BPA is an effective treatment for patients with CTEPH. Several studies have shown that LP evaluated by SPECT was significantly improved after BPA [15–17]. However, LP in some lung segments was not improved after multiple BPA sessions. And there is a lack of studies exploring the factors affecting the perfusion improvement after BPA.
Previous expert documents recommended that the pulmonary vessels located in the lower lobe should be preferred when selecting BPA-targeted vessels [7], but this recommendation is mainly based on clinical experience and lacks the support of research data. When exploring the efficacy of BPA, Wang et al. found that the improvement of pulmonary blood flow in the upper lobe was the least after BPA[14]. However, this study did not explore the relationship between the characteristics of target vessels and postoperative segmental perfusion improvement. Other studies have also found that the therapeutic response of the upper lobe is not as good as that of the middle or lower lobe through other methods. Schoenfeld et al. used MRI to quantify the pulmonary blood flow of each lobe, and the results showed that the improvement of blood flow in the upper lobe was the least significant [18]. Akizuki et al. divided the lung into two parts: the upper-middle lung field and the lower lung field. All patients received two BPA series. In the first series, BPA was performed several times on one lung field. In the second series, BPA was performed several times on the other lung field, and the postoperative efficacy of each series was explored. It was found that the hemodynamic improvement was lighter for BPA in the upper-middle lung field [19]. And the present study was found that the perfusion of vessels located in the upper lobe were least likely to improve after BPA. As for why the distribution of lung lobes affects the efficacy of BPA, the mechanism is still unclear. First, it is speculated that this may be due to the influence of gravity, the improved pulmonary blood flow after treatment is mainly distributed in the lower lung tissue, resulting in the postoperative perfusion of pulmonary vessels in the upper lobe is not easy to improve. Second, during BPA operation, the catheter was difficult to enter the upper lobe pulmonary vessels and could not provide support for balloon dilation, which might lead to poor effect on upper lobe pulmonary vessels.
In addition, this study found that the vessels with less damaged perfusion at baseline was more difficult to improve after operation. Previous studies have also suggested that the vessels with the poorest perfusion should be preferentially selected for BPA [20]. This study provided data support for this. A number of studies have shown that the pathological mechanism of CTEPH includes not only the mechanical obstruction of thrombus, but also peripheral microvasculopathy [21, 22]. Since BPA mainly treats segmental or sub-segmental lesions, there may still be microvascular lesions after surgery, resulting in no significant perfusion improvement in vessels with mild perfusion impairment at baseline.
Previous studies also recommended to select target vessels according to the PAG classification, which is mainly based on the correlation between PAG classification and the success and complication rate of BPA [7, 13]. With the accumulation of experience and the progress of technology, the safety of BPA has been gradually improved. However, there is still a lack of research on the improvement of perfusion after BPA in different PAG types of vessels. This study showed that there was no significant difference in the perfusion improvement of the four types of vessels after BPA. However, Gerges et al. found that the number of successfully treated occlusion lesions had a greater impact on postoperative mPAP and PVR changes than non-occlusion lesions [23]. This was different from the results of this study, which might be related to the small number of occlusions included in this study. Therefore, the relationship between the lesion morphology and postoperative perfusion improvement still needs further exploration.
Due to the physiologically more distribution of blood flow in the right lung, previous studies suggested that lesions located in the right lung should be preferentially selected as the target vessels [20]. However, this study found that the lesion located in the right or the left lung had no significant effect on the perfusion improvement after BPA. Since this was a study in a single center and the number of patients included was small, the relationship between the location of vessels in the left or right lung and the perfusion improvement of them after BPA remains to be explored.
In order to better predict the perfusion improvement of vessels after BPA, the present study established a new scoring system based on the above results by combining the pulmonary lobe where the vessels were located and the PDs at baseline. The results showed that the perfusion improvement rate of the vessels increased significantly with the decrease of the score, indicating that the scoring system could predict the perfusion improvement of the vessels after BPA more effectively than a single factor. Due to the low improvement rate of perfusion after BPA, the lesions located in the upper lobe with mild perfusion injury were not suitable as the target vessels of BPA.
Several limitations should be acknowledged. First, this was a single-center study with a small number of patients, and a study with larger population is needed to verify the reliability of the scoring system proposed in this study. Second, early inexperience might have affected the effectiveness of early BPA procedure. Third, the BPA procedures of intervening leaf-level pulmonary arteries were not included in this study, these processes might affect the perfusion changes of some segments.