Percutaneous coronary intervention (PCI) has been an effective and widely used treatment for patients with coronary artery disease (CAD) since 1979 [1]. However, despite ongoing improvements on drug-eluting stents (DES), in-stent restenosis (ISR) remains a major complication of PCI, accounting for approximately 5% of all complications [2, 3]. Multiple factors, including inflammatory response, patient-specific and procedure-related risk factors, are involved in the development of ISR [4].

Anemia is associated with a greater rate of cardiovascular events [5, 6], and its prevalence is comparatively high in patients with CAD. Previous studies have highlighted links between anemia and a higher risk of death, major adverse cardiovascular events (MACE), myocardial infarction (MI), and bleeding post PCI [7‐10]. Most recently, an in-hospital hemoglobin drop ≥ 3 g/dL was independently associated with increased 1-year mortality among patients with acute coronary syndrome managed invasively [11].

While studies have demonstrated associations between anemia and fatal adverse events post-PCI, there is yet no data on the relationship between anemia and ISR. In this study, we sought to identify and establish an association between anemia and ISR and determine its independent predictive ability of ISR post-PCI in patients with CAD.

Although the association between preprocedural anemia and poor outcomes post-PCI in patients with CAD is common, data on the impact of anemia on ISR at follow-up is scant. In this study, we established a strong association between anemia and the occurrence of ISR, which could be used as an independent predictor of ISR at follow-up.

Catakoglu et al. [16] demonstrated that anemia is a crucial risk factor for predicting nonfatal coronary events, including ST-segment elevation MI, non-ST-segment elevation MI, target vessel revascularization (TVR), and target lesion revascularization (TLR), post-PCI, whereas Geng et al. [17] established no statistically significant relationship between low hemoglobin levels and ISR. In our research, after adjusting for confounding factors, we substantiated a strong link between anemia and ISR, indicating that anemic patients were more likely to develop ISR after undergoing PCI.

How ISR operates in patients with anemia who undergo PCI is not well-known but probably multifactorial. The presence of anemia could lead to a decreased oxygen supply, and hypoxia has been identified with vascular cell proliferation and angiogenesis [18]: both are necessary conditions for the development, maintenance, and expansion of neointimal lesions in restenosis [19‐21]. Compensatory consequences of hypoxia – such as a hyperdynamic state with increased cardiac output, left ventricular hypertrophy, and progressive cardiac enlargement – and a proatherogenic role also contribute to restenosis [22]. Moreover, anemia correlates independently with high platelet reactivity in patients with DES PCI [23], contributing to the development of restenosis [24, 25]. Most recently, a study demonstrated an association between anemia and higher levels of inflammatory markers, such as high sensitive C-reactive protein, fibrinogen, and serum amyloid A, with the authors pointing out that a relationship between anemia and disease outcomes could be caused by underlying inflammation [26]. Because inflammation response is one of the most critical mechanisms for ISR, it is reasonable to infer that anemia could contribute to the progress of ISR. In addition, lower hemoglobin levels are associated with the dysregulation of endothelial cells, causing ISR as well [27, 28].

We also showed in this investigation that higher LDL-c levels, multiple stenting, bifurcation lesion, and smaller stent diameter were linked to post-PCI ISR, which is consistent with previous findings [29‐31].

Smoking is a risk factor for CAD, but we found no noteworthy relationship between smoking and ISR. Interestingly, previous studies have reported data supporting the theory that smoking is associated with a lower rate of restenosis, a finding that remains controversial [32]. Diabetes mellitus is considered the most consistent clinical parameter that increases the risk of restenosis [33, 34], which matches our findings.

Multiple studies have also explored the impact of CKD on post-PCI ISR, with the end-stage renal disease found to possibly increase the risk of restenosis occurrence [35], and mild or moderate CKD having no bearing on the incidence of ISR post-PCI [36], which is inconsistent with our results too, one in which patients with CKD were more likely to have anemia.

The current guidelines stipulate a 6 to 12-month DAPT with a class I recommendation after PCI [37, 38]. The duration of DAPT depends on the balance between ischemia and bleeding risk. However, both prolonged and shortened usage of DAPT in patients with CAD undergoing PCI notably affect cardiovascular events. A multicenter prospective real-world study revealed that prolonged DAPT with low-dose ticagrelor was effective and safe, with low incidences of MACE, MI, and stroke/transient ischemic attack, and no major bleeding [39]. P2Y₁₂ inhibitor monotherapy after coronary revascularisation has also been assessed in several trials, with one meta-analysis recently establishing that P2Y₁₂ inhibitor monotherapy was associated with a similar risk of death, MI, or stroke and lower risk of major bleeding as DAPT, suggesting that shortening DAPT to 1 to 3 months post-PCI could be effective and safe [40]. In this study, we found no difference in the impact of DAPT on patients in the ISR and non-ISR groups, with patients with anemia more likely to stop using aspirin, partly because they were concerned about bleeding.

For restenosis within DES, drug-coated balloon (DCB) and DES are both approved with a class I recommendation [38]. Bioresorbable vascular scaffolds (BVS) have also been proposed as a treatment for ISR, with one study showing that BVS is effective in the treatment of ISR and has an acceptable target lesion failure rate [41]. However, the 1-year follow-up results of a BIORESOLVE-ISR study revealed that BVS increases the rate of device-oriented cardiovascular events compared to DES but has a similar rate to DCB [42]. In our study, 20 patients underwent repeat revascularization: 19 were treated with DCB and one with DES. But our findings and those reported are not sufficient for a generalized outcome. Therefore more studies must be conducted to determine the long-term outcomes of different procedures on ISR treatment.

Radial artery is the recommended default vascular access for PCI [38, 43]; however, crossover from radial to femoral access is sometimes required. One recently published investigation showed that crossover from radial to femoral access diminished the bleeding benefit offered by the radial access site but did not increase the risk of MACE incidence [44]. The authors also developed a simple risk score – MATRIX score – to predict radial crossover in patients with ACS, which could improve outcomes [45]. For patients with ISR, the initial access site selection remains essential to improving management and outcomes. In our cohort, all procedures completed were transradial; the long-term prognosis needs, therefore, to be investigated further.

Despite its promises, our investigation has several limitations. First, it was retrospective in nature. Second, the sample size was relatively small, the follow-up period was fairly short, and the patients involved were from a single hospital; the results, therefore, do not represent a general scenario. Third, the loss of follow-up and the exclusion of patients who refused to be enrolled in the study might have led to a bias. Fourth, our findings were based on a single measurement of preprocedural Hb levels, with Hb during the follow-up not evaluated. Finally, some covariates, such as smoking and LDL-c, might have changed during the follow-up period, in turn affecting our inferences. As a result, larger prospective trials must be conducted to establish anemia’s potential as an independent predictor of ISR.

Patients with baseline anemia before PCI have a higher incidence of ISR after PCI than their nonanemic counterparts. Anemia could, therefore, be used as a predictor of ISR. Our results have important clinical implications, with the assessment of Hb levels also crucial during periprocedures.