The Electrical Activity of the Heart

Chapter 1
The Electrical Activity of the Heart

Basic concepts

The heart is a pump that sends blood to every organ in the human body. This is carried out through an electrical stimulus that originates in the sinus node and is transmitted through the specific conduction system (SCS) to contractile cells.

During the rest period, myocardial cells present an equilibrium between the positive electrical charges outside and the negative charges inside. When they receive the stimulus propagated from the sinus node, the activation process of these cells starts. The activation of myocardial cells is an electro‐ionic mechanism (as explained in detail in Chapter 5) that involves two successive processes: depolarization, or loss of external positive charges that are substituted by negative ones, and repolarization, which represents the recovery of external positive charges.

The process of depolarization in a contractile myocardial cell starts with the formation of a depolarization dipole comprising a pair of charges (−+) that advance through the surface cell like a wave in the sea, leaving behind a wave of negativity (Figure 1.1A). When the entire cell is depolarized (externally negative), a new dipole starting in the same place is formed. This is known as a dipole of repolarization (+−). The process of repolarization, whereby the entire cell surface is supplied with positive charges, is then initiated (Figure 1.1B).

The expression of the dipoles is a vector that has its head in the positive charge and the tail in the negative one. An electrode facing the head of the vector records positivity (+), whereas when it faces the tail, it records negativity (−) (Figures 1.11.3; see also Figures 5.24, 5.25, and 5.28). The deflection originating with the depolarization process is quicker because the process of depolarization is an active one (abrupt entry of Na ions, and later Ca) and the process of repolarization is much slower (exit of K) (see Chapter 5, Section “Transmembrane action potential”).

If what happens in one contractile cell is extrapolated to the left ventricle as the expression of all myocardium, we will see that the repolarization process in this case starts in the opposite place to that of depolarization. This explains why the repolarization of a single contractile cell is represented by a negative wave, whereas the repolarization of the left ventricle expressing the human electrocardiogram (ECG) is represented by a positive wave because the repolarization process of all left ventricles starts in the zone less ischemic, the subepicardium compared with the subendocardium that is the more ischemic zone (Figure 5.28) (see Chapter 5, Section “From cellular electrogram to human ECG”).

How can we record the electrical activity of the heart?

There are various methods used to record the electrical activity of the heart. The best known method, the one we examine in this book, is electrocardiography. An alternative method, rarely used in clinical practice today but very useful in understanding ECG curves and therefore helpful in learning about ECGs, is vectorcardiography.

The latter and other methods will be briefly discussed in Chapter 25. These include, among others, body mapping, late potentials, and esophageal and intracavitary electrocardiography. In addition, normal ECGs can be recorded during exercise and in long recordings (ECG monitoring, Holter technology, telemetry, etc.) (Braunwald et al. 2012; Camm et al. 2006; Fuster et al 2010). For more information about different techniques, consult our books Clinical Arrhythmology (Bayés de Luna and Baranchuk 2017) and Electrocardiography in Ischemic Heart Disease (Fiol‐Sala et al. 2020), and other ECG reference books (Macfarlane and Lawrie 1989; Wagner 2001; Gertsch 2004; Surawicz et al. 2008) (see “Recommended Reading”).

Schematic illustration of depolarization (A) and repolarization (B) of the dipole in an isolated myocardium cell.

Figure 1.1 Depolarization (A) and repolarization (B) of the dipole in an isolated myocardium cell. We see the onset and end of the depolarization and repolarization processes and how this accounts for the positivity and negativity of corresponding waves (see text and Chapter 5).

Schematic illustration of the origin of P, QRS, and T deflections (A, B and C).

Figure 1.2 The origin of P, QRS, and T deflections (A, B and C). When van electrode located in some zone of LV faces the head (+) of a vector of depolarization (P, QRS) or repolarization (T), it records positivity. When an electrode faces the tail of a vector (−), it records negativity. Atrial repolarization is hidden in the QRS (shadow area) (see text and Chapter 5).

What is the surface ECG?

The ECG is the standard technique used for recording the electrical activity of the heart. We can record the process of depolarization and repolarization through recording electrodes (leads) located in various places.

The depolarization process of the heart, atria, and ventricles (see Chapter 5 and Figures 5.16 and 5.18) starts with the formation of a dipole of depolarization (–+), which has a vectorial expression (An illustration of a right arrow.) that advances through the surface of the myocardium and seeds the entire surface of the myocardial cells with negative charges. A recording electrode facing the head of the vector records positivity (Figure 1.2). Later, the repolarization process starts with the formation of a repolarization dipole (+–), which also has a vectorial expression. During this process, the positive charges of the outside surface of the cells are restored.

These two processes relate to specific characteristics of the atria and ventricles (Figure 1.2). The process of atrial depolarization, when recorded on the surface of the body in an area close to the left ventricle (Figure 1.2), presents as a small positive wave called the P wave (An illustration of a wave.). This is the expression of the atrial depolarization dipole (vector). The process of ventricular depolarization, which occurs later when the stimulus arrives at the ventricles, usually presents as three deflections (An illustration of a wave with high peak.), known as the QRS complex, caused by the formation of three consecutive dipoles (vectors). The first vector appears as a small and negative deflection because it represents the depolarization of a small area in the septum the first part of LV to be depolarized, and is usually directed upward and to the right and therefore, recorded from the left ventricle as a small negative deflection (“q wave”). Next, a second important and positive vector is formed, representing the R wave. This is the expression of depolarization in most of the left ventricular mass. The head of this vector faces the recording electrode. Finally, there is a third small vector of ventricular depolarization that depolarizes the upper part of the septum and right ventricle. It is directed upward and to the right and is recorded by the recording electrode in the left ventricle zone as a small negative wave (“s wave”) (Figure 1.2).

After depolarization of the atria and ventricles, the process of repolarization starts. The repolarization of the atria is usually a smooth curve that remains hidden within the QRS complex. The ventricular repolarization curve appears after the QRS as an isoelectric ST segment and a T wave. This T wave is recorded as a positive wave from the left ventricle electrode because the process of ventricular repolarization, as already mentioned and later explained in detail (see Chapter 5, Section “From cellular electrogram to the human ECG” and Figures 5.24 and 5.25), appears very differently from what happens in an isolated contractile cell (see Figure 5.9). Repolarization starts on the opposite side to that of depolarization, because it starts in the zone less ischemic of LV that is the subepicardium. Thus, the recording electrode faces the positive part of the dipole, or head of the vector, and will record a positive deflection, even though the dipole moves away from it (Figure 1.2C; see also Figures 5.24 and 5.25). Therefore, repolarization of the left ventricle in a human ECG (the T wave) is recorded as a positive wave, just as occurs with the depolarization complex (QRS) in leads placed close to the left ventricle surface (An illustration of a wave with high peak.).

The successive recording of the ECG is linear and the distance from one P–QRS–T to another can be measured in time. The frequency of this sequence is related to heart rate.

The heart is a three‐dimensional organ. In order to see its electrical activity on a two‐dimensional piece of paper or screen, it must be projected from at least two planes, the frontal plane (FP) and the horizontal plane (HP) (Figure 1.3).

Schematic illustration of the four locations of a vector and their projection in frontal (FP) and horizontal planes (HP).

Figure 1.3 Four locations of a vector and their projection in frontal (FP) and horizontal planes (HP). A and B have the same projection in FP but not in HP. C and D have the same projection in HP but not in FP. Different positive and negative morphologies appear according to these projections. The locations of the orthogonal leads X, Y, and Z perpendicular to each other are similar to I, aVF, and V2 leads. Vertical lines correspond to the positive hemifields of aVF and V2, and horizontal lines correspond to the positive hemifields of leads I and V6. FP lead I (X) = 0°; VF (Y) = +90°; HP V2 (Z) = +90°; V6 = 0°.

The shape of the ECG varies according to the location (lead) from which the electrical activity is recorded. In general, the electrical activity of the heart is recorded using 12 different leads: six on the FP (I, II, III, aVR, aVL, aVF), located from +120° (III) to −30° (aVL). The aVR is usually recorded in the positive part of the lead that is located in −150° (see Figures 6.10 and 6.11), and six on the HP (V1–V6) located from +120° to 0° (see Chapter 6, Section “Leads” and Figures 6.10 and 6.13).

Each lead has a line that begins where the lead is placed, 0° for lead I or +90° for lead aVF in the FP and 0° for lead V6 and +90° for lead V2 in the HP, for example (see Figure 6.10), and ends at the opposite side of the body, passing through the center of the heart. By tracing each perpendicular line that passes through the center of the heart, we may divide the electrical field of the body into two hemifields for each lead, one positive and one negative (Figure 1.3). A vector that falls into the positive hemifield records positivity, while one that falls into the negative hemifield records negativity. When a vector falls on the line of separation between hemifields, an isodiphasic curve is recorded that is +− or −+ according to the sense of rotation of the curve (loop) (see later) (see Figures 6.14 and 6.16).

The different vectors are recorded as positive or negative depending on whether they are projected onto positive or negative hemifields of different leads (Figures 1.3 and 1.5). This is a key concept for understanding the morphology of ECG curves in different leads and is explained in Chapter 6 in more detail (Figure 6.14).

Oct 9, 2021 | Posted by in CARDIOLOGY | Comments Off on The Electrical Activity of the Heart
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