Biomedical Engineering Reference
In-Depth Information
Fig. 3.1 Schematic depiction of the heart and the blood circulation system. Thick and thin solid
lines represent oxygenated and deoxygenated blood, respectively. RA right atrium, LA left atrium,
RV right ventricle, LV left ventricle, SA sino-atrial, AV atrio-ventricular
In resting conditions, myocardial cells are polarized, i.e., a potential difference
exists across the cellular membrane due to a difference in concentrations of positive
and negative ions. A beat is initiated by an electrical excitation generated at the sino-
atrial node, the natural pacemaker of the heart, located on the top of the right atrium.
As a result of this excitation, a depolarization wavefront propagates from the top to
the bottom of the atria, causing a synchronized atrial contraction and the effective
passage of blood towards the ventricles through the mitral and tricuspid valves.
The depolarization wavefront is delayed at the atrio-ventricular node to allow an
effective blood flow from atria to ventricles, and then rapidly propagates through
the bundle of His, the bundle branches and the Purkinje fibers towards the bottom
of the ventricular myocardium. Finally, the electrical activation propagates from the
apex to the base of the ventricles, making them contract upwards and eject the blood
through the pulmonary and aortic valves towards the lungs and the rest of the body.
The beating process just described corresponds to a healthy electrophysiological
state and is referred to as normal sinus rhythm.
The electrical activity generated in the heart is observable on the body surface.
The electrocardiogram (ECG) records the cardiac electric potentials as measured on
standard locations of the skin resulting in 12 leads: leads I, II and III are potential
differences between electrodes on the wrists and the left foot; the augmented bipolar
leads aVR, aVL and aVF use the same electrodes but take a virtual electrode (the so-
called Wilson's central terminal) as a reference; and the precordial leads V1 to V6
are derived from electrodes across the chest (Fig. 3.2 ). In physiological conditions
(sinus rhythm), the coordinated atrial depolarization and contraction is reflected as
the P wave in the surface ECG. The atrio-ventricular pause is reflected as a an
isoelectric line. Ventricular depolarization (linked to mechanical contraction) and
repolarization (linked to mechanical relaxation) are observed as the QRS complex
and the T-wave, respectively, which is followed by another isoelectric line until the
next cardiac beat. Figure 3.3 illustrates the main inflections of the ECG signal in
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