The anatomical basis

From an anatomical point of view, the heart is made up of four cavities (Figure 4.1): two atria and two ventricles, connected by the atrioventricular (AV) valves. The pulmonary artery originates from the right ventricle and the aorta from the left ventricle. The superior and inferior vena cava are connected to the right atrium and the four pulmonary veins to the left atrium. The pacemaker of the heart is located in the sinus node close to the superior vena cava. The electrical stimulus arrives at the atrial and ventricular myocardium, where the excitation–contraction coupling is performed, through internodal tracts, the AV node, and the intraventricular conduction system (ICS) (see Figure 4.7).

The following is a description of the characteristics of the heart walls and of the specific conduction system (SCS), which are important from an electrocardiographic point of view (Figures 4.1–4.9).

The heart walls: perfusion and innervation (Cerqueira 2002; Fiol‐Sala et al. 2020)

Our best information on the anatomical aspects of the heart walls including necrotic zones and myocardial viability has been obtained from cardiovascular magnetic resonance imaging (CMR) along with the study of the coronary tree performed by both invasive coronariography and the multislice scanner. In addition, CMR and isotopic studies are useful in studying the perfusion of the heart. Another useful technique is echocardiography, which is used to identify heart chamber volume, heart wall thickness, the presence of hypocinetic zones, and myocardial function (ejection fraction).

The left ventricle has four walls (Figure 4.1). They are currently identified as septal, anterior, lateral, and inferior. Historically (Perloff 1964), the term true posterior wall was given to the basal part of the inferior wall that bends upward. However, using imaging techniques such as CMR, it has been demonstrated (Bayés de Luna 2006a; Fiol‐Sala et al. 2020) that the basal part of the inferior wall bends upward in only 25–30% of cases (Figure 4.2) and that a posterior inclination of all inferior (diaphragmatic) wall is present in only 5% of cases in very lean individuals with the heart in a vertical position (Figure 4.2C). The true posterior wall is now known as the inferobasal part of the inferior wall, according to the American Societies of Imaging (Cerqueira 2002). This appears to be a suitable name given that the true posterior wall does not usually exist. From the oblique sagittal view, the anterior and inferior walls (Figure 4.3A) present one part anterior and the other posterior, and also in the horizontal axial plane, one part of the septal and lateral wall is located more anteriorly with respect to the rest of the wall (Figure 4.3B). This is completely contrary to the idea that a true and independent posterior wall exists. As previously stated, it occurs in very few cases (5%) (Figure 4.2C) and when it does occur, the orientation of the necrosis vector also is toward V3–V4, not V1–V2 (Figure 4.3A).

Despite clear findings that the true posterior wall usually does not exist (Cerqueira 2002; Bayés de Luna 2006a), correct heart wall terminology is not always used and has even been questioned (Garcia‐Cosio 2008; Bayés de Luna 2008a; Gorgels and van der Web 2010; Kalinauskiene 2010; Bayés de Luna 2011). It usually takes some time to change a dogma. In their book History of Electrocardiography (1964), Burch and DePasquale state that:

because of efforts by some to advance their own ideas rather than the science itself, progress in electrocardiography has, at times, been hindered. Even obviously simple problems, such as nomenclature, were made difficult because of individual prejudices. Fortunately, truth finally prevails and the best ideas supervene. However, much time and effort is required before the truth is recognized, as is well exemplified by the history of the development of the clinical use of the precordial leads.

This is also what happens with the concept of posterior wall/versus lateral wall. Finally, was accepted in 2015 (Bayés de Luna et al. 2015) the end of the dogma that posterior wall exist, and that the R wave in V1 was due to lateral not posterior MI. It was the result of several studies performed by our group and others that demonstrated (Bayés de Luna et al. 2006a, 2006b; Cino et al. 2006; Rovai et al. 2007; Bayés de Luna 2008a, 2008b; Bayés de Luna et al. 2008; Van der Weg et al. 2009; Fiol‐Sala et al. 2020) that the correlation of ECG with the “in vivo” anatomy provided by CE‐CMR supports this new terminology and the correlation of Q wave myocardial infarctions with these four walls: septal, anterior, lateral, and inferior (see Chapters 13 and 20).

The four walls may be projected onto the heart planes: the short axis (transverse), the vertical long axis (sagittal‐like), and the horizontal long axis, both in CMR imaging (Figure 4.3) and isotopic studies (Colour Plate 1), in addition to echocardiography. These four walls are divided into 17 segments (Figure 4.4), represented in Figure 4.5 in the form of a target. Figure 4.6 shows the perfusion that the different segments receive from the coronary arteries (B–D). It is important to point out that there are some variants in coronary flow distribution as a result of anatomical variants in the coronary arteries. In 80% of cases, the left anterior descending (LAD) artery is long and wraps around the apex, and in around 80% of cases, the right coronary artery (RCA) dominates the left circumflex artery (LCX). Colour Plate 2 shows the normal coronary arteries detected with a multislice scanner.

The left ventricle may be divided into two zones (Figure 4.6): the inferolateral zone encompassing the inferior wall, some of the lower part of the septal wall, and nearly all of the lateral wall, which is perfused by the RCA artery and LCX, and the anteroseptal zone encompassing the anterior wall, an important section of the anterior part of the septal wall and a small part of the mid‐low lateral wall, which is perfused by the LAD artery. The lateral wall is therefore perfused mainly by the LCX and partially by the LAD and RCA. Although the anteroseptal zone is perfused by the LAD, this artery often also supplies blood to the low apical part of the inferior wall (long LAD wrapping around the apex). The RCA perfuses the inferior wall, predominantly the mid‐inferior part of the wall, the lower part of the septum and, in cases of evident RCA dominance, the entire inferior wall and part of the lateral wall. It also shares the perfusion of the right ventricle with the LAD. The LCX supplies blood to the inferolateral zone, especially the inferobasal part of the inferior wall and the lateral wall by its branch, the oblique marginal (OM) artery. The areas perfused by coronary arteries with the areas of shared perfusion are shown in Figure 4.6. The atria are perfused by a branch that usually stems from the LAD proximal part and the AV node receives perfusion mainly from the RCA (90%).