Cardiac output, the amount of blood pumped by the heart in one minute, will be measured in pediatric patients undergoing surgery involving cardiopulmonary bypass (CPB). Cardiac output will be measured after cardiopulmonary bypass using a device that employs ultrasound dilution technology. At times, cardiac output will be measured during a procedure called modified ultrafiltration (MUF). The ultrasound dilution cardiac output measurements will be validated or compared with cardiac output measurements made using other FDA approved techniques and devices.