analyses were carried out using IBM SPSS Statistics for Windows, Version 22.0 (IBM Corp, Armonk, NY). and a p-value less than or equal to 0.05 was considered statistically significant.