Was Einthoven a 21st century visionary?

The electrocardiograph (ECG) signal is a complex time varying repetitive scalar quantity that is commonly measured, recorded, and interpreted in cardiovascular medicine. Since the original description by Wilhelm Einthoven in 1903,r this signal traditionally has been regarded as containing significant information about the heart, including details about cardiac rate, ischemia and other injury, cardiac chamber size, electrical conduction patterns, among others. These types of information all have been derived from the interpretation of short-term portions of the signal, i.e., one or two beats, or at most a short rhythm strip. However, the ECG yields a complex signal that has longterm regularity with additional information that can be derived from longer time period analyses. In the method of signal examination termed Fourier analysis, we consider the ECG signal to be composed of a summation of different repetitive signals, each at its own frequency.’ This is analogous to regarding music as a summation of different notes, each a pure tone, played simultaneously. Regarding the ECG signal in such a manner, we can see that the traditional categories of information have mostly been contained in components of the signal in the range of 1 Hertz (Hz) or faster. There is also information in the frequency range of the order of 0.5-0.05 Hz. The importance of these long-term oscillations to normal physiology was first appreciated with fetal heart rate (HR) monitoring.” However, that application was purely observational, and the individual components of the HR signal were not computed. With the advances in digital signal processing over the last two decades, accurate determination of the intervals between successive heart beats (R-R interval) has become possible, after which various algorithms can be used to assess the frequency and amplitude of the various oscillatory components that make up the signal. Two of these algorithms, the fast Fourier transformation and an autoregressive analysis, have been used by investigators to elucidate two distinct frequency peaks. the high frequency (centered around 0.25 Hz) corresponding to the parasympathetic system and responsible for the respiratory sinus arrhythmia, and the low frequency (centered around 0.1 Hz) corresponding to both parasympathetic and sympathetic influences, as determined in animal studies.’ These long-term ECG components have been studied in several human pathophysiologic states, and a change in the balance between the high and low frequencies has been observed.2.4 In the article by Latson et al.3 in this issue of the Journal oj‘Clinica1 Anesthesia, we see an analysis of these phenomena used in the comparison of several common anesthetic drugs. General anesthesia represents a unique physiologic state during which respiration, and therefore its influence on HR, is controlled. The authors are able to use the full power of the technique by describing the baseline state and observing the change caused by a defined intervention (administration of an anesthetic drug) in an individual patient. Even when anesthetics decreased the total amplitude of HR changes, the ratio