Evaluate cardiac shunt from oxygen saturations.
Qp/Qs = (SaO₂ − SvO₂) / (SpvO₂ − SpaO₂). Normal ratio is 1:1. Left-to-right shunts (ASD, VSD, PDA) have Qp/Qs > 1. Shunts > 1.5:1 generally warrant intervention. Right-to-left shunts result in Qp/Qs < 1. Oxygen saturations are obtained during cardiac catheterization. Can also be estimated by echocardiography or MRI.