This prospective study was conducted in the paediatric intensive care unit of First Hospital of Jilin University, Changchun, China. Study subjects included 61 consecutive patients between January 2019 and May 2019, who were aged less than 18 years. 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 who received MV support for ≥48 h and met the standard criteria for weaning readiness (improvement in the cause of primary disease, PaO2/FiO2 > 200, positive end-expiratory pressure (PEEP) ≤ 5-10 cm H2O, FiO2 ≤ 50%, and hemodynamically stable in the absence of vasopressors) were included in the study. If the child experienced a known neuromuscular disease (such as amyotrophic lateral sclerosis, Guillain-Barre, or myasthenia gravis), cervical spinal cord injury, pneumothorax, death during mechanical ventilation, or if there was an unwillingness of the parents or guardians to participate in the study, then that child was excluded from the study.
All eligibles underwent the spontaneous breathing test (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. Patients who were unable to tolerate spontaneous breathing tests during observation time were classified as failed weaning. Ultrasound measurements and PImax were taken at the fifth minute after the start of SBT. The patient passed the SBT if the exhaled tidal volume was equal to or above 5 mL/kg of the ideal body weight, and if the respiratory rate remained within the targeted range for age (<6 months, 20-50 breaths/min; 6 months-2 yr., 15-45 breaths/min; 2-5 yr., 15-40 breaths/min; >5 yr. 10-35 breaths/min). All patients accepted the Venturi inside the mask for oxygen therapy after passing the SBT. Successful weaning was defined as the ability to maintain spontaneous breathing for >48 h.
The measurement of PImax was occluding the airway at end expiration through a unidirectional valve, and maintained for approximately 10 breaths or 20 s. Finally, the maximum negative pressure displayed by the ventilator was recorded. Body weight (BW) is known as the predictor of PImax in healthy children, therefore, the PImax value was standardised by varying BW (PImax/BW).
All patients were placed in a semi-recumbent position with the head of the bed at a 30-degree angle. Two experienced sonographers performed ultrasound measurements by using the same portable ultrasound machine (Mindray, M7 series, China), and the evaluators were blinded to the results of the SBT prior to measurement. In the present study, only the right hemidiaphragm was measured because the right hemidiaphragm was more feasible and repeatable compared with the left hemidiaphragm. Diaphragm thickness (Tdi) was measured by using a 10 MHz linear probe at the zone of apposition at the right eighth or ninth intercostal space, which is between the anterior axillary and the midaxillary lines. The direction of the ultrasound probe was perpendicular to the diaphragm. At this position, the diaphragmatic ultrasound image was a hypoechoic structure between two echoic lines (the diaphragmatic pleura and the peritoneal membrane) in the B-mode (Fig. 1). In the same position, M-mode ultrasonography was used to measure resting Tdi at end-expiration (Tdi-exp) and end-inspiration (Tdi-insp), respectively (Fig. 2). The Tdi measurement was the inner edge of the peritoneal membrane to the inner edge of the diaphragmatic pleura. The calculation formula of diaphragmatic thickening fraction (DTF) was (Tdi-insp – Tdi-exp) / Tdi-exp. For the measurement of diaphragmatic excursion (DE), a 5 MHz probe was placed at the junction of the right mid-clavicle line and the right subcostal margin, where the probe direction paralleled the diaphragmatic movement. The diaphragmatic movement toward the probe during inspiration was recorded as an upward motion of the M-mode tracing, and the movement was opposite during expiration. In a breathing cycle, the amplitude of DE was the maximum point that moved vertically downward to the lowest point in M-mode (Fig. 3).[22, 23] The DE was continuously measured for 3 times in free breathing, and then the average was taken. DE and BW have significant positive correlations in children. Therefore, DE was standardised by varying BW (DE/BW).
Analyses were carried out using IBM SPSS Statistics for Windows, Version 22.0 (IBM Corp, Armonk, NY). Depending on whether distribution was normal or non-normal, continuous variables were described as mean ± SD or median (interquartile range). Categorical variables were described as n(%). Continuous variables were compared with Student’s t-test or Mann-Whitney U test. Depending on sample size, categorical variables were compared with Chi-squared test or Fisher’s exact test. The correlation analyses were conducted using the Pearson method to test the relationship between DTF, PImax, and DE. To determine the best cut off for DE, DTF and PImax to predict weaning success, the area under the receiver operating characteristic (ROC) curve was calculated. For all final comparisons, a p-value less than or equal to 0.05 was considered statistically significant.