Heart rate variability - a historical perspective

Abstract

Heart rate variability (HRV), the beat-to-beat variation in either heart rate or the duration of the R-R interval - the heart period, has become a popular clinical and investigational tool. The temporal fluctuations in heart rate exhibit a marked synchrony with respiration (increasing during inspiration and decreasing during expiration - the so called respiratory sinus arrhythmia, RSA) and are widely believed to reflect changes in cardiac autonomic regulation. Although the exact contributions of the parasympathetic and the sympathetic divisions of the autonomic nervous system to this variability are controversial and remain the subject of active investigation and debate, a number of time and frequency domain techniques have been developed to provide insight into cardiac autonomic regulation in both health and disease. It is the purpose of this essay to provide an historical overview of the evolution in the concept of HRV. Briefly, pulse rate was first measured by ancient Greek physicians and scientists. However, it was not until the invention of the "Physician's Pulse Watch" (a watch with a second hand that could be stopped) in 1707 that changes in pulse rate could be accurately assessed. The Rev. Stephen Hales (1733) was the first to note that pulse varied with respiration and in 1847 Carl Ludwig was the first to record RSA. With the measurement of the ECG (1895) and advent of digital signal processing techniques in the 1960s, investigation of HRV and its relationship to health and disease has exploded. This essay will conclude with a brief description of time domain, frequency domain, and non-linear dynamic analysis techniques (and their limitations) that are commonly used to measure HRV.

Keywords: autonomic nervous system; frequency domain; heart rate variability; respiratory sinus arrhythmia; time domain.

Figure 1

Heart rate variability: representative electrocardiogram (ECG) recordings from a conscious dog that illustrate beat-to-beat variations in both R–R interval and heart rate.

Figure 2

A timeline of some of the major events in the discovery of heart rate variability (HRV). Please note that the timeline is not drawn to scale.

Figure 3

Portrait of Herophilos (ca. 335–280 BC). He was the first to measure the heart beat using a water clock to time the pulse. Source: Reproduced with permission from the John P. McGovern Historical Collections and Research Center; Houston Academy of Medicine-Texas Medical Center Library; Houston, TX, USA. P-254, color photo; Artist: Joseph F. Doeve, painted in 1953.

Figure 4

Portrait of Galen of Pergamon (131–200 AD). He wrote extensively about the pulse and used it for both the diagnosis and predicting the prognosis of disease. Source: National Library of Medicine (the history of medicine public domain image files). Lithograph by Pierre Roche Vigneron (Paris: Lith de Gregoire et Deneux, ca. 1865).

Figure 5

Portrait of Rev. Stephen Hales (1677–1761). He was the first to report periodic fluctuations in arterial pressure and the beat-to-beat interval that varied with respiration. These pioneering studies were performed on conscious horse. Source: Reproduced with permission from the John P. McGovern Historical Collections and Research Center; Houston Academy of Medicine-Texas Medical Center Library; Houston, TX, USA. P-261, color photo; Artist: Joseph F. Doeve, painted in 1953.

Figure 6

Photograph of Carl Lugwig (1816–1895). He is credited with inventing the smoked drum kymograph and used it to record periodic oscillations in the amplitude and timing of arterial pressure that varied during respiration. Using the dog, he reported that the pulse rate increased during inspiration and decreased during expiration, thereby providing the first documented recordings of the respiratory sinus arrhythmia. Source: National Library of Medicine (the history of medicine public domain image files). Picture made in 1856.

Figure 7

A plot of the Lorenz attractor. (A) represents a the time series for single variable (x) while (B) illustrates the changing relationship between all three variables. The weather model that produced this plot consists of three differential equations: (1) dx/dt = σ(y − x), (2) dy/dt = x(ρ − z) − y, (3) dz/dt = xy − βz. σ = Prandtl number, ratio of fluid viscosity to its thermal conductivity, ρ = Rayleigh number, heat transfer- the temperature difference between the top and the bottom of the gaseous system, and β = a geometric expression, the ratio of width to height of the containing holding the gaseous system. Lorenz used 10 for σ, 28 for ρ and 8/3 for β. x(t) amplitude of the convection current, y(t) temperature diffusion behavior (temperature difference between rising and falling air currents), and z(t) normal temperature deviations. Some applets that can be used to create the Lorenz attractor are found at the following websites: www.cmp.caltech.edu/~mcc/Chaos_Course/Lesson1/Demo8.html, www.geom.uiuc.edu/~worfolk/apps/Lorenz/, http://www.exploratorium.edu/complexity/java/lorenz.html.