Abstract
Heart rate variability (HRV) is a known risk factor for mortality in both healthy and patient populations. There are currently no normative data for short-term measures of HRV. A thorough review of short-term HRV data published since 1996 was therefore performed. Data from studies published after the 1996 Task Force report (i.e., between January 1997 and September 2008) and reporting short-term measures of HRV obtained in normally healthy individuals were collated and factors underlying discrepant values were identified. Forty-four studies met the pre-set inclusion criteria involving 21,438 participants. Values for short-term HRV measures from the literature were lower than Task Force norms. A degree of homogeneity for common measures of HRV in healthy adults was shown across studies. A number of studies demonstrate large interindividual variations (up to 260,000%), particularly for spectral measures. A number of methodological discrepancies underlined disparate values. These include a systematic failure within the literature (a) to recognize the importance of RR data recognition/editing procedures and (b) to question disparate HRV values observed in normally healthy individuals. A need for large-scale population studies and a review of the Task Force recommendations for short-term HRV that covers the full-age spectrum were identified. Data presented should be used to quantify reference ranges for short-term measures of HRV in healthy adult populations but should be undertaken with reference to methodological factors underlying disparate values. Recommendations for the measurement of HRV require updating to include current technologies. (PACE 2010; 1407–1417)
autonomic nervous system, risk factors, norms, homogeneity, populations
Introduction
In 1996, the European Society for Cardiology and the North American Society of Pacing and Electrophysiology supported a Task Force which issued a seminal paper: “Heart rate variability: Standards of measurement, physiological interpretation and clinical use” (Circulation, 1996; 93, 1043–1065). Reference normal values for shortterm measures of heart rate variability (HRV) in healthy adults were published as an appendix to the paper. Some of these values, however, were approximated from studies involving small
sample sizes. As a result, these data are considered as “unsuitable for definite clinical conclusions to be drawn from.”1 The Task Force stressed the need for large prospective population studies to establish normal HRV standards including age and sex subsets. This need was considered greatest for HRV values obtained from short-term recordings. The interest in HRV as a measurement of autonomic function lies in its clinical importance. A reduced HRV is a powerful and independent predictor of an adverse prognosis in patients with heart disease2–4 and in the general population.5,6 Despite the important prognostic power of HRV, it is still not a widely used tool in clinical settings. Key issues relating to this fact include the most appropriate analysis method(s), the recommended length of electrocardiogram recordings, and the conditions in which they should be assessed.7 Arguably an additional key factor is the lack of agreed normative values for HRV, without which classifying “abnormal” HRV remains difficult. In the majority of other clinically health-related measures (e.g., blood pressure, heart rate, forced vital capacity), established norms are routinely compared to provide an indication of current health status. There is no clear explanation why this is not the case for HRV. Since 1996, publications assessing and reporting both 24-hour and short-term HRV in healthy and clinical populations have increased. Pinna and colleagues 8 report an increase in the number of yearly publications from 391 to 584 in the period 2000 to 2006, respectively. Taylor and Studinger9 reported an average of 10 articles related to HRV published weekly during 2005. These “newer” studies provide a potential source of normative data for common HRV measures in healthy populations. Moreover, by comparing values between publications, it may be possible to identify factors contributing to discrepancies in HRV values.
Summary of Main Factors Underlining
Discrepant Values in Short-Term HRV from Healthy Individuals. The measure-by-measure analysis performed for those studies reporting discrepant values revealed a number of underlying factors including:
1. Moderate to high level of participant habitual physical activity;
2. The use of paced breathing protocols, particularly when performed in participants with moderate to high physical activity levels;
3. Where younger participants are measured, values for HRV are typically higher;
4. Poor reporting and/or performance of RR interval error recognition, removal, and/or correction procedures;
5. The use of differing frequency bandwidths and normalization methods for LF and HF spectral measures;
6. Wide variation in HRV measures between healthy participants of the same study;
7. The misclassification of participants as healthy;
8. A failure of studies to recognize the normality/abnormality of values obtained in healthy participants.
Some of the points above (1, 2, 3, and 6) were not unexpected. Of some surprise was the failure to perform error correction procedures by a number of studies and the poor reporting of these procedures by others. The last three summary points are particularly important and highlight the inherent problem of defining a so-called “normal” HRV.
These points are also inter-related in that the failure to question the normality of data when obtained in healthy participants possibly stems from the fact that even in homogonous healthy groups, measures of HRV can display wide interindividual variations (as high as 260,000%, Fagard et al.11; Table IV). It is important, however, to recognize other factors could influence discrepancies between studies. Measures of HRV are influenced by diet (caffeine and alcohol intake) and physical and mental stress. Very few of the studies included here include information on these factors and their impact on values presented cannot be determined. When assessing studies reporting so-called normal HRV, readers should employ close scrutiny of the factors outlined above as well as potential other factors (e.g., diet, stress) related to the individual aspects of each study. With consideration of these factors, the data presented in this study may provide users of HRV with reference ranges by which to determine disparate values for common measures of short-term HRV.
Study Limitations
It is possible that some papers meeting the inclusion criteria for the present study would have been missed by the search strategy employed. Arbitrary selection of the selected search terms may have meant that some studies reporting short-term HRV in healthy adults may have been missed. Alternatively, it could be argued that studies missed despite the comprehensive list of search terms may be too ambiguous in terms of the context in which short-term measures of HRV were used.
Study Recommendations
To facilitate between-study comparisons and aid standardization of measurements, studies need to report the outcomes of RR interval data editing procedures. In addition, measures of stationarity or measures taken to address nonstationary signals should be provided. Moreover, editors and reviewers need to adopt greater diligence in ensuring that papers using HRV provide details of data treatment before accepting the paper(s) for publication. Despite the call for large population-based studies to determine normal HRV standards by the 1996 Task Force paper, there are no studies with participants from the full age spectrum. There is still a need for a population-based study assessing short-term HRV measurements and involving the full age spectrum. A multicenter approach may be the most feasible approach. Such a study would require stringent methodological standards and participant inclusion criteria and awareness of methodological and participant factors known to affect HRV.
There is a need for a revision of current recommendations and standards for the measurement of short-term HRV. These should be made in light of significant developments in the computational power and accuracy of automated RR interval and HRV analysis systems. There is a particular need to stress clarity and transparency by the manufacturers as to the QRS, RR interval, and HRV analysis procedures of new technologies.
Conclusions
Data presented here should be used to quantify reference ranges for short-term measures of HRV in healthy adult populations but should be undertaken with reference to methodological factors underlying disparate values. These include but are not limited to: participant demographic characteristics, including age, sex, and habitual physical activity levels; poor RR interval data editing procedures; poor classification of healthy participants; and a failure to recognize values as disparate. Studies reporting HRV need to recognize the normality of data even when obtained in individuals considered as healthy. The need for large-scale population studies assessing short-term HRV in normally healthy adults still remains. Current recommendations require updating to account for the era of completely automated HRV analysis. Clarification of measurement standards in light of the discrepancies observed between studies is also needed.