# Deep Learning and Big Data in Healthcare: A Double Review for Critical Beginners
## 6 Applications in ECG Processing
### ECG Fundamentals and Databases  
ECG is a set of physiological signals that represent the changes of the electrical activity of the heart over time. It contains information that is used to analyze heart functions like the heart rate and the heart rhythm, and it is frequently used to identify abnormalities in the electrical system of the heart [181]. Few electrodes are placed on certain standardized parts of the body skin in order to record ECG signals, which are basically an electric potential difference between two electrodes. The combination of two electrodes which the electric signal measures forms an imaginary line in the body called a lead (or channel) [182], as shown in Figure 2. From an electrophysiological point of view, the ECG consists of a set of characteristic deflections, so-called waves, which appear near-periodically in time, and each of them is related to one of the basic phases of the cardiac cycle. Accordingly, the P wave corresponds to the overall view of the depolarization of the atria. It is followed by the Q, R, and/or S waves (which jointly form the QRS complex), which represents the energetic depolarization of the ventricles. In addition, finally, the T wave can be observed and brings information about the ventricular repolarization phase and the myocardial tissue recovery. Very infrequently, a U wave can also be detected in the end [182]. One cardiac cycle consists of a P, Q, R, S, T and U wave, corresponding to one ECG beat. From these waves, it is relevant to be able to take morphological and temporal indices, such as the existing peaks (P, Q, R, S, T, and U) and their associated times, the intervals between consecutive peaks (PR, RR, QRS, ST, and QT intervals) and characteristic segments (PR and ST), which are often called jointly the ECG features [183]. There are a few standard configurations of electrodes for the measurement of ECG signals. One of the most used is the 12-lead configuration, in which ten electrodes are used, in such a way that six electrodes are placed on the chest, and four electrodes are placed on the limbs. This configuration allows the acquisition of an ECG signal in 12 leads, namely, three bipolar limb leads (I, II, and III), three monopolar limb leads (aVR, aVL, and aVF), and six monopolar precordial leads (V1, V2, V3, V4, V5 and V6) [53]. The three bipolar leads are the electric potential difference between the right and left arm (I), the right arm and left foot (II), and the left arm and left foot (III). In the cases of the monopolar leads, electric potential differences are generated between the left arm (aVL), the right arm (aVR), or the left foot (aVF), and a mass electrode (usually an average given by a Weathston Central Terminal). The precordial leads are obtained as the difference between each of the six chest electrodes (V1-V6) and the reference mass point. The right foot is normally used for grounding purposes [184]. As previously evoked, DL requires large datasets for its proper training. In addition to this, it is important to enable the comparisons and benchmarking among different DL algorithms, so that researchers are able to replicate algorithms and to evaluate them in a standard manner. To build these kinds of datasets, some authors have pointed out that a cultural change is needed, as it requires the collaboration between institutions and data-shared availabilities [21; 49; 53]. Nevertheless, the field of electrocardiography is one in which a number of databases already exist and can be available for their use for testing some of the hypotheses that are currently being evaluated in the field. We summarize next the most relevant ones of the currently available ECG collections. From the point of view of arrhythmias and patient condition evaluation based on ECG, several databases have been released to the community with different aims and orientations. The _AHA ECG Database_ is probably the one with the most widespread use.