The amount of blood which enters the pulmonary circulation is determined by the degree of fixed outlet obstruction, the tone of the infundibular region and the pulmonary vascular resistance. If either of the latter two increase, more blood is shunted through the VSD and the patient becomes more cyanosed.

The degree of cyanosis will also increase if systemic cardiac output (CO) decreases. A reduction in CO leads to a fall in mixed venous oxygen saturation and a subsequent fall in arterial saturation even if shunt fraction remains unchanged.