Origin of Heart Rate Variability and Turbulence: An Appraisal of Autonomic Modulation of Cardiovascular Function

Federico Lombardi,Phyllis K Stein

doi:10.3389/fphys.2011.00095

Federico Lombardi, Phyllis K Stein

Open Access

https://doi.org/10.3389/fphys.2011.00095

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Abstract

Heart period constantly changes on a beat to beat basis, due to autonomic influences on the sinoatrial node, and changes can be quantified as heart rate variability (HRV). In addition, after a premature ventricular beat, there are reproducible variations in RR interval, also due to baroreflex mediated autonomic influences on the sinoatrial node, that can be measured as heart rate turbulence (HRT). Impaired autonomic function as measured by HRV and HRT has proven to predict adverse outcomes in clinical settings. The ability of reduced HRV and HRT to predict adverse outcomes has been explained by their dependency on vagal mechanisms that could reflect an increased sympathetic and a reduced vagal modulation of sinus node, thus favoring cardiac electrical instability. Analysis of non-linear dynamics of HRV has also been utilized to describe the fractal like characteristic of the variability signal and proven effective in identify patients at risk for sudden cardiac death. Despite the clinical validity of these measures, it has also been evident that the relationship between neural input and sinus node responsiveness is extremely complex and variable in different clinical conditions. Thus, abnormal HRV or HRT on a clinical Holter recordings may reflect non-neural as well as autonomic mechanisms, and this also needs to be taken into account when interpreting any findings. However, under controlled conditions, the computation of the low and high frequency components of HRV and of their normalized powers or ratio seems capable of providing valid information on sympatho-vagal balance in normal subjects, as well as in most patients with a preserved left ventricular function. Thus, analysis of HRV does provide a unique tool to specifically assess autonomic control mechanisms in association with various perturbations. In conclusion, HRV measures are of substantial utility to identify patients with an increased cardiac mortality and to evaluate autonomic control mechanisms, but their ability to capture specific levels of autonomic control may be limited to controlled laboratory studies in relatively healthy subjects.

Highlights

HEART RATE VARIABILITY Heart rate variability (HRV) was the first non-invasive methodology extensively used to evaluate autonomic modulation of the sinus node in normal subjects and in patients with different cardiac and non-cardiac diseases and to identify patients at risk for an increased cardiac mortality
This interpretation is in agreement with experimental evidence indicating a pro-arrhythmic effect of sympatho-excitation (Lown and Verrier, 1976) and with the findings that a reduction of these parameters is associated with an increased cardiac mortality in almost all clinical conditions characterized by an autonomic imbalance, e.g., after myocardial infarction, in patients with heart failure, hypertension, or diabetes (Kleiger et al, 1987; Malik and Camm, 1995; Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996; Fauchier et al, 1997; Nolan et al, 1999; Rashba et al, 2006)
Indices such as the exponent β of 1/f slope for long term analysis (Goldberger, 1996; Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996; Lombardi, 2000) or the scaling exponent α1 for short-term recordings (Peng et al, 1995; Mäkikallio et al, 2005) provide measures of fractal correlation properties of RR intervals at different time scales. These parameters do not reflect a specific component of autonomic modulation and cannot be used to evaluate the presence of increased sympathetic activation or of a reduced vagal tone. They rather reflect the characteristics of heart rate behavior and, in particular, its complexity or randomness which are dependent upon the functional integrity of autonomic control mechanisms and sinus node responsiveness (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996)

Summary

Introduction

HEART RATE VARIABILITY Heart rate variability (HRV) was the first non-invasive methodology extensively used to evaluate autonomic modulation of the sinus node in normal subjects and in patients with different cardiac and non-cardiac diseases and to identify patients at risk for an increased cardiac mortality. The finding that time domain parameters were inversely correlated with aging and influenced by the extent of left ventricular dysfunction made clear that the causes of reduction in HRV and the interpretation of their predictive value were more complex than originally assumed and involved neural and non-neural mechanisms (Malik and Camm, 1995; Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996).

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Conclusion