The Electrocardiogram

The electrocardiogram (ECG) is a visual representation of the electrical impulses generated by the heart with each beat. Ever since Willem Einthoven developed the ECG in the early 1900s, it has become an invaluable tool in the diagnosis of a variety of cardiac conditions.

This page will present the fundamental electrophysiology of the cardiac cycle and the components and appearance of the normal ECG, and will hriefly outline criteria for some abnormal findings.

Basic Cardiac Electrophysiology

The mechanical process of ventricular contraction begins with an electrical impulse generated in a region of the superior right atrium known as the sinoatrial (SA) node (Figure 5-1).

The sinus impulse quickly spreads through the atria resulting in atrial contraction, and then continues through the atrioventricular (AV) node, the His bundle, and the right and left bundle branches, eventually reaching the ventricular myocardium where it results in synchronized biventricular contraction. If ventricular contraction results from impulses that originate in the SA node, then normal sinus rhythm is said to be present. Abnormalities of this process result in various arrhythmias (dysrhythmias).

The Lead System

The electrical impulses in the heart can be recorded by means of electrodes strategically placed on the surface of the body. The standard ECG has 12 leads: 6 limb leads and 6 precordial or chest leads. The limb leads (I, II, III, a VR, a VL, a VF) record cardiac electrical impulses in a vertical or frontal plane (Figure 5-2). The precordial leads (V1-6) are placed over the left chest and record electrical impulses in a horizontal plane.

The ECG is generated by simultaneously recording the electrical activity of the heart at each electrode or pair of electrodes. Any net electrical impulse directed toward the positive aspect of a lead is represented by an upward deflection of the ECG tracing in that lead.

The magnitude of the deflection reflects the strength of the electrical signal, which depends, in part, on the mass of myocardium that is being depolarized, and, in part, on the electrical impedance of interposed tissue.

The normal ECG (Figure 5-3)
Every cardiac cycle's electrical impulse is inscribed on the ECG as a waveform with the following components: P wave, QRS complex and T wave (Figure 5-4).

The P Wave

The onset of the P wave heralds the onset of atrial depolarization. The normal electrical impulse travels horn the SA node and depolarizes the atria in a superior-to- inferior direction, resulting in a P wave on the ECG that is upright (positive) in lead II and downward (negative) in lead a VR. An impulse arising from a site other than the SA node will depolarize the atria in a different direction and results in a different P wave morphology.