Hypoplastic Left Heart Syndrome (HLHS) post Glenn, pre-transplant assessment. This coloured bloodpool model shows the mixing of blue deoxygenated blood and red oxygenated blood. Colouring is not based on scientific data. In hypoplastic left heart syndrome, the left ventricle is very small. The Glenn surgery connects the superior vena cava (SVC) to the pulmonary arteries to allow the deoxygenated blood returning from the upper body to bypass the heart and travel to the lungs directly.