Hypoplastic Left Heat Syndrome (HLHS) is a common congenital heart defect (average 1 of 4300 live births). Problems develop due to the left side of the heart being critically underdeveloped. If born with this syndrome, the heart cannot function effectively to pump blood to both the lungs and the rest of body. Without treatment HLHS is uniformly fatal. Treatment includes a three-stage surgical procedure. Mortality rate after the first stage is 20%-30%. Our hypothesis is that the autonomic nervous system of the infants who do not survive is different from those who survive. We measured 24-hour recordings of ECG in HLHS infants and in age-matched normal infants. We used Principal Dynamic Mode (PDM) analysis as an indication of sympathetic and parasympathetic control of heart rate. We found that the PDM analysis for HLHS does indicate a reduction in sympathetic control of the heart compared to the normal infant . We conclude that the PDM analysis of the autonomic nervous system shows that normal infants versus HLHS infants both differ from one another. "