Understanding the Qp/Qs Ratio
The Qp/Qs ratio represents the relationship between pulmonary blood flow (Qp) and systemic blood flow (Qs), which is crucial for evaluating cardiac shunts and determining appropriate management in congenital heart disease. 1
Definition and Clinical Significance
- Qp/Qs is the ratio of pulmonary blood flow to systemic blood flow, which normally equals 1:1 in healthy individuals without cardiac shunts 1
- In left-to-right shunts, Qp/Qs > 1 indicates greater blood flow to the pulmonary circulation compared to systemic circulation (higher values indicate larger shunts) 1
- In right-to-left shunts, Qp/Qs < 1 indicates greater flow to the systemic circulation (the closer to zero, the larger the shunt) 1
- Quantifying Qp/Qs is vital for determining appropriate management of patients with shunt lesions and is included in most current international guidelines 1
Clinical Interpretation
- Qp/Qs > 1.5:1 is generally considered a hemodynamically significant left-to-right shunt that may warrant intervention 1
- 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 1
- 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 1
- In Eisenmenger syndrome (pulmonary hypertension with reversed shunt), the Qp/Qs ratio becomes < 1 as blood flows from right-to-left 1
Measurement Methods
Invasive Methods
- The reference standard method is invasive oximetry during cardiac catheterization, measuring oxygen saturations in pulmonary and systemic arterial and venous systems 1
- This method should be used when measurement of pulmonary artery pressure and pulmonary vascular resistance are also required (e.g., in Eisenmenger physiology) 1
- Limitations of invasive measurement include:
Non-invasive Methods
Cardiac Magnetic Resonance (CMR) has become the non-invasive reference standard for measuring Qp/Qs using phase-contrast CMR 1
- Allows highly accurate and reproducible quantification without assumptions 1
- Can measure flow in alternative vessels when standard measurements aren't possible 1
- Provides internal quality assurance through multiple measurement methods 1
- Also evaluates ventricular volumes to assess physiological consequences of shunts 1
Doppler echocardiography:
- Uses vessel area and blood flow velocity measurements 1
- Benefits from being the first-line method for anatomically evaluating shunt lesions 1
- However, prone to inaccuracy due to inadequate data acquisition and invalid assumptions 1, 2
- Studies show poor correlation with catheter-derived measurements and tends to overestimate shunt magnitude 2
Common Pitfalls and Caveats
- Echocardiographic evaluation of Qp/Qs, though routinely used, has poor correlation with oximetry-derived values and tends to overestimate left-to-right shunts 2
- Quantification of Qp/Qs is only possible if sampling can be performed distal to the shunt, which is not possible in some extracardiac lesions 1
- Semilunar valve regurgitation can modify the value of Qp/Qs and should be considered when interpreting results 3
- When using ventricular stroke volumes to calculate Qp/Qs, shunt location must be taken into consideration:
- In ASD, RV stroke volume will be twice the LV stroke volume with Qp/Qs of 2:1
- In VSD, LV stroke volume will be twice the RV stroke volume with the same Qp/Qs ratio 1
Clinical Applications
- Qp/Qs is used to determine the need for intervention in congenital heart defects with left-to-right shunts 1
- Surgical repair is recommended when Qp/Qs ≥ 1.5:1 and pulmonary vascular resistance is less than one-third of systemic resistance 1, 4
- In patients with pulmonary hypertension associated with congenital heart disease, the Qp/Qs ratio helps determine whether repair is feasible or contraindicated 1