What is the Qp/Qs Ratio?
The Qp/Qs ratio quantifies the relationship between pulmonary blood flow (Qp) and systemic blood flow (Qs). In a normal heart without any shunting, the amount of blood flowing through the lungs equals the amount flowing through the body, giving a Qp/Qs ratio of approximately 1.0.
When an abnormal communication exists between the left and right sides of the heart (such as an atrial septal defect or ventricular septal defect), blood can shunt from one side to the other, causing an imbalance. This ratio is critical for deciding whether surgical or percutaneous closure of a defect is indicated.
The Qp/Qs ratio can be measured invasively during cardiac catheterization using oxygen saturation data (the Fick principle) or non-invasively using Doppler echocardiography.
Calculation Formulas
Oxygen Saturation Method (Fick Principle)
This method uses oxygen saturation values from four locations in the cardiovascular system, obtained during cardiac catheterization:
Where:
- SaO2 = Systemic arterial oxygen saturation (typically from aorta or femoral artery)
- SvO2 = Mixed venous oxygen saturation (from SVC, IVC, or right atrium)
- SpvO2 = Pulmonary venous oxygen saturation (assumed ~98% if not directly measured)
- SpaO2 = Pulmonary arterial oxygen saturation
Doppler Echocardiography Method
This non-invasive method calculates flow volumes through the left and right ventricular outflow tracts:
Interpreting Results
| Qp/Qs Ratio | Interpretation | Clinical Action |
|---|---|---|
| 1.0 | Normal — No shunt | No intervention needed |
| 1.0 – 1.4 | Small left-to-right shunt | Usually observed; follow-up recommended |
| 1.5 – 1.9 | Significant left-to-right shunt | Consider closure; evaluate RV volume overload |
| ≥ 2.0 | Large left-to-right shunt | Closure generally indicated |
| < 1.0 | Right-to-left shunt | Evaluate for Eisenmenger physiology; closure may be contraindicated |
Cardiac Shunt Diagram
Types of Cardiac Shunts
A cardiac shunt is an abnormal pathway that allows blood to flow between the left and right sides of the heart, or between the systemic and pulmonary circulations:
- Left-to-right shunt (Qp/Qs > 1): Oxygenated blood recirculates through the lungs. Causes include ASD, VSD, PDA, and partial anomalous pulmonary venous return. Leads to pulmonary overcirculation and right heart volume overload.
- Right-to-left shunt (Qp/Qs < 1): Deoxygenated blood bypasses the lungs and enters systemic circulation, causing cyanosis. Seen in Tetralogy of Fallot, Eisenmenger syndrome, and severe pulmonary hypertension.
- Bidirectional shunt: Blood flows in both directions depending on the cardiac cycle phase. This may occur in transitional or balanced situations.
Atrial Septal Defect (ASD) and Ventricular Septal Defect (VSD)
Atrial Septal Defect (ASD)
An ASD is a hole in the atrial septum allowing blood to flow from the left atrium to the right atrium. The most common type is the secundum ASD, accounting for about 75% of all ASDs. Small ASDs may close spontaneously in childhood, but larger defects require closure.
Hemodynamic significance is generally defined as a Qp/Qs ratio ≥ 1.5 with evidence of right ventricular volume overload. Closure can be performed percutaneously (Amplatzer device) for secundum ASDs or surgically for other types.
Ventricular Septal Defect (VSD)
A VSD is a hole in the ventricular septum and is the most common congenital heart defect, accounting for approximately 20-30% of all congenital heart disease. VSDs are classified by location: perimembranous (most common), muscular, inlet, and outlet (supracristal).
Small restrictive VSDs with Qp/Qs < 1.5 are typically managed conservatively. Moderate to large VSDs with Qp/Qs ≥ 2.0 generally require surgical closure to prevent pulmonary vascular disease.
Eisenmenger Syndrome
Eisenmenger syndrome occurs when a long-standing left-to-right shunt causes progressive pulmonary vascular remodeling and irreversible pulmonary hypertension. Eventually, pulmonary vascular resistance exceeds systemic vascular resistance, and the shunt reverses to become right-to-left (or bidirectional).
Key features include:
- Central cyanosis (SpO2 typically 80-90%)
- Erythrocytosis (secondary polycythemia)
- Digital clubbing
- Right heart failure in advanced stages
- Qp/Qs approaches or falls below 1.0
Once Eisenmenger physiology is established, surgical closure of the defect is contraindicated as the right ventricle depends on the shunt as a "pop-off valve." Management includes pulmonary vasodilator therapy and ultimately consideration for heart-lung transplantation.
Worked Example
Oxygen Saturation Method
A patient with a suspected ASD undergoes cardiac catheterization. The following saturations are obtained:
- SaO2 = 97%, SvO2 = 72%, SpvO2 = 99%, SpaO2 = 82%
This indicates a moderate left-to-right shunt approaching the threshold for intervention (1.5). Further evaluation with imaging and clinical correlation is needed.
Doppler Method
Echocardiographic measurements: LVOT diameter = 2.0 cm, LVOT VTI = 22 cm, RVOT diameter = 2.5 cm, RVOT VTI = 20 cm.
Qp = π × (2.5/2)² × 20 = 3.14 × 1.5625 × 20 = 98.2 mL
Qp/Qs = 98.2 / 69.1 = 1.42
Frequently Asked Questions
What Qp/Qs ratio indicates a need for surgery?
Generally, a Qp/Qs ≥ 1.5 with evidence of right ventricular volume overload is considered significant enough to warrant closure for ASDs. For VSDs, a ratio ≥ 2.0 is often the threshold. However, clinical decisions incorporate multiple factors including symptoms, right heart size, and pulmonary pressures.
Can Qp/Qs be measured non-invasively?
Yes. Doppler echocardiography provides a reliable non-invasive estimate of Qp/Qs by measuring flow through the left and right ventricular outflow tracts. Cardiac MRI is another non-invasive modality that can accurately quantify shunt ratios.
Why might SaO2 and SpvO2 differ in a left-to-right shunt?
In a pure left-to-right shunt, SaO2 should approximately equal SpvO2 because there is no mixing of deoxygenated blood into the systemic circulation. However, in the presence of lung disease or a concurrent right-to-left shunt, these values may differ.
What is the assumed SpvO2 if not directly measured?
When direct pulmonary venous sampling is not available, SpvO2 is commonly assumed to be 98% in patients breathing room air, or the same as SaO2 in the absence of a right-to-left shunt.