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. 2023 May 27;28(4):e13066. doi: 10.1111/anec.13066

Value of frontal QRS axis for risk stratification of individuals with prolonged PR interval

Xiaodi Cao 1, Zhe Wang 1, Zhang Fang 1, Chuanchuan Yu 2, Linsheng Shi 3,4,✉
PMCID: PMC10335622  PMID: 37243938

Abstract

Background

There is ongoing controversy regarding the prognostic value of PR prolongation among individuals free of cardiovascular diseases. It is necessary to risk‐stratify this population according to other electrocardiographic parameters.

Methods

This study is based on the Third National Health and Nutrition Examination Survey. Cox proportional hazard models were constructed and Kaplan–Meier method was used.

Results

A total of 6188 participants (58.1 ± 13.1 years; 55% women) were included. The median frontal QRS axis of the entire study population was 37° (IQR: 11–60°). PR prolongation was present in 7.6% of the participants, of whom 61.2% had QRS axis ≤37°. In a multivariable‐adjusted model, mortality risk was highest in the group with concomitant prolonged PR interval and QRS axis ≤37° (hazard ratio [HR]: 1.20; 95% confidence interval [CI]: 1.04–1.39). In models with similar adjustment where population were reclassified depending on PR prolongation and QRS axis, prolonged PR interval and QRS axis ≤37° was still associated with increased risk of mortality (HR: 1.18; 95% CI: 1.03–1.36) compared with normal PR interval.

Conclusions

QRS axis is an important factor for risk stratification in population with PR prolongation. The extent to which this population with PR prolongation and QRS axis ≤37° is at higher risk of death compared with the population without PR prolongation.

Keywords: electrocardiogram indicators, PR interval, QRS axis, risk stratification

The QRS axis is an important risk stratification factor for prolongation of the PR interval in people without cardiovascular disease. Evaluation of the QRS axis is particularly necessary in patients whose electrocardiogram suggests prolongation of the PR interval.

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1. INTRODUCTION

The PR interval measured from the surface electrocardiogram (ECG) is considered an indicator reflecting atrial and atrioventricular node myocyte depolarization and conduction (Schwartzman, 2004). PR prolongation, usually defined as PR interval >200 ms, is a relatively common electrocardiographic finding with the prevalence of 2%–6% in the general population (Aro et al., 2014; Cheng et al., 2013; Kwok et al., 2016). However, the prognosis value of PR prolongation is controversial due to many factors can influence the PR interval (Holmqvist & Daubert, 2013). Previous studies have demonstrated that PR prolongation is associated with the occurrence of atrial fibrillation and left ventricle (LV) dysfunction (Cheng et al., 2015; Smith et al., 2017; Uhm et al., 2014). The prolonged PR interval is also related to an increased risk of death in patients with cardiovascular disease (CVD) including heart failure (HF), coronary artery disease, and hypertrophic cardiomyopathy (HCM) (Crisel et al., 2011; Friedman et al., 2016; Higuchi et al., 2020). But in the general population, especially those without CVD, the association between PR interval and adverse outcomes is not significant, so abnormal PR interval has not received widespread attention in clinical work (Aro et al., 2014; Magnani et al., 2013; Vad et al., 2021).

The P‐wave duration is a crucial component of the PR interval, and the link between P‐wave duration prolongation and adverse outcomes, including increased risk of atrial arrhythmias and mortality, is well established (Magnani et al., 2011). Previous studies using data from the Third National Health and Nutrition Examination Survey (NHANES III) have found that the prognostic significance associated with PR prolongation depends largely on the duration of P wave (Soliman et al., 2014), but information related to P waves has rarely been reported in studies of PR interval prolongation. Admittedly, hemodynamic abnormalities have indeed been found in people with prolonged PR time, resulting in impaired effective left ventricular diastolic filling time (Schnittger et al., 1988). Another important hemodynamic effect of prolonged PR is diastolic mitral regurgitation (MR). Increased MR, decreased left ventricular compliance, and impaired relaxation are inseparable (Ishikawa et al., 1992; Zile et al., 1991, 1993).

Based on the previous results, we proposed that LV diastolic function may be a significant factor in determining risk stratification in people with prolonged PR interval. The QRS axis is an important ECG parameter that reflects the diastolic function of the LV (Kurisu et al., 2018), and the QRS loop reflects the depolarization process of the ventricle. The spatial QRS‐T angle has been utilized for stratify heart disease risk, while the frontal and spatial QRS‐T angle have been used to predict future cardiac events (including sudden death, all‐cause mortality, and further heart disease morbidity) (Aro et al., 2012; Kors et al., 2003; Pavri et al., 2008). However, there is a lack of research on risk stratification using the QRS axis in patients with long PR intervals. Therefore, we designed this study to identify those with poor prognosis for PR interval prolongation in people without CVD by using QRS axis.

2. METHODS

NHANES III is a program of studies conducted by the National Center for Health Statistics (NCHS) designed to assess the prevalence of disease, disease risk factors, and nutritional status of civilian non‐institutionalized US population. Linked mortality information through April 26, 2022, was provided by the NCHS of the Centers for Disease Control and Prevention (CDC). The NCHS of CDC conducts statistical and epidemiological activities under the authority granted by the Public Health Service Act. NCHS survey data are protected by Federal confidentiality laws. Data collection was reviewed and approved by the National Center for Health Statistics Research Ethics Review Board and signed informed consent forms were obtained from study participants. All data in this study are publicly available at https://www.cdc.gov/nchs/nhanes/index.htm.

2.1. Study population

In NHANES III, 6990 participants (aged 40–90 years) with normal sinus rhythm and no evidence of bundle‐branch block or intraventricular conduction delay (QRS ≥120 ms) were recruited. Exclusion criteria included: (1) Participants with CVD, including HF (N = 306), myocardial infarction (N = 262), and stroke (N = 185). (2) Electrocardiographic data were unreliable (N = 41) and follow‐up data were missing (N = 8). A total of 6188 participants were included in the study. Eligible subjects were divided into four groups based on different combinations of the PR interval and QRS axis (Figure 1).

FIGURE 1.

FIGURE 1

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The flow chart of study case selection.

2.2. Electrocardiography

During participant examinations, standard 12‐lead ECGs were recorded by trained technicians on the Marquette MAC 12 system (Marquette Medical Systems, Inc.). Computerized analysis of the ECG data was performed using Minnesota and Nova codes. PR prolongation on the ECG is defined as PR interval >200 ms. The population was divided into two groups according to the presence or absence of PR prolongation, and then the two groups were subdivided according to the median of the QRS axis (QRS axis median = 37°) in the total population. That is, the population was divided into normal PR interval with QRS axis >37°, normal PR interval with QRS axis ≤37°, prolonged PR interval with QRS axis >37°, and prolonged PR interval with QRS axis ≤37°. Other parameters included in the analysis included P‐wave duration, heart rate, P‐wave frontal axis, P‐wave terminal force in V1, QT interval, and QRS duration. The QT interval was corrected by Bazette's formula.

2.3. Other variables

Data on age, sex, race, and household income were obtained in household interviews. Ethnicities were classified as white and others. Income was divided into <$20 K/year and ≥$20 K/year. Smoking status was obtained through the corresponding questionnaires and analyzed as a 2‐level categorical variable (current and other). Body mass index (BMI) was defined as body weight (kg) divided by height (m) squared. The history of some diseases, such as cancer and diabetes mellitus (DM), was self‐reported. Family history of CVD and use of antihypertensive as well as atrioventricular nodal drugs, including β‐blockers or dihydropyridine calcium blockers, were also obtained through self‐reporting. Blood pressure was the average of six measurements (three in‐home measurements and three mobile center measurements). Serum total cholesterol and high‐density lipoprotein cholesterol (HDL‐c) levels were determined enzymatically.

2.4. Statistical analysis

Continuous variables were expressed as mean ± standard deviation or median values [first quartile, third quartile], and categorical variables were expressed as proportions. For between‐group comparisons, the ANOVA test and the Kruskal–Wallis test were used for continuous variables, while the χ 2 test was used for comparison of categorical variables. We then performed the Kaplan–Meier (KM) method and log‐rank tests to determine survival differences between the four groups. The Cox proportional hazards model was applied to calculate the hazard ratio (HR) and 95% confidence interval (CI) for each group in the cohort. In the adjusted full model, we regrouped the study population by combining all individuals with normal PR intervals and calculating the median HR for each group. We performed three sensitivity analyses to assess the robustness of the findings. Subgroup analyses were also performed in clinically relevant subgroups and interactions were tested. Missing data for covariates were interpreted using multiple imputation methods. All data analysis was performed using R statistical software (version 4.2.1).

3. RESULTS

Among the 6188 participants in NHANES III (mean age 58.1 ± 13.1 years; 55.0% female), 4463 (72.1%) were white. PR prolongation occurred in 7.6% (N = 472) of participants, of which 61.2% had QRS axis ≤37°. Table 1 showed subject characteristics stratified by PR extension and QRS axis. Participants with prolonged PR and QRS axis ≤37° were more likely than other groups, especially the normal PR interval, to be older, male, low‐income, with prolonged P‐wave duration, and taking antihypertensive drugs and AV nodal drugs.

TABLE 1.

Baseline characteristics of the study population.

Characteristics Normal PR interval (N = 5716) Prolonged PR interval (N = 472) p‐Value
QRS axis >37° QRS axis ≤37° QRS axis >37° QRS axis ≤37°
(N = 2891) (N = 2825) (N = 183) (N = 289)
Age, years 55.1 ± 12.2 60.0 ± 13.1 60.1 ± 13.1 67.8 ± 13.3 <.001
Female sex, % 1672 (57.8) 1534 (54.3) 79 (43.2) 119 (41.2) <.001
White, % 2104 (72.8) 2046 (72.4) 113 (61.7) 201 (69.6) .009
Total annual income <$20 K, % 1181 (41.5) 1345 (48.5) 74 (40.9) 139 (49.6) <.001
Current smoke, % 797 (27.6) 511 (18.1) 55 (30.1) 40 (13.8) <.001
Body mass index, kg/m2 26.6 ± 5.4 28.5 ± 5.4 27.1 ± 5.7 29.1 ± 5.8 <.001
Diabetes mellitus, % 214 (7.4) 353 (12.5) 16 (8.7) 35 (12.1) <.001
History of CVD, % 384 (13.5) 297 (10.7) 21 (11.6) 28 (10.0) .007
Antihypertensive medications use, % 539 (18.8) 888 (31.5) 67 (36.6) 131 (45.6) <.001
Cancer, % 247 (8.5) 283 (10.0) 18 (9.8) 53 (18.3) <.001
AV nodal drug use a , % 134 (4.6) 186 (6.6) 21 (11.5) 36 (12.5) <.001
HDL‐c, mg/dL 52.7 ± 16.5 50.6 ± 16.2 51.7 ± 17.9 49.9 ± 16.1 <.001
Diastolic blood pressure, mmHg 76.8 ± 11.7 79.0 ± 12.2 77.1 ± 13.8 76.3 ± 12.7 <.001
Systolic blood pressure, mmHg 129.2 ± 19.9 136.2 ± 20.2 132.8 ± 20.1 138.4 ± 21.9 <.001
Corrected QT interval, ms 428.7 ± 22.7 432.2 ± 23.6 423.6 ± 25.0 425.7 ± 25.5 <.001
Heart rate, bpm 68.9 ± 11.2 68.7 ± 11.5 65.0 ± 10.0 63.9 ± 11.4 <.001
PR interval, ms 156.4 ± 21.6 159.7 ± 20.3 215.3 ± 14.3 220.0 ± 20.6 <.001
QRS duration, ms 94.6 ± 9.8 95.7 ± 9.7 95.5 ± 10.9 96.1 ± 10.6 <.001
P‐wave duration, ms 108.6 ± 12.8 111.7 ± 13.5 123.7 ± 14.4 127.9 ± 15.9 <.001
Negative P‐wave V1, uV −33.0 [−51.0, −18.0] −39.0 [−55.0, −24.0] −41.0 [−60.0, −27.0] −48.0 [−64.0, −33.0] <.001
P‐wave axis, ° 67.0 [53.0, 77.0] 59.0 [44.0, 70.0] 66.0 [48.5, 77.0] 55.0 [41.0, 68.0] <.001
QRS axis, ° 60.0 [49.0, 73.0] 12.0 [−6.0, 25.0] 60.0 [49.5, 72.0] 5.0 [−11.0, 22.0] <.001
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Note: Values are expressed as mean ± SD, N (%), or median values [first quartile, third quartile].

Abbreviations: CVD: cardiovascular disease; HDL‐c, high‐density lipoprotein cholesterol.

a

AV nodal drug use: β‐blocker or dihydropyridine calcium blocker.

A total of 3541 deaths occurred during the median follow‐up of 298 months (IQR: 16–69 months). Mortality was highest in participants with prolonged PR interval and QRS axis ≤37° (5.23/100 person‐years), while mortality was lowest in those with normal PR interval and QRS axis >37° (2.29/100 person‐years). Mortality was lower on the QRS axis >37° on PR prolongation than on the QRS axis ≤37° without extension (2.82 vs. 3.11 per 100 person‐years). Figure 2 showed the K‐M survival curves for the four groups, all with statistically significant differences between groups.

FIGURE 2.

FIGURE 2

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Kaplan–Meier survival curves. Group 1: Normal PR interval and QRS axis >37°. Group 2: Normal PR interval and QRS axis ≤37°. Group 3: Prolonged PR interval and QRS axis >37°. Group 4: Prolonged PR interval and QRS axis ≤37°.

First, we included PR interval prolongation and QRS axis ≤37° as two separate variables into the same model. In the unadjusted model, both were associated with an increased risk of all‐cause mortality (PR prolonged: [HR: 1.59; 95% CI: 1.43–1.78]; QRS axis ≤37°: [HR: 1.45; 95% CI: 1.35–1.55]). However, in the fully adjusted model (adjusted for age, sex, race, income, smoking status, BMI, diabetes, family history of CVD, antihypertensive drug use, AV node drug use, cancer, HDL cholesterol, heart rate, diastolic blood pressure, systolic blood pressure, QRS duration, P‐wave duration, corrected QT interval, and negative P‐wave V1), we found a weakened association between the two and death (PR prolongation: [HR: 1.03; 95% CI: 0.92–1.15]; QRS axis ≤37°: [HR: 1.01; 95% CI: 0.94–1.15]).

Then, we combined the PR interval and QRS axis into four groups (normal PR interval and QRS axis >37°, normal PR interval and QRS axis ≤37° [reference group], PR interval prolongation and QRS axis >37°, PR interval prolongation and QRS axis ≤37°), incorporating them into the Cox proportional hazard model for analysis (Table 2). We found that prolonged PR interval and QRS axis ≤37° group was associated with a 1.8‐fold increased risk of death in unadjusted models compared with normal PR interval and QRS axis ≤37° group. After adjusting for all risk factors and potential confounders, the risk of death remained highest with prolonged PR interval and QRS axis ≤37° group (HR: 1.20; 95% CI: 1.04–1.39). The risk of death was lowest when the PR interval was prolonged and the QRS axis >37° (HR: 0.80; 95% CI: 0.65–0.98). The results were shown in Figure 3.

TABLE 2.

Hazard ratios (HRs) and 95% confidence intervals (CIs) for all‐cause mortality by PR interval and QRS axis.

PR interval QRS axis HR (95% CI)
Model 1 a Model 2 b Model 3 c
Normal >37° 0.71 (0.67–0.77) 1.03 (0.96–1.11) 1.05 (0.97–1.13)
Normal ≤37° 1 (Ref.) 1 (Ref.) 1 (Ref.)
Prolonged >37° 0.89 (0.73–1.09) 0.79 (0.65–0.97) 0.80 (0.65–0.98)
Prolonged ≤37° 1.82 (1.59–2.08) 1.21 (1.05–1.39) 1.20 (1.04–1.39)
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a

Model 1: unadjusted.

b

Model 2: adjusted for age, sex, race, income, smoke status, and body mass index (BMI).

c

Model 3: adjustment for age, sex, race, income, smoke status, BMI, diabetes mellitus, family history of cardiovascular disease, antihypertensive medications use, AV nodal drug use, cancer, high‐density lipoprote cholesterol, heart rate, diastolic blood pressure, systolic blood pressure, QRS duration, P‐wave duration, corrected QT interval, and Negative P‐wave V1.

FIGURE 3.

FIGURE 3

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Pairwise comparisons of hazard ratios for all‐cause mortality between the four groups in Model 3. *p < 0.05; **p < 0.01; ***p < 0.001. Model 3: Adjustment for age, sex, race, income, smoke status, body mass index, diabetes mellitus, family history of cardiovascular disease, antihypertensive medications use, AV nodal drug use, cancer, high‐density lipoprote cholesterol, heart rate, diastolic blood pressure, systolic blood pressure, QRS duration, P‐wave duration, corrected QT interval, and Negative P‐wave V1.

Finally, we replaced the 4‐level variables with 3‐level variables (normal PR interval [reference group], PR interval prolongation and QRS axis >37°, PR interval prolongation and QRS axis ≤37°) into the Cox proportional hazard model. Sensitivity analysis and subgroup analysis were performed (Table 3). We found that prolonged PR interval and QRS axis ≤37° group was associated with an increased risk of death compared with the normal PR interval (HR: 1.18; 95% CI: 1.03–1.36). In the sensitivity analysis, the results were similar to our main analysis. In subgroup analyses, age, sex, race, DM, and antihypertensive drug use were stratified to examine the association between different combinations of PR interval and QRS axis and mortality. Similar directions were observed in each subgroup, and there was no significant interaction between the components of each subgroup.

TABLE 3.

Hazard ratios and 95% confidence intervals for all‐cause mortality by QRS axis.

Population Normal PR interval Prolonged PR interval p for interaction
QRS axis >37° QRS axis ≤37°
Total 1 (Ref.) 0.78 (0.64–0.95) 1.18 (1.03–1.36)
Sensitivity analysis
P/PR ratio <0.7 1 (Ref.) 0.77 (0.62–0.95) 1.18 (1.00–1.38)
Exclude events within the 1st year 1 (Ref.) 0.77 (0.63–0.95) 1.18 (1.02–1.36)
Exclude patients taking AV node drugs 1 (Ref.) 0.79 (0.63–0.97) 1.23 (1.05–1.43)
Subgroup analysis
Sex .434
Men 1 (Ref.) 0.81 (0.62–1.04) 1.22 (1.01–1.47)
Women 1 (Ref.) 0.72 (0.52–1.00) 1.09 (0.87–1.36)
Race .156
Whites 1 (Ref.) 0.85 (0.67–1.08) 1.22 (1.04–1.45)
Others 1 (Ref.) 0.66 (0.46–0.94) 1.11 (0.84–1.46)
Age .489
<50 1 (Ref.) 0.87 (0.46–1.65) 1.23 (0.62–1.65)
≥50 1 (Ref.) 0.78 (0.63–0.96) 1.17 (1.01–1.35)
Diabetes mellitus .406
Yes 1 (Ref.) 0.93 (0.52–1.66) 1.21 (0.81–1.79)
No 1 (Ref.) 0.75 (0.61–0.93) 1.20 (1.03–1.40)
Antihypertensive medications use .189
Yes 1 (Ref.) 0.83 (0.62–1.12) 1.09 (0.89–1.35)
No 1 (Ref.) 0.74 (0.56–0.97) 1.31 (1.08–1.60)
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4. DISCUSSION

Based on our research, we propose that QRS axis is an important factor in risk stratification in populations with prolonged PR interval. People with prolonged PR interval and QRS axis ≤37° have a higher risk of death than those without prolongation of the PR interval. These associations were not related to P‐wave duration and other known risk factors.

Our results are similar to previous studies in which prolongation of the PR interval is not an independent risk factor for poor prognosis in people without CVD (Aro et al., 2014; Magnani et al., 2013). Studies of the PR interval in the general population have found that prolonged PR is associated with an increased risk of AF and HF compared with the normal PR interval (Cheng et al., 2015; Nielsen et al., 2013; Uhm et al., 2014). Previous studies had observed diastolic MR in patients with prolonged PR interval (Salden et al., 2018). However, in patients with a normal PR interval, hemodynamic changes due to this regurgitation may be attenuated, resulting in decreased left atrial pressure and increased left ventricular preload (Nishimura et al., 1995). Therefore, it is necessary to stratify risks according to this change.

Previous studies have pointed out that prolongation of the PR interval is an important risk factor for early death in some special populations. In a study of patients with HCM, PR interval ≥200 ms was associated with HCM‐related death, including a composite endpoint of sudden death or fatal arrhythmia (Higuchi et al., 2020). In subanalysis of two trials of cardiac resynchronization therapy (CRT) in patients with HF, individuals with prolonged PR intervals had a higher risk of all‐cause mortality than individuals with normal PR intervals (Lin et al., 2017; Olshansky et al., 2012). Diastolic dysfunction (DD) is a prominent clinical feature in patients with HCM or HF (Badano et al., 2004; Zile & Brutsaert, 2002), and the superposition of prolonged PR and DD may produce greater damage (Wan et al., 2014). One possible mechanism is DD‐associated mechanical left ventricular desynchronization, further exacerbating atrioventricular coupling abnormalities (Kuznetsova et al., 2013). Several studies investigating the value of the clinical benefit of restoring AV coupling in patients with prolonged PR interval confirmed that CRT reduces the risk of all‐cause mortality in patients with HF, but this benefit is limited to individuals with prolonged PR (Kutyifa et al., 2014; Stockburger et al., 2016). Our study complements the mechanism of the interaction between left ventricular function and the PR interval described above.

The QRS axis as a parameter obtained directly from ECG reports may be a surrogate marker of left ventricular diastolic blood pressure (Kurisu et al., 2018). However, we found that the association between PR interval prolongation and QRS axis and death was not significant when considered separately, which may indicate that neither can be used alone as a criterion for assessing prognosis in people without CVD. Previous studies had also suggested that considering both the abnormal QRS axis and the prolonged PR interval may help further identify abnormal left ventricular function (Aron & Hertzeanu, 1988). In this study, we detected a statistically significant interaction between the two. As the QRS axis shifts to the left, the risk of all‐cause mortality continues to increase in people with prolonged PR interval. Therefore, we believe that the QRS axis can be an important factor in risk stratification in populations with prolonged PR interval and compare the difference in prognosis between populations stratified by their combination. This result is also in line with our hypothesis that when people combine QRS axis shift left and PR interval prolongation at the same time, the prognosis is worse.

P‐wave duration has been noted as a more sensitive marker for assessing cardiovascular risk compared with PR segments (Soliman et al., 2014), as prolonged P‐wave duration reflects structural and electrophysiological abnormalities in the atrium (Chen et al., 2022). In sensitivity analysis, the combination of prolonged PR interval and QRS ≤37° increased the risk of adverse outcomes, even in populations with low P/PR ratios. In addition, MR associated with prolongation of the PR interval can also induce chronic traction and conduction abnormalities in the atrium and lead to prolonged P‐wave time limits (John et al., 2010; Schiros et al., 2015). Due to the lack of relevant studies, it is impossible to establish their causality. Our study provides new clues that the QRS axis may be another factor in risk stratification in patients with prolonged PR interval beyond the duration of P waves. The ultimate goal of this study is to assess risk assessment of people with prolonged PR intervals by ECG alterations. For high‐risk patients, close follow‐up and aggressive early intervention may be required.

5. CONCLUSION

The QRS axis is an important risk stratification factor for prolongation of the PR interval in people without CVD. People with prolonged PR and QRS axis ≤37° have a higher risk of death compared with those without PR prolongation. Evaluation of the QRS axis is particularly necessary in patients whose ECG suggests prolongation of the PR interval.

6. AUTHOR CONTRIBUTIONS

Cao, Wang, and Shi were involved in concept and design. Cao, Wang, and Yu were involved in acquisition, analysis, or interpretation of data and statistical analysis Cao, Wang, and Fang were involved in drafting of the manuscript. All authors were involved in the critical revision of the manuscript for important intellectual content.

CONFLICT OF INTEREST STATEMENT

All authors have no conflict of interest with this article.

7. ETHICS STATEMENT

Data collection was rigorously reviewed and approved by the NCHS Research Ethics Review Committee to ensure strict adherence to ethical standards. Prior to the commencement of data collection, all study participants provided informed consent by signing a consent form, demonstrating their voluntary participation. The study meticulously adhered to all pertinent laws and regulations governing the protection of human subjects, guaranteeing the utmost respect for their rights and welfare.

ACKNOWLEDGMENTS

Grant of Jiangsu Commission of Health (Grant No. LKM2022060), Grant of Nantong Commission of Health (Grant No. MA2021013), Science Foundation of Nantong City (Grant No. JC2021139), and Research innovation team project from Kangda college of Nanjing Medical School (Grant No. KD2022KYCXTD009).

Cao, X. , Wang, Z. , Fang, Z. , Yu, C. , & Shi, L. (2023). Value of frontal QRS axis for risk stratification of individuals with prolonged PR interval. Annals of Noninvasive Electrocardiology, 28, e13066. 10.1111/anec.13066

Xiaodi Cao and Zhe Wang contributed equally to this work.

DATA AVAILABILITY STATEMENT

Our data are obtained from the publicly available database NHANES. All data are publicly available at https://www.cdc.gov/nchs/nhanes/index.htm.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

Our data are obtained from the publicly available database NHANES. All data are publicly available at https://www.cdc.gov/nchs/nhanes/index.htm.

Articles from Annals of Noninvasive Electrocardiology are provided here courtesy of International Society for Holter and Noninvasive Electrocardiology, Inc. and Wiley Periodicals, Inc.

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