If a child has a single ventricle why do they need a? Sleep study?

Cardiac abnormality. This is a congenital issue, with a single ventricle there is a high probability of hypoxemia and a sleep study can help to quantify this and its sleep stage dependence. The treatment of this is multidisciplinary and specialists are absolutely warranted.
Not related. Typically, infants with any of the various types of single ventricle do not need a sleep study. However, occasionally, there are benefits of understanding the various contributions to heart and lung physiology during sleep by doing a formal sleep study. A sleep study can be helpful to understand if either central apnea (brain) or obstructive apnea is occurring during sleep.