This study was performed at the Department of Cardiovascular Medicine of the Second Xiangya Hospital, Central South University, China. Two-hundred consecutive CHD patients diagnosed with PAH (PAH-CHD group) from July 2017 to June 2019 were enrolled. The inclusion criteria were: 1) a diagnosis of PAH, defined as an mPAP ≥25 mmHg, pulmonary artery wedge pressure (PAWP) ≤15 mmHg, and PVR ≥3 Wood; 2) all measurements were performed via right heart catheterization (RHC). The exclusion criteria were: 1) any other cause of PH, including connective tissue disease, left heart disease, lung disease and/or hypoxia, and chronic thromboembolic disease; 2) patients with gout, liver disease, kidney disease, hypertension, coronary heart disease, an age <18 years old, missing data, a history of CHD surgery, or prior use of PAH-specific drugs. Twenty-five CHD patients with normal pulmonary arterial pressures (non-PAH-CHD group) and 40 healthy adults (normal group) were used as controls. The inclusion criteria for the healthy adult group were age older than 18 years old and no history of hypertension, obesity, diabetes, coronary heart disease, stroke, or other cardiovascular disease. The study protocol was approved by the ethics committee of The Second Xiangya Hospital of Central South University, and all participants provided informed consent before enrollment. The flow chart for this study was described in Figure 1.

Baseline clinical information, including gender, age, body mass index (BMI), systolic blood pressure (SBP), diastolic blood pressure (DBP), arterial oxygen saturation (SaO₂), heart rate, WHO functional class, and previous medical history were collected. Blood samples were drawn from the unilateral cubital vein after an overnight fast and prior to RHC. Medications, including diuretics, cardiac inotropes, and targeted drugs, were initiated based on an adequate risk status assessment. This protocol was implemented to reduce the influence of medication regimen on baseline serum UA level to the greatest extent possible. A complete blood count was analyzed using a SYSMEX Blood Analyzer XN10 (Kobe, Japan). Serum UA levels were measured using the uricase-peroxidase method. Other biochemical parameters, including creatinine (Cr), alanine aminotransferase (ALT), aspartate aminotransferase (AST), direct bilirubin (DBIL), total bilirubin (TBIL), D-Dimer, and NT-proBNP, were analyzed using a Hitachi Automatic Biochemical Analyzer 7,600-020 (Tokyo, Japan) in the laboratory at The Second Xiangya Hospital of Central South University. The reference range for UA was 142.0–416.0 μmol/L.

An experienced cardiologist performed transthoracic echocardiography (TTE) on all patients using a commercially available device (Vivid 7, Vingmed, GE, United States of America). Using left ventricular (LV) long axis, LV short axis, and apical 4-chamber views, we evaluated and recorded the right atrial end-systolic diameter (RAS), right ventricular end-diastolic diameter (RVD), internal diameter of the pulmonary artery (PA), aortic inner diameter (AO), and left ventricular ejection fraction (LVEF). Systolic tricuspid regurgitation velocity (TRV) was calculated using continuous-wave Doppler echocardiography, and the peak TRV was used to estimate the peak tricuspid regurgitation pressure gradient (PTR) using a simplified Bernoulli equation. Systolic PA pressure (sPAP) was estimated by adding PTR to the estimated right atrial pressure based on the size and change in diameter of the inferior vena cava during respiration.

With the exception of healthy subjects, all patients underwent RHC via their right femoral vein at the Interventional Catheterization Centre of The Second Xiangya Hospital of Central South University. We used a Siemens Axiom Artis X-ray device (SIEMENS Co., Munich, Germany). Baseline haemodynamic parameters, including systolic pulmonary arterial pressure (SPAP), diastolic pulmonary arterial pressure (DPAP), mPAP, mean right atrial pressure (mRAP), PAWP, and oximetry samples of various heart chambers, were measured using a Swan-Ganz catheter. Cardiac index (CI) was calculated as CO based on Fick^’s principle divided by body surface area. PVR was calculated using the following formula: (mPAP-PAWP)/CO. We used Wood units as the units for PVR. ES was defined as Qp/Qs < 1.0 and PVR >10 Wood. All studied patients were diagnosed with PAH-CHD via RHC according to the following standard criteria: an mPAP≥25 mmHg and PVR>3 Wood units at rest in the presence of a normal PAWP (≤15 mmHg).

The pre-treatment risk assessment for the 200 PAH-CHD patients was based on the comprehensive risk stratification recommended by the European Heart Journal in 2018 (Kylhammar et al., 2018). This risk assessment strategy includes eight variables: World Health Organization functional class (FC), 6-min walking distance (6MWD), CI, right atrial pressure, NT-proBNP, mixed venous oxygen saturation (SvO₂), right atrial area, and the presence of a pericardial effusion. All of the aforementioned variables were a part of the risk assessment instrument proposed by the ESC/ERS 2015 guidelines (Galiè et al., 2015). Each variable was categorized as low, intermediate, or high risk based on pre-specified values and given one, two, or three points, respectively. We then divided the sum of all grades by the number of available variables for each patient, resulting in a mean grade. The risk group for each patient was defined by the mean grade rounded off to the nearest integer. PAH-CHD patients were categorized into three risk groups: low (PAH-CHD-L group), intermediate (PAH-CHD-M group), or high (PAH-CHD-H group).

Surgical indications at our center were a pulmonary-to-systemic flow ratio (Qp/Qs) ≥1.5 and a PVR ≤ 3Wood. The 138 PAH-CHD patients who were not candidates for transcatheter closure or surgery received individualized treatments. In addition to conventional treatment, including oxygen, diuretics, or digitalis, patients with a positive acute pulmonary vasodilator test received a calcium channel blocker. Patients with a negative acute pulmonary vasodilator test received at least one disease-specific drug, such as an endothelin receptor antagonist (ERA), phosphodiesterase type 5 inhibitor (PDE-5i), or prostacyclin analogue (PGI₂). We followed all PAH-CHD patients who received PAH-specific drug therapies. Twenty PAH-CHD patients underwent repeated RHC after an average follow-up period of 7 ± 1 month. Additional information, including biochemical indicators and echocardiographic parameters, were also collected for these patients.

Statistical analyses were performed using the Statistical Package for Social Science version 22.0 for Windows (SPSS Inc., Chicago, IL, United States). Quantitative data (Clinical features, biochemical indicators, echocardiography parameters, and haemodynamic variables) were described as means and standard deviations (normal or approximately normal distributions) or medians and interquartile ranges (IQR) (non-normal distributions). Normally distributed data were compared using a one-way analysis of variance for repeated measurement data, and non-normal distributions were compared using Friedman’s M test. Qualitative data and ranked data (WHO FC) were described as a number and percentage. Ranked data were compared using Friedman’s M test, and qualitative data were compared using Cochran’s Q test. Correlation coefficients between two variables were calculated using Pearson^’s correlation. A receiver operating characteristic (ROC) curve was used to confirm the serum UA level that provided the best diagnostic significance for intermediate to high-risk PAH-CHD patients. A paired t-test was performed to compare baseline and post-treatment echocardiographic, haemodynamic, and serological features. Two-sided p-values less than 0.05 were considered statistically significant in all analyses.

This study included 200 PAH-CHD patients (mean age 37.7 ± 14.1 years; range 15–72 years; 145 female). The PAH-CHD-L group had 94 patients (47.0%), the PAH-CHD-M group 88 (44.0%), and the PAH-CHD-H group 18 (9.0%). All PAH-CHD patients received standard therapy with diuretics. Compared with the nPAH-CHD group and the control group, the body weight and BMI of the PAH-CHD group were lower (p < 0.05). There was no statistically significant difference in BMI between the low, medium, and high-risk patients in the PAH-CHD group (p > 0.05). There were no significant differences in baseline ALT, AST, total bile acid (TBA), blood Urea Nitrogen (BUN), Cr, international standardized ratio (INR), and D-Dimer levels between groups. Airect bilirubin (DBIL), NT-proBNP, and platelet levels were higher in the PAH-CHD group than in the normal control and nPAH-CHD groups (p < 0.05). Subgroup analysis showed that NT-proBNP and platelet levels in the PAH-CHD-M and PAH-CHD-H groups were significantly higher than in the PAH-CHD-L group (p < 0.05), but there was no statistical difference between the two groups (p > 0.05). The red blood cell distribution width (RDW) in the PAH-CHD group was equivalent to that of the non-PAH-CHD and control groups (p > 0.05). However, our subgroup analysis showed that the RDW level of the PAH-CHD-H group was higher than that of the PAH-CHD-M group (p < 0.05), but equivalent to the PAH-CHD-L group (Table 1).

The mean RAS, RVD, PA, and TRV of the PAH-CHD (including all subgroups) and nPAH-CHD groups were significantly higher than those of the control group (p < 0.05). The mean PA and TRV levels in the PAH-CHD group (including all subgroups) were both higher than those of the nPAH-CHD group (p < 0.05) (Table 1).

Compared with the nPAH-CHD group, the PAH-CHD group (including all subgroups) had significantly increased pulmonary artery systolic pressure (SPAP), pulmonary artery diastolic pressure (DPAP), pulmonary artery mean pressure (mPAP), right ventricular mean pressure (mRVP), and total pulmonary resistance (PVR) (p < 0.05). The levels of SPAP and mPAP in the PAH-CHD-H group were significantly higher than those of the PAH-CHD-M and PAH-CHD-L groups (p < 0.05), but there was no statistically significant difference in SPAP between the PAH-CHD-M and PAH-CHD-L groups (p > 0.05). The mean DPAP and mRVP of the PAH-CHD-H and PAH-CHD-M groups were higher than those of the PAH-CHD-L group (p < 0.05). However, there was no statistically significant difference in DPAP and mRVP between the PAH-CHD-H and the PAH-CHD-M group (p > 0.05). SvO₂ was significantly lower in the PAH-CHD group (including all subgroups), and decreased with increased PAH risk stratification level (p < 0.05) (Table 1).

Baseline UA levels were significantly higher in PAH-CHD patients than in CHD patients with normal pulmonary pressures or normal control subjects (Table 1). Patients in the intermediate- and high-risk groups had significantly higher UA levels than low-risk patients (Figure 2). A total of 40 patients presented with hyperuricemia (UA > 416.0 μmol/L), including 2 non-PAH-CHD patients, 10 PAH-CHD-L patients, 20 PAH-CHD-M patients, and 8 PAH-CHD-H patients, accounting for 8.0%, 10.6%, 22.7%, and 44.4% of each group. As no significant difference in UA levels was observed between the PAH-CHD-L and non-PAH-CHD groups, we performed ROC analysis and identified a cutoff serum UA level of 330.9 μmol/L to achieve the maximum Youden index {sensitivity [65.1%]-[1-specificity (71.4%)]}. The area under the curve (AUC) was 0.706 (95% CI 0.638-0.773) for predicting intermediate-high risk PAH (Figure 3).

Table 2 presents correlations between serum UA levels and different variables. Serum UA levels were positively correlated with mPAP, SPAP, WHO functional class, NT-proBNP, and PVR, and a negatively correlated with SvO₂, and SaO₂ (Figure 4) the influence of serum UA levels on clinical indicators.

The optimal cutoff value for UA diagnosis for medium to high-risk PAH-CHD patients was calculated using ROC curve analysis. This study divided PAH-CHD patients into UA > 330.9 μmol and UA ≤ 330.9 μmol groups. Differences in patient characteristics, biochemical indicators, imaging measurements, and hemodynamic parameters were compared between the two groups. Cardiac function grading, NT-proBNP, RAS, RVD, mean PA, and PVR were significantly increased in the higher UA level group than the UA ≤ 330.9 μmol group, while SaO₂, SvO₂ and right CI were significantly decreased in the higher UA level group (p < 0.05). These results indicate that high levels of UA can indicate more severe PAH and a worse prognosis in PAH-CHD patients (Table 3).

All PAH-CHD patients were treated with one or two targeted drugs. Twenty patients followed up for an average of 7 ± 1 month and underwent repeat RHC. During that follow-up period, 10 of the 20 patients received an ERA combined with a PDE-5i, 5 received an ERA with PGI2, 1 received a PDE-5i with PGI2, 1 received PGI2 monotherapy, and the remaining three received ERA monotherapy. After at least 6 months of therapy, serum UA level and other clinical variables were re-examined. Changes in serum UA levels, echocardiographic data, and haemodynamic parameters are presented in Table 4. Haemodynamic and echocardiographic parameters, such as RAS, RVD, sPAP, mPAP, and PVR were significantly decreased, while CI and mixed SvO2 significantly improved after vasodilator treatment. Serum UA levels significantly decreased after vasodilator treatment, from 352.7 ± 97.5 μmol/L to 294.4 ± 56.8 μmol/L (p = 0.001). However, no significant decrease in mRAP was observed after treatment.

UA levels are be affected by a diverse range of factors, such as gender, age, race, and diet. Hyperuricemia is common in PAH (Nagaya et al., 1999; Voelkel et al., 2000; Dimitroulas et al., 2011). Nagaya et al. (1999) found that increased serum UA levels in idiopathic PAH patients are negatively correlated with CO, and constitute an independent risk factor for long-term mortality. Dimitroulas et al. (2011) reported that systemic sclerosis patients with PAH had higher serum UA levels than those without PAH, and that serum UA levels correlated with 6-min walking and other functional capacity tests. A study conducted on patients with primary and secondary PAH found that serum UA levels were higher in patients with severe PAH (Voelkel et al., 2000).

The results of our research are in agreement with these studies. We found that serum UA levels were significantly higher in patients with PAH-CHD than in non-PAH-CHD patients or healthy subjects. We also noted that hyperuricemia in the present study was mainly present in high-risk PAH-CHD patients.

Although the exact mechanism as to why PAH patients have elevated serum UA levels is unclear, tissue ischaemia and/or hypoxia may play an important role. CHD patients with ES have hypoxic exacerbations due to right-to-left shunting, which may lead to increased serum UA levels (Fathallah and Krasuki, 2018). We hypothesized that the differences in serum UA levels observed within the PAH-CHD group may be due to the higher incidence of ES in the intermediate and high-risk groups. SaO₂ levels were lowest in PAH-CHD-H patients, and UA levels were negatively correlated with SaO₂, SvO₂, and CI, suggesting that uric acid overproduction reflects damage to oxidative metabolism.

Reduced CI and renal perfusion may also be responsible for the elevated serum UA levels. A study by Ross et al. (1986) found that elevated UA levels in ES patients were due to inappropriately low uric acid excretion and enhanced urate reabsorption. Bendayan et al. (2003) reported that hyperuricemia in patients with worsening PAH is caused by impaired renal excretion of UA, which was associated with decreased CO and renal perfusion pressure (Prince et al., 2012). Although serum UA levels were negatively correlated with right CI in the present work, no statistically significant association was found with either serum Cr or LVEF in any of the five groups. These results suggest that patients with normal intrarenal dynamics can appropriately manage elevated UA levels.

Hoeper et al. (1999) found a strong association between serum UA levels and mRAP. This was not the case in our study, although intermediate-risk PAH-CHD patients higher mRAP than CHD patients with normal pulmonary artery pressures. Interestingly, no increases in mRAP were observed in high-risk PAH-CHD patients, which may be explained by right-to-left shunting reducing right atrial pressure.

Diuretic therapy is known to increase serum UA levels by stimulating urate reabsorption in the proximal tubule. All PAH patients received diuretics in our study. However, their baseline renal function was normal, so no significant differences were found between groups. We therefore believe that diuretic use cannot explain the elevated UA level observed in PAH-CHD patients.

While the degree to which elevated UA levels contribute to the development of PAH is unknown, several mechanisms may be proposed to this regard. First, persistent hyperuricemia may result in endothelial dysfunction, which in turn may lead to PAH progression through the promotion of oxidative stress. Besides being a key enzyme for UA production, xanthine oxidoreductase (XOR) is closely related to vascular oxidative stress, plausibly through the generation of reactive oxygen species (ROS) (Berry and Hare, 2004). ROS produced by increased XOR activity may exceeds cellular antioxidant capacity, inducing oxidative stress and endothelial dysfunction (Cai and Harrison, 2000). Second, disruption of NO signaling pathways by UA may be related to the pathobiology of PAH. Pulmonary vascular endothelial cells can induce pulmonary vessel vasodilation by synthesizing NO through the L-arginine-eNOS pathway, a complex process that is catalyzed by eNOS with arginine as a substrate (Zharikov et al., 2008). Third, elevated UA levels stimulate the release of a variety of inflammatory mediators and induce smooth muscle cell proliferation, thereby promoting the development and progression of pulmonary vascular disease (Rao et al., 1991; Kanellis et al., 2003; Kang et al., 2005). The present study lacked a long follow-up time or the use of cardiovascular events as endpoints, but it did find that serum UA levels significantly correlated with prognostic parameters. UA levels may therefore indirectly reflect hemodynamic status, thereby assessing the severity of PAH-CHD.

All 20 patients who received PAH-specific therapy for at least 6 months demonstrated significant improvements in their WHO functional class, echocardiographic parameters, and haemodynamic variables. Serum UA levels decreased and mixed SvO₂ and right heart CI increased. Oya et al. found that serum UA levels vary in ES patients, with reduced PVR after treatment with PGI₂ (Oya et al., 2000). These results suggest that serum UA levels may serve as a useful indicator of disease progression and treatment efficacy in PAH-CHD patients.

PAH-CHD is a heterogeneous patient population with various phenotypes of pulmonary vascular disease that range from increased pulmonary blood flow to Eisenmenger physiology with shunt reversal due to supra-systemic pulmonary pressures and right-to-left shunting. There is now additional evidence that PAH-targeted therapies are of benefit to patients with PAH-CHD, and that these therapies are commonly used in this patient population (Rosenkranz et al., 2015; Hidayati et al., 2020; Kaemmerer et al., 2021). This study followed 20 PAH-CHD patients who were re-catherized after at least 6 months of targeted drug treatment. We found that right heart size was significantly improved and hemodynamic indicators, including PVR and PAP, were significantly decreased. This suggests that targeted drug treatment can improve the symptoms and reduce the incidence of cardiovascular events in patients with PAH-CHD. We look forward to a further confirming the role of targeted drugs in congenital heart disease with PH with a larger scale clinical study.

Serum UA levels are simple to measure in a non-invasive and inexpensive manner. Several additional biochemical markers, including BNP, TNT, ET-1, and CRP, have been proposed, (Nagaya et al., 1999; Leuchte et al., 2007; Castillo-Martínez et al., 2016; Kylhammar et al., 2018), but serum UA may be superior in that it performs as a predictor of the disease severity and mortality of patients with PAH-CHD over long-term follow-up. The present work also found that serum UA levels decreased following PAH-specific drug therapy. Based on these findings, we suggest that serum UA levels be repeatedly measured to evaluate the treatment response of PAH-CHD patients in both the outpatient and inpatient settings.

The findings of the present work have to be interpreted in the context of its limitations. First, this is a single center study. Second, although this study had a relatively large sample size compared with related studies in the literature, it was still small for stratified analysis. A larger multi-center prospective study is necessary to further refine our findings. Third, only a few patients underwent repeated RHC after treatment with PAH-specific drugs. The current data can only partially reflect the relationship between decreased UA levels and improved haemodynamic indicators. Fourth, this study lacked a long follow-up time and the use of cardiovascular events as endpoints. We also failed to confirm that whether serum UA levels could act as an independent risk factor for CHD patients with PAH.

In conclusion, we associated serum UA levels with clinical and haemodynamic severity in PAH-CHD patients. Serum UA levels may be a practical biomarker for assessing risk stratification in patients with PAH, and for evaluating treatment response to PAH-specific drugs. Further studies with larger sample sizes are necessary to confirm the results of this study.