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This study demonstrated that patients with rheumatoid arthritis (RA) without classic cardiovascular risk factors and with moderate disease burden generally have preserved endothelial function. However, those with elevated levels of total cholesterol and low‑density lipoprotein cholesterol (LDL‑C) displayed lowered flow‑mediated dilation (FMD), suggesting that RA in young and middle‑aged individuals may exacerbate harmful effects of lipoproteins on macrovascular endothelial function. In contrast, microvascular response in the skin was enhanced in RA patients with higher LDL‑C levels. In addition, RA patients with increased C‑reactive protein levels exhibited a better FMD response and higher maximal shear rate and shear stress. These findings highlight a heterogeneous endothelial response in the skin microcirculation and conduit vessels of RA patients with a relatively low disease burden. Furthermore, our study underscores the importance of strict lipid profile control in RA patients and suggests that compensatory vascular adaptations—detectable before overt vascular dysfunction—may serve as early diagnostic markers.

Introduction

Rheumatoid arthritis (RA) is the most prevalent autoimmune arthritis. It is not only responsible for pain, disability, and decreased quality of life, but also carries a substantial cardiovascular risk translated into increased cardiovascular morbidity and mortality shared by several other autoimmune diseases.¹ In patients with RA,² acceleration of atherosclerosis development and early signs of vascular remodeling are well‑established findings, even in younger individuals without overt cardiovascular disease. The clinical importance of the cardiovascular burden of RA is frequently compared to that of diabetes,³ but the approach to monitoring the cardiovascular risk in RA and diabetes is different. Recently, researchers have attempted to incorporate the presence of RA into the next generation of the Systematic Coronary Risk Evaluation models.⁴

Given the key role of endothelial dysfunction (ED) with its prothrombotic, proinflammatory, and pro‑oxidant phenotype⁵ in the development of atherosclerosis and cardiovascular diseases, there have been numerous studies characterizing endothelial function in patients with RA that aimed to find the link between RA‑induced ED and the increased prevalence of cardiovascular diseases in this population. ED was repeatedly detected in patients with RA; however, it is still uncertain which factors are responsible for its occurrence.^6,7 Classic cardiovascular risk factors and systemic inflammation were proposed to be causally involved. In fact, numerous reports demonstrated that classic cardiovascular risk factors were prevalent in patients with arthritis. RA was more common in smokers,^8,9 arterial hypertension affected 40% of patients with RA, and hypercholesterolemia was found in 31% of this population.¹⁰ Importantly, up to 30% of patients with RA had their modifiable cardiovascular risk factors poorly managed in the outpatient setting.¹¹

On the other hand, multiple proinflammatory cytokines that play a pathogenic role in RA were reported to be involved in ED and atherosclerosis progression,¹² suggesting that systemic inflammation can contribute significantly to the development of ED even in the absence of classic cardiovascular risk factors. Indeed, upregulation of various proinflammatory cytokines, such as interleukin 6 (IL‑6), tumor necrosis factor (TNF), members of the IL‑1 family (IL‑1, IL‑18, and IL‑33), and IL‑17, was demonstrated in patients with RA.¹³ In several studies, the presence of ED correlated with elevated levels of inflammatory biomarkers, proinflammatory cytokines, and moderate‑to‑high disease activity.¹⁴ These findings supported the idea that maintaining remission should provide an improvement in ED and vascular inflammation and, subsequently, a reduction in cardiovascular risk. In fact, a number of studies reported that some anti‑inflammatory therapies reduced cardiovascular risk in RA patients.¹⁵ However, even after successful biological treatment (in terms of decreased disease activity), the residual cardiovascular risk appeared to persist, as evidenced in a large Scandinavian cohort of patients with RA.¹⁶ Furthermore, ED was present even in young, otherwise healthy patients with low‑activity RA,¹⁷ suggesting that low disease activity in young patients with RA may still be associated with the impaired endothelial function.

Taken together, the evidence overwhelmingly suggests that RA can lead to ED, but the mechanisms involved remain unclear. It is still unexplained to what extent ED is due to the specific inflammatory burden linked to RA or whether it is rather related to comorbidities. These uncertainties stem from the design of most previous studies that detected ED in patients with RA. These studies involved heterogeneous groups, observed over a long‑term disease trajectory, and utilized a variety of methods to assess the endothelium, and only in a few reports were macro- and microvascular endothelial functions evaluated simultaneously.

To address the knowledge gap concerning ED in RA, this study aimed to determine whether young and middle‑aged patients with RA, who were unaffected by classic cardiovascular risk factors, would exhibit ED. Furthermore, to comprehensively analyze the endothelial function, we used 3 noninvasive in vivo methods to assess the endothelial function in the conduit artery and in the microcirculation, that is, flow‑mediated dilation (FMD) in the brachial artery, reactive hyperemia peripheral arterial tonometry (RH‑PAT) in the peripheral microcirculation, and flow‑mediated skin fluorescence (FMSF) in the skin microcirculation, with multiple parameters to quantify functional endothelial responses in FMD-, RH‑PAT-, and FMSF‑based techniques.

Patients and methods

In this cross‑sectional study, patients were recruited at the University Hospital in Kraków, Poland. The study was approved by the Ethics Committee of the Jagiellonian University (1072.6120.162.2020) and conducted in accordance with the Declaration of Helsinki. The patients were men and women of Polish ethnicity, aged between 18 and 50 years, diagnosed with RA classified according to the 2010 American College of Rheumatology / European Alliance of Associations for Rheumatology criteria. Healthy controls were volunteers recruited among the University Hospital staff matched for age and sex with the RA patients.

Results

A total of 124 patients (64 with RA and 60 matched controls) were examined. Basic and laboratory data of the patients with RA and healthy controls are shown in Table 1. Clinical characteristics of the patients with RA are presented in Table 2. As many as 49 patients with RA (76.6%) were seropositive, 23 patients (36%) had early RA, and 41 (64%) had established RA. The average disease activity was moderate. A total of 65.6% of the patients were on methotrexate, 40.6% were treated with biologic DMARDs (TNF or IL‑6 inhibitors). The endothelial function parameters in the patients with RA and healthy controls are shown in Table 3. Clinical characteristics of the RA patients with and without macrovascular endothelial dysfunction are presented in Table 4. There were no significant differences in the endothelial function parameters between the patients with RA and healthy controls. However, the individuals with RA had increased AI of 3 (–9 to 10) vs –6.5 (–16 to 2; P = 0.003). The patients with RA categorized by disease activity did not show differences in the endothelial function parameters. Interestingly, the subset of patients with FMD below 7% showed higher mean (SD) TC level (5.09 [0.85] vs 4.47 [0.77] mmol/l; P = 0.005), and LDL‑C level (2.92 [0.86] vs 2.42 [0.75] mmol/l; P = 0.02; Figure 1). On the other hand, the subgroup with FMD above 7% (n = 29) had higher median (IQR) CRP level (1.11 [1–8.46] vs 1 [1–1] mg/l; P = 0.03). The patients with plasma CRP level of 2 mg/l or above had higher FMD/SR max [au] than the patients with normal CRP levels (0.0103 [0.0033–0.0123] vs 0.0059 [0.0027–0.0078]; P = 0.02; Figure 1). Notably, there was a significant weak correlation between CRP and FMD%, and a weak‑to‑moderate correlation for CRP vs FMD parameters normalized for SR max and SS max (Table 5). RH‑PAT or FMSF did not discriminate the patients in terms of their lipid profile or inflammatory status. However, the analysis of the microcirculation oscillation contribution showed a higher median (IQR) percentage of neural oscillations in the RA group than the controls (17.38% [10.88%–28.38%] vs 13.44% [8.44%–21.52%]; P = 0.02; Supplementary material, Table S1). Interestingly, in the analysis of FMSF parameters, high RHR% was more frequent in the patients with RA and LDL‑C level equal to or above 2.5 mmol/l than in the patients with RA and LDL‑C level below 2.5 mmol/l (Figure 2). The patients with RA exhibited higher median (IQR) value of the neurogenic component of the microcirculation oscillations than the control group (17.38% [10.88%–28.38%] vs 13.44% [8.44%–21.52%]; P = 0.02; Supplementary material, Table S1).

Discussion

Despite multiple reports demonstrating ED in RA patients it is still unclear whether ED in RA is due to the specific inflammatory burden linked to RA or is more related to comorbidities present in a majority of RA patients.²² In this study, we demonstrated that young and middle‑aged patients with RA, presenting with a moderate inflammatory burden in the absence of classic cardiovascular risk factors, exhibit largely preserved endothelial function, in both the conduit artery and microcirculation. Interestingly, the subgroup of patients with higher plasma LDL‑C level, but within the range where pharmacotherapy is not typically indicated based on current recommendations, showed impaired endothelial function measured by FMD. These results suggest that RA may exacerbate the harmful effects of lipoproteins on macrovascular endothelial function. In contrast, microvascular responses in the skin were enhanced in the RA patients with higher plasma LDL‑C level. Furthermore, the RA patients with increased CRP levels exhibited a better FMD response and higher SR max and SS max.

Taken together, these findings emphasize a heterogenous endothelial response observed in the skin, peripheral circulation, and the conduit vessels in RA of moderate disease burden. Furthermore, our study underscores the importance of strict lipid profile control in RA patients. Finally, we uncovered compensatory vascular adaptations in the FMD and FMSF tests that were detectable before overt vascular dysfunction, and may possibly serve as early diagnostic markers.

Although several reports on endothelial function in patients with RA have been published,^2,7,8 our study is distinguished by its homogeneous cohort of young and middle‑aged patients (≤50 years old) who were rigorously screened to exclude classic cardiovascular risk factors and had limited exposure to GCs and NSAIDs. Most previous studies included patients with various classic cardiovascular risk factors.^14,23 While methods such as FMD and RH‑PAT have been widely used,^14,17 the novelty of our approach lies in expanding traditional FMD% diameter response by incorporating additional parameters that provide insights into changes in SR and SS. These parameters, which are modified by the inflammatory state—likely due to altered blood viscosity²⁴—were also found to be altered in our work. To the best of our knowledge, these aspects of the FMD response have not been evaluated previously in patients with RA. Notably, altered SS and blood velocity during RH of FMD response had a stronger independent relationship with cardiovascular risk factors including age, blood pressure, and LDL‑C concentration, as compared with FMD% alone, supporting the value of these measurements in the patients with RA.²⁵

From a methodological standpoint, it is worth adding that this study is the first to employ the FMSF method for assessing the functional status of skin microcirculation in RA. Previously, this technique has been applied in patients with diabetes,²⁶ coronary artery disease,¹⁹ and other conditions.²⁷ Remarkably, our findings suggest that young and middle‑aged patients with RA exhibit compensatory microcirculatory changes. A similar pattern has been reported in young patients with type 1 diabetes.²⁸

Importantly, our study showed that none of the vascular tests were associated with RA disease activity, aligning with the current view that there is no direct relationship between micro- or macrovascular ED and disease activity in RA.²⁹ This observation implies that, despite a favorable clinical response to RA treatment, patients remain at a risk for accelerated atherosclerosis. In fact, a recent study reported a residually increased incidence of myocardial infarction among RA patients successfully treated with biologics,¹⁶ and other evidence indicates that cardiac damage may persist even after achieving rheumatologic therapeutic goals.³⁰ The increased arterial stiffness observed in our relatively healthy group of patients with RA suggests ongoing subclinical vascular pathology, indicating that effective anti‑inflammatory treatment may not fully protect against macrovascular ED. This is further supported by other reports detecting ED in young patients with RA despite standard treatment.^17,31

An important finding of our work is that the subgroup of RA patients with higher LDL‑C level showed impaired endothelial function measured by FMD. This result advocates for closer lipid profile monitoring and suggests that the threshold levels of TC and LDL‑C currently deemed safe in RA, for example, the level of LDL‑C below 2.6 mmol/l (100 mg/dl) in patients with RA under 50 years old, without other cardiovascular risk factors, and irrespective of the disease activity, may need to be revisited. This recommendation would mirror current strategies for young and middle‑aged patients with diabetes.³² Our findings are particularly relevant in light of the underdiagnosis of hypercholesterolemia in RA³³ and the promising outcomes of trials demonstrating the benefits of strict control of traditional cardiovascular risk factors.³⁴ Furthermore, our results seem to challenge the “lipid paradox,” that is, a concept of an inverse relationship between systemic inflammation and lipid levels in patients with RA.^35,36

Several studies have found correlations between macrovascular ED measured by FMD³⁷ and microvascular dysfunction assessed with laser Doppler flowmetry^38,39 with CRP and systemic inflammation in RA patients. However, these findings were not consistently confirmed by other reports.⁴⁰ This inconsistency may be influenced by the heterogeneity of inflammatory biomarkers assessed in RA.⁴¹ Interestingly, Holmes et al⁴² reported that RA patients with elevated CRP level showed higher FMD% values, which aligns with our findings. These results suggest that, in patients without cardiovascular risk confounders, a compensatory vascular response leads to a higher FMD%, representing an early endothelial reaction to detectable systemic inflammation. CRP directly decreases the expression and bioactivity of endothelial nitric oxide synthase (eNOS),⁴³ which may be due to increased blood viscosity related to systemic inflammation. In fact, the increased peak SR and increased SS observed in RA patients may stimulate nitric oxide–dependent vasodilation.⁴⁴ In this context, detecting the initial inflammatory macrovascular endothelial response in RA should involve not only FMD% measurements, but also normalization for SR max. Noteworthy, hyperemic shear stress is a major correlate of brachial artery flow‑mediated dilation and the major stimulus for the FMD response correlating with risk factors in the Framingham Heart Study.⁴⁵ On the other hand, increased FMD in RA patients with elevated CRP level may represent compensatory endothelial response described in other settings.^3,5,46 Interestingly, in a rat model of adjuvant‑induced arthritis, increased eNOS activity was reported in early arthritis.⁴⁷

It is plausible that this compensatory vascular response precedes the overt endothelial dysfunction, which eventually becomes more prevalent in patients with inflammatory arthropathies as cardiovascular risk factors accumulate.^48,49 It is therefore speculated that the gradual loss of the compensatory endothelial response may drive the transition from an adaptive to a maladaptive endothelial phenotype as the inflammatory disease progresses, and, if monitored, may have a diagnostic value.

Measurement of skin microcirculation by FMSF in the RA patients provided interesting observations. First, higher neuronal oscillations during the recovery phase were observed in the RA group, possibly reflecting autonomic system dysregulation associated with RA that warrants further investigation.⁵⁰ Second, the RA patients with LDL‑C levels exceeding 2.5 mmol/l displayed a paradoxically augmented RHR. These results may indicate a compensatory response in the skin microcirculation of the young and middle‑aged RA patients, similarly to a compensatory increase in FMD response.

Regarding microvascular ED in the patients with RA measured in the peripheral vasculature (RH‑PAT), our conclusions remain elusive. Although no impaired RH‑PAT response was detected, the RA patients exhibited higher AI parameters. This finding corroborates the results of Pieringer et al,⁵¹ who also reported increased AI in patients with RA without classic cardiovascular risk factors. Consequently, the elevated AI—indicative of increased arterial stiffness despite preserved endothelial function and absence of additional cardiovascular burden—may represent an intrinsic characteristics of RA.