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Abstract
ECG is a reflection of electrical events occurring in the heart during each contraction. Its interpretation requires a good understanding of the ionic basis responsible for both the cardiac resting membrane potential and the cardiac action potential.
The cardiac action potential is the resultant of spontaneous depolarization and repolarization of the SA node, the natural pacemaker of the heart which fires faster creating electrical impulses that are transmitted through the different parts of the conduction system of the heart; the atrial muscle, AV node, bundle of His, bundle branches, Purkinjee fibers; resulting in depolarization and repolarization of the myocardium.
Regulation of the conduction through the heart is normally under the control of sympathetic and parasympathetic systems.
The standard 12-lead ECG represents the electrical potential differences; recorded on the body’s surface during depolarization and repolarization.
Different leads pick up different components of electrical currents and the sum of these currents is the resultant vector determining the cardiac axis, and producing the deflection in the electrocardiograph stylus the magnitude of which depends on the angle the vector forms with the ECG lead.
A complex ECG waveform the P-QRS-T-U waveform recorded during cardiac action potential represents the depolarization and repolarization of the different parts of the myocardium; the atria, the atrioventricular septum, and the ventricles.
The complex ECG waveform usually occurs at a regular rate, thus the heart beats regularly and the heart rate could be determined by calculating the number of waveforms in one minute.