Qp Qs Ratio: Formula & Meaning

The Qp/Qs ratio is a Doppler echocardiography measurement that compares pulmonary blood flow (Qp) to systemic blood flow (Qs). You enter the velocity-time integral (VTI) and cross-sectional area for both the pulmonary and systemic outflow tracts, and the calculator returns the dimensionless ratio, which helps quantify the size of a left-to-right shunt.

Formula

The Qp/Qs ratio is calculated using the continuity equation applied to both the pulmonary and systemic circulations. The formula is:

Qp/Qs = (RVOT area × RVOT VTI) / (LVOT area × LVOT VTI)

Where:

• RVOT area = Ï€ × (RVOT diameter / 2)², measured in cm² (right ventricular outflow tract)
• RVOT VTI = velocity-time integral of the pulsed-wave Doppler through the RVOT, in cm
• LVOT area = Ï€ × (LVOT diameter / 2)², measured in cm² (left ventricular outflow tract)
• LVOT VTI = velocity-time integral through the LVOT, in cm

The VTI represents the distance blood travels with each heartbeat (stroke distance). Multiplying by the cross-sectional area gives the stroke volume. The ratio of stroke volumes (pulmonary to systemic) equals the Qp/Qs ratio. A ratio of 1.0 means equal flows; greater than 1.0 indicates a left-to-right shunt.

Worked Example

Using the default values from the calculator:

• LVOT VTI = 18 cm, LVOT diameter = 2.0 cm
• RVOT VTI = 14 cm, RVOT diameter = 2.2 cm

First, calculate the cross-sectional areas:

LVOT area = π × (2.0/2)² = π × 1.0² = 3.1416 cm²

RVOT area = π × (2.2/2)² = π × 1.1² = 3.8013 cm²

Then compute the flows (stroke volume × heart rate, but for the ratio heart rate cancels out):

Qp = RVOT area × RVOT VTI = 3.8013 × 14 = 53.22 cm³ per beat

Qs = LVOT area × LVOT VTI = 3.1416 × 18 = 56.55 cm³ per beat

Qp/Qs = 53.22 / 56.55 = 0.94

The calculator returns 0.94 (rounded to two decimals), indicating slightly less pulmonary than systemic flow – within the normal range (no significant shunt). The absolute flow values (Qp and Qs in L/min) are also shown, assuming a heart rate of 60 bpm (the calculator multiplies by 60/1000).

When to Use the Qp/Qs Ratio

Cardiologists and echocardiographers use the Qp/Qs ratio primarily to evaluate suspected left-to-right shunts, such as:

• Atrial septal defect (ASD)
• Ventricular septal defect (VSD)
• Patent ductus arteriosus (PDA)
• Partial anomalous pulmonary venous return

A ratio above 1.5 is often considered significant enough to consider surgical or percutaneous closure, especially if the patient is symptomatic. Ratios between 1.0 and 1.5 are considered small shunts that may be monitored. The ratio is also used to assess the hemodynamic significance of a shunt before and after intervention.

How to Read the Result

The Qp/Qs ratio is a dimensionless number:

• Qp/Qs = 1.0: Normal – pulmonary and systemic flows are equal.
• Qp/Qs > 1.0: Left-to-right shunt – pulmonary flow exceeds systemic flow. The higher the number, the larger the shunt.
• Qp/Qs < 1.0: Right-to-left shunt – systemic flow exceeds pulmonary flow, which can occur in Eisenmenger syndrome or with right-sided obstructive lesions.

In clinical practice, a ratio of 1.5–2.0 is moderate, and >2.0 is large. However, the ratio alone does not determine the need for closure; symptoms, pulmonary artery pressures, and right heart size also matter.

Common Mistakes and Pitfalls

Several measurement errors can skew the Qp/Qs ratio:

• Mixing up diameters: Using the LVOT diameter for the RVOT area (or vice versa) will invert the ratio. Always double-check which measurement goes where.
• Incorrect VTI tracing: The VTI must be traced along the modal (brightest) edge of the Doppler envelope, not the outer edge. A slightly different tracing can change the VTI by 10–20%.
• Unit errors: Diameters are in cm, but if you accidentally use mm, the area becomes 100 times larger. The calculator expects cm for both diameter and VTI.
• Assuming a circular orifice: The LVOT and RVOT are not perfectly circular, but the formula assumes they are. This is a known limitation of the method.
• Ignoring heart rate: The ratio itself is independent of heart rate because both flows are multiplied by the same rate. However, if you calculate absolute Qp and Qs, you need the heart rate – the calculator assumes 60 bpm for the L/min output, which is a simplification.

Limitations of the Formula

The Qp/Qs ratio derived from echocardiography is an estimate, not a gold-standard measurement. Cardiac MRI and cardiac catheterization (using the Fick method or thermodilution) are more accurate. The echo method has several limitations:

• Geometric assumption: The outflow tracts are assumed to be circular, but they are often elliptical, leading to area errors.
• VTI measurement variability: The VTI depends on the angle of interrogation; if the Doppler beam is not parallel to flow, the VTI is underestimated.
• Intra-observer variability: Different sonographers may trace slightly different envelopes, causing variability.
• Not valid in complex shunts: If there are multiple shunts or valvular regurgitation, the simple ratio may not reflect the true hemodynamics.

Despite these limitations, the Qp/Qs ratio remains a quick, non-invasive screening tool that is widely used in everyday practice.

How to Measure VTI and Diameter Correctly

Accurate measurement is critical. Here are the standard steps:

1. LVOT diameter: Measure in the parasternal long-axis view, at the level of the aortic annulus, during systole, from inner edge to inner edge.
2. LVOT VTI: Place a pulsed-wave Doppler sample volume just proximal to the aortic valve in the apical five-chamber view. Trace the spectral envelope.
3. RVOT diameter: Measure in the parasternal short-axis view at the level of the pulmonary valve, during systole.
4. RVOT VTI: Place the sample volume just proximal to the pulmonary valve in the same view, and trace the envelope.

Take the average of three consecutive beats (five if the patient is in atrial fibrillation) to reduce beat-to-beat variability.

Frequently Asked Questions

What is a normal Qp/Qs ratio?

A normal Qp/Qs ratio is approximately 1.0, meaning pulmonary and systemic blood flows are equal. In healthy individuals, the ratio ranges from 0.9 to 1.1. Values above 1.5 suggest a hemodynamically significant left-to-right shunt.

What does a Qp/Qs ratio of 2.0 mean?

A ratio of 2.0 means the pulmonary blood flow is twice the systemic flow. This indicates a large left-to-right shunt, often seen with moderate-to-large atrial or ventricular septal defects. Such a shunt usually warrants consideration for closure, especially if the patient has symptoms or right heart dilation.

How is Qp/Qs calculated in cardiac catheterization?

In catheterization, Qp and Qs are calculated using the Fick principle: Q = VO₂ / (CaO₂ − CvO₂), where VO₂ is oxygen consumption, CaO₂ is arterial oxygen content, and CvO₂ is mixed venous oxygen content. For Qp, the mixed venous sample is taken from the pulmonary artery (or right ventricle if no shunt), and for Qs, the arterial sample is from the aorta. The ratio is then Qp/Qs.

Can Qp/Qs be less than 1?

Yes, a Qp/Qs less than 1 indicates a right-to-left shunt, where systemic flow exceeds pulmonary flow. This can occur in conditions like Eisenmenger syndrome, where pulmonary hypertension causes reversed shunting, or in tetralogy of Fallot with right-to-left shunting.

Does the Qp/Qs ratio require heart rate?

No, the ratio itself does not require heart rate because both Qp and Qs are multiplied by the same heart rate, so it cancels out. However, if you want to report absolute flow in L/min, you need the heart rate. The calculator assumes 60 bpm for the L/min outputs, which may not match the patient’s actual heart rate.

What is the difference between Qp/Qs and shunt fraction?

Qp/Qs is the ratio of pulmonary to systemic flow. The shunt fraction (Qp − Qs)/Qp represents the proportion of pulmonary flow that is shunted. For example, a Qp/Qs of 2.0 corresponds to a shunt fraction of 50%. Both are used to quantify shunts, but Qp/Qs is more commonly reported in echo reports.

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in lvot_vti | LVOT VTI | cm | 18 | 0 | 60
in lvot_d | LVOT diameter | cm | 2.0 | 0.5 | 4.0
in rvot_vti | RVOT VTI | cm | 14 | 0 | 60
in rvot_d | RVOT diameter | cm | 2.2 | 0.5 | 4.0
out qp_qs | Qp/Qs ratio | - | (rvot_vti * pi * (rvot_d/2)^2) / (lvot_vti * pi * (lvot_d/2)^2) | 2
out qp | Pulmonary flow | L/min | (rvot_vti * pi * (rvot_d/2)^2) * 60 / 1000 | 2
out qs | Systemic flow | L/min | (lvot_vti * pi * (lvot_d/2)^2) * 60 / 1000 | 2