Abdominal aortic aneurysm (AAA) is common in the older population, and its prevalence ranges from 4% to 7% [1]. Endovascular aneurysm repair (EVAR) is a minimally invasive technique for treating AAA and exhibits the advantages of being less invasive and being associated with lower procedure-related mortality or complications compared with open surgery [2, 3, 4, 5].

Anaemia has been considered to be a part of the natural disease process in patients with AAA and was linked to peri-operative complications [6]. Previous studies have indicated that patients with pre-operative anaemia who undergo cardiac surgery have higher risks of developing adverse cardiac events than those without anaemia [7, 8]. Furthermore, the presence of a low haemoglobin (Hb) level or anaemia before EVAR has been reported to be associated with high mortality and adverse cardiac events [7, 9]. Diehm et al. [9] showed that long-term survival was significantly lower in patients with pre-operative anaemia than in those without anaemia and that the Hb level was inversely related to the maximum diameter of the AAA. Furthermore, AAA size has been documented to be independently associated with an increased risk of peri-operative complications [10]. During EVAR, the stent graft is firmly positioned at the upper and lower edges of AAA, which results in the narrowing of the lumen and indicates that the aneurysm is successfully excluded from systematic circulation. However, some blood is isolated outside the covered stent cavity, a process similar to acute haemorrhage, which causes a decrease in the Hb level or even anaemia [11]. With the advancements in segmentation software, the volumetric analysis of AAA has become feasible [12]. No study has so far reported post-operative Hb alteration or anaemia in relation to aneurysm volume change after EVAR.

Computed tomography (CT) is a routine imaging modality in the follow-up of patients with EVAR, and its primary role is to monitor the changes in the aneurysm size and detect endoleaks that occur commonly in patients after EVAR [13, 14, 15]. Usually, aneurysm diameter is measured and compared on pre- and post-EVAR CT images to determine the success of EVAR procedures. However, volumetric analysis has been shown to be more accurate than maximal diameter growth in determining whether surgical intervention is needed [16, 17]. In addition, a follow-up study has observed that lumen volume was superior to maximal diameter in predicting aneurysm enlargement [12].

This study aimed to investigate the alterations in the Hb level and the AAA volume (indicating the AAA lumen) before and after EVAR and examine the correlation between AAA volume loss and Hb level 24 h after the procedure. The attributes of AAA volume, pre-operative Hb level and history were analysed to determine the risk factors for anaemia 24 h after EVAR. Although several reports analysing various biomarkers in relation to their roles in EVAR outcomes have been published so far, this study adds valuable information to the existing literature by exploring the post-operative Hb level in relation to AAA volume change following EVAR. Most of the current studies have documented the association between pre-operative Hb level and EVAR outcomes and analysed AAA diameter changes.

In this study, the Hb level declined significantly 24 h after EVAR, and 59.5% of the patients experienced anaemia. Moreover, the post-operative Hb level was negatively correlated with AAA volume loss. In the prediction of anaemia 24 h after EVAR, the results of binary logical regression showed that hypertension was a risk factor, whereas pre-operative Hb level was a protective factor. The area under the receiver operating characteristic curve for this model was 0.840.

In this study, the results demonstrated that the Hb level decreased or anaemia occurred after EVAR. This occurrence may be related to the process of EVAR as the aortic lumen is narrower than that before the procedure because of the formation of the new conduit. This conduit is introduced by the endovascular stent graft inside the aneurysm, where the aneurysm is excluded from systemic circulation and the blood between the aortic wall and the stent graft is isolated from circulation. This process is similar to acute haemorrhage. These findings are consistent with those from a previous study [19], which indicated that acute blood loss resulted in a decline in the Hb level or even anaemia. Moreover, according to the auto-resuscitation theory, after blood loss, the interstitial fluid moves into the capillaries to compensate for the decreased plasma volume [20]. This process begins at the onset of blood loss, and the plasma volume is replenished in a short time of 1 h [21, 22, 23]. Taken together, the Hb level decreases after blood loss, but the plasma volume is replenished by interstitial fluid compensation. This process results in a decrease in the Hb level.

Post-operative Hb decline or anaemia was observed after EVAR in this study, which agrees with a previous study [24]. Furthermore, the results showed that decrease in the Hb level after vascular surgery, including EVAR, is an independent risk factor for developing 30-day cardiovascular events. A decrease of 10 g/L in the Hb level was associated with a 20% increased risk of a 30-day cardiovascular event. The mean decrease in the Hb level in their study group was 22 g/L, whereas in our group, it was 14 g/L. Despite the relatively low decrease in the Hb level in our study and that by Valentijn et al. [24], when compared with the 50% decrease from baseline as shown by Karkouti et al. [25], the potential risk of post-operative decrease in the Hb level and its association with cardiovascular events should not be ignored. Blood transfusions are beneficial in preventing mortality and morbidity in patients who suffer from intraoperative haemorrhage and in those who are severely anaemic in the peri-operative period [26]. The recommended threshold for red blood cell transfusion is Hb level of 70 g/L [27]. A previous study found that patients with mild anaemia receiving peri-operative transfusion had significantly higher odds of mortality and in-hospital complications (ORs of 5.7 and 4.3, respectively) than those who did not receive transfusion (mild anaemia was defined as a Hb level of 100–120 g/L in women and 100–130 g/L in men) [28]. In addition, another study reported that transfusions were rarely beneficial when Hb levels were $>$100 g/L [29]. In our study, the mean post-operative Hb level was 124 g/L, which exceeded 100 g/L and did not meet the blood transfusion criteria. Thus, we inferred that transfusions would not benefit the patients. In sum, the significance of anaemia or decreased Hb level should not be neglected because it is a reliable marker for predicting the risk of post-operative cardiac events, yet transfusion could make it worse when the level is $>$100 g/L.

Furthermore, our results indicated that post-operative Hb level was negatively correlated with AAA volume loss. In addition, no correlation was observed between AAA volume loss and decrease in the Hb level or pre-operative AAA volume and decrease in the Hb level. This lack of correlation could be ascribed to the following reasons: First, as per the auto-resuscitation theory mentioned above, after blood loss, the interstitial fluid moves into the capillaries to compensate for the decrease in the plasma volume [20]. This compensation ability varies from patient to patient. In addition, the fluid infusion and urine volume during the surgery differ among patients. This may be the reason for the absence of correlation between AAA volume loss and decrease in the Hb level. Second, the AAA volume loss depends on the volume of the aneurysm and the size of the stent, which is determined by its diameter and length. In general, the diameter of the stent is based on the diameter of the aneurysm neck. Therefore, a greater pre-operative AAA volume would be associated with more blood loss only if the diameter of the aneurysm neck and the length of the stent are similar. These findings suggest that aneurysm volume loss may be responsible for a lower post-operative Hb level 24 h after EVAR.

Finally, the predictor model results showed that hypertension was a risk factor for anaemia 24 h after EVAR. A previous study indicated that hypertension is a risk factor for cardiovascular disease [30] and AAA because high blood pressure causes an increase in the diameter [31]. Most AAAs contain a complex structure of fibrin, inflammatory cells, platelets and red blood cells in the aneurysmal sac, known as intraluminal thrombus [32]. An association is present between endothelial dysfunction and hypertension [33]. Endothelial dysfunction may lead to tissue swelling, chronic inflammation and thrombosis [34]. During thrombosis, red blood cells in the circulating blood decrease, leading to a decrease in the Hb level. In addition, according to Bernoulli’s principle, as the pressure inside the aneurysm increases, the velocity of the flow decreases [35]. This slow blood flow would result in more blood isolating outside the graft during EVAR. Furthermore, from the pathological perspective, during hypertension, blood vessels become stiffer owing to distension [36]. This decreased vascular elasticity causes more blood to remain in the vessel, especially in the aneurysm, which can augment the pressure and further reduce the blood flow. Thus, hypertension would result in more blood being isolated outside the graft, thus leading to increased blood loss during EVAR.

The effect of radiation dose on red blood cell counts and Hb level alterations remains to be determined. Previous studies reported that red blood cell counts and Hb levels decreased significantly as a result of prolonged radiation exposure [37, 38]. However, Tian et al. [39], in their recent study, analysed red blood cell counts and Hb level alterations in medical workers with exposure to low-dose radiation and found no significant changes in red blood cell counts and Hb levels in both sexes with extended service. Our results showed significant increase in the radiation dose following EVAR treatment, but the Hb levels decreased 24 h after the procedure. Thus, increased radiation dose may not contribute to the decrease in Hb levels owing to the fact that the Hb recording time was earlier than that of CT scanning.

The strengths of this study are as follows: (1) The idea was innovative as we correlated the post-operative Hb level with the AAA volume loss after EVAR. Furthermore, we identified the risk factor for post-operative anaemia 24 h after EVAR. (2) The cost of the research is low because of its retrospective nature.

However, this study has some limitations that should be addressed. First, the number of patients was small. In the future study, we intend to enrol more patients. Moreover, the mean AAA diameter was 50 mm in this study group. Inclusion of AAA cases with a larger diameter is necessary to validate our findings. Second, the urine volume and infusion administration were not calculated in detail because of the retrospective nature of this study. The operation record was not complete, but complete surgical records containing data on these two volumes were roughly equal during the procedure. Third, we did not follow-up patients in this group regardless of the presence of anaemia 24 h after EVAR. This gap needs to be addressed in future studies preferably by comparing major adverse cardiac events in patients who developed anaemia with those who did not. A previous study signified that the decrease in Hb levels in the first 24 h after the surgery (Day 0 to Day 1) underestimated the ultimate lowest Hb level on Day 2. A further decrease of 4.4 g/L in Hb levels was noted between Day 1 and Day 2, but no significant changes were found from Day 2 to Day 3, Day 3 to Day 4 and Day 4 to Day 5 [40]. Therefore, future work should investigate whether the degree of Hb decline is more severe in patients with anaemia in the first 24 h after EVAR and whether they are likely to develop chronic anaemia following EVAR treatment.

Our results indicate that the Hb level decreased and even reached the diagnostic level for anaemia 24 h after EVAR. Furthermore, the post-operative Hb level was negatively correlated with the aneurysm volume loss. For predicting anaemia 24 h after EVAR, a regression model was developed, which indicated that history of hypertension was a risk factor and that the pre-operative Hb level exerted a protective effect. The model suggested that aneurysm volume loss is worthy of attention owing to its negative association with the Hb level 24 h after EVAR. Furthermore, history of hypertension should be included in the post-operative risk assessment of patients with AAA undergoing EVAR treatment as it is a risk factor for anaemia 24 h after the procedure.

AAA, abdominal aortic aneurysm; AUC, area under the curve; CI, confidence interval; CTA, computed tomography angiography; DLP, dose length product; EVAR, endovascular aneurysm repair; Hb, haemoglobin; OR, odds ratio; VL, volume of the lumen.

The datasets generated and/or analyzed during the current study are not publicly available due to ethical restrictions but are available from the corresponding author on reasonable request.

YingL, ZS and YuL designed the research study. YingL and HZ performed the research. YingL, HZ and JZ acquired the data. YingL and ZS analyzed the data. YingL wrote the original manuscript, YingL, HZ and YuL reviewed and edited manuscript. SX, LX, LS and YuL interpreted the data. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

This study was approved by the Beijing Anzhen Hospital Ethics Committee (no. 2022023X). Patient consent was waived due to the retrospective nature of the study.

Not applicable.

This research received no external funding.

The author declares no conflict of interest. Zhonghua Sun is serving as one of the Editorial Board members and Guest editors of this journal. We declare that Zhonghua Sun had no involvement in the peer review of this article and has no access to information regarding its peer review. Full responsibility for the editorial process for this article was delegated to Carmela Rita Balistreri.