How to Calculate PASP on an Echo: A Deep Dive

So, you need to calculate Pulmonary Artery Systolic Pressure (PASP) using an echocardiogram. Let’s cut to the chase: PASP is typically estimated using the modified Bernoulli equation applied to the tricuspid regurgitation (TR) jet. Specifically, PASP = 4(TR velocity)^2 + Right Atrial Pressure (RAP). This estimation relies on Doppler measurements obtained during the echo. Accurate assessment hinges on skillful acquisition of the TR jet and a reliable estimation of RAP. We’ll unpack each component of this calculation meticulously.

Decoding the PASP Calculation

The beauty, and potential pitfall, of estimating PASP from an echocardiogram lies in its indirect nature. We’re not directly measuring pulmonary artery pressure, but rather inferring it from the velocity of blood leaking backwards through a leaky tricuspid valve (TR).

Understanding the Modified Bernoulli Equation

The core of the PASP calculation is the modified Bernoulli equation:

PASP = 4(TR velocity)^2 + RAP

Let’s break down each component:

• TR Velocity: This is the peak velocity of the tricuspid regurgitation jet measured using continuous-wave Doppler. The higher the velocity, the greater the pressure difference between the right ventricle and right atrium, and thus the higher the estimated PASP. Accurate measurement of this peak velocity is critical. Underestimating the peak velocity will lead to underestimation of PASP.

• (TR Velocity)^2: This is the peak velocity squared. This highlights the exponential relationship between the velocity and the pressure gradient.

• 4(TR Velocity)^2: This calculation represents the pressure gradient across the tricuspid valve. This gradient reflects the pressure difference between the right ventricle and the right atrium during systole.

• RAP (Right Atrial Pressure): This is an estimation of the pressure within the right atrium. This is arguably the weakest link in the chain, as it relies on indirect assessment of inferior vena cava (IVC) diameter and its respiratory variation.

Estimating Right Atrial Pressure (RAP)

The RAP estimation is where the art meets the science of echocardiography. We rely on assessing the IVC diameter and its collapsibility during respiration. Here’s a widely used, though not perfect, guideline:

• IVC Diameter ≤ 2.1 cm AND >50% collapse with respiration: RAP is estimated at 3 mmHg (Range: 0-5 mmHg).

• IVC Diameter > 2.1 cm AND <50% collapse with respiration: RAP is estimated at 15 mmHg (Range: 10-20 mmHg).

• IVC Diameter ≤ 2.1 cm OR >50% collapse with respiration: RAP is estimated at 8 mmHg (Range: 5-10 mmHg). Use this when the IVC diameter and collapsibility are discordant (e.g., small IVC but doesn’t collapse well).

Important Considerations for RAP Estimation:

• Ventilated Patients: In patients on mechanical ventilation, IVC assessment for RAP estimation becomes unreliable due to altered intrathoracic pressure dynamics.

• Tricuspid Regurgitation Severity: Severe TR can falsely elevate RAP estimates.

• Other Conditions: Ascites, constrictive pericarditis, and right heart failure can impact IVC dynamics and RAP estimation.

Putting It All Together: Example Calculation

Let’s say we measure a TR velocity of 3 m/s and estimate RAP to be 5 mmHg.

PASP = 4(3 m/s)^2 + 5 mmHg

PASP = 4(9) + 5 mmHg

PASP = 36 + 5 mmHg

PASP = 41 mmHg

Caveats and Limitations

It’s crucial to remember that this PASP calculation is an estimation, not a direct measurement. Several factors can impact accuracy:

• Image Quality: Poor image quality can hinder accurate TR jet visualization and velocity measurement.

• Angle Dependency: Doppler measurements are angle-dependent. The ultrasound beam should be aligned as parallel as possible to the TR jet to avoid underestimation of the velocity.

• Tricuspid Stenosis: If tricuspid stenosis is present, the PASP estimation will be less accurate.

• Pulmonary Stenosis: Co-existing pulmonary stenosis also affects the accuracy of PASP.

• Technical Expertise: Accurate TR jet acquisition requires skill and experience.

Frequently Asked Questions (FAQs)

1. What is considered a normal PASP range?

While ranges may vary slightly between labs, generally, a PASP less than 36 mmHg is considered normal. PASP between 36 and 50 mmHg is considered borderline or mild pulmonary hypertension. PASP above 50 mmHg is considered moderate to severe pulmonary hypertension. Remember to interpret the PASP within the clinical context.

2. Can you have pulmonary hypertension with a normal PASP on echo?

Yes, it’s possible. The echo provides an estimate of PASP at rest. Patients with exercise-induced pulmonary hypertension may have a normal PASP at rest but elevated PASP during exercise. A right heart catheterization is the gold standard for diagnosing pulmonary hypertension and measuring pulmonary artery pressures directly.

3. What other echo findings suggest pulmonary hypertension?

Besides elevated PASP, other echocardiographic findings suggestive of pulmonary hypertension include right ventricular enlargement, right ventricular dysfunction, flattened interventricular septum, dilated pulmonary artery, and increased tricuspid regurgitation velocity.

4. How accurate is PASP estimation compared to right heart catheterization?

The accuracy of PASP estimation varies. While echo is a valuable screening tool, it can underestimate or overestimate PASP compared to right heart catheterization. A meta-analysis can show the correlation between the echo estimation and the gold standard measurement. Right heart catheterization remains the gold standard for diagnosing pulmonary hypertension and should be considered when clinical suspicion is high despite a normal or borderline PASP on echo.

5. What if I can’t find a tricuspid regurgitation jet?

The absence of a TR jet makes PASP estimation impossible. In such cases, other findings suggestive of pulmonary hypertension should be carefully evaluated. Clinical suspicion should guide further investigation, including considering a right heart catheterization if appropriate.

6. How does pulmonary regurgitation affect PASP estimation?

Pulmonary regurgitation (PR) does not directly affect the PASP calculation. However, it provides an opportunity to estimate pulmonary artery diastolic pressure (PADP). The modified Bernoulli equation can be applied to the peak PR velocity to estimate the pressure difference between the pulmonary artery and the right ventricle during diastole. Subtracting this gradient from the estimated right ventricular diastolic pressure (often assumed to be similar to RAP) yields an estimation of PADP.

7. What are the common pitfalls in PASP estimation?

Common pitfalls include underestimation of TR velocity due to poor image quality or angle dependency, inaccurate RAP estimation, and failure to consider coexisting cardiac conditions. Always correlate the echocardiographic findings with the clinical context.

8. How does COPD affect PASP estimation?

Chronic obstructive pulmonary disease (COPD) can increase the risk of pulmonary hypertension. However, COPD can also make accurate echocardiographic assessment more challenging due to hyperinflation of the lungs and poor acoustic windows.

9. Is PASP estimation useful in children?

Yes, PASP estimation is useful in children, but normal values may differ based on age and size. Pediatric cardiologists have specific reference ranges. The same limitations regarding accuracy apply as in adults.

10. Can medications affect PASP?

Yes, certain medications can affect PASP. Pulmonary vasodilators (e.g., sildenafil, tadalafil) can lower PASP, while some vasoconstrictors can increase it. It’s important to consider a patient’s medication list when interpreting PASP results.

11. How do I document the PASP estimation in my echo report?

Clearly document the TR velocity, the method used for RAP estimation (IVC diameter and collapsibility), the estimated RAP value, and the calculated PASP. Also, mention any limitations encountered during the study.

12. What’s the future of PASP assessment on echo?

Advances in echocardiographic technology, such as 3D echocardiography and strain imaging, may improve the accuracy and reliability of PASP assessment in the future. These techniques offer the potential to better assess right ventricular function and pulmonary artery pressures. Machine learning algorithms can potentially assist in the accuracy of PASP estimation.