(Circulation. 2000;102:1239.)
© 2000 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Division of Human Nutrition and Epidemiology (J.D., E.S.), Wageningen Agricultural University, Wageningen, the Netherlands; Institute for Research in Extramural Medicine (J.D.), Vrije Universiteit Amsterdam (the Netherlands); Division of Epidemiology (R.C., A.F., P.H.), School of Public Health, University of Minnesota, Minn; Department of Epidemiology (D.L.), School of Public Health, University of North Carolina (Chapel Hill); and Department of Cardiology (C.S.), University Hospital, Leiden, the Netherlands.
Correspondence to Dr Jacqueline M. Dekker, Institute for Research in Extramural Medicine, Faculty of Medicine Vrije Universiteit, Van der Boechorststraat 7, 1081 BT Amsterdam, the Netherlands. E-mail JM.Dekker.EMGO{at}med.vu.nl
| Abstract |
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Methods and ResultsWe studied the predictive value of HRV for CHD and death from several causes in a population study of 14 672 men and women without CHD, aged 45 to 65, by using the case-cohort design. At baseline, in 1987 to 1989, 2-minute rhythm strips were recorded. Time-domain measures of HRV were determined in a random sample of 900 subjects, for all subjects with incident CHD (395 subjects), and for all deaths (443 subjects) that occurred through 1993. Relative rates of incident CHD and cause-specific death in tertiles of HRV were computed with Poisson regression for the case-cohort design. Subjects with low HRV had an adverse cardiovascular risk profile and an elevated risk of incident CHD and death. The increased risk of death could not be attributed to a specific cause and could not be explained by other risk factors.
ConclusionsLow HRV was associated with increased risk of CHD and death from several causes. It is hypothesized that low HRV is a marker of less favorable health.
Key Words: heart rate nervous system, autonomic coronary disease mortality heart diseases
| Introduction |
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| Methods |
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In the home interview, questions were asked about health behaviors,
sociodemographics, and disease history. The clinical examination
included an ECG recording. After a 5- to 10-minute rest period, during
which electrodes were placed, each subject had a standard 12-lead ECG
and a 2-minute rhythm strip recorded. Plasma total
cholesterol, HDL cholesterol, LDL
cholesterol, and triglyceride levels were
determined with standard methodology. Serum insulin was measured with a
radioimmunoassay (125I Insulin Kit; Cambridge
Medical Diagnostics). Serum glucose was assessed with the
hexokinase method. Prevalent diabetes mellitus was defined as a fasting
glucose level of
140 mg/dL (
7.8 mmol/L), a nonfasting glucose
level of
200 mg/dL (
11.1 mmol/L), or a history of, or
treatment for, diabetes. Three seated blood pressure measurements were
taken on the right arm with a random-zero sphygmomanometer. The mean of
the last 2 measurements was used. Hypertension was defined as
systolic blood pressure of
140 mm Hg,
diastolic blood pressure of
90 mm Hg, or the use of
antihypertensive agents. The ratio of waist (umbilical level) and hip
(maximum buttocks) circumferences was calculated as a measure of fat
distribution. The average carotid intima-media thickness was assessed
with a standardized B-mode ultrasonic technique.11
Study Design and Methods of Follow-Up
Because it was not feasible to determine HRV for all
participants, the case-cohort design was applied, in which data are
collected in the complete cohort before disease occurs. At a later
stage, the available data are processed for the cases that occurred
during follow-up and in a random sample (including a number of future
cases), which is used to reflect the total cohort. This design provides
estimates of relative risks.12
For the present study, a random sample of 900 subjects was selected in 1995 from the 14 672 subjects without prevalent coronary heart disease (CHD) at baseline, combined with all incident CHD cases and all deaths that occurred through 1993. Prevalent CHD was defined as self-reported history of physician-diagnosed heart attack, 12-lead Minnesota Code evidence of prior myocardial infarction (MI), prior cardiovascular surgery, or prior coronary angioplasty.
The method of follow-up has been described previously.13 Briefly, interviewers contacted participants annually by telephone to identify hospitalizations and deaths. For hospitalized patients with potential acute CHD events, trained abstractors recorded the presenting signs and symptoms, including chest pain, cardiac enzyme levels, and related clinical information. As many as three 12-lead ECGs were visually coded with the Minnesota Code, and waveform evolution was evaluated with side-by-side comparisons.14 Out-of hospital deaths were investigated by means of the death certificate, an interview with the next-of-kin, and questionnaires completed by the patients physicians. Coroner reports and autopsy reports, when available, were used for validation.
CHD incidence was defined as definite or probable MI, cardiac revascularization procedures (excluding thrombolytic therapy), or definite CHD death. Cause-specific death was based on the underlying cause given on the death certificate: cardiovascular disease (CVD) (ICD-9 codes 390 to 459) and cancer (ICD-9 codes 140 to 240). After the exclusion of subjects with missing data, the study population consisted of 856 subjects in the random sample combined with 395 incident CHD cases (29 were also part of the sample) and 443 deaths (21 were also part of the sample), of whom 140 died from CVD and 209 died from cancer. Of the subjects who died from cancer, 36 reported a history of cancer at baseline.
Data analysis
In 1995 to 1996, the duration of all normal sinus intervals (R-R
intervals) was measured at the University of Minnesota ECG Coding
Center by coders who were blinded with respect to disease occurrence
during follow up. A digitizing tablet (Calcomp) and a personal computer
were used. Coders mounted the 2-minute rhythm strip on the tablet and
used the mouse to successively mark the top of all of the R peaks. The
X-Y coordinates were transmitted to the personal computer,
and the duration of the R-R interval was computed on the basis of the
difference between coordinates. To reduce measurement error, the
procedure was performed in duplicate, and the mean of the 2
measurements of each R-R interval was used for the analyses.
Abnormal beats were marked by the coders. The 2 intervals after a
single abnormal beat were excluded for analysis of HRV.
Subjects with >2 abnormal beats in the rhythm strip were excluded from
the analyses (36 in the random sample, 98 cases), because the
presence of multiple ventricular extrasystoles is known to
be associated with elevated mortality risk,15 and it
is accompanied with higher HRV.
Four measures of HRV were determined: the standard deviation of all R-R intervals (SDNN), the mean of absolute successive differences (rMSSD), the standard deviation of absolute differences between successive intervals (SDSD), and the percentage of intervals differing by >50 ms from the preceding interval (pNN50). In addition, heart rate was calculated from the mean duration of R-R intervals. HRV could not be determined for 4 subjects in the random sample, and rhythm strips were missing for 3 subjects in the random sample and for 23 cases.
The intercoder and intracoder variabilities were evaluated in a set of 10 rhythm strips, which were measured 3 times by each coder. HRV measures were used as the dependent variables. For the SDNN, the between-subject variance was 310 ms2, the between-coder variance was 0.04 ms2, and the intercoder variance was 3.24 ms2. The correlation of all HRV measures between coders was >0.99.
Subjects were categorized a priori into 3 groups according to HRV and heart rate. Categories were based on the tertile cut points of the distribution of subjects in the random sample. The cut points were 23.9 and 35.4 ms for SDNN, 14.7 and 22.3 ms for rMSSD, 11.3 and 16.4 ms for SDSD, 0.8% and 5.9% for pNN50, and 63 and 72 bpm for heart rate.
Relative incidence rates of CHD, total mortality, and cause-specific mortality, in categories of all measures of HRV and of heart rate, were computed with Poisson regression for the case-cohort design.12 In line with previous work, the intermediate category was taken as the reference.5 Subjects with a self-reported history of cancer at baseline were excluded from the analyses of cancer risk.
Two models were used for the regression analyses. The first model adjusted for age, sex, race group, and ARIC recruitment center; the multivariable model also adjusted for variables that were associated with HRV: current smoking, cigarette-years, triglycerides, HDL cholesterol, diabetes, hypertension, body mass index, waist-to-hip ratio, and carotid intima-media thickness. For cancer mortality, current smoking and cigarette-years were added in the multivariable model.
| Results |
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In Table 1
, population
characteristics are shown in the random sample in tertiles of SDNN.
Age, male sex, heart rate, body mass index, ratio of waist to hip
circumferences, insulin level, triglyceride level, HDL
cholesterol level, blood pressure, carotid intima-media
thickness, and the prevalence of hypertension and diabetes differed
significantly for the HRV categories, with subjects with low HRV in
general having a worse cardiovascular risk profile.
Therefore, these variables were considered possible confounding or
mediating factors.
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In the survival analysis (Table 2
), low HRV and high heart rate were
associated with elevated risk of all end points. The age-, sex-, race-,
and field centeradjusted relative risks of
cardiovascular mortality in the lowest compared with
the intermediary tertile of SDNN was 2.10 (95% CI 1.21 to 3.64) For
rMSSD, SDSD, and pNN50, the relative risks of all end points were
higher than those observed for SDNN. The relative risks were less
favorable in the high than in the intermediate tertile. A U shape was
observed for cancer mortality; the relative rates in the lowest and
highest tertiles of pNN50 were 1.95 (1.27 to 2.92) and 1.53 (0.97 to
2.41), respectively. Adjustment for possible confounders slightly
decreased the increased risks for the low SDNN and high heart rate
categories and increased the risks in the lowest tertiles of rMSSD,
SDSD, and pNN50 and in the high tertiles of all measures of HRV.
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Adjustment for insulin or glucose level in subjects without diabetes
did not affect the associations. Stratified analysis was
performed to evaluate possible confounding and interaction.
Stratification did not show appreciably different relative risk
estimates within subgroups of heart rate, age, sex, race, hypertension,
diabetes, smoking, or body mass index. Furthermore, when
analyses were restricted to subjects without hypertension,
diabetes, cancer, or symptomatic heart disease at baseline,
the associations with death remained virtually unchanged (Table 3
).
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| Discussion |
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Methodological Issues
In the present study, the case-cohort design was used. To
evaluate whether the cohort sample was a representative
sample, we compared mean values for cardiovascular risk
factors of the cohort sample versus those of the full cohort. None of
these values were significantly different, and they never differed by
>1%. It is therefore likely that the distributions of measures of HRV
in the cohort sample also are representative of the
distributions in the full cohort, thus justifying the estimates of
relative risks.
The use of certain medication was considered as a possible confounding factor, because many drugs affect autonomic nervous system function. The analyses for subjects without chronic diseases (subjects who used antihypertensive, glucose-lowering, or other cardioactive medication were excluded) showed similar results. When the analyses were restricted to persons not using any medication, the associations did not change.
Two-minute rhythm strips were used for the determination of HRV. It is known from 24-hour Holter recordings that HRV changes during the day.1 However, short-term HRV measurements during the daytime are correlated with 24-hour HRV measures, and in patients with myocardial infarction, such short-term HRV measurements predict future mortality rates.3
Heart rate was correlated with measures of HRV and associated with mortality risk and therefore may be considered a possible confounding variable. Because both heart rate and HRV reflect autonomic function, the inclusion of both parameters was regarded as an overadjustment. When the coefficient of variation was used, SDNN divided by mean R-R interval, the age-, sex-, race-, and field centeradjusted relative risks of the lowest versus the intermediate tertile were 1.82 (1.07 to 3.09) for CVD mortality and 1.45 (1.04 to 2.03) for CHD incidence.
Previous Studies
Previous work in the ARIC study, with spectral analysis of
heart rate, has shown low HRV to predict incident CHD.7
The results of the present study confirmed this association over a
longer follow-up period with the use of different methodology (time
domain) and a different subset of the ARIC cohort. The
consistency of these results strengthens confidence that
low HRV is an important predictor of CHD events. In addition, Tsuji et
al6 reported low HRV, as determined in 2-hour ambulatory
ECG recordings, to be a risk indicator for incident CHD in the
elderly participants of the original Framingham Heart Study, combined
with the younger subjects from the Framingham Offspring Study. In the
elderly cohort, low HRV was a strong predictor of death,4
but because of the limited number of cases, the relation with specific
causes of death could not be analyzed. In the Zutphen
Study,5 a prospective study in middle-aged and elderly
Dutch men, HRV was determined from 15- to 30-second recordings.
A strong association between low HRV and death from all causes,
including cancer, was observed. The results of the present study
are in line with these findings. Furthermore, we were able to show that
the predictive value of HRV for death cannot be attributed to known
underlying disease. The higher risk of death in the high-HRV group
observed in the present study was also reported in the Zutphen
Study.
Mechanisms
Heart rate and HRV are influenced by the autonomic nervous system.
The rMSSD, SDSD, and pNN50 mostly reflect fast
breathingrelated beat-to-beat changes and are measures of
parasympathetic involvement in circulatory control.2 The
SDNN in a 2-minute recording contains both Mayer waves
(10-second fluctuations) and fast fluctuations in heart rate and
reflects sympathetic as well as parasympathetic
involvement.2
In general, with sympathetic predominance, HRV is reduced, and the risk of (fatal) arrhythmias is elevated,1 which may explain mortality risk in patients with myocardial infarction who have low HRV.3 However, this cannot account for the observed adverse cardiovascular risk profile and greater incidence of nonfatal CHD in subjects with low HRV in the present study, so other explanations are needed.
Many other factors affect autonomic nervous system function and, thus, HRV. HRV decreases with age,7 high insulin level,16 reduced baroreflex sensitivity,17 physical inactivity,18 rapid and shallow breathing,19 smoking,20 depression,21 atherosclerosis,9 obstructive sleep apnea,22 and diabetic autonomic neuropathy.1 Autonomic nervous system function affects all organ systems, including the immune system. Direct effects of sympathetic activity on the function, number, and subset distribution of circulating lymphocytes have been described.23 24 All of these mechanisms may have contributed to the observed associations. However, our measures of age, insulin, smoking, hypertension, physical activity, atherosclerosis, diabetes, and cancer did not explain these relationships. In addition, the association was present in subjects without known disease. This suggests that low HRV precedes manifest disease. Possibly, sympathetic predominance, as reflected in low HRV and high heart rate, may be indicative of less favorable general health, with HRV being a more sensitive indicator than heart rate.
In conclusion, in a population-based study of middle-aged men and women, high heart rate and, especially, low HRV were predictive of increased mortality rates. For HRV, this relation could not be attributed to cardiovascular risk factors or to underlying disease. It may be hypothesized that low HRV is an indicator of poor general health.
| Acknowledgments |
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| Footnotes |
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Received January 7, 2000; revision received March 27, 2000; accepted April 13, 2000.
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