Echocardiography and data analysis

Echocardiography (IE33 Ultrasound System; Philips Medical Systems, Andover, MA, USA) was performed in all patients, preoperatively the day before the surgery (PREOP), on the day of surgery in the first 3 hours after admission to the paediatric cardiology intensive care unit (EARLY) and at hospital discharge (DISCHARGE).

Examinations were performed with the child in the decubitus position. S8-1 or S5-1 matrix probes were used, depending on the age of the patient. The frame rate was kept between 70 and 100 Hz, and an electrocardiogram was connected to detect systole and diastole. Examinations started with conventional bidimensional echocardiography.

Echocardiographic assessment of right and left ventricular global function

Data on variables of RV function were collected. TAPSE was acquired by the conventional M-mode method from the lateral point of the tricuspid valve in a standard apical four-chamber view , and was expressed according to the z score because of the important effect of age . Tricuspid annular peak systolic velocity (TA Sa) was obtained from a modified apical four-chamber view in TDI . Left ventricular ejection fraction (LVEF) was measured from the apical four-chamber view by the monoplane Simpson method, and fractional shortening was obtained by the M-mode method from the parasternal long-axis view .

We focused on substantial factors that could influence the evolution of RV variables. Volume overload is known to influence RV variables, so we assessed RV function in patients with preoperative volume overload (those with atrial septal defect or pulmonary and associated tricuspid regurgitation) and patients without preoperative volume overload. Also, the type of surgery can be linked to RV dysfunction, so we compared patients who required surgery with and without CBP.

2D strain

At the end of the examination, three loops were recorded in the apical four-chamber view, focused on the left ventricle and then the right ventricle. A modified apical view was used to obtain a complete image of the right ventricle . Special attention was given to optimize gain and compression, and to obtain a central and complete picture of the left ventricle and then of the free wall of the right ventricle. Examinations were recorded on Digital Imaging and Communications in Medicine (DICOM) compact discs for off-line analysis using QLab 10 software (Philips Medical Systems, Andover, MA, USA).

To measure longitudinal global left ventricular (LV) and RV peak systolic strain, the endocardial border was traced manually by the operator. Basal points were placed just above the atrioventricular valve. The software automatically generated a second line at the level of the epicardium, delineating a region of interest, including the entire wall between endocardial and epicardial border. The accuracy of the tracking of the region of interest during the cardiac cycle was checked visually on a dynamic loop. In case of discrepancy, manual corrections were applied. The measurement of strain was performed automatically. The analysis was considered as acceptable if the software validated the measurement for each cardiac segment. If a segment was not accepted, the limit of the region of interest was retraced. The strain values were averaged to determine global longitudinal strain. RV peak systolic strain (RV-PSS) was calculated by averaging only strain curves from the RV free wall (three-segment model: basal, median and apical), excluding the septum to avoid interaction with LV function .

Reproducibility

Interobserver and intraobserver variabilities of longitudinal LV and RV strain variables were tested in 30 examinations from 10 randomly selected subjects for each echocardiographic assessment (PREOP, EARLY and DISCHARGE). Loop acquisitions were performed by the same operator in all patients. To assess interobserver variability, the dataset was analysed off-line by a second operator with the same level of experience, blinded to the previous results. To evaluate intraobserver variability, the first operator repeated the measurements at the end of the data analysis process, blinded to the previous results.

Statistical analysis

Data collected were demographic variables, including body weight, length, sex, body surface area, age, heart rate and type of CHD. Surgery data included CPB time, ultrafiltration rate and aortic cross-clamp time. Postoperative data included mechanical ventilation duration, lactate peak, troponin peak and inotrope duration. Echocardiographic variables included z score of TAPSE, RV-PSS, TA Sa, LVEF, LV fractional shortening and LV peak systolic strain (LV-PSS).

Data are expressed as median and interquartile range (IQR) or number (percentage). Because the number of subjects in the study was small, and many of the variables did not fit normal distributions, non-parametric tests were used to compare data. In particular, the Mann-Whitney U test was used to compare RV-PSS among subgroups, according to CHD type. Spearman’s coefficients were used to assess correlations between systolic function variables. Relationships between RV and LV 2D strain and outcome or myocardial injury were further assessed by linear regression analysis. The Wilcoxon matched-pairs signed-rank test was used to assess the evolution of systolic function variables after surgery compared with preoperative data. Intraobserver and interobserver variabilities for the different strain variables were assessed using the Bland-Altman approach, with calculation of the mean bias (average difference between measurements) and the lower and upper limits of agreement (95% limits of agreement of mean bias) .

Statistical analysis was performed using GraphPad Prism 5.0 (GraphPad Software,Inc., La Jolla, CA, USA). P values < 0.05 were considered to indicate statistical significance.

Study population

Of the 37 CHD operations, six (16.2%) were performed in the neonatal period. Demographics, surgery data and distribution of CHD are reported in Table 1 . Median age was 19 months (IQR 5–63 months). Median weight was 9.2 kg (IQR 5.3–18.0 kg). There were no surgical deaths, no deep hypothermic cardiac arrest and no pacing.

Table 1

General data.

Variable
Age (month)	19 (5–63)
Female sex	18 (48.6)
Weight (kg)	9.2 (5.3–18.0)
BSA (m 2 )	0.41 (0.30–0.72)
Heart rate	117.0 (91.0–140.5)
Type of congenital heart disease
ASD	10 (27.0)
VSD	8 (21.6)
ToF	6 (16.2)
Coarctation	5 (13.5)
TGA	2 (5.4)
Subaortic membrane	2 (5.4)
PA-VSD	1 (2.7)
AVSD	1 (2.7)
Pulmonary tricuspid regurgitation	1 (2.7)
Double aortic arch	1 (2.7)
CBP duration (minutes)	70 (43–93)
Clamping duration (minutes)	52 (22–72)
Mechanical ventilation duration (hours)	10 (4–72)
Ultrafiltration (mL/kg / min)	0.62 (0.23–0.95)
Lactate peak (mmol/L)	2.9 (2.2–5.0)
Troponin peak (μg/L)	27.9 (10.5–55.9)
Inotrope duration (day)	2.6 ± 3.5
Days in ICU	3 (2–7.3)
Discharge echography delay (day)	6 (4–8)

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