2Department of Cardiology, First Affiliated Hospital of Huzhou University (the First People's Hospital of Huzhou), Huzhou, Zhejiang Province, China
Abstract
Background: Cardiovascular disease (CVD) is a leading cause of death in older adults and is closely associated with inflammation. The aggregate index of systemic inflammation (AISI), a novel biomarker, may predict CVD mortality in this population. To analyze the association between AISI levels and CVD mortality in the older population.
Methods: This study was based on the National Health and Nutrition Examination Survey (NHANES) database. By constructing weighted Kaplan–Meier (K-M) survival curves and Cox proportional hazards models, the link between AISI levels and CVD mortality rate were analyzed in the elderly. The restricted cubic spline (RCS) was applied to elucidate the non-linear link. A random survival forest model was constructed to assess the predictive value of multiple variables.
Results: One thousand three hundred nineteen CVD death events were recorded. The weighted K-M survival curve manifested that the CVD mortality risk was considerably higher in the highest tertile group than in the lowest tertile. In the model with full adjustments, each one-unit increase in AISI was associated with a 1.52-fold higher risk of death
(HR = 1.52, 95% CI: 1.30-1.76, P < .001), and a non-linear relationship was detected (P-non-linear = .0001). When AISI was above the threshold of 263.43, the CVD mortality risk was significantly elevated (HR = 1.99, 95% CI: 1.59-2.49, P < .001). No significance was observed below this threshold. AISI had the highest predictive value for CVD mortality in the elderly.
Conclusion: The AISI is an effective indicator for predicting the CVD mortality risk in the elderly, especially when AISI reaches high levels.
#This work was equally contributed to by the authors.
Highlights
- This article reveals for the first time that there is a significant non-linear positive correlation between aggregate index of systemic inflammation (AISI) and the risk of cardiovascular disease (CVD) death in the elderly population, and the threshold point is determined to be 263.43.
- This article innovatively uses Cox regression and random survival forest models to mutually verify the reliability of AISI as an independent predictor of CVD mortality risk in the elderly population.
- The AISI has the potential to serve as a simple and easily accessible auxiliary tool for timely identification of high-risk CVD populations in the elderly population in clinical practice.
Introduction
The health status and disease management of the elderly have become a focus of public health due to the growing worldwide aging population. Cardiovascular disease (CVD) contributes to the largest global death toll among the elderly,
In recent years, the aggregate index of systemic inflammation (AISI) has received widespread attention as an emerging comprehensive inflammatory indicator, consisting of neutrophil count, platelet count, monocyte count, and lymphocyte count.
Therefore, the mechanism by which AISI affects CVD mortality may work synergistically through 3 core pathological pathways. Congenital immune overactivation (neutrophils and monocytes) directly leads to endothelial damage and progression of atherosclerosis.
The objective of this study was to explore the link between AISI and CVD mortality in the elderly by analyzing a large sample size of data from the National Health and Nutrition Examination Survey (NHANES). Based on existing research progress, the hypothesis was proposed that there is an independent positive correlation between AISI and CVD mortality rate in the elderly population, and this correlation exhibits a non-linear threshold effect. It was intended to reveal the potential value of AISI in predicting CVD mortality in the elderly and provide a theoretical basis for future clinical applications.
Methods
Study Population from National Health and Nutrition Examination Survey
The population data used in this study were available through the NHANES database,
The link between AISI and CVD mortality in the elderly was probed using survey data from 10 cycles from 1999 to 2018 (n = 101 316). The exclusion criteria are as follows: (1) excluding participants younger than 60 (n = 82 229); (2) excluding participants with lacking or invalid AISI data (n = 2164); and (3) excluding participants with missing data on other covariate variables (n = 3401). A total of 13 522 elderly subjects were studied. The specific process of subject screening is displayed in
Independent Variable
The calculation of AISI was based on indicators in the whole blood cell count. The blood sampling followed the NHANES standardized protocol: venous blood samples were taken on an empty stomach for more than 8 hours and were collected at a mobile examination center. The complete blood cell count was measured using a Beckman Coulter automatic blood analyzer to ensure data reliability.
AISI = neutrophil count × platelet count × monocyte count/lymphocyte count.
Dependent Variable
The National Death Index records as of December 31, 2019 were the reference for us to determine the mortality outcomes. The underlying causes of mortality were assessed by the International Statistical Classification of Diseases, 10th Revision (ICD-10).
Variables
Covariates in this investigation included gender, body mass index (BMI), ethnicity, education level, poverty income ratio (PIR), red blood cell (RBC) count, diabetes, smoking, white blood cell (WBC) count, alcohol consumption, hypertension, hemoglobin, and mean RBC volume. Subjects fell into 3 distinct PIR categories: low income (PIR ≤ 1.3), moderate income (1.3 < PIR ≤ 3.5), and high income (PIR > 3.5).20 Body mass index was calculated as weight (kg) divided by the square of height (m), and classified as obese (>30 kg/m2), overweight (25-30 kg/m2), and underweight/healthy weight (< 25 kg/m2).21 Smoking status was grouped into 3 groups based on the smoking history and current smoking behavior of the subjects: never smokers (reporting a total of less than 100 cigarettes smoked), former smokers (reporting a total of 100 or more cigarettes smoked but currently not smoking), and current smokers (reporting a total of 100 or more cigarettes smoked and still smoking).
Statistical Analysis
We completed all statistical analyses in this work by using R software (V4.4.1). The
The
In the model with adjustment for all covariates, a restricted cubic spline (RCS) analysis was conducted by utilizing the
The
Time-dependent ROC curves were calculated using the
Sensitivity analysis was conducted to ensure the robustness of the results: (1) As NHANES aims to represent the health status of the non-institutionalized American population, participants with acute or chronic inflammatory conditions were not excluded from this study. However, in order to assess the impact of related confounding factors, participants with hepatitis B virus infection, self-reported cancer, or rheumatoid arthritis were excluded, and 8632 participants were included for weighted Cox regression analysis. (2) After removing blood parameters, Model 3 was constructed for weighted Cox regression analysis. Model 3 adjusted for gender, race, BMI, educational level, PIR, smoking, drinking, diabetes, and hypertension.
Results
Baseline Characteristics
To probe into the relationship between AISI and CVD mortality in the elderly, 13 522 samples from NHANES 1999-2018 were included. Of these, 1319 (8.5%) died from CVD. According to the AISI-weighted tertiles of the subjects, there were 364 deaths (6.5%) in Group T1, 413 deaths (7.9%) in Group T2, and 542 deaths (11.1%) in Group T3. Compared to participants in the lowest tertile of AISI, those with higher AISI were always male, former and current smokers, obese (BMI ≥ 30), and individuals with comorbidities such as diabetes and hypertension. Additionally, the levels of WBC count, RBC count, and hemoglobin in subjects with higher AISI were considerably higher than those in subjects with the lowest tertile (
Relation Between Aggregate Index of Systemic Inflammation and Cardiovascular Disease Mortality in the Elderly
The weighted K-M survival curve manifested the survival probability trend in AISI-weighted tertile groups. The mortality risk was highest in the T3 group over time (
Nonlinear Relationship Between Aggregate Index of Systemic Inflammation and Cardiovascular Disease Mortality Rate in the Elderly Population
Further, the threshold effect model was applied and RCS to probe into the nonlinear link between AISI and CVD mortality rate in the elderly, revealing a significant overall trend between AISI and the CVD mortality risk (
The threshold effect results implied no statistically significant link with the risk of CVD mortality in the elderly when AISI < 263.43 (
Prognostic Value of Aggregate Index of Systemic Inflammation
We developed an RSF model to examine the value of AISI in predicting CVD mortality in the elderly. The results demonstrated that AISI was the most effective predictor of CVD mortality in the elderly compared to other variables (
Subsequently, the predictive performance of the model was tested using the ROC curves. The model had AUC values of 0.729, 0.711, and 0.749 for predicting CVD mortality in elderly individuals at 3, 5, and 10 years, respectively, suggesting that the RSF model possessed good predictive ability (
The Incremental Value of Aggregate Index of Systemic Inflammation Prediction Model
We used a weighted Cox model (Model 3) to construct models with and without AISI and evaluated predictive performance using Harrell’s C-index and time-dependent AUC (3, 5, and 10 years). The results showed that the C-index of the model containing AISI was 0.741, which was higher than that of 0.733 in the model without AISI. The time-dependent AUC showed that the model containing AISI had better predictive performance at 3 years (0.765 vs. 0.751), 5 years (0.757 vs. 0.745), and 10 years (0.753 vs. 0.747) (
In addition, the AISI model showed a statistically significant improvement in predictive ability compared to the baseline model (without AISI). The estimated value of IDI was greater than 0, and at 10 years, the model’s integrated discriminative ability was improved by a net 1.5% (IDI = 0.015,
Discussion
In this nationally representative large-scale study, a significant positive relation between AISI and CVD mortality in the elderly population was revealed for the first time. The threshold effect analysis further demonstrated that when AISI reached or exceeded 263.43, the elderly had a considerably elevated risk of CVD mortality. In addition, the RSF model also verified AISI as a strong indicator for predicting CVD mortality in the elderly.
Mounting studies have revealed that chronic inflammation is essential for the pathogenesis of atherosclerosis and other CVDs.
Inflammatory biomarkers are positively linked with the risk and mortality rate of CVD. A prospective study demonstrated that higher levels of CRP are associated with an elevated risk of heart failure in CVD patients.
It was considered that individuals with higher AISI levels often have more complications (such as diabetes and obesity), so the association between AISI and CVD mortality may be disturbed by such factors. To verify whether AISI has an independent predictive effect on comorbidities, a multicollinearity diagnosis was performed using the variance inflation factor, and the results confirmed that there was no serious collinearity issue between AISI and other comorbidities. Further subgroup analysis revealed that AISI maintained stable predictive ability in different clinical feature populations, supporting its robustness as an independent predictor. Based on the above analysis, AISI is not just a substitute indicator for known risk factors, but more likely an independent risk marker that can reflect the potential immune-inflammatory status of the body.
The threshold effect analysis uncovered that the CVD mortality in the elderly population significantly rose when AISI reached or exceeded 263.43 (HR = 1.99), indicating that for every unit increase in AISI, the relative risk of CVD occurrence increased by 99%. No significant statistical significance was detected below this threshold. This suggested that the CVD mortality rate in the elderly population significantly rose only when the inflammatory state reached a certain level. The weighted K-M survival curves also confirmed this finding, demonstrating a significantly elevated risk of CVD death only in the highest tertile (T3) group, with no obvious difference in CVD mortality risk observed between the T1 and T2 groups. These findings echo a former study revealing that as AISI exceeds 507.45, the mortality rate of stroke patients significantly rises with the increase of AISI values.
A hallmark of aging is systemic chronic inflammation, a phenomenon known as inflammation.
Although this study offered evidence supporting AISI as an indicator for predicting CVD mortality risk in the elderly, certain shortcomings persist. First, though this prospective cohort study had a large sample size and lasted for a long follow-up period, a causal relationship cannot be established due to the nature of observational studies. Individuals with preclinical or undiagnosed subclinical CVD may already have more severe systemic inflammatory states in their bodies, leading to elevated levels of AISI. In this case, the increase in AISI may be a result of potential CVD rather than the cause, which could affect the interpretability of the research results. Secondly, the research focuses on the elderly population in the United States, and caution should be exercised when generalizing the research results to other healthcare environments, younger populations, or populations in other countries, as there may be differences in lifestyle, access to medical resources, and disease prevalence among different regions or age groups. Thirdly, NHANES only provides a single record of laboratory blood cell count and cannot evaluate changes in AISI over time, which may not reflect the average level of long-term follow-up. In future research, measurements of AISI can be repeated at multiple time points to explore the relationship between the dynamic changes of AISI and the risk of CVD in elderly people. Lastly, although the independent predictive value of AISI has been validated through various statistical methods, there may still be other confounding factors that have not been considered, such as dietary details, genetic factors, etc., that may affect the results. Therefore, future large-scale clinical trials are recommended to validate the findings of this study.
Conclusion and Recommendations
This study found that AISI is an independent predictor of CVD mortality risk in the elderly population, especially for those at high AISI levels (AISI ≥ 263.43). Therefore, AISI has the potential to serve as an objective and easily accessible auxiliary tool for the timely identification of high-risk populations in clinical practice, and can also be integrated into other CVD risk assessment models, which may optimize the predictive value of prediction models.
Supplementary Materials
Footnotes
Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.
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