CHAPTER 1
Anatomical and Electrophysiological Bases

1.1.  The Heart Walls

The heart has four cavities, two atria and two ventricles, comprised mainly of contractile cells called cardiomyocytes. The electrical stimulus originating in the sinus node (SN) is distributed through the entire heart by means of a specific conduction system (SCS).

The left ventricle (LV) has four walls: anterior, septal, inferior, and lateral. Figure 1.1 shows the three segments of the anterior and inferior walls, the five segments of the septal and lateral walls, and the apex segment (segment 17). Magnetic resonance imaging has now shown that the previously-named posterior wall corresponds to the inferobasal segment of the inferior wall (segment 4 in Fig. 1.1) (Bayés de Luna et al., 2006a; Bayés de Luna A and Fiol-Sala, 2008). [A]

Figure 1.1  (A) Segments into which the left ventricle is divided according to the transverse (short-axis) sections performed at the basal (B), medial (M), and apical (A) levels. The basal and medial sections delineate into six segments each, while the apical section shows four segments. Together with the apex, they constitute the 17 segments into which the left ventricle can be divided, according to the classification performed by the American Imaging Societies (Cerqueira et al., 2002). Also shown is the view of the 17 segments with the heart open in a horizontal long-axis plane (B) and vertical long-axis (sagittal-like) plane (C). In D, the 17 segments and the four walls of the heart are shown in a ‘bull's-eye view’. RV = right ventricle.

1.2.  Coronary Circulation (Fig. 1.2)

Based on coronary perfusion, the heart is divided into two zones: the anteroseptal zone, perfused by the left anterior descending artery (LAD) (Fig. 1.2A) and the inferolateral zone, perfused by the right coronary artery (RCA) and circumflex artery (CX) (Figs 1.2C and 1.2D). The heart has areas of shared perfusion (shown in grey in Fig. 1.2A) in which one of the two arteries dominates. For example, segment 17 (apex) is perfused by the LAD, if long; otherwise by the RCA and even partially by the CX. [B]

Figure 1.2  According to the anatomical variants of coronary circulation, the areas of shared variable perfusion are shown in grey (A). The perfusion of these segments by the corresponding coronary arteries (B–D) can be seen in the ‘bull's-eye’ images. For example, the apex (segment 17) is usually perfused by the LAD but sometimes by the RCA, or even the LCX. Segments 3 and 9 are shared by LAD and RCA, and also the small part of the mid-low lateral wall is shared by LAD and LCX. Segments 4, 10 and 15 correspond to the RCA or the LCX, depending on which of them is dominant (the RCA in >80% of the cases). Segment 15 often receives blood from LAD.

1.3.  The Specific Conduction System (Fig. 1.3)

Electrical stimuli pass through the internodal pathways (Bachmann, Wenckebach and Thorel bundles), from the sinus node to the AV node and the His bundle. From there stimuli reach the ventricles through the ventricular conduction system: the right branch (RB) and the trunk of the left branch (LB), and its divisions (superoanterior and inferoposterior fascicles and the middle fibers that exist between them) (Figs 1.3A and 1.3B). [C]

Figure 1.3  (A) Right lateral view of the specific conduction system. 1, 2 and 3: internodal tracts; 4: AV node; 5: bundle of His; 6: left branch; 7: right branch with its ramifications; Ao: aorta; AVN: AV node; CS: coronary sinus; FO: fossa ovalis; IVC: inferior cava vein; SN: sinus node; SVC: superior cava vein. (B) Left lateral view of LV: see the superoanterior (SA) division (1), the inferoposterior (IP) division (2) and the middle fibers (3) (quadrifascicular theory) or rather a quadruple input of ventricular activation theory. (C) Structure of the AV junction extending further than the AV node (the compact node). The zone shaded in gray is included in the AV junction, and may be involved in the reentry circuits exclusive to the AV junction: CFB: central fibrous body; N: compact AV node; PHB: bundle of His—penetrating portion; RHB: bundle of His—ramifying portion; LB: left branch; RB: right branch. Slow conduction (α) and rapid conduction (β) pathways; 1–4: entry of fibers of internodal pathways into the AV node: NH: nodal-His transition zone; CS: coronary sinus. (D) The open left ventricle shows the three points of LV activation according to Durrer (see text).

Figure 1.3C shows in grey the structures that the AV junction encompass. Figure 1.3D shows the three activation entry points in the left ventricle (LV) (Durrer et al., 1970).

1.4.  The Ultrastructure of Cardiac Cells

• There are two types of cell in the heart:
  1. Contractile cells or cardiomyocytes, which are responsible for cardiac pump function (contractile). Under normal conditions these cells do not have automatic capacity and cannot generate stimuli. [D]
  2. Specific cells (cells of specific system of conduction), which are in charge of impulse formation (automaticity) and impulse transmission to the contractile myocardium.
• Contractile cells (Fig. 1.4) are comprised of the following:
  1. The contractile system made up of myofibers in which the contractile unit is the sarcomere (Figs 1.4A, 1.4B2, and 1.4B3), a structure that can contract and relax. The energy for this activity is provided by mitochondria.
  2. The cellular activation–relaxation system consisting of the cellular membrane formed by a lipid bilayer (sarcolemma) (Figs 1.4B1 and 1.4B2). Ions (Na+, K+, and especially Ca++), responsible for activation, depolarization and repolarization, systole, and cellular rest (diastole) phases, flow through channels that exist in this membrane.
  3. The transverse tubular (T) system (Fig. 1.4B2), which allows electrical excitation to enter the cell, and the sarcoplasmatic reticulum (Fig. 1.4 B2) which contains the calcium necessary for cellular contraction.
  4. The specific cells, non-contractile, are of three types: (a) P cells, the ones with more automaticity, located especially in sinus node 4; (b) Purkinje cells, with less automaticity, located especially in the His bundle, bundle branches and Purkinje network; and (c) transitional cells.

Figure 1.4  (A) Microphotography of a sarcomere where actin and myosin filaments are observed (see B-3). (B-1) Structure of the cellular membrane (or sarcolemma) showing an ionic channel. (B-2) Section of a myocardial contractile cell including all different elements. (B-3) Enlarged sarcomere scheme.

1.5.  The Electrophysiology of Cardiac Cells

1.5.1.  Transmembrane Diastolic Potential (TDP) and Transmembrane Action Potential (TAP) in Automatic and Contractile Cells

All contractile cardiac cells at rest show equilib­­rium between the external electrical charges and the internal negative charges (Fig. 1.5A). When a micro-electrode is located inside a contractile cell at rest while a second micro-electrode remains in the exterior (Fig. 1.5B), a difference in transmembrane potential, called the transmembrane diastolic potential (TDP), is produced. Under normal conditions the voltage is –90 mV (Fig. 1.5B). [E]

Figure 1.5  (A) The predominant negative charges inside the cell are due to the presence of significant non-diffusible anions which outweigh the ions with a positive charge, especially K+. (B) Two microelectrodes placed at the surface of a myocardial fiber record a horizontal reference line during the resting phase (zero line), signifying no potential differences on the cellular surface. When one of the two electrodes is introduced inside the cell, the reference line shifts downwards (−90 mV). This line (the DP) is stable in contractile cells and has a more or less ascending slope in the specific conduction system cells (Figs 1.6 and 1.7).

As contractile cells are not automatic; TDP is rectilinear (Fig. 1.6). This means that during the diastolic phase, an equilibrium between the potassium outward ionic current, and the sodium and calcium inward ionic current takes place.

Figure 1.6  Transmembrane diastolic or resting potential (TDP) and transmembrane action potential (TAP) of contractile cells.

When the contractile cell receives a stimulus transmitted from a neighboring cell, sodium current enters the cell rapidly. This creates a stimulus upon reaching the threshold potential, which forms the transmembrane action potential (TAP) (Fig. 1.6).

Thus, in the contractile cells the formation of TAP (Fig. 1.6), which is the basis of cellular activation (depolarization and repolarization), takes place because a stimulus (a) transmitted from a neighboring cell, originating from a rapid entry of sodium, reaches the threshold potential (TP) and results in a TAP (b and c are stimuli under threshold potential) (Fig. 1.8B). The TAP has four phases: phase 0 that is the depolarization, the loss of external electrical charges, and phases 1 to 3 involve repolarization, the recovery of these charges. [F]

Cells of the...