Resting heart rate and incident atrial fibrillation: A stratified Mendelian randomization in the AFGen consortium

Background

Both elevated and low resting heart rates are associated with atrial fibrillation (AF), suggesting a U-shaped relationship. However, evidence for a U-shaped causal association between genetically-determined resting heart rate and incident AF is limited. We investigated potential directional changes of the causal association between genetically-determined resting heart rate and incident AF.

Method and results

Seven cohorts of the AFGen consortium contributed data to this meta-analysis. All participants were of European ancestry with known AF status, genotype information, and a heart rate measurement from a baseline electrocardiogram (ECG). Three strata of instrumental variable-free resting heart rate were used to assess possible non-linear associations between genetically-determined resting heart rate and the logarithm of the incident AF hazard rate: <65; 65–75; and >75 beats per minute (bpm). Mendelian randomization analyses using a weighted resting heart rate polygenic risk score were performed for each stratum.

We studied 38,981 individuals (mean age 59±10 years, 54% women) with a mean resting heart rate of 67±11 bpm. During a mean follow-up of 13±5 years, 4,779 (12%) individuals developed AF. A U-shaped association between the resting heart rate and the incident AF-hazard ratio was observed. Genetically-determined resting heart rate was inversely associated with incident AF for instrumental variable-free resting heart rates below 65 bpm (hazard ratio for genetically-determined resting heart rate, 0.96; 95% confidence interval, 0.94–0.99; p = 0.01). Genetically-determined resting heart rate was not associated with incident AF in the other two strata.

Conclusions

For resting heart rates below 65 bpm, our results support an inverse causal association between genetically-determined resting heart rate and incident AF.

Electrocardiographic parameters and the risk of new-onset atrial fibrillation in the general population

Funding Acknowledgements

Type of funding sources: Public Institution(s). Main funding source(s): The Rotterdam Study is funded by Erasmus Medical Center and Erasmus University, Rotterdam, Netherlands Organization for the Health Research and Development (ZonMw), the Research Institute for Diseases in the Elderly (RIDE), the Ministry of Education, Culture and Science, the Ministry for Health, Welfare and Sports, the European Commission (DG XII), and the Municipality of Rotterdam. This study is further supported by the Senior Scientist Grant from Dutch Heart Foundation (03-004-2021-T050).

Background

The (shape of the) association and sex-differences between electrocardiographic parameters and new-onset atrial fibrillation (AF) remain incompletely understood.

Purpose

To investigate the association between electrocardiographic parameters and new-onset atrial fibrillation among men and women in the general population.

Methods

12,212 participants free of AF from a large population-based cohort study were included. Up to five repeated measurements of electrocardiographic parameters including PR, QRS, QT, QT corrected for heart rate (QTc), JT, RR interval, and heart rate were assessed at baseline and follow-up examinations. Cox proportional hazards models and joint models, both adjusted for cardiovascular risk factors, were used to determine the (shape of) association between baseline and longitudinal electrocardiographic parameters with new-onset AF. Additionally, we evaluated potential sex-differences.

Results

During a median follow-up of 9.3 years, 1,282 incident AF cases occurred among 12,212 participants (mean age 64.9 years, 58.2% women). Penalized cubic splines revealed that associations between baseline electrocardiographic measures and risk of new-onset AF were generally U-shaped (Figure 1). Sex-differences in terms of the shape of the various associations were most apparent for baseline PR, QT, QTc, RR, and heart rate in relation to new-onset AF. Longitudinal measures of PR (hazard ratio (HR), 95% confidence interval (CI), 1.43, 1.02-2.04, p=0.0393), and QTc interval (HR, 95% CI, 5.23, 2.18-12.45, p=0.0002) were significantly associated with new-onset AF. Sex-stratified analyses showed that the longitudinal associations were more prominent among men.

Conclusions

Baseline electrocardiographic measures and risk of new-onset AF were generally U-shaped. Longitudinal electrocardiographic measures of PR, and QTc interval were significantly associated with new-onset AF, more pronounced in men. Our findings imply that different thresholds of electrocardiographic parameters might translate to a differential risk of AF among men and women, and that treatment options targeting specific electrocardiographic parameters might prevent AF in the general population, in particular in men.

Heart rate variability and atrial fibrillation in the general population: a longitudinal and mendelian randomization study

Funding Acknowledgements

Type of funding sources: Public Institution(s). Main funding source(s): The Rotterdam Study is funded by Erasmus Medical Center and Erasmus University, Rotterdam, Netherlands Organization for the Health Research and Development (ZonMw), the Research Institute for Diseases in the Elderly (RIDE), the Ministry of Education, Culture and Science, the Ministry for Health, Welfare and Sports, the European Commission (DG XII), and the Municipality of Rotterdam. This study is further supported by the Gender and prevention grant (555003017) from ZonMw.

Background

Sex-differences and the causality of the association between heart rate variability (HRV) and atrial fibrillation (AF) remain unclear.

Purpose

To investigate the sex-differences and the causality of the association between heart rate variability and atrial fibrillation.

Methods

12,334 participants free of AF from a large population-based cohort study were included. Measures of HRV including the standard deviation of normal RR-intervals (SDNN), SDNN corrected for heart rate (SDNNc), RR-interval differences (RMSSD), RMSSD corrected for heart rate (RMSSDc), and heart rate were assessed at baseline and follow-up examinations. Joint models, adjusted for cardiovascular risk factors, were used to determine the association between longitudinal measures of HRV with new-onset AF. Additionally, we evaluated sex-differences. Genetic variants for HRV were used as instrumental variables in a Mendelian randomization (MR) analysis using GWAS summary-level data.

Results

During a median follow-up of 9.4 years, 1,302 incident AF cases occurred. In joint models, higher SDNN (hazard ratio (HR), 95% confidence interval (CI), 1.24, 1.04-1.47, p=0.0213), and higher RMSSD (HR, 95% CI, 1.33, 1.13-1.54, p=0.0010) were significantly associated with new-onset AF. Sex-stratified analyses showed that the associations were mostly prominent among women. In MR analyses, genetically determined decreases in SDNN (odds ratio (OR), 95% CI, 1.60, 1.27-2.02, p=8.36x10-05), and RMSSD (OR, 95% CI, 1.56, 1.31-1.86, p= 6.32x10-07) were significantly associated with increased AF risk.

Conclusions

Longitudinal measures of uncorrected HRV were significantly associated with new-onset AF, in particular among women. MR analyses supported the causal relationship between uncorrected measures of HRV with AF. Our findings indicate that measures to modulate HRV might prevent AF in the general population, especially among women.

Disentangling the association between kidney function and atrial fibrillation: a bidirectional Mendelian randomization study

Background

Observational studies suggest that kidney function and atrial fibrillation (AF) are bidirectionally associated. Whether this bidirectional association is causal remains unclear.

Purpose

To investigate the causality of the bidirectional association between kidney function and AF.

Methods

Genetic variants associated with different measures of kidney function including estimated glomerular filtration rate (eGFR) based on creatinine (eGFRcreat), blood urea nitrogen (BUN), chronic kidney disease (CKD, eGFR &lt;60ml/min/1.73m2), eGFR based on cystatin (eGFRcys), urine albumin-to-creatinine ratio (UACR) and microalbuminuria (MA, UACR &gt;30mg/g) were retrieved from multiple Genome-Wide Association Studies (GWAS). These GWAS were all part of the Chronic Kidney Disease Genetics (CKDGen) Consortium (n=24,063–1,040,070). Genetic variants associated with AF were retrieved from a GWAS on AF (n=1,030,836). We used two-sample MR analyses to assess the potential causality of the bidirectional association between kidney function and AF.

Results

MR analyses supported a causal effect of genetically predicted BUN, CKD and MA on AF risk (for BUN: n=18 SNPs, outlier corrected odds ratio (OR): 2.05, per 1 unit increase of BUN (mg/dL), 95% CI: 1.30–3.25, p-value = 2.13E-03. For CKD: n=9 SNPs, outlier corrected OR: 1.10, 95% CI: 1.04–1.17, p-value = 1.97E-03. For MA: n=5 SNPs, outlier corrected OR: 1.26, 95% CI: 1.10–1.46, p-value = 1.38E-03). MR analyses also supported a causal effect of genetically predicted AF on eGFRcreat (n=97 SNPs, outlier corrected OR: 0.998, per 1 unit increase of log transformed eGFRcreat (ml/min/1.73m2), 95% CI: 0.997–0.999, p-value = 6.78E-03), CKD risk (n=107 SNPs, outlier corrected OR: 1.06, 95% CI: 1.03–1.09, p-value = 2.97E-04) and MA risk (n=83 SNPs, outlier corrected OR: 1.07, 95% CI: 1.04–1.09, p-value = 2.49E-08). A suggestive causal effect of genetically predicted AF on eGFRcys was found (n=103 SNPs, outlier corrected OR: 0.993, per 1 unit increase of log transformed eGFRcys (ml/min/1.73m2), 95% CI: 0.986–0.999, p-value = 4.60E-02). MR analyses did not support a significant causal effect of the other kidney function measures on AF risk and vice versa. Moreover, sensitivity analyses, including weighted median estimator (WME), MR-Egger and the MR pleiotropy residual sum and outlier test (MR-PRESSO) indicated that these findings were robust. Furthermore, the associations did not change when genetic variants associated with coronary artery disease and heart failure were excluded.

Conclusions

MR analyses supported a bidirectional causal association between kidney function and AF. Our findings carry the potential for identification of important therapeutic targets for both conditions with implications for secondary prevention.

Funding Acknowledgement

Type of funding sources: Public Institution(s). Main funding source(s): Erasmus Medical Center and Erasmus University, Rotterdam Forest plot with the MR effect estimatesBidirectional MR: Kidney function and AF

Pharmacogenomics study of thiazide diuretics and QT interval in multi-ethnic populations: the cohorts for heart and aging research in genomic epidemiology

Thiazide diuretics, commonly used antihypertensives, may cause QT interval (QT) prolongation, a risk factor for highly fatal and difficult to predict ventricular arrhythmias. We examined whether common single-nucleotide polymorphisms (SNPs) modified the association between thiazide use and QT or its component parts (QRS interval, JT interval) by performing ancestry-specific, trans-ethnic and cross-phenotype genome-wide analyses of European (66%), African American (15%) and Hispanic (19%) populations (N=78 199), leveraging longitudinal data, incorporating corrected standard errors to account for underestimation of interaction estimate variances and evaluating evidence for pathway enrichment. Although no loci achieved genome-wide significance (P<5 × 10-8), we found suggestive evidence (P<5 × 10-6) for SNPs modifying the thiazide-QT association at 22 loci, including ion transport loci (for example, NELL1, KCNQ3). The biologic plausibility of our suggestive results and simulations demonstrating modest power to detect interaction effects at genome-wide significant levels indicate that larger studies and innovative statistical methods are warranted in future efforts evaluating thiazide-SNP interactions.

Interference Removal with an Improved Incremental Estimation Filter

Mains interference in digitized electrocardiographic signals is often removed with a nonlinear filter, the so-called incremental estimaton filter. A prerequisite for proper functioning of the filter is that the variation of the signal is considerably slower than that of the disturbing interference. This condition will often not be true, especially during wave deflections. As a result, the filter will not fully remove the interference when present, or, alternatively, it may generate a sine wave in the absence of interference. These effects are analyzed and different solutions proposed, one of which is the use of new, very simple filters based on the incremental estimation technique.

Reconstruction of Repetitive Signals

A technique is presented for the reconstruction of signals that suffered sampling-frequency decimation. Two assumptions are made: the original signal has to be repetitive, and no anti-aliasing filter has been used before frequency decimation. The performance of the technique is assessed by using test signals of which the original signal is known.

Methodology of the Modular ECG Analysis System MEANS

The methodology, used in the Modular ECG Analysis System (MEANS) is described. MEANS consists of modules for signal analysis and diagnostic classification. The basic structure of the modular interpretation system remained intact over a period of 20 years, while all modules underwent many changes as a function of experience and insight, and the continuously changing information technology. The article describes the advantages of a modular approach to decision-support systems, the most important ones being easier maintenance of the software package and separate optimization and testing of each module. The overall evaluation of MEANS was done in the CSE study. Evaluation results for modules and for the entire system are presented.

The Delphi Method to Validate Diagnostic Knowledge in Computerized ECG Interpretation

We investigated the applicability of the Delphi method for increasing the agreement among multiple cardiologists on, firstly, their classifications of a set of electrocardiograms and, secondly, their reasons for these classifications. Five cardiologists were requested to judge the computer classifications of a set of thirty ECGs. If a cardiologist disagreed with the computer classification, he had to provide a new classification and a reason for this change. The results of this first round were compiled and anonymously fed back to the cardiologists. In a second round the cardiologists were asked once again to judge the ECGs and to rate the reasons provided in the first round. The level of agreement was estimated by means of the kappa statistic. The Delphi procedure substantially increased the agreement on the classifications among the cardiologists. The final agreement was very high and comparable with the intra-observer agreement. There was also a high level of agreement on the reasons provided by the cardiologists. However, their use in improving the program’s performance is hampered by the qualitative nature of many of the reasons. Suggestions are given for a more formalized elicitation of knowledge.

Classification Methods for Computerized Interpretation of the Electrocardiogram

Two methods for diagnostic classification of the electrocardiogram are described: a heuristic one and a statistical one. In the heuristic approach, the cardiologist provides the knowledge to construct a classifier, usually a decision tree. In the statistical approach, probability densities of diagnostic features are estimated from a learning set of ECGs and multivariate techniques are used to attain diagnostic classification. The relative merits of both approaches with respect to criteria selection, comprehensibility, flexibility, combined diseases, and performance are described. Optimization of heuristic classifiers is discussed. It is concluded that heuristic classifiers are more comprehensible than statistical ones; encounter less difficulties in dealing with combined categories; are flexible in the sense that new categories may readily be added or that existing ones may be refined stepwise. Statistical classifiers, on the other hand, are more easily adapted to another operating environment and require less involvement of cardiologists. Further research is needed to establish differences in performance between both methods. In relation to performance testing the issue is raised whether the ECG should be classified using as much prior information as possible, or whether it should be classified on itself, explicitly discarding information other than age and sex, while only afterwards other information will be used to reach a final diagnosis. Consequences of taking one of both positions are discussed.

Signal Analysis for ECG Interpretation

In ECG interpretation usually two main areas are discerned: the signal analysis and the diagnostic classification. This article reviews the major developments in the first area. ECG signal analysis itself is subdivided into the stages data acquisition, data transformation, feature selection, and data reduction. These stages are consecutively reviewed, while in the data transformation stage digital filtering, detection, wave typing, beat selection, and boundary recognition are discussed.

Antidepressants and heart-rate variability in older adults: a population-based study

BACKGROUND

Tricyclic antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs) may be associated with lower heart rate variability (HRV), a condition associated with increased mortality risk. We aimed to investigate the association between TCAs, SSRIs and HRV in a population-based study.

METHOD

In the prospective Rotterdam Study cohort, up to five electrocardiograms (ECGs) per participant were recorded (1991-2012). Two HRV variables were studied based on 10-s ECG recordings: standard deviation of normal-to-normal RR intervals (SDNN) and root mean square of successive RR interval differences (RMSSD). We compared the HRV on ECGs recorded during use of antidepressants with the HRV on ECGs recorded during non-use of any antidepressant. Additionally, we analysed the change in HRV on consecutive ECGs. Those who started or stopped using antidepressants before the second ECG were compared with non-users on two ECGs.

RESULTS

We included 23 647 ECGs from 11 729 participants (59% women, mean age 64.6 years at baseline). Compared to ECGs recorded during non-use of antidepressants (n = 22 971), SDNN and RMSSD were lower in ECGs recorded during use of TCAs (n = 296) and SSRIs (n = 380). Participants who started using TCAs before the second ECG had a decrease in HRV and those who stopped had an increase in HRV compared to consistent non-users (p < 0.001). Starting or stopping SSRIs was not associated with HRV changes.

CONCLUSION

TCAs were associated with a lower HRV in all analyses, indicating a real drug effect. For SSRIs the results are mixed, indicating a weaker association, possibly due to other factors.

Drug-gene interactions and the search for missing heritability: a cross-sectional pharmacogenomics study of the QT interval

Variability in response to drug use is common and heritable, suggesting that genome-wide pharmacogenomics studies may help explain the 'missing heritability' of complex traits. Here, we describe four independent analyses in 33 781 participants of European ancestry from 10 cohorts that were designed to identify genetic variants modifying the effects of drugs on QT interval duration (QT). Each analysis cross-sectionally examined four therapeutic classes: thiazide diuretics (prevalence of use=13.0%), tri/tetracyclic antidepressants (2.6%), sulfonylurea hypoglycemic agents (2.9%) and QT-prolonging drugs as classified by the University of Arizona Center for Education and Research on Therapeutics (4.4%). Drug-gene interactions were estimated using covariable-adjusted linear regression and results were combined with fixed-effects meta-analysis. Although drug-single-nucleotide polymorphism (SNP) interactions were biologically plausible and variables were well-measured, findings from the four cross-sectional meta-analyses were null (Pinteraction>5.0 × 10(-8)). Simulations suggested that additional efforts, including longitudinal modeling to increase statistical power, are likely needed to identify potentially important pharmacogenomic effects.

Mapping the domain of medical informatics

OBJECTIVES

The domain of medical informatics (MI) is not well defined. It covers a wide range of research topics. Our objective is to characterize the field of MI by means of the scientific literature in this domain.

METHODS

We used titles and abstracts from MEDLINE records of papers published between July 1993 and July 2008, and extracted uni-, bi- and trigrams as features. Starting with the ISI category of medical informatics, we applied a semi-automated procedure to identify the set of journals and proceedings pertaining to MI. A clustering algorithm was subsequently applied to the articles from this set of publications.

RESULTS

MI literature can be divided into three subdomains: 1) the organization, application, and evaluation of health information systems, 2) medical knowledge representation, and 3) signal and data analysis. Over the last fifteen years, the field has remained relatively stable, although most journals have shifted their focus somewhat.

CONCLUSIONS

We identified the scientific literature pertaining to the field of MI, and the main areas of research. We were able to show trends in the field, and the positioning of different journals within this field.

Unrecognized myocardial infarction and the risk of stroke: The Rotterdam Study

OBJECTIVE

To investigate the relationship between unrecognized myocardial infarction and the risk of stroke in a population-based cohort study.

METHODS

We followed 6,439 participants from the Rotterdam Study for stroke until January 2002. Participants were free from stroke, and presence of myocardial infarction was assessed at baseline (1990-1993). We calculated hazard ratios of stroke for persons with unrecognized or recognized myocardial infarction compared with persons without myocardial infarction. Analyses were adjusted for age, sex, and cardiovascular risk factors.

RESULTS

In 52,915 person-years of follow-up, 505 strokes occurred. Recognized myocardial infarction was only borderline associated with an increased risk of stroke. Unrecognized myocardial infarction increased the risk of stroke by 76% (age- and sex-adjusted hazard ratio 1.76, 95% CI 1.31 to 2.37). Stratification by sex showed that the increased risk was only found in men (hazard ratio for men 2.53, 95% CI 1.68 to 3.81; hazard ratio for women 1.27, 95% CI 0.82 to 1.96). After adjusting for cardiovascular risk factors at baseline, the risk remained significantly increased in men (hazard ratio for stroke 2.13, 95% CI 1.35 to 3.36). Subtyping of strokes revealed that unrecognized myocardial infarction was particularly associated with cortical ischemic strokes (hazard ratio for men 3.57, 95% CI 1.79 to 7.12).

CONCLUSIONS

Men with unrecognized myocardial infarction have an increased risk of stroke.

Computerized ST depression analysis improves prediction of all-cause and cardiovascular mortality: the strong heart study

BACKGROUND

Nonspecific ST depression assessed by standard visual Minnesota coding (MC) has been demonstrated to predict risk. Although computer analysis has been applied to digital ECGs for MC, the prognostic value of computerized MC and computerized ST depression analyses have not been examined in relation to standard visual MC.

METHODS

The predictive value of nonspecific ST depression as determined by visual and computerized MC codes 4.2 or 4.3 was compared with computer-measured ST depression >or= 50 microV in 2,127 American Indian participants in the first Strong Heart Study examination. Computerized MC and ST depression were determined using separate computerized-ECG analysis programs and visual MC was performed by an experienced ECG core laboratory.

RESULTS

The prevalence of MC 4.2 or 4.3 by computer was higher than by visual analysis (6.4 vs 4.4%, P < 0.001). After mean follow-up of 3.7 +/- 0.9 years, there were 73 cardiovascular deaths and 227 deaths from all causes. In univariate Cox analyses, visual MC (relative risk [RR] 4.8, 95% confidence interval [CI] 2.6-9.1), computerized MC (RR 6.0, 95% CI 3.5-10.3), and computer-measured ST depression (RR 7.6, 95% CI 4.5-12.9) were all significant predictors of cardiovascular death. In separate multivariate Cox regression analyses that included age, sex, diabetes, HDL and LDL cholesterol, body mass index, systolic and diastolic blood pressure, microalbuminuria, smoking, and the presence of coronary heart disease, computerized MC (RR 3.0, 95% CI 1.6-5.6) and computer-measured ST depression (RR 3.1, 95% CI 1.7-5.7), but not visual MC, remained significant predictors of cardiovascular mortality. When both computerized MC and computer-measured ST depression were entered into the multivariate Cox regression, each variable provided independent risk stratification (RR 2.1, 95% CI 1.0-4.4, and RR 2.1, 95% CI 1.0-4.4, respectively). Similarly, computerized MC and computer-measured ST depression, but not visual MC, were independent predictors of all-cause mortality after controlling for standard risk factors.

CONCLUSIONS

Computer analysis of the ECG, using computerized MC and computer-measured ST depression, provides independent and additive risk stratification for cardiovascular and all-cause mortality, and improves risk stratification compared with visual MC. These findings support the use of routine computer analysis of ST depression on the rest ECG for assessment of risk and suggest that computerized MC can replace visual MC for this purpose.

Association between cardiac disorders and a decades-previous history of diphtheria

A long, prior history of diphtheria is common among middle-aged and elder European adults. The aim of the present study was to determine whether the risk of reduced ventricular function and impaired intraventricular conduction is increased in individuals with a history of diphtheria. A study population of 2,480 subjects (1,222 women) aged 45 years or older who were recruited for the Study of Health in Pomerania were available for the present analyses. Left ventricular function was assessed by echocardiography. Intraventricular conduction blocks were diagnosed using electrocardiograms. Multivariable analyses revealed that individuals with a history of diphtheria had neither an increased odds for reduced fractional shortening (OR 1.21, 95% CI 0.69-2.11; p=0.51) nor an increased odds for intraventricular conduction blocks (OR 0.90, 95% CI 0.55-1.46; p=0.67). However, regression models revealed two-way interactions between the exposure variable and hypertension with respect to both endpoints. A history of diphtheria increased the odds for both endpoints in normotensive but not in hypertensive individuals. The findings show that a history of diphtheria several decades previously in a patient is a risk marker for reduced cardiac function and impaired intraventricular conduction in individuals at low risk for these disorders.

Co-occurrence based meta-analysis of scientific texts: retrieving biological relationships between genes

MOTIVATION

The advent of high-throughput experiments in molecular biology creates a need for methods to efficiently extract and use information for large numbers of genes. Recently, the associative concept space (ACS) has been developed for the representation of information extracted from biomedical literature. The ACS is a Euclidean space in which thesaurus concepts are positioned and the distances between concepts indicates their relatedness. The ACS uses co-occurrence of concepts as a source of information. In this paper we evaluate how well the system can retrieve functionally related genes and we compare its performance with a simple gene co-occurrence method.

RESULTS

To assess the performance of the ACS we composed a test set of five groups of functionally related genes. With the ACS good scores were obtained for four of the five groups. When compared to the gene co-occurrence method, the ACS is capable of revealing more functional biological relations and can achieve results with less literature available per gene. Hierarchical clustering was performed on the ACS output, as a potential aid to users, and was found to provide useful clusters. Our results suggest that the algorithm can be of value for researchers studying large numbers of genes.

AVAILABILITY

The ACS program is available upon request from the authors.

Prevalence of macrovascular disease amongst type 2 diabetic patients detected by targeted screening and patients newly diagnosed in general practice: the Hoorn Screening Study

OBJECTIVES

Screening for type 2 diabetes has been recommended and targeted screening might be an efficient way to screen. The aim was to investigate whether diabetic patients identified by a targeted screening procedure differ from newly diagnosed diabetic patients in general practice with regard to the prevalence of macrovascular complications.

DESIGN

Cross-sectional population-based study.

SETTING

Population study, primary care.

SUBJECTS

Diabetic patients identified by a population-based targeted screening procedure (SDM patients), consisting of a screening questionnaire and a fasting capillary glucose measurement followed by diagnostic testing, were compared with newly diagnosed diabetic patients in general practice (GPDM patients). Ischaemic heart disease and prior myocardial infarction were assessed by ECG recording. Peripheral arterial disease was assessed by the ankle-arm index. Intima-media thickness of the right common carotid artery was measured with ultrasound.

RESULTS

A total of 195 SDM patients and 60 GPDM patients participated in the medical examination. The prevalence of MI was 13.3% (95% CI 9.3-18.8%) and 3.4% (1.0-11.7%) in SDM patients and GPDM patients respectively. The prevalence of ischaemic heart disease was 39.5% (95% CI 32.9-46.5%) in SDM patients and 24.1% (15.0-36.5%) in GPDM patients. The prevalence of peripheral arterial disease was similar in both groups: 10.6% (95% CI 6.9-15.9%) and 10.2% (4.7-20.5%) respectively. Mean intima-media thickness was 0.85 mm (+/-0.17) in SDM patients and 0.90 mm (+/-0.20) in GPDM patients. The difference in intima-media thickness was not statistically significant.

CONCLUSIONS

Targeted screening identified patients with a prevalence of macrovascular complications similar to that of patients detected in general practice, but with a lower degree of hyperglycaemia.

Distribution of information in biomedical abstracts and full-text publications

MOTIVATION

Full-text documents potentially hold more information than their abstracts, but require more resources for processing. We investigated the added value of full text over abstracts in terms of information content and occurrences of gene symbol--gene name combinations that can resolve gene-symbol ambiguity.

RESULTS

We analyzed a set of 3902 biomedical full-text articles. Different keyword measures indicate that information density is highest in abstracts, but that the information coverage in full texts is much greater than in abstracts. Analysis of five different standard sections of articles shows that the highest information coverage is located in the results section. Still, 30-40% of the information mentioned in each section is unique to that section. Only 30% of the gene symbols in the abstract are accompanied by their corresponding names, and a further 8% of the gene names are found in the full text. In the full text, only 18% of the gene symbols are accompanied by their gene names.

The coming of age of computerized ECG processing: can it replace the cardiologist in epidemiological studies and clinical trials?

In spite of decades of research and widespread use of computer programs for the analysis of electrocardiograms (ECGs), the accuracy and usefulness of computerized ECG processing has been questioned. To determine whether ECG computer programs can replace cardiologists in epidemiological studies and clinical trials, we reviewed the literature for evidence, concentrating on one influential ECG measurement, viz. QT interval duration, and one classification method, the Minnesota Code, which is the de facto standard for ECG coding. We compared interobserver variabilities of cardiologists with differences between computer programs and cardiologists, in order not to prejudice against the computer. Studies that contain this type of information indicate that interobserver variabilities are at least as large as differences between computer and cardiologist. This suggests that ECG computer programs perform at least equally well as human observers in ECG measurement and coding, and can replace the cardiologist in epidemiological studies and clinical trials.

PEDMEANS: a computer program for the interpretation of pediatric electrocardiograms

The interpretation pediatric electrocardiograms (ECGs) is complicated because of the strong age-dependency of the diagnostic criteria. We wanted to develop and evaluate a computer program for the interpretation of pediatric 12-lead ECGs. Continuous age-dependent normal limits were established based on ECGs from 1,912 healthy Dutch children. Additionally, a reference interpretation was obtained for 1,718 ECGs recorded at the Sophia Children's Hospital. The total set of ECGs was divided in a training set of 1076 ECGs and a test set of 642 ECGs. All ECGs were recorded at a sampling rate of 1,200 Hz. Based on the normal limits and the training set, diagnostic rules were formalized in an iterative process by using expert interviews and automatic rule induction. The resultant rules were evaluated on the test set. The performance of the program, on our study population, appears to justify its use in a clinical setting. Preferably, the program should also be evaluated in other clinical centers.

Correction of ECG variations caused by body position changes and electrode placement during ST-T monitoring

BACKGROUND

Electrocardiogram variations (ECG) due to body position changes and electrode placements are common problems of continuous ST-T monitoring. Body position changes may cause QRS and ST-T changes and trigger false alarms. Placement of arm and leg electrodes in a coronary care unit environment is usually near the thorax instead of standard position at the wrists and ankles. This may affect the limb leads and complicate diagnostic interpretation. The purpose of this study was to assess the effects of these sources of ECG variation and to correct for them. Continuous 12-lead ECG recordings were obtained from 160 patients admitted to the coronary care unit. Each patient underwent a body position test (supine, left-lateral, and upright position). Scalar and spatial approaches were investigated for reconstruction of the ECG in supine position. The scalar approach uses linear regression. The spatial approach transforms the ECG into a derived vestorcardiogram. The spatial QRS-loop is then rotated and scaled to match the vector loop in supine position and transformed back to a 12-lead ECG.

MATERIALS AND METHODS

To assess the effect of electrode placement, monitoring and standard limb leads were simultaneously recorded in a group of 80 patients. To map the monitoring leads to standard leads, general and patient-specific reconstruction coefficients were derived by linear regression from half of the patients and tested on the other half. Similarity between the reference and reconstructed ECGs was measured by correlation, similarity coefficient [(SC=1-RMS(residual error)/RMS(signal)], and difference in frontal QRS-Axis.

RESULTS AND CONCLUSION

Only 14% (23 of 160) of the patients showed marked ECG changes (ST elevations, QRS-axis shifts, T-wave inversions). The scalar method (median correlation > 0.994, SC > 0.902, QRS axis difference 0 degrees) performed better than spatial (median correlation 0.946, SC > 0.792, QRS axis difference 0 degrees). Monitoring leads can be mapped to standard limb leads in good to excellent approximaiton. General reconstruction (median correlation 0.993 and SC 0.764) performed slightly worse than patient-specific reconstruction (median correlation 0.997 and SC 0.908).

Employment of intra-individual variability to improve computerized ECG interpretation

One of the reasons for the limited practical utility of computer programs for interpretation of electrocardiograms (ECGs) is their susceptibility to intra-individual variability. Two of the most prominent sources of intra-individual variability in ECGs, electrode placement variations and respiration, were studied for their effects on computerized ECG interpretation. Previous research has shown that the effects of intra-individual variability on computerized ECG interpretation depend largely on the individual ECG. To enable the assessment of chest electrode position variations for individual standard 12- lead ECGs, ECGs resulting from simulations of such position variations were interpreted. Variability due to respiration was assessed by interpreting all individual ECG beats instead of an averaged beat. In this paper two methods are presented that employ information about the intra-individual variability in individual ECGs. The first method provides an estimate of the reliability of the interpretation, the second attempts to improve the interpretation itself. In the first method we quantified the variation in interpretation caused by the two sources of intra-individual variability with the use of a stability index, a high index value indicating a low variation in interpretation. This index was subsequently studied using two sets of ECGs. For the first set a â clinical' reference interpretation was obtained from discharge letters. For the second set three cardiologists provided a â cardiologists' reference. The performance of subgroups of ECGs having stability indices higher than a particular value was computed. It appeared that for the â cardiologists' reference, the interpretations of ECGs with a high stability index were more often correct. No effect was found for the â clinical' reference. In the second method we attempted to improve the original interpretation by combining the alternative interpretations into a new interpretation. This was done by taking the median or the average of the quantified alternatives. These combined interpretations proved to perform better than the original interpretation when a cardiologist's interpretation was taken as a reference. This paper shows that intra-individual ECG variability can be used to improve original interpretations. This can be done without having to record multiple ECGs, provided that a model is available to simulate intra-individual variability. The presented methods do not depend on the classification algorithm that is used. They can be used both during classifier design to correct imperfections, and in routine use of the classifier to produce more representative classifications.

Minimum bandwidth requirements for recording of pediatric electrocardiograms

BACKGROUND

Previous studies that determined the frequency content of the pediatric ECG had their limitations: the study population was small or the sampling frequency used by the recording system was low. Therefore, current bandwidth recommendations for recording pediatric ECGs are not well founded. We wanted to establish minimum bandwidth requirements using a large set of pediatric ECGs recorded at a high sampling rate.

METHODS AND RESULTS

For 2169 children aged 1 day to 16 years, a 12-lead ECG was recorded at a sampling rate of 1200 Hz. The averaged beats of each ECG were passed through digital filters with different cut off points (50 to 300 Hz in 25-Hz steps). We measured the absolute errors in maximum QRS amplitude for each simulated bandwidth and determined the percentage of records with an error >25 microV. We found that in any lead, a bandwidth of 250 Hz yields amplitude errors <25 microV in >95% of the children <1 year. For older children, a gradual decrease in ECG frequency content was demonstrated.

CONCLUSIONS

We recommend a minimum bandwidth of 250 Hz to record pediatric ECGs. This bandwidth is considerably higher than the previous recommendation of 150 Hz from the American Heart Association.

Accurate automatic detection of electrode interchange in the electrocardiogram

In recording an electrocardiogram (ECG), an interchange of electrodes may easily go unnoticed. Automatic detection would be desirable, but current algorithms, when dealing with more than left arm-right arm reversal, have moderate sensitivity. We propose a novel approach that uses the redundancy of information in the standard 12-lead ECG. We assume that each of the 8 independent electrocardiographic leads can be reconstructed from the 7 others in reasonable approximation. The correlation between any electrocardiographic lead and its reconstruction should be higher if the electrodes are correctly placed than when some interchange were present. The difference in correlation should have discriminative power. This was verified on a set of 3,305 ECGs for 14 common electrode interchange errors. The material was split in a learning and test set, and general reconstruction coefficients were computed from the learning set. For each interchange, electrode-error ECGs were derived by rearranging leads of the unaltered ECGs. Correlations between the actual leads and their reconstructions were computed for all ECGs. From the differences in lead correlation, decision rules were derived for each kind of interchange. All 14 rules had specificities of > or =99.5% in the test set. Sensitivities were > or =93% for 11 rules, and left arm-left leg electrode reversal scored low.

Minimal lead sets for reconstruction of 12-lead electrocardiograms

It may not always be possible to record all precordial leads of the standard 12-lead electrocardiogram (ECG). Especially in monitoring situations, a minimal lead set from which the 12-lead ECG can be reconstructed, would be valuable. This article assesses how well missing precordial leads could be synthesized from the remaining leads of the 12-lead ECG. A total of 2,372 diagnostic 12-lead ECG recordings were obtained from subjects with chest pain suggestive for acute myocardial infarction. Representative average beats were computed from the digital 12-lead ECG recordings with our Modular ECG Analysis System. The recordings were divided into a learning set and a test set. We considered all lead sets with one or more precordial leads removed, but always including limb leads I and II. By using the learning set, general reconstruction coefficients were computed to synthesize the missing precordial leads to each lead set. Performance of the synthesis was assessed by cross correlation between the original and the reconstructed leads. Also, patient-specific reconstruction coefficients were derived for each ECG in the test set and correlations were determined. High correlation coefficients were found with both reconstruction techniques. For different sizes of lead sets, the best patient-specific reconstructions had higher correlation values than the general reconstructions. For example, when 2 precordial leads were excluded, the best patient-specific median correlation was 0.994 compared to 0.963 for the best general reconstruction correlation. General reconstruction allows synthesis of 2 or 3 excluded precordial leads in good approximation. When patient-specific reconstruction can be applied, a minimal lead set including the limb leads and only 2 precordial leads suffices.

New normal limits for the paediatric electrocardiogram

AIMS

Previous studies that determined the normal limits for the paediatric ECG had their imperfections: ECGs were recorded at a relatively low sampling rate, ECG measurements were conducted manually, or normal limits were presented for only a limited set of parameters. The aim of this study was to establish an up-to-date and complete set of clinically relevant normal limits for the paediatric ECG.

METHODS AND RESULTS

ECGs from 1912 healthy Dutch children (age 11 days to 16 years) were recorded at a sampling rate of 1200 Hz. The digitally stored ECGs were analysed using a well-validated ECG computer program. The normal limits of all clinically relevant ECG measurements were determined for nine age groups. Clinically significant differences were shown to exist, compared with previously established normal limits. Sex differences could be demonstrated for QRS duration and several amplitude measurements.

CONCLUSIONS

These new normal limits differ substantially from those commonly used and suggest that diagnostic criteria for the paediatric ECG should be adjusted.

Within-subject electrocardiographic differences at equal heart rates: role of the autonomic nervous system

Various combinations of sympathetic and vagal tone can yield the same heart rate, while ventricular electrophysiology differs. To demonstrate this in humans, we studied healthy volunteers in the sitting position with horizontal legs. First, heart rate was increased by lowering the legs to 60 degrees and back. Thereafter, heart rate was increased by handgrip. In each subject, a leg-lowering angle was selected at which heart rate matched best with heart rate in the third handgrip minute. Thirteen subjects had a heart rate match better than 1%. Heart rate (control: 65.2+/-9.0 bpm) increased to 72.1+/-8.7 (leg lowering) and to 72.1+/-8.8 (handgrip) bpm. QRS azimuth, QRS duration, maximal T vector, T azimuth, T elevation, ST duration, QRS-T angle and QT interval differed significantly (P<0.05) between leg lowering and handgrip (QT interval 418+/-15 versus 435+/-21 ms). Also, septal dispersion of repolarization, assessed as the time difference between the apex and the end of the T wave in the V2 and V3 leads, differed significantly (V2: 96.7+/-19.3 versus 110.0+/-23.3 ms, P<0.01; V3: 88.7+/-19.3 versus 97.3+/-23.3 ms; P<0.01). Hence, leg lowering and handgrip cause different ventricular depolarization and repolarization. The hypertensive handgrip manoeuvre entails a longer QT interval and probably an increased septal dispersion of repolarization.

Microalbuminuria is independently associated with ischaemic electrocardiographic abnormalities in a large non-diabetic population. The PREVEND (Prevention of REnal and Vascular ENdstage Disease) study

AIM

To assess the value of microalbuminuria as an indicator of increased cardiovascular risk in a non-diabetic population.

METHODS AND RESULTS

7579 non-diabetic subjects were studied with ages ranging from 28 to 75 years selected from a population based cohort. Using computerized Minnesota coding, ischaemic electrocardiographic abnormalities were divided into three categories: infarct patterns, major ischaemia, and minor ischaemia. Urinary albumin excretion was measured as the mean of two 24-h urine collections. Cardiovascular risk indicators were defined as an age above 60 years, male sex, hypertension, hypercholesterolaemia, smoking, obesity and a positive cardiovascular family history. Microalbuminuria was associated with age, sex, blood pressure, serum cholesterol, serum glucose, body mass index and all three categories of electrocardiographic abnormalities. In a multivariate model, adjusted for established cardiovascular risk indicators, microalbuminuria was independently associated with infarct patterns (OR [95% CI] 1.61 [1.12-2.32]), major ischaemia (OR 1.43 [1.08-1.91]) and minor ischaemia (OR 1.32 [1.03-1.68]).

CONCLUSIONS

The independent association between microalbuminuria and ischaemic electrocardiographic abnormalities suggests that microalbuminuria has additional value to conventional risk indicators in predicting cardiovascular disease in non-diabetics. Assessment of microalbuminuria could be an instrument to identify those at an increased risk for coronary vascular disease in an early stage.

Atrial fibrillation and the risk of cerebral white matter lesions

BACKGROUND

Cerebral white matter lesions are often observed on MRI scans of elderly nondemented and demented persons. Their pathogenesis is not fully understood but cerebral hypoperfusion may be involved. Atrial fibrillation is a common finding in elderly subjects and may lead to a reduced cardiac output with cerebral hypoperfusion. The authors investigated the association between atrial fibrillation and the presence of white matter lesions.

METHODS

From 1995 through 1996, the authors randomly sampled 1077 subjects from two ongoing prospective population-based studies. From each participant, an electrocardiogram (ECG) was recorded; atrial fibrillation and left ventricular hypertrophy were diagnosed with a computer program. For one of the two groups (553 subjects), earlier ECGs were available (mean follow-up 4.7 years). All subjects underwent 1.5-T MRI scanning; white matter lesions were separately rated for the periventricular and subcortical regions.

RESULTS

The prevalence of atrial fibrillation was 1.9% among subjects younger than 75 years and 5.5% in subjects older than 75 years. The total number of subjects with atrial fibrillation was 28. Subjects with atrial fibrillation had severe periventricular white matter lesions more than twice as often as subjects who did not (RR 2.2; 95% CI 1.0 to 5.2) but had no increased risk of subcortical white matter lesions (RR 1.1; 95% CI 0.4 to 2.6). For seven subjects with atrial fibrillation both at baseline and at follow up, these relative risks were 6.3 (95% CI 1.1 to 37.1) and 0.7 (95% CI 0.1 to 3.7).

CONCLUSIONS

Atrial fibrillation is associated with periventricular white matter lesions, but not with subcortical white matter lesions.

Comparison of computer-assigned Minnesota Codes with the visual standard method for new coronary heart disease events

The Minnesota Code is the most widely used electrocardiogram (ECG) classification system for epidemiologic studies and has been incorporated into several Computer algorithms. The authors compared the Modular ECG Analysis System (MC-MEANS) and NOVACODE computer ECG findings with the Visual coding standard for agreement and prognostic associations with coronary heart disease (CHD) events occurring during follow-up from 1987 to 1995 in 2,116 individuals participating in the Atherosclerosis Risk in Communities (ARIC) Study. The exact agreement between Visual and computer findings was greater than 90% for all Minnesota Code categories except Q-code, which was 77% for MC-MEANS and 81% for NOVACODE. Approximately 60% of all Q-codes were assigned by computer methods only. Among the 2,116 participants, there were 246 (11.6%) new coronary events. Unadjusted relative risks for codes assigned by the three methods were similar. When computer methods disagreed on code severity, the CHD occurrence rates for MC-MEANS-detected severer code versus NOVACODE-detected severer code were 21% and 7%, respectively. This study provides clear evidence that computers assign more and severer Minnesota Codes with similar prognostic importance as does the Visual method; it also alerts researchers to potential problems in pooling Minnesota Code data read by different methods.

Both decreased and increased heart rate variability on the standard 10-second electrocardiogram predict cardiac mortality in the elderly: the Rotterdam Study

Decreased heart rate variability has been associated with an adverse prognosis in patients after myocardial infarction. Studies carried out in the population at large show contradictory results. The authors examined the association between heart rate variability on a standard 10-second electrocardiogram and cardiac and all-cause mortality in the Rotterdam Study, a population-based cohort study of men and women aged > or =55 years, using data collected between 1990 and 1996 (mean follow-up = 4 years). Heart rate variability, taken as the standard deviation of normal R-R intervals (SDNN), was computed by means of the Modular ECG Analysis System. After exclusion of subjects with arrhythmia and those with fewer than six normal R-R intervals, the study population consisted of 2,088 men and 3,184 women. Cox's proportional hazards model was used to examine the age- and sex-adjusted risk for cardiac, noncardiac, and total mortality in relation to quartiles of SDNN, using the third quartile of SDNN as the reference category. Subjects in the lowest quartile of SDNN relative to those in the third quartile had an 80 percent age- and sex-adjusted increased risk for cardiac mortality (hazard ratio = 1.8; 95% confidence interval: 1.0, 3.2). Interestingly, for subjects in the highest quartile of SDNN, an even more pronounced risk for cardiac mortality was present (hazard ratio = 2.3; 95% confidence interval: 1.3, 4.0). Additional adjustment for possible confounders did not materially change the risk estimates. The authors conclude that heart rate variability measured on the standard 10-second electrocardiogram can be used to identify older men and women with an increased risk for cardiac mortality. In the elderly, increased heart rate variability is an even stronger indicator of cardiac mortality than decreased heart rate variability. Further studies are needed to confirm these findings and to elucidate their physiologic meaning.

QT dispersion as an attribute of T-loop morphology

BACKGROUND

The suggestion that increased QT dispersion (QTD) is due to increased differences in local action potential durations within the myocardium is wanting. An alternative explanation was sought by relating QTD to vectorcardiographic T-loop morphology.

METHODS AND RESULTS

The T loop is characterized by its amplitude and width (defined as the spatial angle between the mean vectors of the first and second halves of the loop). We reasoned that small, wide ("pathological") T loops produce larger QTD than large, narrow ("normal") loops. To quantify the relationship between QTD and T-loop morphology, we used a program for automated analysis of ECGs and a database of 1220 standard simultaneous 12-lead ECGs. For each ECG, QT durations, QTD, and T-loop parameters were computed. T-loop amplitude and width were dichotomized, with 250 microV (small versus large amplitudes) and 30 degrees (narrow versus wide loops) taken as thresholds. Over all 1220 ECGs, QTDs were smallest for large, narrow T loops (54.2+/-27.1 ms) and largest for small, wide loops (69. 5+/-33.5 ms; P<0.001).

CONCLUSIONS

QTD is an attribute of T-loop morphology, as expressed by T-loop amplitude and width.

T-loop morphology as a marker of cardiac events in the elderly

ST-T wave changes of electrocardiographic (ECG) leads have long been recognized as predictors of future cardiac events, but they only imperfectly characterize T-loop morphology. Using vectorcardiographic (VCG) parameters, we investigated the predictive value of T-loop abnormality for fatal and nonfatal cardiac events in a prospective cohort study among 5,815 elderly. Separately, the predictive value of an easily obtainable T-loop parameter, the T axis, was also assessed. Measurements were determined by a computer program, using VCGs reconstructed from the standard 12-lead ECGs. During the 3 to 6 (mean 4) years of follow-up, 166 fatal and 193 nonfatal cardiac events occurred. Subjects with an abnormal T-loop morphology had increased risks for fatal cardiac events (hazard ratio 4.3; 95% CI 3.0-6.4) and nonfatal cardiac events (3.0; 1.9-4.8). Risks associated with an abnormal T axis alone were only slightly lower. Additional adjustment for established cardiovascular risk indicators resulted in lower, but still highly significant risks. Both T-loop and T-axis abnormalities appear to be strong, independent risk indicators of cardiac events in the elderly.

Prolonged QT interval predicts cardiac and all-cause mortality in the elderly. The Rotterdam Study

AIMS

To examine the association between heart-rate corrected QT prolongation and cardiac and all-cause mortality in the population-based Rotterdam Study among men and women aged 55 years or older and to compare the prognostic value of the QT interval, using different formulas to correct for heart rate.

METHODS AND RESULTS

After exclusion of participants with arrhythmias or bundle branch block on the ECG, the study population consisted of 2083 men and 3158 women. The QT interval was computed by the Modular ECG Analysis System (MEANS). Data were analysed using Cox' proportional hazards model. Participants in the highest quartile of the heart-rate corrected QT interval had about a 70% age- and sex-adjusted increased risk for both all-cause mortality (hazard ratio (HR) 1.8; 95% CI:1.3-2.4) and cardiac mortality (HR 1.7; 95% CI:1.0-2.7) compared to those in the lowest quartile. In women, the increased risk associated with prolonged QT for cardiac death was more pronounced than in men. These risk estimates did not change after adjustment for potential confounders, including history of myocardial infarction, hypertension and diabetes mellitus.

CONCLUSION

A prolonged heart-rate corrected QT interval is an independent predictor for cardiac and all-cause mortality in older men and women. The risk associated with prolonged QT is hardly affected by the heart-rate correction formula used.

Measurement error as a source of QT dispersion: a computerised analysis

OBJECTIVE

To establish a general method to estimate the measuring error in QT dispersion (QTD) determination, and to assess this error using a computer program for automated measurement of QTD.

SUBJECTS

Measurements were done on 1220 standard simultaneous 12 lead electrocardiograms.

DESIGN

The computer program was validated against two observers on a random subset of 100 electrocardiograms. Simple laws of physics require that at least five of the six extremity leads have the same QT duration. This allows the direct assessment of the error in measuring QTD derived from five extremity leads (QTD5). It also enables ST-T amplitude dependent distributions of measurement error in determining QT duration to be established. These QT error distributions were then used to estimate the error in measuring QTD from all 12 leads (QTD12).

MAIN OUTCOME MEASURES

Mean and standard deviation of error in measuring QTD duration, QTD5, and QTD12.

RESULTS

Performance of the program was comparable to that of observers. Errors in measuring QT duration (measured QT minus reference QT) fell from a mean (SD) of 6.9 (17.1) ms for ST-T amplitudes < 50 microV to -1.4 (6.3) ms for amplitudes > 350 microV. Measurement errors of QTD5 and QTD12 were 20.4 (11.5) ms and 29.4 (14.9) ms.

CONCLUSIONS

The fact that no QTD can exist between five of the six extremity leads provides a means of estimating QTD measurement error. Measuring error of QT duration is dependent on ST-T amplitude. QTD measurement error is large compared with typical QTD values reported.

T axis as an indicator of risk of cardiac events in elderly people

BACKGROUND

The T axis was postulated to be a general marker of repolarisation abnormality, indicative of subclinical myocardial damage. The aim of this investigation was to assess the prognostic importance of the T axis for fatal and non-fatal cardiac events, in a prospective cohort study of men and women aged 55 years and older.

METHODS

2352 men and 3429 women from the population-based Rotterdam Study took part in the study. Electrocardiograms were done, and T axes were categorised as normal, borderline, or abnormal. Data were analysed with Cox's proportional-hazards models; adjustment for age and sex was done where appropriate.

FINDINGS

During 3-6 (mean 4) years of follow-up of the 5781 participants, 165 (2.9%) fatal and 192 (3.3%) non-fatal cardiac events occurred. Participants with an abnormal T axis (n=609) had an increased risk of cardiac death (hazard ratio 3.9 [95% CI 2.8-5.6]), sudden cardiac death (4.4 [2.6-7.4]), non-fatal cardiac events (2.7 [1.9-3.9]), and combined fatal or non-fatal cardiac events (3.2 [2.5-4.1]); p<0.001 for each. Additional adjustment for established cardiovascular risk factors resulted in lower, but still significant risk for all endpoints. The risk associated with an abnormal T axis was higher than those for any other cardiovascular risk factor. Additional subgroup analyses indicated that the risk of cardiac death was not substantially modified by age, sex, or history of myocardial infarction.

INTERPRETATION

The T axis is a strong and independent risk indicator of fatal and non-fatal cardiac events in the elderly.

Reproducibility of computerized ECG measurements and coding in a nonhospitalized elderly population

The standard 12-lead electrocardiogram (ECG) is used in many epidemiologic studies to diagnose and predict cardiovascular disease. In view of this, knowledge about the reproducibility of ECG measurements and coding is essential. Minute-to-minute, day-to-day, and year-to-year variability of ECG measurements, composite scores, and Minnesota Code classification were assessed by use of a computer program, in 101 nonhospitalized elderly men and women. Interval ECG measurements were more reproducible than amplitude measurements. The best reproducibility was found for the overall QTc interval (coefficient of variation 3.1%, 4.0%, and 5.2% for the minute-to-minute, day-to-day, and year-to-year groups, respectively) and the poorest was found for the Cardiac Infarction Injury Score (coefficient of variation 67.1%, 78.5%, and 94.3%, respectively). Minnesota Code discrepancies occurred in 16%, 19%, and 22% of the ECGs in the minute-to-minute, day-to-day, and year-to-year groups, respectively. Reproducibility within specific code categories was much better. Overall, variability tended to increase with time. In the routine setting, electrode positioning had relatively little effect on total variability.

QTc dispersion predicts cardiac mortality in the elderly: the Rotterdam Study

BACKGROUND

Increased QTc dispersion has been associated with an increased risk for ventricular arrhythmias and cardiac death in selected patient populations. We examined the association between computerized QTc-dispersion measurements and mortality in a prospective analysis of the population-based Rotterdam Study among men and women aged > or = 55 years.

METHODS AND RESULTS

QTc dispersion was computed with the use of the Modular ECG Analysis System as the difference between the maximum and minimum QTc intervals in 12 and 8 leads (ie, the 6 precordial leads, the shortest extremity lead, and the median of the 5 other extremity leads). After exclusion of those without a digitally stored ECG, the population consisted of 2358 men and 3454 women. During the 3 to 6.5 years (mean, 4 years) of follow-up, 568 subjects (9.8%) died. The degree of QTc dispersion was categorized into tertiles. Data were analyzed using the Cox proportional hazards model, with adjustment for age. For QTc dispersion in 8 leads, those in the highest tertile relative to the lowest tertile had a twofold risk for cardiac death (hazard ratio, 2.5; 95% confidence interval [CI], 1.6 to 4.0) and sudden cardiac death (hazard ratio, 1.9; 95% CI, 1.0 to 3.7) and a 40% increased risk for total mortality (hazard ratio, 1.4; 95% CI, 1.2 to 1.8). Additional adjustment for potential confounders, including history of myocardial infarction, hypertension, and overall QTc, did not materially change the risk estimates. Hazard ratios for QTc dispersion in 12 leads were comparable to those found for QTc dispersion in 8 leads.

CONCLUSIONS

QTc dispersion is an important predictor of cardiac mortality in older men and women.

Prolonged QT interval: a tricky diagnosis?

Prolonged heart-rate adjusted QT intervals on the electrocardiogram (ECG) are associated with an increased risk for coronary heart disease and sudden death. However, the diagnosis of the prolonged QT interval is hampered by lack of standards. We studied variations in the prevalence of prolonged QT, based on different common definitions, in a large nonhospitalized population, and compared our results with other studies applying the same definitions. The study population consisted of 2,200 men and 3,366 women participants of the Rotterdam Study, > or =55 years old. The QT interval was computed by our Modular ECG Analysis System (MEANS). Three different formulas to adjust QT for heart rate were used: Bazett's formula (QTc), a linear regression equation (QTlr), and the QT index (QTI). Prolonged QT occurred frequently in both men and women, and its prevalence increased with age. Women had longer heart-rate adjusted QT intervals than men (mean QTc 433 ms vs 422 ms), and mean values for QTlr were lower than for QTc (mean QTlr 422 ms in women and 412 ms in men). Prevalence was highest for prolonged QTlr (31% in men and 26% in women) and lowest for prolonged QTI (6% in men and 9% in women). Comparison with other studies applying the same correction formulas showed large discrepancies in prevalence estimates of prolonged QTc and QTlr, and to a lesser degree of prolonged QTI, possibly due to differences in measurement techniques. Future research is needed to relate QT interval to prognosis, to obtain measurement technique specific reference values of heart-rate adjusted QT measurements, and to obtain age- and sex-specific threshold values for prolonged QT. Such data are needed to use the QT interval with confidence.

Prevalence, determinants, and misclassification of myocardial infarction in the elderly

We evaluated the prevalence, determinants, and misclassification of different types of myocardial infarction in 3,272 men and women age 55 years or older. We defined self-reported myocardial infarction with electro-cardiographic evidence as "typical myocardial infarction." We defined self-reported myocardial infarction without electrocardiographic evidence, but verified with additional clinical information, as "non-Q-wave myocardial infarction." Finally, we defined myocardial infarction detected by electrocardiogram that was not self-reported as "silent myocardial infarction," after verification of absence of symptoms. Overall, the prevalence of typical myocardial infarction was 4.1% [95% confidence interval (CI) = 3.5-4.9], of non-Q-wave myocardial infarction 2.8% (95% CI = 2.2-3.4), and of silent myocardial infarction 3.9% (95% CI = 3.2-4.5). Silent myocardial infarction was more prevalent in women, hypertensives, cigarette smokers, and those with higher post-load blood glucose. Self-reported myocardial infarction without electrocardiographic characteristics could be verified as myocardial infarction by means of additional clinical information in 56% of the cases. We conclude that myocardial infarction occurs frequently in the elderly without typical symptoms or electrocardiographic changes. As all these manifestations of myocardial infarction convey an increased risk of symptomatic heart disease or death, they require further attention. Misclassification due to limited sources of information can be considerable and should be taken into account in the design and interpretation of epidemiologic studies.