The mechanical contraction of individual heart cells is initiated by action potentials (APs), electrical impulses that travel across a physiologically determined pathway across the cardiac muscle. In a healthy heart, the trigger for each cardiac cycle is in the upper right area of the right atrium, the sinoatrial (SA) node (Fig. 22.2). This area contains cells that spontaneously can generate APs. The frequency at which action potentials are generated by this natural pacemaker has a natural baseline value, but this frequency usually is controlled by the autonomic nervous system. From the SA node, APs travel across a pre-programmed pathway to adjacent cardiac cells; first, the atrial myocardial cells are activated in such a way that they contract from the top to the atrioventricular areas. Contrary to the mechanical barrier that prevents blood to flow from right to left, APs travel from right to left atria and ventricles. However, there is an electrical barrier between both atria and the ventricles: APs cannot cross the atrioventricular junction, except in one area called the atrioventricular (AV) node . This structure is located in the muscle wall between the two atria and consists of specialized tissue that introduces a delay in the conduction of APs. Once passed through the atrioventricular barrier, APs are conducted via the bundle of His and the bundle branches to the bottom of the right and left ventricles, respectively. Through small branches called Purkinje fibers, the ventricular myocardium is innervated, basically from bottom to the atrioventricular areas. After the electrophysiological innervation of cardiac muscle cells, there is always a resting or refractory period which needs to be finished in other to initiate a new AP.

Action potentials not only act as a trigger for the mechanical contraction of cardiac muscle cells but also cause electrical currents that spread across all conductive tissues surrounding the heart. Most pericardial tissue can be considered as a bioelectrical universe that instantaneously conducts the local currents generated in and around individual cardiac cells toward the natural electrical conductive barrier that surround every subject: skin that is surrounded by air. Because air cannot conduct electrical current, the currents that are generated in the myocardium also cause currents that flow across the skin. Because skin has a finite electrical resistance, Ohms law applies, and voltage differences are generated across different locations at the skin. Because of the highly collective and synchronized manner, many cardiac cells are activated and deactivated during each heart cycle; all the small contributions of individual cells add up and generate a measurable signal at the skin surface, in particular at skin positions close to the heart. This signal, the electrocardiogram (ECG), varies over time because the activation of the various heart regions varies over time and consequently the distance between active and “silent” cardiac regions is time-varying. The basic rule of thumb is that the closer an active region is to any skin position, the larger its contribution in the net electrical voltage that can be measured at such a location. This typically is done using adhesive sensors called electrodes.

Figure 22.3 shows an example of a 10-s episode of an ECG recording obtained at nine different skin positions . A detailed description is beyond the scope of this text, but one can easily see that the morphology of the various recordings differs significantly. Nevertheless there are a number of common characteristics: each recording shows a characteristic prominent sequence of sharp peaks and valleys, some recordings having the opposite polarity when compared to others. This is the QRS complex, which is associated with the electrical activation of the ventricles. Although the amplitude (peak-to-peak value) may differ remarkably, its time of occurrence for each recording is exactly the same. This is a result of the laws of physics.

A second characteristic that regularly appears in ECG recordings and is of relevance for HRV analysis is illustrated by the large, slow fluctuations that occur in all recordings around 5 s from the beginning of this trace and in the lower three recordings at the end of this epoch depicted in Fig. 22.3. This is an example of the many types of artifacts that may be encountered during ECG recordings: such large fluctuations are usually a consequence of movement, poor electrode contact, or even loose electrodes. Other manifestations of artifacts may be presence of irregular waveforms (usually referred to as noise), flat recordings, or extremely high amplitudes.

Because different skin areas have different relative positions to the bioelectric heart activity , the morphology of an ECG strongly depends on the position of an electrode. This becomes apparent from illustrated in Fig. 22.3. Two standard electrode configurations, or montages, are commonly used in clinical practice. The oldest electrode configuration was defined by the first pioneer in ECG recordings, Willem Einthoven, who in the early 1900s used four basins filled with saline in which the two lower arms and legs where immersed. The basins acted as electrodes and were connected to a galvanometer that recorded the voltage differences between the four “leads.” Today, adhesive electrodes are used that directly are wired to modern polygraphs, but the leads still are referred to as RA, LA, RL, and LL for right and left arm/leg, respectively (sometimes the term “foot” is used instead of “leg”). Any electrophysiological recording requires a reference voltage; often the right leg electrode is used for this purpose, but many alternatives can be found in literature. For standard extensive cardiological assessments , the wrists and ankles still are used, but for clinical studies the positions of the four Einthoven leads often are chosen at more convenient positions such as just below the left/right claviculae and at left and right upper legs or hips. In addition to the voltage difference between the “active” leads and the reference (RL) electrode, one also can determine the difference between two active leads, e.g., LA-RA, LL-RA, and LL-LA. In compliance with Einthoven’s naming conventions, these three signals are coded with roman numerals: I, II, and III, respectively.

The second type of electrode montage uses six electrodes which are placed at the so-called precordial positions, skin locations close to the heart at well-defined positions at the chest. The positioning is not only chosen such that they can be used for any gender but also allow observation of the electrical heart activity from different directions. The naming convention for these electrode positions is V1…V6. Figure 22.3 also shows example of the signals acquired from these electrodes.

In most clinical and health HRV assessments , a subset of the ten standard electrode positions is used; the precordial positions usually are preferred because these ECG recordings have the most prominent QRS complex and consequently allow the most robust R-peak detection. When a minimal number of electrodes is required, often lead LA-RA is used (positions at left and right front shoulders, just below the claviculae) or V1–V6, the latter having the largest amplitudes [22] but are not always practical considering client-friendliness.

In cardiology, ECG recordings are indispensable for the initial diagnostic of a host of cardiac diseases. HRV, however, can only be done reliably and in a sensible manner when no clear deficits of heart rate control by the autonomic nervous system and AP conduction problems across the myocardium exist. Under these conditions, each QRS complex can always be associated with one single “trigger” in the SA node. Therefore, the first step in ECG-based HRV analysis is the detection of each individual and consecutive QRS complex and careful determination of its time of occurrence and detection of the next QRS complex.

From a signal analysis perspective, QRS detection is a pattern recognition task. Trained human experts are known to be extremely skilled in such a task, but automation using computer algorithms in practice turns out to be a very difficult task, in particular when dealing with long-duration and/or ambulatory conditions. A host of proposals have been made, but to our experience no algorithm has shown to be as good or even superior to human pattern recognition. A useful and recommended strategy is to preprocess the data by a modern QRS detection algorithm, such as the one based on filter banks proposed by Afonso et al. [1], and subsequently inspect, and if necessary correct, the algorithmically determined fiducial points.

Given accurate determination of the fiducial QRS complex points, the R-R (inter-beat) intervals can be calculated. These intervals are also referred to as normal-to-normal (NN) intervals or heart periods . Actually, a more correct term for what is usually referred to as heart rate variability would be heart period variability, because usually most calculations are performed on heart periods not rates.¹ The heart periods can be plotted against time in the cardiotachogram (Fig. 22.4b). The calculations on heart periods can be classified into four categories: time domain measures, frequency domain measures, time-frequency domain methods, and nonlinear measures. Software for many of these measures is freely available in several packages such as Kubios (http://​kubios.​uku.​fi/​), Physionet (http://​www.​physionet.​org/​), and RHRV (http://​cran.​r-project.​org/​web/​packages/​RHRV/​).