2.1 | Imaging techniques
Gastric emptying was assessed using 2 methods at a time when infants were receiving at least 120 mL/kg/day of enteral feedings and no intravenous fluids. The first method is to calculate the maximum antral cross-sectional area (ACSA) by measuring the longest anteroposterior and transverse diameters of the gastric antrum by real-time US (Zonare, Mindray, Shenzhen, China) [8]. The second method is to calculate the spheroid gastric volume by measuring the largest anteroposterior, longitudinal and transverse diameters of the stomach filled with milk with the US transducer positioned on the lower left lateral chest and upper abdomen [9,10]. Each infant had a total of 4 measurement points: before feeding (0 hour), at 1 hour, 2 hours and 3 hours after the initiation of the feeding (Figure 1 ). Infants were kept in a flat supine position during the study period. Based on the consecutive measurements, ACSAs and spheroid volumes were used to calculate percent change in ACSA and percent change in spheroid gastric volume between two consecutive feedings as proxies for percentage of milk emptied per minute. Early gastric emptying was defined as the change in volume determination from 1 hour to 2 hours after the initiation of the feeding. Late gastric emptying defined as the change in volume from 2 hours to 3 hours after the initiation of the feeding. Values obtained by both US methods were compared between the two modes of CPAP.
Gastric residual volume percentage was calculated by dividing the gastric volume at 1 hour (immediately prior to the next feeding) by the gastric volume at 1 hour (after completion of the study feeding). Value obtained was expressed as a percentage.
A lung ultrasound score (LUS) was also determined at the 1-hour measurement point in order to ensure that the CPAP groups did not differ in the severity of their respiratory disease. The LUS score is assigned based on observations reflecting the efficacy of aeration at 6 different lung fields [12]. A recent meta-analysis has validated it for assessing the severity of RDS with high sensitivity and specificity [11].
Ultrasound gel was prewarmed and transducer sanitized before each exam. During the exam, infants were comforted with a pacifier. If an infant had oxygen desaturation below 80% or bradycardia below 80 bpm, the exam was stopped to allow the infant to recover. No sedation was used during the US exams.