Height, weight, and vital signs (including oxygen saturation by pulse oximetry) are measured on Day 1. Resting energy expenditure, oxygen consumption (VO2), carbon dioxide production (VCO2), and resting respiratory exchange quotient (RQ) are measured using open circuit respiratory calorimetry on Day 1. Patients undergo assessment of total daily energy expenditure using the doubly labeled water method comprised of oral deuterium and oral oxygen O 18 with the next scheduled feeding on Day 1. Urine samples are collected prior to isotope administration, then serially for approximately 12 hours after isotope administration on Day 1, and then daily on Days 2-7. These samples are analyzed by mass spectrometry. On Day 1, patients also undergo echocardiogram to confirm size of defect and measure the degree of pulmonary/systemic blood flow ratio and pulmonary artery pressures.