How to Measure Pulmonary Blood Flow (Qp) by Echocardiography

How to Measure Pulmonary Blood Flow (Qp) by Echocardiography: Step-by-Step Guide

Pulmonary blood flow (Qp) is an important echocardiographic parameter used to assess blood flow through the pulmonary circulation. It is particularly useful in evaluating congenital heart disease, intracardiac shunts, and conditions associated with abnormal pulmonary blood flow.

In Doppler echocardiography, pulmonary stroke volume can be calculated by multiplying the cross-sectional area of the pulmonary valve annulus by the velocity-time integral (VTI) of blood flow in the right ventricular outflow tract (RVOT).

Key Formula
Pulmonary Stroke Volume (Qp per beat) = Pulmonary Valve Annular Area × Pulmonary VTI

Qp = π × (D/2)² × VTI

D = Pulmonary valve annulus diameter
VTI = Velocity-time integral in cm
Result = Stroke volume in mL/beat

1. Measure the Pulmonary Valve Annulus Diameter

The first step is to measure the diameter of the pulmonary valve annulus. Because the diameter is squared when calculating the cross-sectional area, even a small measurement error can significantly affect the calculated stroke volume.

Echocardiographic View

• Obtain the parasternal short-axis (PSAX) view at the level of the great arteries.
• Visualize the pulmonary valve and RVOT clearly.
• Optimize image quality before taking the measurement.

Measurement Technique

1. Freeze the image during mid-systole, when the pulmonary valve is open.
2. Zoom in on the pulmonary valve annulus.
3. Measure the annular diameter from inner edge to inner edge.
4. Record the diameter in millimeters, then convert it to centimeters before calculating the area if VTI is measured in centimeters.

Clinical Tip: Ensure that the annulus is visualized in an appropriate plane and that the measurement is perpendicular to the direction of blood flow. An oblique measurement may overestimate the diameter and calculated area.

2. Calculate the Pulmonary Valve Annular Area

Once the pulmonary valve annulus diameter has been measured, calculate its cross-sectional area using the circular area formula.

Area = π × (Diameter/2)²

Alternatively:

Area = 0.785 × Diameter²

When the diameter is measured in centimeters, the calculated area is expressed in cm².

Worked Example

Suppose the pulmonary valve annulus diameter is 2.0 cm.

Area = π × (2.0/2)²

Area = π × 1²

Pulmonary valve annular area = 3.14 cm²

Important: The pulmonary valve annulus is often treated as circular for this calculation. If the annulus is elliptical, the circular assumption can introduce error. Three-dimensional imaging or CT may provide additional anatomical information when clinically indicated.

3. Measure Pulmonary VTI Using Pulsed-Wave Doppler

The pulmonary velocity-time integral (VTI) represents the distance travelled by blood during one systolic ejection. It is obtained by tracing the Doppler velocity envelope.

Recommended View

Use a parasternal short-axis view at the level of the great arteries to assess the RVOT and pulmonary valve. Other suitable imaging windows may be used when they provide better Doppler alignment.

Doppler Technique

1. Activate pulsed-wave (PW) Doppler.
2. Place the sample volume in the RVOT, just proximal to the pulmonary valve.
3. Align the Doppler beam as parallel as possible to blood flow. A Doppler angle below 20° is a useful practical target.
4. Optimize the spectral Doppler scale and gain to obtain a clear envelope.
5. Trace the outer border of the complete systolic Doppler envelope.
6. Measure several representative beats and average the results when appropriate, particularly in patients with irregular rhythms.

What Does VTI Represent?
VTI is the integral of blood velocity over the ejection period. It represents the distance travelled by the blood column during one cardiac cycle's systolic ejection and is expressed in centimeters.

Common Sources of Error

• Incorrect sample-volume position.
• Poor Doppler alignment with blood flow.
• Incomplete tracing of the Doppler envelope.
• Inadequate spectral Doppler signal quality.
• Failure to account for beat-to-beat variability in irregular rhythms.

4. Calculate Pulmonary Stroke Volume

After obtaining the pulmonary annular area and pulmonary VTI, multiply the two measurements to calculate pulmonary stroke volume.

Pulmonary Stroke Volume = Area × VTI

Qp per beat = π × (D/2)² × VTI

Worked Example

Assume the following measurements:

• Pulmonary valve annulus diameter = 2.0 cm
• Pulmonary valve annular area = 3.14 cm²
• Pulmonary VTI = 20 cm

Therefore:

Pulmonary stroke volume = 3.14 × 20

Pulmonary stroke volume = 62.8 mL/beat

This represents the estimated volume of blood ejected into the pulmonary artery with each heartbeat.

Terminology Note: Strictly speaking, the product of annular area and VTI gives pulmonary stroke volume in mL/beat. Pulmonary blood flow expressed as a volume per minute is calculated by multiplying stroke volume by heart rate.

Pulmonary blood flow (mL/min) = Pulmonary stroke volume (mL/beat) × Heart rate (beats/min)

5. Understanding Qp and the Qp:Qs Ratio

In clinical echocardiography, Qp is commonly used to describe pulmonary blood flow, while Qs represents systemic blood flow. The ratio of pulmonary to systemic blood flow is particularly useful when assessing intracardiac shunts.

Qp:Qs = Pulmonary blood flow / Systemic blood flow

• Qp:Qs = 1:1: Pulmonary and systemic blood flow are approximately equal.
• Qp:Qs > 1: Pulmonary blood flow exceeds systemic blood flow, as may occur with a significant left-to-right shunt.
• Qp:Qs < 1: Systemic blood flow exceeds pulmonary blood flow, as may occur with a right-to-left shunt.

A Qp:Qs ratio near 1 does not exclude every type of shunt or haemodynamic abnormality. The findings must be interpreted in the context of the clinical presentation, cardiac anatomy, and other echocardiographic measurements.

6. Important Clinical Considerations

• Diameter measurement matters: Errors in annular diameter are magnified because the diameter is squared in the area calculation.
• Use consistent measurement locations: The diameter and VTI should represent corresponding flow through the same anatomical level as closely as possible.
• Consider valve disease: Pulmonary stenosis, significant pulmonary regurgitation, and abnormal RVOT anatomy may affect interpretation.
• Assess rhythm: In atrial fibrillation or frequent ectopy, select representative beats and average measurements as appropriate.
• Integrate the findings: Interpret calculated flow alongside chamber dimensions, ventricular function, Doppler findings, and other relevant haemodynamic data.
• Recognize limitations: Errors in Doppler alignment, annular geometry, and measurement reproducibility can influence the calculated result.

7. Quick Revision: Pulmonary Blood Flow Measurement

Step	Measurement	Key Point
1	Pulmonary annulus diameter	PSAX view; inner edge to inner edge during mid-systole
2	Annular area	π × (D/2)²
3	Pulmonary VTI	PW Doppler in the RVOT, just proximal to the pulmonary valve
4	Pulmonary stroke volume	Annular area × pulmonary VTI
5	Pulmonary blood flow per minute	Stroke volume × heart rate

Conclusion

Estimating pulmonary blood flow by echocardiography requires accurate measurement of the pulmonary valve annulus diameter and pulmonary VTI. Multiplying the calculated annular area by VTI provides pulmonary stroke volume, while multiplying stroke volume by heart rate gives pulmonary blood flow per minute.

Careful image acquisition, correct Doppler sample placement, and appropriate interpretation are essential for reliable results. When evaluating suspected intracardiac shunts, pulmonary flow should be considered alongside systemic flow to estimate the Qp:Qs ratio.

References and Further Reading

1. Mitchell C, Rahko PS, Blauwet LA, et al. Guidelines for Performing a Comprehensive Transthoracic Echocardiographic Examination in Adults: Recommendations from the American Society of Echocardiography. Journal of the American Society of Echocardiography. 2019;32:1–64.
2. Rudski LG, Lai WW, Afilalo J, et al. Guidelines for the Echocardiographic Assessment of the Right Heart in Adults. Journal of the American Society of Echocardiography. 2010;23:685–713.

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