Electrocardiographic predictors of coronary heart disease and sudden cardiac deaths in men and women free from cardiovascular disease in the Atherosclerosis Risk in Communities study

Association between electrocardiographic QRS transition zone and mortality from all causes and cardiovascular diseases: A systematic review and meta-analysis

AIMS

To investigate whether clockwise rotation (CWR) and counterclockwise rotation (CCWR) of electrocardiographic QRS transition zone is associated with mortality from all causes and cardiovascular diseases (CVD).

METHODS

Studies were identified from searching of PubMed, EMBASE and the reference lists of relevant papers. Summary multivariate-adjusted hazard ratios (HRs) and the corresponding 95% confidence intervals (CIs) and 95% prediction intervals (PIs) were computed through meta-analysis.

RESULTS

A total of five observational cohort studies fulfilled the inclusion criteria, which included 47,252 participants from the general population (8.8% CWR; 44.1% CCWR). Pooling data revealed that CWR was significantly associated with the increased risk of all-cause mortality (HR, 1.18; 95% CI: 1.12-1.24; 95% PI: 1.03-1.37) and CVD mortality (HR, 1.18; 95% CI: 1.08-1.29; 95% PI: 0.98-1.42) compared to NR pattern, with low heterogeneity among studies (P = 0.29, I² = 20%; P = 0.37; I² = 7%; respectively). However, CCWR was inversely associated with the risk of all-cause mortality (HR, 0.92; 95% CI: 0.89-0.95; 95% PI: 0.80-1.05) with low heterogeneity (P = 0.14; I² = 43%), while no significant association existed between CCWR and CVD mortality (HR, 0.89; 95% CI: 0.77-1.02; 95% PI: 0.53-1.48) with high heterogeneity (P < 0.01; I² = 78%).

CONCLUSION

Our meta-analysis demonstrated CWR was positively associated with higher risk of mortality from all-cause and CVD, while CCWR was negatively associated with the risk of all-cause mortality and no significant association with CVD mortality. These findings suggested that QRS transition zone carries important prognostic value, more attention should be paid in clinical practice.

Importance of lead aVR on predicting adverse cardiac events in patients with noncompaction cardiomyopathy

Background

Noncompaction cardiomyopathy (NCCM) is a relatively rare cardiac abnormality with high rates of mortality and morbidity. T‐wave amplitudes during ventricular repolarization in lead aVR (TaVR) have been reported to be associated with the prognosis of various cardiovascular diseases. This study sought to investigate the prevalence and prognostic role of positive TaVR in patients with NCCM.

Methods

We evaluated consecutive 161 patients with NCCM (65.8% men, mean age 42.5 ± 15.2 years old). Presentation electrocardiogram was assessed regarding classical parameters as well as T‐wave amplitudes in lead aVR. The primary endpoint was defined as composite lethal arrhythmic events, including sudden cardiac death, ventricular fibrillation, or sustained ventricular tachycardia or appropriate implantable cardioverter–defibrillator shock. Heart failure requiring hospitalization, cardiovascular death, and all‐cause mortality were also investigated as secondary endpoints.

Results

Patients with positive TaVR showed higher rates for arrhythmic events, hospitalization for heart failure, and death compared with patients without it. In multivariate Cox model, after adjusting for other known clinical and electrocardiographic risk factors, the positive TaVR was found to be a strong independent predictor of primary endpoint (HR: 4.8, 95% CI: 1.2–19.3; p = .025) and all‐cause death (HR: 3.5, 95% CI: 1.0–12.1; p = .045).

Conclusion

Our findings revealed that positive TaVR is significantly and independently associated with adverse outcomes in NCCM patients. This unique ECG criterion in the often ignored lead provides incremental information beyond what is available with other traditional risk factors.

Usefulness of positive T wave in lead aVR in predicting arrhythmic events and mortality in patients with hypertrophic cardiomyopathy

BACKGROUND

Positive T wave in lead aVR (TaVR) has been associated with increased risk of adverse events in patients with various cardiovascular diseases.

OBJECTIVE

The purpose of this study was to investigate the prevalence and prognostic significance of positive TaVR in patients with hypertrophic cardiomyopathy (HCM).

METHODS

This study investigated 421 consecutive patients with HCM (177 women; age 51.1 ± 14.9 years). Admission electrocardiogram was examined for the presence of a positive TaVR. The primary endpoint was defined as a composite of major arrhythmic events (MAEs), which included sudden cardiac death, sustained ventricular tachycardia or fibrillation, or appropriate implantable cardioverter-defibrillator therapy. Cardiovascular mortality and all-cause death were evaluated as secondary endpoints.

RESULTS

During median follow-up period of 6.0 years (interquartile range 4.0-11.6 years), 53 patients (12.6%) experienced the primary endpoint. On multivariable competing analysis, after adjusting for other confounding factors, the presence of positive TaVR was found to be an independent and strong predictor of the primary composite endpoint. Time-dependent receiver operating characteristic analysis, net reclassification index, and integrated discrimination improvement showed that the addition of positive TaVR to conventional HCM risk factors improved prediction of arrhythmic events. However, in subgroup analysis, a positive TaVR lost statistical significance in patients with apical HCM but remained significant in patients with all other hypertrophy patterns.

CONCLUSION

Positive TaVR is associated with MAE in HCM patients, independent of and incremental to traditional risk factors.

Distinguishing between those dying suddenly or not suddenly from coronary heart disease: long-term prospective results from the Northwick Park Heart Study

AIM

To establish whether ECG findings are associated with subsequent risk of sudden death from coronary heart disease (CHD).

METHODS AND RESULTS

Potential risk factors for CHD were measured at entry to the first Northwick Park Heart Study of 2167 men. ECG findings were coded as high or low risk for CHD according to definitions in the Minnesota code. Sudden or non-sudden deaths were defined as occurring in less than or more than 24 hours, respectively. The only factor independently associated with sudden death among the 262 men dying of CHD was high-risk ECG. Of 184 sudden CHD deaths, 34 men (18.5%) had had high-risk ECGs at entry to the study compared with 5 (6.4%) of 78 men who experienced non-sudden deaths (adjusted OR 3.94 (95% CI 1.33 to 11.67)) (p=0.006). Findings were also compared among all 2167 men, where high-risk ECGs were again associated with sudden death. T-wave changes were the main abnormalities associated with a high risk of sudden death.

CONCLUSIONS

In a group of men who had not previously experienced major episodes of CHD but who subsequently died from it, there was strong evidence that high-risk ECG changes, mainly T-wave abnormalities, differentiated between those who later died sudden deaths and those who survived for >24 hours.

An Electrocardiogram-Based Risk Equation for Incident Cardiovascular Disease From the National Health and Nutrition Examination Survey

Importance

Electrocardiography (ECG) may detect subclinical cardiovascular disease (CVD) in asymptomatic individuals, but its role in assessing adverse events beyond traditional risk factors is not clear. Interval and vector data that are commonly available on modern ECGs may offer independent prognostic information that improves risk classification.

Objectives

To derive and validate a CVD risk equation based on ECG metrics and to determine its incremental benefit in addition to the Framingham risk score (FRS).

Design, Setting, and Participants

This study included 3640 randomly selected community-based adults aged 40 to 74 years without known CVD from the First National Health and Nutrition Examination Survey (NHANES I) cohort (1971-1975) and 6329 from the NHANES III cohort (1988-1994). Participants were sampled from across the United States. A risk score to assess incident nonfatal and fatal CVD events was derived based on computer-generated ECG data, including frontal P, R, and T axes; heart rate; and PR, QRS, and QT intervals from NHANES I. The most prognostic variables, along with age and sex, were incorporated into the NHANES ECG risk equation. The equation was evaluated in the NHANES III cohort for an independent validation. Follow-up in the NHANES III cohort was completed on December 31, 2006. Data for this study were analyzed from August 11, 2015, to May 20, 2016.

Main Outcomes and Measures

The primary end point was CVD death. Secondary outcomes included 10-year ischemic heart disease and all-cause death.

Results

The final study sample included 9969 participants (4714 men [47.3%]; 5255 women [52.7%]; mean [SD] age, 55.3 [10.1] years) from both cohorts. Frontal T axis, heart rate, and heart rate-corrected QT interval were the most significant ECG factors in the NHANES I cohort. In the validation cohort (NHANES III), the equation provided for prognostic information for fatal CVD with a hazard ratio (HR) of 3.23 (95% CI, 2.82-3.72); the C statistic was 0.79 (95% CI, 0.76-0.81). When added to the FRS in Cox proportional hazards regression models, the categorical (1%, 5%, and 10% cutoffs) net reclassification improvement was 24%. When the FRS and ECG scores were combined in a single model, the C statistic improved by 0.04 (95% CI, 0.02-0.06) to 0.80 (95% CI, 0.77-0.82). Similar improvements were noted when the ECG score was added to the pooled cohort equation. When the equation for prognostic information about ischemic heart disease and all-cause death was evaluated, the results were similar.

Conclusions and Relevance

An ECG risk score based on age, sex, heart rate, frontal T axis, and QT interval assesses the risk for CVD and compares favorably with the FRS alone in an independent cohort of asymptomatic individuals. Although the ECG risk equation is low cost, further research is needed to ascertain whether this additional step in risk stratification may improve prevention efforts and reduce CVD events.

T-wave reversal in the augmented unipolar right arm electrocardiographic lead is associated with increased risk of sudden death

BACKGROUND

Repolarization abnormalities are associated with ventricular arrhythmias, and published studies report that a reversal of T wave polarity (positive or flat T wave) in lead aVR may be linked to increased cardiovascular mortality. We evaluated whether a positive or flat T wave in aVR is a risk marker for sudden cardiac death (SCD).

METHODS

SCD cases from the Oregon Sudden Unexpected Death Study (catchment population ~1 million) were compared to geographic controls with coronary artery disease and no history of SCD. Archived electrocardiograms performed prior and unrelated to the SCD event were evaluated.

RESULTS

SCD cases (n = 691, 67.6 ± 14.9 years, 69% male) were more likely than controls (n = 663, 66.2 ± 11.6 years, 67% male) to have diabetes (40 vs 32%; p < 0.01), left ventricular ejection fraction (LVEF) ≤35% (27 vs 11 %; p < 0.01), prolonged QTc (≥450 ms; 54 vs 28%; p < 0.01) and positive (19 vs 13%; p < 0.01) or flat T wave (14 vs 7%; p < 0.01) in aVR. On multivariable analysis, a positive/flat T wave in aVR was independently associated with SCD (OR 1.9, 95% CI 1.3-2.8, p < 0.01). However, a positive T wave alone lost statistical significance in patients with LVEF ≤ 35% and QTc ≥ 450 ms. In a subgroup analysis among patients with normal LVEF, QTc, and no diabetes, a positive T wave in aVR (but not a flat T wave) remained associated with SCD (OR 2.8, 95% CI 1.2-6.1, p < 0.01).

CONCLUSIONS

A positive or flat T wave in lead aVR was associated with SCD in subsets of patients. This simple ECG marker in this often-ignored lead may contribute to enhancement of SCD risk stratification, and warrants further evaluation.

Prediction of Pulmonary Arterial Hypertension in Chronic Obstructive Lung Disease from Three-Dimensional Vectorcardiographic Parameters

AIM

The objective of our study was to assess diagnostic value of three-dimensional (3D) vectorcardiographic (VCG) parameters in detecting pulmonary arterial hypertension (PAH) in chronic obstructive lung disease (COLD) with and without right ventricular hypertrophy (RVH).

METHODS

The study group of 62 patients with COPD was stratified on the basis of color Doppler echocardiographic findings into three subgroups: non-PAH (n = 23), PAH without RVH (n = 22), and PAH with RVH (n = 17). Pairwise differences between the subgroups were evaluated by one-way analysis of variance, and Pearson correlation analysis was used to evaluate the significance of the correlations between pulmonary arterial systolic pressure (PASP) and various VCG parameters.

RESULTS

The azimuth of the QRS vector decreased from -24° in the non-PAH group to -62° in PAH without RVH and to -140° in PAH with RVH (P < 0.01 for pairwise differences between all three groups). Similar significant decrease was observed for the azimuth of the ventricular gradient (VG) vector. Spatial QRS/T angle increased from 69° in the non-PAH group to 115° in PAH without RVH (P < 0.01). In the PAH group with RVH, QRS/T angle was 94° (P < 0.05 for difference from the non-PAH group). There was a significant correlation between PASP and QRS/T angle (r = 0.89, P < 0.05) and between PASP and the azimuth of the VG vector (r = 0.86, P < 0.05). PASP increase from linear regression model was 0.8 mmHg for a QRS/T angle increase by 10° and 1.3 mmHg for each 10° increase in the azimuth of the VG vector.

CONCLUSION

3DVCG parameters are potentially useful for predicting PASP in COLD patients, and possibly also for differentiation between COLD patients with PAH and RVH from those without RVH.

Electrocardiographic Predictors of Cardiovascular Mortality

Cardiovascular diseases are the main causes of mortality. Sudden cardiac death may also appear in athletes, due to underlying congenital or inherited cardiac abnormalities. The electrocardiogram is used in clinical practice and clinical trials, as a valid, reliable, accessible, inexpensive method. The aim of the present paper was to review electrocardiographic (ECG) signs associated with cardiovascular mortality and the mechanisms underlying those associations, providing a brief description of the main studies in this area, and consider their implication for clinical practice in the general population and athletes. The main ECG parameters associated with cardiovascular mortality in the present paper are the P wave (duration, interatrial block, and deep terminal negativity of the P wave in V1), prolonged QT and Tpeak-Tend intervals, QRS duration and fragmentation, bundle branch block, ST segment depression and elevation, T waves (inverted, T wave axes), spatial angles between QRS and T vectors, premature ventricular contractions, and ECG hypertrophy criteria.

A wide QRS/T angle in bundle branch blocks is associated with increased risk for coronary heart disease and all-cause mortality in the Atherosclerosis Risk in Communities (ARIC) Study

BACKGROUND

Repolarization abnormality in bundle branch blocks (BBB) is traditionally ignored. This study evaluated the prognostic value of QRS/T angle for mortality in the presence and absence of BBB.

METHODS AND RESULTS

Total 15,408 participants (mean age 54 years, 55.2% women, 26.9% blacks, 2.8% with BBB) were from the Arteriosclerosis Risk in Communities Study. Sex stratified Cox regression models were used to compute hazard ratios (HRs) with 95% confidence intervals (CIs) for coronary heart disease (CHD) and all-cause mortality for wide spatial QRS/T angle with and without BBB including right BBB (RBBB), left BBB (LBBB) and indetermined-type ventricular conduction defect (IVCD) and RBBB combined with left anterior fascicular block. During a median 22-year follow-up, 4767 deaths occurred, 728 of them CHD deaths. Using the No-BBB with QRS/T angle below median value as gender-specific reference groups, the mortality risk increase was significant for both women and men with No-BBB and QRS/T angle above the median value. In the pooled ICVD/LBBB group, the risk for CHD death was increased 15.9-fold in women and 6.04 fold in men, and for all-cause deaths 3.01-fold in women and 1.84-fold in men. However, the mortality risk in isolated RBBB group was only significantly increased in women but not in men.

CONCLUSION

A wide spatial QRS/T angle in BBB is associated with increased risk for CHD and all-cause mortality over and above the predictive value for BBB alone. The risk for women is as high as or higher than that in men.

Preoperative frontal QRS-T angle is an independent correlate of hospital length of stay and predictor of haemodynamic support requirement following off-pump coronary artery bypass graft surgery

OBJECTIVES

With the adoption of novel operative techniques and aggressive care protocols that facilitate earlier extubation and mobilization of patients, postoperative length of stay (LOS) following coronary artery bypass graft surgery (CABG) has declined. However, there is paucity of information regarding preoperative electrocardiographic predictors of LOS following CABG. In this study, we investigated whether frontal QRS-T angle, which is an abnormal repolarization marker in prediction of various cardiovascular events, was an independent correlate of postoperative hospital LOS for off-pump CABG. Furthermore, we evaluated independent predictors of vasopressor agent/intra-aortic balloon pump (IABP) support requirement following off-pump CABG.

METHODS

In this observational study, 78 patients with stable angina, who were scheduled for elective coronary artery bypass surgery following diagnosis of obstructive coronary artery disease by conventional angiography, were enrolled.

RESULTS

Left ventricular ejection fraction (LVEF) was significantly lower and vasopressor agent/IABP support requirement and incidence of sustained atrial or ventricular arrhythmias was higher in patients with wide QRS-T angle (P < 0.05). Postoperative hospital LOS was also longer in this group. From the preoperative characteristics, wide frontal QRS-T angle was found to be an independent correlate of postoperative hospital LOS (B ± SD: 11.97 ± 0.62, P ≤ 0.01). Wide frontal QRS-T angle was also found to be an independent predictor of vasopressor agent/IABP support requirement postoperatively (OR: 7.87, P ≤ 0.01).

CONCLUSIONS

Prediction of the hospital LOS and patient outcome following CABG is of great importance. Being easily obtainable via standard 12-lead electrocardiogram and its low cost may make frontal QRS-T angle a beneficial marker for reducing both patient-based morbidity and economic burden.

Usefulness of electrocardiographic QRS/T angles with versus without bundle branch blocks to predict heart failure (from the Atherosclerosis Risk in Communities Study)

Repolarization abnormalities in the setting of bundle branch blocks (BBB) are generally ignored. We used Cox regression models to determine hazard ratios (HRs) with 95% confidence intervals (CIs) for incident heart failure (HF) associated with wide spatial and frontal QRS/T angle (upper twenty-fifth percentile of each) in men and women with and without BBB. This analysis included 14,478 participants (54.6% women, 26.4% blacks, 377 [2.6%] with BBB) from the Atherosclerosis Risk in Communities Study who were free of HF at baseline. Using No-BBB with normal spatial QRS/T angle as the reference group, the risk for HF in multivariable adjusted models was increased 51% for No-BBB with wide spatial QRS/T angle (HR 1.51, 95% CI 1.37 to 1.66), 48% for BBB with normal spatial QRS/T angle (HR 1.48, 95% CI 1.17 to 1.88), and the risk for incident HF was increased more than threefold for BBB with wide spatial QRS/T angle (HR 3.37, 95% CI 2.47 to 4.60). The results were consistent across subgroups by gender. Similar results were observed for the frontal plane QRS/T angle. In the pooled BBB group excluding right BBB, a positive T wave in lead aVR and heart rate 70 bpm and higher were also potent predictors of incident HF similar to the QRS/T angles. In conclusion, both BBB and wide QRS/T angles are predictive of HF, and concomitant presence of both carries a much higher risk than for either predictor alone. These findings suggest that repolarization abnormalities in the setting of BBB should not be considered benign or an expected consequence of BBB.

Electrocardiographic repolarization-related variables as predictors of coronary heart disease death in the women's health initiative study

BACKGROUND

We evaluated 25 repolarization-related ECG variables for the risk of coronary heart disease (CHD) death in 52 994 postmenopausal women from the Women's Health Initiative study.

METHODS AND RESULTS

Hazard ratios from Cox regression were computed for subgroups of women with and without cardiovascular disease (CVD). During the average follow-up of 16.9 years, 941 CHD deaths occurred. Based on electrophysiological considerations, 2 sets of ECG variables with low correlations were considered as candidates for independent predictors of CHD death: Set 1, Ѳ(Tp|Tref), the spatial angle between T peak (Tp) and normal T reference (Tref) vectors; Ѳ(Tinit|Tterm), the angle between the initial and terminal T vectors; STJ depression in V6 and rate-adjusted QTp interval (QTpa); and Set 2, TaVR and TV1 amplitudes, heart rate, and QRS duration. Strong independent predictors with over 2-fold increased risk for CHD death in women with and without CVD were Ѳ(Tp|Tref) >42° from Set 1 and TaVR amplitude >-100 μV from Set 2. The risk for these CHD death predictors remained significant after multivariable adjustment for demographic/clinical factors. Other significant predictors for CHD death in fully adjusted risk models were Ѳ(Tinit|Tterm) >30°, TV1 >175 μV, and QRS duration >100 ms.

CONCLUSIONS

Ѳ(Tp|Tref) angle and TaVR amplitude are associated with CHD mortality in postmenopausal women. The use of these measures to identify high-risk women for further diagnostic evaluation or more intense preventive intervention warrants further study.

CLINICAL TRIAL REGISTRATION URL

http://www.clinicaltrials.gov. Unique identifier: NCT00000611.

Abnormal electrocardiographic QRS transition zone and risk of mortality in individuals free of cardiovascular disease

AIMS

We examined the prognostic significance of abnormal electrocardiographic QRS transition zone (clockwise and counterclockwise horizontal rotations) in individuals free of cardiovascular disease (CVD).

METHODS AND RESULTS

A total of 5541 adults (age 53 ± 10.4 years, 54% women, 24% non-Hispanic black, 25% Hispanic) without CVD or any major electrocardiogram (ECG) abnormalities from the US Third National Health and Nutrition Examination Survey were included in this analysis. Clockwise and counterclockwise horizontal rotations were defined from standard 12-lead ECG using Minnesota ECG Classification. Mortality and cause of death were assessed through 2006. At baseline, 282 participants had clockwise rotation and 3500 had counterclockwise rotation. During a median follow of 14.6 years, 1229 deaths occurred of which 415 were due to CVD. In multivariable-adjusted Cox proportional hazard analysis and compared with normal rotation, clockwise rotation was significantly associated with increased risk of all-cause mortality {hazard ratio (HR) [95% confidence interval (CI)]: 1.43 (1.15-1.78); P = 0.002} and CVD mortality [HR (95% CI): 1.61 (1.09, 2.37) P = 0.016]. In contrast, counterclockwise rotation was associated with significantly lower risk of all-cause mortality [HR (95% CI): 0.86 (0.76, 0.97); P = 0.017] and non-significant association with CVD mortality [HR (95% CI): 1.07 (0.86, 1.33); P = 0.549]. These results were consistent in subgroup analysis stratified by age, sex, and race.

CONCLUSION

In a diverse community-based population free of CVD and compared with normal rotation, clockwise rotation was associated with increased risk of all-cause and CVD mortality while counterclockwise rotation was associated with lower risk of all-cause mortality and non-significant association with CVD mortality. These findings call for attention to these often neglected ECG markers, and probably call for revising the current definition of normal rotation.

Short-term repeatability of electrocardiographic Tpeak-Tend and QT intervals

BACKGROUND

The electrocardiographic (ECG) Tpeak-Tend interval (TpTe) is associated with arrhythmias and sudden cardiac death. TpTe offers a supplementary measure for the QT interval (QT), yet its repeatability has not been established.

PURPOSE

Evaluate short-term repeatability of TpTe and QT.

METHODS

Four ECGs were obtained on sixty participants. The sources of variation, intra-class correlation coefficient (ICC) - an index of reproducibility - and minimal detectable change (MDC) were estimated for TpTe and QT. The impact of repeated measurements on repeatability was estimated for a hypothetical clinical trial designed to detect drug-induced prolongation of TpTe and QT.

RESULTS

We used heart rate-adjusted QT [(QT)a] but TpTe in the study group was rate-invariant. The ICC [95% confidence interval (CI)] was 0.77 (0.69, 0.85) for TpTe, 0.75 (0.65, 0.85) for QT and 0.60 (0.47, 0.73) for (QT)a. The MDC (ms) was 21, 32 and 26 for TpTe, QT and (QT)a respectively.

CONCLUSION

TpTe has excellent repeatability supporting its use as a supplement to QT in observational and clinical studies.

Race- and sex-associated differences in rate-adjusted QT, QTpeak, ST elevation and other regional measures of repolarization: the Atherosclerosis Risk in Communities (ARIC) Study

BACKGROUND

Data are limited about race-and sex-associated differences in prognostically important ECG measures of regional repolarization.

METHODS AND RESULTS

The normal reference group from the Atherosclerosis Risk in Communities (ARIC) study included 8,676 white and African-American men and women aged 40-65 years. Exclusion criteria included cardiovascular disease, hypertension, diabetes and major ECG abnormalities. Notable sex differences (p<0.001) were observed in the upper 98% limits for rate-adjusted QTend (QTea) which was 435 ms in white and African-American men and 445 ms in white and African-American women, and for left ventricular epicardial repolarization time (RTepi) which was 345 ms in white and African-American men and 465 ms in white and African-American women. These sex differences reflect earlier onset and end of repolarization in men than in women. Upper normal limits for STJ amplitude in V2-V3 were 100 μV in white and African-American women, 150 μV in white men and 200 μV in African-American men (p<0.001 for sex differences), and for other chest leads, aVL and aVF 50 μV in white women, 100 μV in African-American women, 100 μV in white men and 150 μV in African-American men (p<0.001 for sex and race differences).

CONCLUSIONS

Shorter QTea and RTepi in men than in women reflect earlier onset and end of repolarization in men. STJ amplitudes in African-American men were higher than in other subgroups by race and sex. These sex and race differences need to be considered in clinical and epidemiological applications of normal standards.