From the Guidelines
When there is right ventricular outflow tract (RVOT) obstruction, the standard Bernoulli equation must be modified to calculate pulmonary artery systolic pressure (PASP) as PASP = Right ventricular systolic pressure (RVSP) - Pressure gradient across the RVOT. This differs from the typical PASP calculation (PASP = RVSP = 4v² + RAP, where v is tricuspid regurgitation velocity and RAP is right atrial pressure) because RVOT obstruction creates a pressure difference between the right ventricle and pulmonary artery, as noted in the 2018 AHA/ACC guideline for the management of adults with congenital heart disease 1. To apply this in practice, first measure RVSP using tricuspid regurgitation velocity (RVSP = 4v² + RAP), then measure the pressure gradient across the RVOT obstruction using the modified Bernoulli equation (4v²), where v is the peak velocity across the RVOT, which can be categorized as mild (<36 mm Hg or <3 m/s), moderate (36-64 mm Hg or 3-4 m/s), or severe (≥64 mm Hg or >4 m/s) 1. Finally, subtract this gradient from RVSP to obtain the true PASP. This calculation is essential because failing to account for RVOT obstruction would lead to overestimation of pulmonary artery pressure, potentially resulting in incorrect clinical decisions regarding pulmonary hypertension management. Key points to consider in the assessment of RVOT obstruction include:
- Estimations of RV systolic pressure by TR velocity as part of the echocardiographic assessment of RV obstruction, since Doppler measurements across the RV obstruction itself may be unreliable 1
- The severity of RVOT obstruction, which is crucial for determining the appropriate management strategy.
From the Research
Calculation of Pulmonary Artery Systolic Pressure (PASP)
To calculate PASP in the presence of Right Ventricular Outflow Tract (RVOT) obstruction, several methods can be used. The following are some of the equations and methods that have been studied:
- The study 2 found that combining myocardial isovolumic relaxation time (IVRT) and isovolumic contraction velocity (IVCV) in a formula can predict PASP accurately.
- The study 3 derived equations to estimate pulmonary vascular resistance (PVR) using the ratio of estimated pulmonary artery systolic pressure (PASPDoppler) to right ventricular outflow tract velocity time integral (VTI). Although this study does not provide a direct equation for PASP, it highlights the importance of considering RVOT measurements in estimating PASP.
- The study 4 compared three different formulas for Doppler estimation of PASP, including:
- Method 1: Doppler gradient + 10 mm Hg
- Method 2: Right ventricular-right atrial Doppler gradient x 1.1 + 14
- Method 3: Doppler gradient + 6,9, or 16 mm Hg based on the inferior vena cava collapsibility index (IVCCI) These methods can be used to estimate PASP, but the study found that the correlation between Doppler and catheter measurements of PASP was similar with the three methods.
RVOT Obstruction Considerations
In the presence of RVOT obstruction, the calculation of PASP may be more complex. The study 5 found that the timing of onset and peak of the RVOT systolic spectral signal appears to be useful in characterizing the severity of PASP. However, more studies are needed to determine the clinical utility of prospectively measuring RVOT in patients with RVOT obstruction.
Limitations and Future Studies
While these studies provide some insights into the calculation of PASP, more research is needed to fully understand the relationship between RVOT obstruction and PASP. The studies 2, 3, 5, 6, 4 highlight the importance of considering multiple factors, including RVOT measurements, IVCCI, and myocardial velocities, in estimating PASP. However, the optimal method for calculating PASP in the presence of RVOT obstruction remains unclear and requires further investigation.