This study demonstrates that the index of mPAPEcho / LVIDd represents a simpler and better method of estimating markedly elevated PVR in pre-capillary PH patients.
A number of echocardiographic methods have been proposed for the noninvasive estimation of PVR [1–2, 9–12]. However, most methods acquired formulas containing various empirical constants, which would affect the accuracy of the estimating PVR results. The guidelines of the American Society of Echocardiography recommended the noninvasive estimation of PVR should not be used as a substitute for invasive evaluation of PVR. However, pulmonary vascular resistance still needs to be evaluated in PH patients, especially continuous preoperative and postoperative monitoring in IPAH and CTEPH patients. PVR higher than 1200 dyn·s·cm− 5 (15WU) is considered to be severe and is correlated with an increased risk of post-operative mortality [13–14]. We proposed a simple method to help clinically identify patients with extremely high PVR values (> 15 WU) instead of generating a regression equation compared to previously reported methods.
Our results showed that both Abbas et al  and our new methods correlate similarly for patients with lower PVR values (PVR ≤ 15 WU). However, for PVR > 15 WU, the correlation appeared stronger with mPAPEcho / LVIDd compared with TRV2/TVIRVOT. Moreover, the AUC of mPAPEcho / LVIDd is higher than the AUC of TRV2 /TVIRVOT to determine PVR > 15 WU. The new method has its own advantages. The underlying explanations are as following:
The echocardiographic estimation of mean PAP is more consistent with PVR formula. The mean PAP reflects the driving pressure that is required for pulmonary blood flow more accurately than systolic PAP. PR is reported to occur in almost 75% of the population . Dilation of the annulus and main pulmonary artery are usually accompanied by PH patients. PR secondary to PH is common, thus it is easy to obtain the complete PR Doppler signal. It has been reported that the early-diastolic PA-RV pressure gradients derived from the peak early-diastolic PR velocity is useful for estimating the mean PA pressure [3, 16–17]. In our study, TR could be obtained in 117 of 129 patients (90%). Among these patients, 10 had severe TR, 41 had not well-defined TR signal quality, which may lead to overestimation or underestimation of pulmonary arterial pressure [18–20]. Abbas et al. method [1–2] and Haddad et al. method using TRV to calculate systolic PAP and Lindquist et al. method using TRV to calculate mean PAP may be inaccurate in patients under these circumstances. Extremely severe PR is rare in patients with PH. Therefore, we consider that the mean PAP estimated by PR Doppler signal is more reliable when PR is present. TRV can be measured to estimate systolic PAP when PR is absent.
Compared to the method of Abbas et al  and Kaga et al , the new method also takes into account the contribution of RAP to estimate mPAP. Current guidelines have suggested that RAP predicts prognosis of PAH . For patients with PVR > 15 WU, RAP will also be higher. So, the estimated RAP will contribute more to mPAPEcho in patients with PVR > 15 WU. Although Haddad et al.  took RAP into account, according to the size and collapse index of the IVC, RAP was estimated to be 10 mmHg, 15mmHg, or 20mmHg, which would lead to overestimation of RAP.
PH results in the reduced of pulmonary blood flow, which also causes the reduced of the blood flow back to the left atrial and ventricular. In addition, there is a leftward displacement of the interventricular septum during diastole of LV in sever PH patients, due to the presence of right ventricular pressure overload [21–22], which will further lead to the compression of the LV cavity. Grzegorz et al.  reported that LVEDV in IPAH patients correlated inversely with PVR and LVEDV is reduced in PH patients in proportion to reduced pulmonary flow. Therefore, LVEDV represents pulmonary blood flow in pre-capillary PH and is significantly reduced in patients with extremely high PVR. LVIDd, as an alternative indicator of LVEDV, is easier to obtain and more suitable for routine applications. Moreover, LVIDd had better repeatability and accuracy than TVIROVT. In patients with PH, the dilatation of main pulmonary artery caused pulmonary valve to move forward. When obtaining the flow velocity in the RV outflow tract, the Doppler sampling line had a larger angle with the direction of blood flow, which would affect the accuracy of the measurement of TVIROVT. In addition, when the velocity of the RV outflow tract was low, the contour of the spectrum was not clearly displayed, which would affect the repeatability of the measurement.
Our novel index has utility for predicting patients with markedly elevated PVR, which is of great value in etiological diagnosis, disease severity and continuous monitoring of PVR before and after treatment.