Page 108 - Computational Modeling in Biomedical Engineering and Medical Physics

Electrical activity of the heart 97

Myocardium cells (30 100 μm in length, 8 20 μm in diameter) differ significantly
from nerve cells or striated muscle tissue cells. They are tightly packed and their cell-
to-cell connection is intimate: the ionic channels continue from the membrane of a
cell to the membrane of the neighboring cell, and the ions flow directly in between
without reaching the extracellular space. Depolarization diffuses from cell to cell
through the myocardium, producing peaks of transmembrane voltage, called the fast
action potential (fast AP) (Fig. 4.2, right). Several types of sources that model the AP
are discussed later (Morega, 1999).
1
The AP presents five stages: Phase 0: Na channels are activated, the membrane depo-
1
larizes fast, the AP increases abruptly. Phase 1: a flux of K ions exit the cell, accompanied
by a temporarily decrease of AP and the Ca 21 channels begin to open. Phase 2: the bal-
21 1
ance between the inward Ca and outward K fluxes corresponds to the AP plateau.
Phase 3: at the end of Phase 2, the interior of the cell becomes highly electropositive, and
1
the potential approaches the electrochemical equilibrium for Na (1 61 mV). Phase 4: K
ions flux exceeds the Ca 21 ions flux, accompanied by repolarization. Phase 5: return to
1
initial state (resting potential), due mainly to the K ions flux (Morega, 1999).
The electrical activity of the heart is monitored and measured using the electrocardio-
gram (ECG) recording (Ettinger et al., 1974; Luo and Johnston, 2010), which provides
information related to the AP dynamics duringthe cardiaccycles. Thebodytissues have
good electrical conductivity (the body is commonly considered a volume conductor under
low frequency electric field stress; Woosley et al., 1985; Plonsey, 1963)and theelectricalsig-
nal variations recorded on its surface are given by the projection of the APs of the myocar-
dial fibers. ECG signals observe the course of the depolarization wave and they give good
information to identify cardiac arrhythmias (Einthoven, 1908). In Einthoven’s electrocar-
diographic model, the cardiac source is a current dipole with its origin inside the heart
(Section 4.1), and variable orientation and magnitude (Fig. 4.3). Usually, the electrodes
describing the “Einthoven triangle” are positioned on the right shoulder (UD), left (US),
and abdomen (A) (Malmivuo and Plonsey, 1995; Berne et al., 1998). However, taking
advantage of the good electric conductivity of the body (the volume conductor concept), it
is customary to adopt one more convenient electrodes positioning system: on the wrists of
the right and left hands, and on the left leg, respectively.
The morphology of the ECG signal is related to depolarization wave propagation.
P wave is related to the RA depolarization. The pulse lasts 70 80 ms, and then the
ECG signal returns to the reference level. QRS complex is related to the ventricular
depolarization. It starts B220 ms from the beginning of the cardiac cycle and it lasts
B80 ms, when the ECG signal returns to the reference level for B120 ms. T wave,
with a duration of B200 ms, corresponds to the ventricular repolarization. U wave,
not always present, corresponds to the repolarization of a certain type of ventricular
cells with slow electrical activity, but their connection with the electrophysiological
phenomena of the heart is not yet precisely established.