In the immediate post-operative course of young, congenital heart repaired patients, non-invasive cardiac output (CO) measurements were critical. In pediatric intensive care units (PICUs), the adoption of the Ultrasonic Cardiac Output Monitor (USCOM) was growing. Researchers sought to see how well the USCOM device and echocardiography agreed on CO measurements for a study. Future observational research was conducted in a pediatric cardiac intensive care unit (PCICU). About 2 senior performers used USCOM and echocardiography to collect paired CO measurements in young, mechanically ventilated, immediate post-operative patients to rule out an unrepaired or residual intra-cardiac shunt. Percentage error and Bland-Altman analysis were used to analyze the agreement between echocardiography and USCOM. On 61 patients, 131 comparative scans were performed: average age somewhere between 94 and 111 days, weight somewhere between 4.7 and 2.1 kg, vaso-inotropic score somewhere between 15.3 and 11, and STAT score 3–4. (46%). The percent difference in the USCOM cardiac index (CI) was −9.6% (45.6), and the velocity-time-integral (VTI) was 8.9% (34.7). Bland- Altman analyses revealed low agreement when comparing USCOM to echocardiography for CI, stroke volume (SV), VTI, and aorta diameter (AO) measures. Because USCOM underestimates CI compared to an echocardiogram, it should be utilized with caution as a total estimate or proxy of CI in neonates and babies in the immediate postoperative phase of congenital heart surgery.