DISCUSSION
In this study, we have showed that BSA-index of left atrial maximum volume (LAVImax) and left atrial conduit strain (LAScd) measured by 4D auto LAQ were useful noninvasive parameters in distinguishing pre- and post-capillary PH: LAVImax and LAScd were higher in post-capillary PH group, and their cutoff values were 35.69ml/m2(sensitivity 86%, specificity74%) and -9% (sensitivity 80%, specificity 70%).
In our studies, most patients in post-capillary PH group were belong to group 2 PH according to the NICE classification, along with higher serum NT-Pro BNP, hemoglobin and HbA1c, which were in line with previous studies[1, 6-8, 18]. It was indicated that the clinical diagnosis and grouping were reliable, although the PASP and PAWP were estimated by echocardiography in our studies. Both groups had similarly structure and function of right heart chamber, with enlarged PA and RA, right ventricular-pulmonary artery uncoupling and decreasing RV systolic function. However, the LVEF was lower, while LVMI and E/e’ were higher in post-capillary PH group, which indicating that the LV systolic and diastolic function were depressed. These findings supported that post-capillary PH predominantly classified into group 2 PH in our study.
As we know, LA size is a maker of LV filling pressure and diastolic function. The LA and the pulmonary veins and venules are the proximal part of the pulmonary circulation. Therefore, regardless of any cause in LA pressure rising, it would be reflected on the pulmonary venous circulation pressure, and following on the PAWP. Continuous increasing of LA pressure could cause LA and LV enlargement, myocardial stretch and significant rise of stiffness[19, 20]. Consequently, LA volume and strain were important in differentiating pre- and post-capillary PH except for PAWP. Recent studies had showed that LA maximum area access by CT, LA maximum volume index evaluated by MRI and echocardiography were useful parameters for distinguishing pre- and post-capillary PH[8, 21, 22]. These were consistent with our results, which demonstrated that LAVImax measured by 4D auto LAQ was a valuable indication. However, Csaba et al. showed that LAVImin measured by transthoracic 3DE discriminated better than LAVImax between pre- and post-capillary PH[6]. In that paper, the software of 3DE were not specifically dedicated to 3D left atrial measurements, and the strain of LA wasn’t taken into for analysis, which may explain the reason inconsistency with our study. Besides, when analysing the ability of LAVImin for differentiating pre- and post-capillary PH, the AUC was also high by ROC analysis (AUC=0.844, P <0.001), yet LAVImin wasn’t an independent factor for differentiating both groups when adding the LA strain parameters into multivariate logistic regression analysis.
4D auto LAQ is specially designed for LA 3D analysis which could determine the LA volume of different periods in diastole as well as LA strains conveniently and highly reproducibly. In our study, it was demonstrated that LAScd was lower, while ePLAGS were higher in post-capillary PH, and they were also useful parameters for distinguishing pre- and post-capillary PH. Ashwin et al. also found that ePLAGS assessed by 2D echocardiography accurately differentiated pre-capillary from post-capillary PH[7], and this was in line with our study. On the basis of Frank Starling law, the enlargement of LA is associated with myocardial thinning, restrained contraction function and hence compromised deformation. We found that LASr and LAScd were obviously decreased in patients of post-capillary PH in our study, which could be seen as directly reflecting LA myocardial dysfunction, in adhering with previous study that LA Strain was the strongest predictor of cavity pressure.[23] Nevertheless, LAScd was one of the most independent factors for differentiating pre- and post-capillary PH by regress analysis. Maybe because the LA conduit strain connected reservoir strain and LA contraction strain, and it can also be reflected the LV filling pressure as well as LASr. In addition, LA storage and channel function (DEI and PEI) were higher in post-capillary PH group, but LA active systolic function (AEI) was not in this study, which could be evidenced this suppose.
Nevertheless, there are a few limitations. Firstly, this was a single-center study with a relatively small sample size. Secondly, the PASP and PAWP were measured from echocardiography and not obtained by invasively, while these noninvasive parameters were significantly correlated with invasive measurements[2]. Finally, we didn’t analyse the variability of parameters of LA structure, function and stain between 4D auto LAQ and 2D echocardiography. LA myocardial function assessed by 2D strain analysis is much more complex than that of the thicker LV, so LA volume and function should be calculated with another software.