Heart Rate Variability Measurement

Thank you to Megan L. Jester and Emily J. Jones for this article on heart rate variability measurement. You can read the second article in this series here.

Prevention and Treatment of Cardiovascular Disease Across the Lifespan

Heart rate variability is a complex physiological phenomenon linked to autonomic nervous system (ANS) innervation of the heart and how well it functions.^1-7 Measuring heart rate variability is easily accessible, non-invasive, and may provide insight into the influence of the ANS on childhood diseases and their progression to adult-onset diseases including cardiovascular disease.^1-3 Heart rate variability measures the instant variations between consecutive heartbeats and the subsequent variations caused by inhalation and exhalation¹ and is defined by the duration of the R-wave intervals of the QRS complex on an electrocardiograph.  

Today, heart rate variability is primarily used as a risk measurement tool based on a rich history. Hales (1733) first observed heart rate variability as a respiratory pattern in a horse’s blood pressure (BP).⁸ Ludwig (1847) is believed to have first identified respiratory sinus arrhythmia by observing changes in a dog’s pulse during respiration.⁸ The term heart rate variability was coined in 1965 when researchers noted interbeat changes before a change in heart rate variability during fetal distress.⁹ In the mid-1970s, the use of heart rate variability as a measurement tool was further refined when researchers identified embedded rhythms between RR intervals (the time elapsed between two successive R-waves of the QRS signal).⁹ Power spectral analysis was initiated in the early 1980s to quantify heart rate variability findings, and the clinical importance of heart rate variability was validated as a strong predictor of mortality following an acute myocardial infarction.⁹ 

Autonomic Balance and Heart Rate Variability 

Autonomic function plays an important, underlying role in an individual’s overall health. As a reminder, the ANS comprises two systems: the parasympathetic nervous system and the sympathetic nervous system, which both regulate heart rate and rhythms and other system functions to maintain homeostasis. The vagus nerve controls parasympathetic nervous system action, and the post-ganglionic paraspinous nerve controls sympathetic nervous system action.⁷ During a state of rest, vagal tone dominates heart rate control as well as any variations that result from vagal modulation.⁹ When the parasympathetic nervous system and sympathetic nervous system are in harmony with each other, an autonomic state termed sympathovagal balance occurs.¹⁰ Sympathovagal balance helps regulate blood pressure and cardiac output, leading to adequate blood perfusion¹¹ and homeostasis. The degree of heart rate variability may represent adaptability of the cardiovascular system,¹² and, importantly, an imbalance between sympathetic and parasympathetic control may lead to early morbidity and mortality.  

Measuring Heart Rate Variability 

The two main domains for measuring heart rate variability, time and frequency, are often reported independently but typically correlate with one another. Various mathematical methods are used to calculate the indices within each domain.¹³ According to the Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology (henceforth the Task Force),⁹ time-domain indices generally reflect parasympathetic outflow and are measured from either the heart rate at a given point in time or the successive normal RR intervals.¹² The Task Force recommends recording at least 18 hours of continuous data, including overnight hours.⁹ Two subsets of time-domain indices are measured: statistical, which is recorded over a 24-hour period and is more complicated to analyze; and geometrical, which converts a series of RR intervals into geometric patterns.  

Frequency-domain indices reflect how power, or variance, distributes by way of frequency⁹ and reflects sympathovagal balance. The RR interval is divided into sinusoidal waveforms based on pre-established frequency bandwidths.⁶ The frequency-domain subsets comprise both non-parametric and parametric variables and are analyzed by either short-term recordings of 1–5 minutes or long-term recordings of 24 hours.^8–9 The Task Force recommends the use of frequency-domain indices as opposed to time-domain indices for short-term recordings, and recordings of 5 minutes are recommended unless the study design states otherwise.⁹  

Heart Rate Variability in Children and Adults 

Autonomic function in children is still a complex mystery because of a child’s growth trajectory and its potential influence. Maturation is the most significant predictor of heart rate variability deviations within children and between children and adults.⁷ With increasing age, the heart and respiratory rates should decrease while the beat-to-beat variability increases.⁶ Age and sex may influence the respiratory rate, both of which are associated with parasympathetic nervous system activity.⁸ Therefore, the reliability of heart rate variability may be compromised when HRV indices are compared among individuals across a broad age range.⁶ Finally, as maturation occurs, heart and respiratory rates decrease, and BP becomes less labile, which may contribute to more consistent heart rate variability measurements.⁶ 

For all individuals, autonomic withdrawal from reduced ventricular performance, in conjunction with sympathetic nervous system activation and sympathovagal imbalance leads to a reduced overall heart rate variability,^{9, 13} an early indicator of cardiovascular complications for physiological disorders including obesity, and type 1 and type 2 diabetes.⁶ In 1996, the Task Force⁹ defined a wide range of HRV parameters and recommended longitudinal studies to measure and establish HRV guidelines for sex and for multiple age ranges, including children. Over 25 years later, the psychometric properties for measuring HRV specifically in children are not well-established compared to adults,⁶ presenting opportunities for nurse scientists and clinicians to define and contribute to this growing body of knowledge. 

Key Takeaways for Cardiovascular Nurses 

1. Heart rate variability measurement may help guide prevention and treatment strategies for cardiovascular disease. 

2. Our work continues to revise the current heart rate variability guidelines for adults and to establish guidelines for children.  

3. Part two of this series will focus on current devices and methods to examine the association between heart rate variability and cardiovascular disease across the lifespan and recommendations for future research and practice. 

References

1. Tarvainen MP, Lipponen J, Niskanen JP, Ranta-aho PO. Kubios HRV software user’s guide. Kubios- heart rate variability. Published November 3, 2021. Accessed April 11, 2022.
2. Khattab K, Khattab AA, Ortak J, Richardt G, Bonnemeier H. Iyengar yoga increases cardiac parasympathetic nervous modulation among healthy yoga practitioners. Evid Based Complement and Altern Med. 2007;4(4):511-517. doi:10.1093/ecam/nem087
3. Mazurak N, Sauer H, Weimer K, et al. Effect of a weight reduction program on baseline and stress-induced heart rate variability in children with obesity. Obesity. 2016;24(2):439-445. doi:10.1002/oby.21355
4. Papp ME, Lindfors P, Storck N, et al. Increased heart rate variability but no effect on blood pressure from 8 weeks of hatha yoga – a pilot study. BMC Res Notes. 2013;6(1):59-64. doi:10.1186/1756-0500-6-59
5. Vinay AV, Venkatesh D, Ambarish V. Impact of short-term practice of yoga on heart rate variability. Int J Yoga. 2016;9(1):62-66. doi:10.4103/0973-6131.171714
6. Weiner OM, McGrath JJ. Test-retest reliability of pediatric heart rate variability: A meta-analysis. J Psychophysiol. 2017;31(1):6-28. doi:10.1027/0269-8803/a000161
7. Williams T, Tang X, Gilmore G, et al. Measures of and changes in heart rate variability in pediatric heart transplant recipients. Pediatr Transplantation. 2017;21(4)1-8. doi:10.1111/petr.12894
8. Berntson GG, Thomas Bigger J, Eckberg DL, et al. Heart rate variability: Origins, methods, and interpretive caveats. Psychophysiology. 1997;34(6):623-648. doi:10.1111/j.1469-8986.1997.tb02140.x
9. Electrophysiology TF. Heart rate variability. Circulation. 1996;93(5):1043-1065. doi:10.1161/01.cir.93.5.1043
10. Pace M, Dumortier L, Favre-Juvin A, et al. Heart rate variability during sleep in children with autism spectrum disorders. Physiol Behav. 2016;167:309-312. doi:10.1016/j.physbeh.2016.09.027
11. Costa HA, Silva-Filho AC, Dias CJ, et al. Cardiovascular response of an acute exergame session in prepubertal obese children. Games Health J. 2017;6(3):159-164. doi:10.1089/g4h.2016.0081
12. Cohen M, Tyagi A. Yoga and heart rate variability: A comprehensive review of the literature. Int J Yoga. 2016;9(2):97-113. doi:10.4103/0973-6131.183712
13. Seppälä S, Laitinen T, Tarvainen MP, et al. Normal values for heart rate variability parameters in children 6-8 years of age: The panic study. Clin Physiol Funct Imaging. 2013;34(4):290-296. doi:10.1111/cpf.12096