Qp/Qs Calculator - Pulmonary to Systemic Flow Ratio
The Qp/Qs calculator quantifies the ratio of pulmonary blood flow (Qp) to systemic blood flow (Qs) across an intracardiac or great-vessel shunt. A ratio of 1.0 means balanced flow; values above 1.0 indicate a left-to-right shunt, values below 1.0 a right-to-left shunt. Choose the Fick oxygen saturation method (used during cardiac catheterization) or the echocardiographic LVOT/RVOT Doppler method, enter your measurements, and get an instant result with clinical interpretation.
Formula
Worked example
Fick method: SaO2 = 98%, SvO2 = 70%, PvO2 = 99%, PAO2 = 80%. Numerator = 98 - 70 = 28. Denominator = 99 - 80 = 19. Qp/Qs = 28 / 19 = 1.47, consistent with a mild-to-moderate left-to-right shunt. Echo method: LVOT 22 mm, VTI 20 cm, RVOT 26 mm, VTI 22 cm. Qs = pi x (1.1)^2 x 20 = 76.0 cm3. Qp = pi x (1.3)^2 x 22 = 116.9 cm3. Qp/Qs = 116.9/76.0 = 1.54.
What is the Qp/Qs ratio?
Qp/Qs expresses the ratio of pulmonary blood flow (Qp) to systemic blood flow (Qs) in patients with intracardiac or great-vessel shunts. In a normal heart these two flows are equal, so the ratio is 1.0. When there is a hole in the septum or a patent ductus arteriosus, blood may pass from the high-pressure left side to the right side (left-to-right shunt), causing Qp to exceed Qs and the ratio to rise above 1.0. Conversely, in cyanotic conditions or advanced pulmonary hypertension, right-side pressure may exceed left-side pressure, forcing deoxygenated blood into the systemic circulation (right-to-left shunt), and the ratio falls below 1.0. The ratio is used to decide whether the shunt is large enough to warrant closure and to monitor change over time.
Two methods: Fick principle and echocardiographic Doppler
The Fick principle method uses oxygen saturations sampled during cardiac catheterization from four sites: the systemic artery (SaO2), the mixed venous blood (SvO2, usually from the pulmonary artery or calculated as (3 x SVC + IVC) / 4), the pulmonary vein (PvO2, often assumed 98-100% when direct sampling is unavailable), and the pulmonary artery (PAO2). The formula is Qp/Qs = (SaO2 - SvO2) / (PvO2 - PAO2). This is considered the reference standard because it is independent of heart rate and does not require geometric assumptions. The echocardiographic method calculates stroke volume from outflow-tract area and velocity time integral at both the left (LVOT) and right (RVOT) ventricular outflow tracts, using the formula Qp/Qs = (RVOT VTI x RVOT area) / (LVOT VTI x LVOT area). It is non-invasive and widely used for serial assessment, but small errors in diameter measurement propagate into the area calculation because the area depends on the square of the radius.
Clinical decision thresholds
Most guidelines use a Qp/Qs of 1.5 as the lower boundary for recommending closure of an atrial or ventricular septal defect in patients without significant pulmonary hypertension. When the ratio reaches 2.0 or above, the shunt is considered large and closure is generally indicated unless pulmonary vascular resistance is already prohibitively elevated (Eisenmenger physiology). For right-to-left shunts, the priority shifts from volume overload to assessing pulmonary vascular resistance: a ratio below 0.8 warrants formal measurement of pulmonary vascular resistance and sometimes vasoreactivity testing with oxygen, nitric oxide, or prostacyclin. Closure of a shunt in the setting of severe irreversible pulmonary hypertension can be fatal.
Factors that influence the ratio and sources of error
For the Fick method, the accuracy of SvO2 is critical; streaming of blood from the superior and inferior vena cava means a single-sample SvO2 from the right atrium can be misleading, which is why the weighted formula (3 x SVC + IVC) / 4 is preferred. For the echo method, assuming a circular outflow tract when it is actually elliptical introduces systematic error; diameter should be measured at the same phase of the cardiac cycle each time. Both methods are affected by any condition that changes the oxygen-carrying capacity of blood, including anemia or polycythemia. Inter-atrial shunts are more sensitive to ventricular compliance differences, while inter-ventricular and great-vessel shunts depend primarily on the ratio of pulmonary to systemic vascular resistance. Management of Qp/Qs therefore includes both anatomical closure and manipulation of vascular resistances with oxygen, nitric oxide, or vasopressors in acute settings.
Qp/Qs clinical interpretation guide
| Qp/Qs range | Shunt type | Clinical significance | Typical decision |
|---|---|---|---|
| < 0.5 | Severe right-to-left | Critical desaturation | Urgent evaluation |
| 0.5 - 0.8 | Moderate right-to-left | Pulmonary hypertension likely | Vasoreactivity testing |
| 0.8 - 0.9 | Mild right-to-left | Possible elevated PVR | Close monitoring |
| 0.9 - 1.1 | None / balanced | Normal circulation | No intervention |
| 1.1 - 1.5 | Mild left-to-right | Volume load, often tolerated | Observe; repeat assessment |
| 1.5 - 2.0 | Moderate left-to-right | Right-heart volume overload | Consider closure |
| > 2.0 | Large left-to-right | Significant overload | Closure indicated |
Standard thresholds used in adult and pediatric cardiology for shunt classification and intervention guidance.
Frequently asked questions
What is a normal Qp/Qs value?
A Qp/Qs of 1.0 is ideal, indicating that pulmonary and systemic blood flows are equal with no net shunting. Values between 0.9 and 1.1 are considered within normal variation. Anything below 0.9 suggests a net right-to-left shunt and anything above 1.1 suggests a net left-to-right shunt, though the clinical significance depends on the degree of deviation.
At what Qp/Qs is intervention recommended?
The conventional threshold for considering closure of an ASD or VSD in adults is a Qp/Qs of 1.5 or greater, provided pulmonary vascular resistance is not severely elevated. A ratio of 2.0 or above generally makes the case for closure more compelling. These are guidelines, not absolute rules: symptoms, chamber dilation, and pulmonary pressure all factor into the final decision.
Can Qp/Qs be measured non-invasively?
Yes. The echocardiographic LVOT/RVOT Doppler method is widely used as a non-invasive alternative to catheterization-based Fick measurements. It is less accurate when outflow tracts are not clearly circular or when Doppler alignment is suboptimal, but it is reproducible enough for serial monitoring and initial screening. Cardiac MRI phase-contrast flow quantification is another non-invasive option and is considered highly accurate.
What happens if the pulmonary venous saturation (PvO2) is assumed rather than measured?
PvO2 is often assumed to be 98-100% in patients without lung disease, because the pulmonary veins normally carry fully oxygenated blood. In patients with pulmonary disease or ventilation-perfusion mismatch, PvO2 can be lower, and using 100% would overestimate the denominator and underestimate the true Qp/Qs. Direct sampling during catheterization is more accurate in these cases.
What is Eisenmenger syndrome and how does Qp/Qs change?
Eisenmenger syndrome develops when a chronic large left-to-right shunt causes progressive pulmonary vascular disease, eventually reversing the shunt direction. As pulmonary vascular resistance rises toward or above systemic levels, Qp/Qs falls from above 1.0 toward or below 1.0. Once the syndrome is established, closing the shunt is dangerous because the defect is then acting as a pressure relief valve. Patients become cyanotic and polycythemic.
Why does the echo method use velocity time integral rather than peak velocity?
The velocity time integral (VTI) represents the area under the Doppler velocity-time curve for a complete cardiac cycle and is proportional to the stroke distance (how far a column of blood travels per beat). Multiplying VTI by the outflow-tract cross-sectional area gives the stroke volume. Peak velocity alone does not account for the shape of the velocity envelope, so it cannot be used to calculate volumetric flow.
Does anemia affect the Qp/Qs calculation?
Anemia does not directly affect the oxygen saturation percentages used in the Fick formula, so the ratio itself is not directly distorted. However, anemia increases cardiac output and can increase shunt flow through a fixed defect, so the clinical magnitude of the shunt may be greater than the ratio alone implies. Hemoglobin concentration is used separately to convert saturations to actual oxygen content when calculating absolute flows (liters per minute) rather than just the ratio.