Understanding the Qp/Qs Ratio in Congenital Heart Disease

Definition and Clinical Significance

The Qp/Qs ratio represents the relationship between pulmonary blood flow and systemic blood flow, which is crucial for evaluating cardiac shunts and determining appropriate management in congenital heart disease, according to the American Heart Association 1, 2
Qp/Qs is the ratio of pulmonary blood flow to systemic blood flow, which normally equals 1:1 in healthy individuals without cardiac shunts, as stated by the American College of Cardiology 3
In left-to-right shunts, Qp/Qs > 1 indicates greater blood flow to the pulmonary circulation compared to systemic circulation, with higher values indicating larger shunts, as reported by the American Heart Association 1
In right-to-left shunts, Qp/Qs < 1 indicates greater flow to the systemic circulation, with the closer to zero indicating larger shunts, according to the American Heart Association 1
Quantifying Qp/Qs is vital for determining appropriate management of patients with shunt lesions and is included in most current international guidelines, including those from the American College of Cardiology 1

A Qp/Qs ratio > 1.5:1 is generally considered a hemodynamically significant left-to-right shunt that may warrant intervention, as recommended by the American College of Cardiology 4
Surgical or device closure is recommended for atrial septal defects when Qp/Qs ≥ 1.5:1, with right atrial and RV enlargement, provided pulmonary vascular resistance is less than one-third of systemic resistance, according to the American College of Cardiology 4
Closure should not be performed when pulmonary arterial systolic pressure is greater than two-thirds systemic or when pulmonary vascular resistance is greater than two-thirds systemic, as stated by the American College of Cardiology 4
In Eisenmenger syndrome, the Qp/Qs ratio becomes < 1 as blood flows from right-to-left, as reported by the European Respiratory Society 5

The reference standard method for measuring Qp/Qs is invasive oximetry during cardiac catheterization, measuring oxygen saturations in pulmonary and systemic arterial and venous systems, as stated by the American Heart Association 1
Cardiac Magnetic Resonance (CMR) has become the non-invasive reference standard for measuring Qp/Qs using phase-contrast CMR, allowing highly accurate and reproducible quantification without assumptions, according to the American Heart Association 6
Doppler echocardiography uses vessel area and blood flow velocity measurements, but is prone to inaccuracy due to inadequate data acquisition and invalid assumptions, as reported by the American Heart Association 1

Qp/Qs is used to determine the need for intervention in congenital heart defects with left-to-right shunts, as recommended by the American College of Cardiology 4
Surgical repair is recommended when Qp/Qs ≥ 1.5:1 and pulmonary vascular resistance is less than one-third of systemic resistance, according to the American College of Cardiology 4
In patients with pulmonary hypertension associated with congenital heart disease, the Qp/Qs ratio helps determine whether repair is feasible or contraindicated, as reported by the European Respiratory Society and the European Society of Cardiology 5, 8