We estimated that the median time to the primary end point for participants with the ventricular assist device would be 100 days, and the median time to the primary end point for the propensity-score–matched control group of children receiving support with ECMO would be 5 days. On the basis of these assumptions, we calculated that the inclusion of 24 participants in each ventricular-assist cohort would provide more than 99% power, with a two-sided alpha level of 0.05, to test the hypothesis that survival with the ventricular assist device would be significantly longer than survival with ECMO.
All comparisons between the ventricular-assist and ECMO groups were performed on an intention-to-treat basis. Cumulative event rates were calculated according to the Kaplan–Meier method. For the ventricular-assist group, the time to an event was measured from the time of implantation of the ventricular assist device, regardless of whether another form of mechanical support had been in use before implantation. For the ECMO group, the time to an event was measured from the time of implantation of the ECMO device. The between-group difference in the time to the occurrence of the primary end point was assessed by means of the log-rank test within each of the two study cohorts. The duration of support with the device was compared with the use of the Wilcoxon median two-sample test.
The primary efficacy outcome was also evaluated with the use of a competing-risk analysis. The proportion of participants having each of the competing outcomes at each time point was plotted. Outcomes at 30 days and at the end of device support for the participant who received support for the longest time were compared between groups with the use of chi-square tests.
The primary safety end point was calculated as the number of serious adverse events per day during circulatory support with the ventricular assist device. A Poisson exact confidence interval was calculated, and the critical-value method was used for significance testing. Success was prospectively defined as less than 0.25 events per day for the upper bound of the 95% Poisson exact confidence interval. A two-tailed Fisher's exact test was used to compare the proportion of participants in each functional-status category at each time point with the proportion in each category before the devices were implanted.
All reported P values are two-sided. A P value of less than 0.05 was considered to indicate statistical significance, without adjustment for multiple comparisons.