1 INTRODUCTION

Anemia, defined as a decrease in hemoglobin or hematocrit, is one of the most common diseases in the world, affecting about one-quarter of the global population.1 It is prevalent among women, the elderly, and people with chronic diseases (such as heart failure, diabetes mellitus, etc.),1 the same groups that are at high risk of atrial fibrillation (AF).

One recent large-scale community-based epidemiological survey conducted in China showed that the weighted AF prevalence was 1.8% among adults over 45 years old.2 Since the prevalence of AF and anemia increases with age, the occurrence of both is likely to be more dominant in the future as a result of an aging population. Studies have found that anemia is a common comorbidity among patients presenting with AF, but the estimated prevalence varies widely (from 12% to 37%) with most of the surveys conducted in selected AF populations.3-6

Chronic anemia can induce ventricular remodeling and cardiac dysfunction7 with a 40% increased risk of all-cause mortality among the general population.8 AF aggravates hemodynamic instability and gradually leads to changes in heart structure, which is relevant given that heart failure is one of the most important causes of death in AF.9 Also, compared with nonanemic AF, anemia may affect the decision whether or not to prescribe oral anticoagulant (OAC) as well as the choice of OAC.5 Recently, studies have shown that anemia was associated with an 80% increased risk of death in AF patients10 and the risk of subsequent major bleeding increased by about twofold in those on OAC.3-5, 11 However, others concluded that anemia was only associated with an increased risk of noncardiovascular death but not cardiovascular (CV) death,12 and not related to bleeding.13, 14

Currently, the limit of hemoglobin up to which anemia is clinically important has not been well-investigated. Treatment of anemia in AF still lacks clear targets and specific therapy, though some anemias (such as iron deficiency) are relatively easy to correct compared with other comorbidities. For the associations of hemoglobin levels with prognosis of Asian patients in AF, data are more limited and contradictory, most are focused on the population at high risk of stroke or on OAC and had excluded patients with moderate to severe anemia (hemoglobin <10 g/dl).

Since these two diseases have overlapping effects on the CV system, we aimed to assess the associations of anemia with all-cause mortality, CV death, and major bleeding in patients with AF by using large prospective real-world data.

2 METHODS

The rationale and design of the Chinese Atrial Fibrillation Registry (CAFR) study have been previously published.15 In brief, this is an ongoing, prospective, multicenter, hospital-based cohort study led by Beijing Anzhen Hospital. Thirty-one tertiary and nontertiary hospitals in Beijing that can provide AF diagnosis and treatment services participate in this study. The CAFR registry was approved by the ethics committee of Beijing Anzhen Hospital, and all patients have signed informed consent before enrollment.

In this study, we selected both inpatient and outpatient AF patients recruited from August 2011 to December 2018. We excluded patients if they meet any of the following: (1) age<18 years; (2) follow-up time less than 6 months; (3) transient AF caused by reversible cause; (4) AF in the setting of concomitant valvular heart disease such as mitral stenosis, mitral valve prosthesis, and so on; and (5) missing baseline hemoglobin values. Finally, 18,106 participants were included in this analysis. Patients were divided into three groups according to their baseline hemoglobin value at the time of enrollment (see the flowchart in Figure 1).

Enrollment of patients.

AF, atrial fibrillation; M to S, moderate to severe.

2.1 Data collection

The following clinical data were collected for each patient enrolled: basic sociodemographic information (age, sex, body mass index [BMI], and level of education), combined CV risk factors (current smoking and drinking), type of AF, medical history (hypertension, chronic heart failure [CHF], established coronary artery disease [CAD], previous stroke, previous bleeding, chronic kidney disease [CKD], diabetes, etc.), CHA2DS2-VASc score, HAS-BLED score, results of laboratory and echocardiography tests, combined medications, and prior ablation therapy.

2.2 Follow-up and clinical outcomes

Each enrolled patient was followed up in the 1st, 3rd, and 6th months, then every 6 months thereafter by trained staff under outpatient settings or through a telephone interview. Information relating to the occurrence of adverse events was collected during follow-up. The follow-up period was the time from enrollment to the occurrence of endpoint events or the last follow-up time without events.

The primary endpoint was all-cause death and the secondary endpoints were the time to CV death or major bleeding. CV deaths included deaths caused by myocardial infarction, sudden cardiac death, heart failure, and other CV diseases. Major bleeding was defined according to the International Society on Thrombosis and Haemostasis (ISTH) criteria,16 that is, clinically overt bleeding accompanied by transfusion of at least 2 units of whole blood or packed red cells, bleeding that required hospitalization or caused permanent dysfunction, or bleeding in important anatomical area or organ, such as intracranial, intraspinal, intraocular, retroperitoneal, pericardial, and so on, or fatal bleeding. All events were adjudicated by an independent endpoint committee.

2.3 Definition of anemia

Baseline hemoglobin values were derived from the latest blood test result of each patient (<1 year). The World Health Organization's (WHO) criteria of anemia17 were used to diagnose and classify anemia. We divided the enrolled patients into three groups: mild anemia group (110≤Hb<129 g/L for male, 110≤Hb<119 g/L for female); moderate to severe anemia group (Hb≤109 g/L); and nonanemic group (Hb130g/L or higher for male; Hb120 g/L or higher for female) according to the severity of anemia.

2.4 Statistical analysis

Continuous variables were presented as mean ± standard deviation (SD) or median (interquartile range [IQR]) and compared among groups using one-way analysis of variance (ANOVA) tests or Kruskal–Wallis tests where appropriate. Categorical variables were expressed as numbers (percentages) and compared using χ2 tests. Event rates were presented as the number of events of primary and secondary endpoints per 100 patient-years during follow-up. Kaplan-Meier curves of event-free survival of all-cause death, CV death, or major bleeding were plotted by groups, and compared using the log-rank tests. Step-wised models (Model 0, Model 1, and Model 2) were performed by Cox proportional hazards regressions, adjusted for different sets of confounders, and the associations of anemia with clinical outcomes were presented with hazard ratios (HR) and their 95% confidence intervals (CIs). In Model 0, no confounder was adjusted; in Model 1, age and sex were adjusted; in Model 2, additional confounders were added to Model 1, including BMI, current smoking, education status, AF type, hypertension, CHF, CAD, stroke/transient ischemic attack/systemic embolism history, peripheral artery disease, bleeding history, CKD, diabetes, COPD, liver dysfunction, OACs, statins, antiplatelet drugs, angiotensin-converting enzyme inhibitors + angiotensin Ⅱ receptor blockers, and ablation history. A fully adjusted Cox model (with the same confounders in Model 2) with restricted cubic splines were plotted to examine the non-linear associations between Hb levels and all-cause death and CV death, respectively. Knots were placed at the 25th, 50th, and 75th percentile of the distribution of Hb value. Subgroup analyses were conducted to explore the interaction effects of anemia on the risk of all-cause death stratified by age, sex, CKD, CHF, bleeding history, OACs, and ablation therapy.

In sensitive analysis, Fine and Gray models were completed to analyze the associations between anemia severity with CV death or major bleeding, considering non-CV death or nonmajor bleeding as competing events, respectively. In all analyses, a two-sided p-value <.05 was considered statistically significant. Statistical analyses were performed using SAS version 9.4 (SAS Institute).

3 RESULT

In the current analysis, we excluded 5732 patients missing baseline Hb values, and baseline characteristics of included and excluded subjects are summarized in Table S1. Finally, a total of 18,106 AF patients with available baseline Hb values were enrolled in the present analysis, of which 15,606 patients (86.2%) were free of anemia, 1800 (9.9%) were diagnosed with mild anemia, and 700 (3.9%) with moderate to severe anemia. Baseline characteristics according to the severity of anemia are shown in Table 1. In general, patients with anemia were older, more likely to be female, had lower BMI, had more comorbidities. Both CHA2DS2-VASc and HAS-BLED scores were significantly higher in patients with anemia. OACs prescription and ablation therapy were less prevented in anemic patients.

Table 1. Baseline characteristics of patients among three study groups Patient characteristics at baseline No anemia (N = 15,606) Mild anemia (N = 1800) M to S anemia (N = 700) p value Demographics Age, years 62.5 ± 11.7 70.7 ± 10.9 72.7 ± 11.9 <.001 <65 8627 (55.3) 464 (25.8) 144 (20.6) <.001 65–74 4425 (28.4) 556 (30.9) 179 (25.6) <.001 ≥75 2554 (16.4) 780 (43.3) 377 (53.9) <.001 Female, n (%) 5558 (35.6) 774 (43.0) 470 (67.1) <.001 Personal characteristics BMI, kg/m² 25.7 ± 3.6 24.5 ± 3.9 24.5 ± 4.2 <.001 Normal (<24) 4307 (27.6) 722 (40.1) 286 (40.9) <.001 Overweight (24–28) 8054 (51.6) 846 (47.0) 328 (46.9) <.001 Obese (BMI ≥ 28) 3245 (20.8) 232 (12.9) 86 (12.3) <.001 Current smoking, n (%) 2654 (17.0) 201 (11.2) 48 (6.7) <.001 Current drinking, n (%) 3189 (20.4) 200 (11.1) 46 (6.6) <.001 Highly Educated, n (%) 4274 (27.4) 404 (22.4) 108 (15.4) <.001 AF type, n (%) Newly diagnosed 869 (5.6) 191 (10.6) 78 (11.1) <.001 Paroxysmal AF 9204 (59.0) 1046(58.1) 376(53.7) <.001 Persistent AF 5533 (35.5) 563 (31.3) 246 (35.1) <.001 AF duration ≥1 year, n (%) 8984 (57.6) 983 (54.6) 365 (52.1) 0.002 Comorbidities, n (%) Hypertension 9348 (59.9) 1273 (70.7) 500 (71.4) <.001 Chronic heart failure 1893 (12.1) 443 (24.6) 305 (43.6) <.001 Established CAD 2185 (14.0) 424 (23.6) 185 (26.4) <.001 Ischemic stroke/TIA/SE 2016 (12.9) 392 (21.8) 160(22.9) <.001 Peripheral artery disease 110 (0.7) 34 (1.9) 14 (2.0) <.001 Bleeding history 542 (3.5) 96 (5.3) 42 (6.0) <.001 CKD 1258 (8.1) 401 (22.3) 268 (38.3) <.001 Diabetes 3739 (24.0) 513 (28.5) 252 (36.0) <.001 COPD 125 (0.8) 25 (1.4) 18 (2.6) <.001 Liver dysfunction (TBIL > 34.2μmol/L or AST > 120 U/L or ALT > 165 U/L) 464 (3.0) 46 (2.6) 36 (5.1) 0.002 Hyperthyroidism/Hypothyroidism 764 (4.9) 92 (5.1) 38 (5.4) 0.766 CHA2DS2-VASc score 2.3 ± 1.7 3.4 ± 1.9 4.2 ± 1.9 <.001 ≥2, n (%) 9468 (60.7) 1522 (84.6) 636 (90.9) <.001 HAS-BLED score 1.6 ± 1.1 2.4 ± 1.3 3.4 ± 1.3 <.001 ≥3, n (%) 3234 (20.7) 834 (46.3) 541 (77.3) <.001 Laboratory analysis and echocardiography Hemoglobin(g/L) 146.7 ± 13.8 119.1 ± 5.7 97.9 ± 11.3 <.001 Heart rate (bpm) 78.9 ± 19.8 78.7 ± 21.5 80.2 ± 21.2 0.011 Left atrial diameter (mm) 40.4 ± 6.2 41.2 ± 7.4 41.8 ± 7.6 <.001 Left ventricular end diastolic dimension (mm) 48.4 ± 5.6 48.8 ± 6.4 48.2 ± 6.5 0.061 LVEF (%) 62.5 ± 8.4 61.8 ± 9.6 61.4 ± 9.7 0.019 Treatment, n (%) Antiarrhythmic drugs 6515 (41.8) 558 (31.0) 155 (22.1) <.001 Ventricular rate control drugs 6045 (38.7) 918 (51.0) 395 (56.4) <.001 Anticoagulant drugs 10853 (69.5) 933 (51.8) 267 (38.1) <.001 Antiplatelet drugs 3066 (19.7) 657 (36.5) 293 (41.9) <.001 Statins 5784 (37.1) 813 (45.2) 329 (47.0) <.001 ACEIs/ARBs 5038 (32.3) 751 (41.7) 291 (41.6) <.001 Ablation therapy 9764(62.6) 711 (39.5) 182 (26.0) <.001 Note: CKD was defined as eGFR< 60 ml/min·1.73 m2 (estimated by CKD-EPI equation). Abbreviations: ARBs, angiotensin Ⅱ receptor blockers; ACEIs, angiotensin-converting enzyme inhibitors; AST, aspartate amino transferase; ALT, alanine amino transferase; AF, atrial fibrillation; BMI, body mass index; CAD, coronary artery disease; COPD, chronic obstructive pulmonary disease; CKD, chronic kidney disease; LVEF, left ventricular ejection fraction; M to S, moderate to severe; SE, systemic embolism; TIA, transient ischemic attack; TBIL, total bilirubin. 3.1 Anemia and all-cause death

The average follow-up period was 4.01 years and a total of 1700 deaths were recorded. The incidence of all-cause death is displayed as event-free survival curves in Figure 2A. The crude event rates per 100 person-years were gradually increased in the no anemia, mild anemia, and moderate to severe anemia groups. Compared with the no anemia group (1.78; 95% CI: 1.68–1.89), the incidence of all-cause deaths in the mild anemia group increased by nearly twofold (4.86; 95% CI: 4.38–5.39, p < .0001), and quadrupled in the moderate to severe anemia group (8.90; 95% CI: 7.85–10.19, p < .0001) (Table 2).

Kaplan–Meier curves for the event-free survival among three study groups. (A) All-cause death, (B) cardiovascular death, and (C) major bleeding. M to S, moderate to severe

Table 2. Event rates and associations of anemia with clinical outcomes Model 0b Model 1c Model 2d Endpoints Number of events (event ratea) HR (95% CI) p value HR (95% CI) p value HR (95% CI) p value All-cause death No anemia 1116/1700 (1.78) 1.00 (reference) 1.00 (reference) 1.00 (reference) Mild anemia 358/1700 (4.86) 2.71 (2.41–3.05) <.001 1.41 (1.25–1.60) <.001 1.22 (1.08–1.38) .002 M to S anemia 226/1700 (8.90) 5.07 (4.39– 5.85) <.001 2.41 (2.08–2.80) <.001 1.53 (1.31–1.77) <.001 CV death No anemia 606/929 (0.97) 1.00 (reference) 1.00 (reference) 1.00 (reference) Mild anemia 210/929 (2.85) 2.90 (2.48–3.40) <.001 1.54 (1.31–1.82) <.001 1.29 (1.10–1.52) .002 M to S anemia 113/929 (4.47) 4.65 (3.80–5.68) <.001 2.21 (1.80–2.73) <.001 1.27 (1.03–1.57) .025 Major bleeding No anemia 323/389 (0.51) 1.00 (reference) 1.00 (reference) 1.00 (reference) Mild anemia 47/389 (0.62) 1.18 (0.87–1.60) .297 0.90 (0.66–1.23) .506 0.91 (0.67–1.25) .568 M to S anemia 19/389 (0.71) 1.42 (0.89–2.25) .141 1.00 (0.62–1.60) .995 1.07 (0.66–1.73) .777 Abbreviations: ARBs, angiotensin Ⅱ receptor blockers; ACEIs, angiotensin-converting enzyme inhibitors; BMI, body mass index; CAD, coronary artery disease; CHF, chronic heart failure; CAD, coronary artery disease; COPD, chronic obstructive pulmonary disease; CKD, chronic kidney disease; CV, cardiovascular; HR, hazard ratio; CI, confidence interval; M to S, moderate to severe; OAC, oral anticoagulant; TIA, transient ischemic attack.

On Multivariate Cox regression analysis from the fully adjusted model (Model 2), baseline anemia was significantly related to increased risk for all-cause death (mild; adjusted HR: 1.22, 95% CI: 1.08–1.38; moderate to severe; adjusted HR: 1.53, 95% CI: 1.31–1.77) in AF patients, compared with those without anemia (Table 2). Notably, a dose-dependent relationship of anemia with all-cause death was observed. When Hb was considered as a continuous variable, as shown in Figure 3A, decreasing Hb was associated with an increased risk of all-cause death. The cutoff value was 143 g/L.

Graphic representation of the HR (95% CI) for all-cause death according to baseline hemoglobin levels. Adjusted for age, sex, BMI, current smoking, education status, AF type, hypertension, CHF, CAD, stroke/TIA/SE history, peripheral artery disease, bleeding history, CKD, diabetes, COPD, liver dysfunction, OACs, statins, antiplatelet drugs, ACEIs+ARBs, and ablation therapy. ARBs, angiotensin Ⅱ receptor blockers; ACEIs, angiotensin-converting enzyme inhibitors; AF, atrial fibrillation; CAD, coronary artery disease; CHF, chronic heart failure; CAD, coronary artery disease; COPD, chronic obstructive pulmonary disease; TIA, transient ischemic attack

Subgroup analyses stratified separately by age, sex, CHF, CKD, OACs, and ablation therapy were conducted across different groups (Table S2). Interaction analysis revealed a significant interaction with age for the all-cause death. Though anemia was associated with worsening outcomes in both age groups, the association was consistently stronger in younger patients (age < 65). In the moderate to severe anemia group, patients without CHF had higher risks of all-cause death compared with CHF patients, while no difference between subgroups was found in the mild anemia group. The association between anemia and the all-cause death did not differ according to sex, kidney function, and treatment allocation (p for interaction >.05, Table S2).

3.2 Anemia and CV death

During the follow-up period, a total of 929 CV deaths were recorded. A significant association was also observed between anemia and CV death. When compared with no anemia, mild anemia (adjusted HR: 1.29, 95% CI: 1.10–1.52), and moderate to severe anemia (adjusted HR: 1.27, 95% CI: 1.03–1.57) were significantly associated with a higher risk of CV death (Table 2). The same findings were observed in the analyses using Hb as a continuous variable (Figure 3B).

3.3 Anemia and major bleeding

A total of 389 major bleeding events occurred. The event rates and 95% CIs were 0.51 (95% CI: 0.46–0.57), 0.62(95% CI: 0.47–0.83), 0.71(95% CI: 0.46–1.12) per 100 person-years for the no anemia group, mild anemia group, and moderate to severe anemia group, respectively (Table 2). Upon both univariate and multivariate analysis, there were no significant statistical correlations between anemia and major bleeding in subjects (both p-value＞.1; Table 2).

3.4 Sensitivity analyses

When death was considered a competing risk, the results remained unchanged. Lower hemoglobin was still associated with a significantly higher risk of CV death. The association between anemic status and major bleeding also remained statistically insignificant (Table S3).

4 DISCUSSION

Our principal findings in this analysis from a contemporary nationwide AF registry are summarized as follows: (i) anemia was present in 13.8% of the studied population, which indicates anemia as common comorbidity in AF patients; and (ii) anemia is associated with a significant increase in the adjusted risk of both all-cause death and CV death wherein progressive degrees of anemia portended worse outcomes, but not for major bleeding in AF patients.

4.1 Anemia and death

Baseline hemoglobin level or anemia is associated with a 41% increase in all-cause mortality among the general population8 while a 78% increase in AF population,10 which indicate anemia may exacerbate AF progression through chronic cardiac remodeling. In our analysis, baseline anemia was statistically associated with both all-cause death and CV death in AF patients in which lower hemoglobin is strongly related to poor survival outcomes. Interestingly, we were able to demonstrate a stronger association in the younger age group compared with older patients for all-cause death. This conclusion is consistent with results in post-hoc analysis of RE-LY4 and ARISTOTLE3 trials.

Currently, we are unable to explain the etiology of this interaction, but we speculate that younger patients potentially experienced prolonged severity and duration of anemia, while anemia is more physiological in older patients. Also, normal Hb distribution varies not only with sex but also with ethnicity and physiological status. Although there is still no uniform standard, new lower limits of normal Hb values have been proposed according to age.18 Thus, normality ranges of Hb may be interpreted differently in the young and elderly.

There are pathophysiological explanations to the hypothesis that the presence of anemia could directly lead to worse outcomes among AF patients. In a perennial anemic state, hypoxemia and hypoperfusion trigger an increase in heart rate and stroke volume to increase cardiac output in a reflex response, leading to cardiac remodeling. Dilation of the left chambers secondary to anemia results in increased systolic wall stress and compensatory hypertrophy.19 Indeed, anemia may be an independent predictor of subclinical myocardial damage, after adjusting for confounders.20 In addition, anemia has been suggested to be related to the inflammation response,21 which could be involved in affecting the prognosis of AF through inflammation-induced alteration of electrophysiological properties, remodeling of cardiac structure, and enhancement of fibrosis.22 All these factors could gradually lead to heart failure and potentially worsen myocardial oxygen supply.