Measurement of pulmonary vascular resistance (PVR) is traditionally done by cardiac catheterization. It remains the (un)disputed gold standard, despite numerous assumptions, errors in measurement, and lack of reproducibility.
PVR by Echocardiography
Recently, echocardiographic calculation of PVR gained importance. Resistance is pressure divided by flow. Pressure is measured by Doppler, flow is measured by the cross-sectional area of RVOT times the TVI. We can arrive at PVR quickly. As simple as that.
Still, many institutions and purists ( Who have huge trust in cath derived, Oxygen diluted data) ) won’t accept this as standard . They fail to realise echo methodology carries less limitation, if not similar limitations as in traditional cath method. However, a significant advantage is, it is more real -time, can be repeated any number of times, and documents a baseline PVR, and at least is useful for follow-up.
There are two formulas used.
1.Abbas Formula
PVR = (TRV / RVOT VTI) × 10 + 0.16
2.Haddad formula :
PVR= (TRV²/VTI_RVOT)
*TRV = Tricuspid Regurgitation Velocity (m/s)
*RVOT VTI = Right Ventricular Outflow Tract Velocity Time Integral (cm)
Comparing Abbas vs Haddad
The Abbas formula is better validated and widely used in clinical practice, as it was specifically designed to correlate with catheter-based pulmonary vascular resistance measurements. It provides reasonably accurate estimates, especially for screening pulmonary hypertension, though it tends to under-estimate PVR at higher values (>8–10 Wood units).
In contrast, the Haddad formula ) is simpler but less rigorously validated and is more commonly applied in research settings focused on right ventricular-pulmonary artery coupling rather than direct PVR estimation.
Haddad’s method may be less reliable in patients with significant tricuspid or pulmonary valve abnormalities. Therefore, Abbas remains the preferred formula for routine clinical application. There is still hope to improve Haddad equation.
How to improve upon Haddad equation* ?
The Haddad equation for estimating PVR (TRV²/RVOT VTI) lacks calibration and overlooks key hemodynamic variables. It can be improved by introducing empirically derived correction factors to correlate with catheter-based values. Incorporating right atrial pressure (RAP), RV functional indices like TAPSE or RV strain, and adjusting for heart rate or rhythm variability can enhance accuracy. Averaging VTI across multiple cardiac cycles could also stabilize measurements. Additionally, machine learning on large datasets and AI-enhanced model could outperform the current linear Haddad formula for non-invasive PVR estimation.
* This is a fresh area of study , young fellows should come forward to do.
Final message
As discussed earlier, Abbas remains the preferred formula. But, the real issue is cardiologists refusing to accept any Echo-derived PVR and incorporate it, in the day to day practice. We have accepted EF % as the gold standard for LV function in spite of some serious lacunae. PVR carries the same story. Cath-derived data, in all likelihood, is enjoying pseudo-sanctity. It is time we should embrace one of these Echo formulas regularly and make life simple for both ourselves and the patients (who are often tiny babies or children). I think it can be done without compromise on scientific purity.
Reference
Dr.S.Venkatesan MD 