Most left to right shunts cause oxygenated blood to pass from the left to the right side of the heart, meaning that these are usually ‘acyanotic’ lesions i.e. patients will have normal SpO2. Some might struggle with excess volume loading of the right heart and a degree of pulmonary oedema. In left to right shunt Qp > Qs
Types of left to right shunt:
Atrial septal defect
Ventricular septal defect
Atrio-ventricular septal defect
Patent ductus arteriosus
Aorto-pulmonary window
A large VSD or AVSD can demonstrate more complex balancing between Qp:Qs.
A large reduction in SVR, or a pulmonary hypertensive crisis are two examples that cause a left-to-right (Qp>Qs) shunt to reverse to a right-to-left shunt (Qp<Qs), resulting in cyanosis
Southampton Cardiac Anaesthesia 