Electrocardiography and the Electrocardiogram (ECG)

Contraction and pumping of blood by the heart is triggered by an electrical signal that originates in the upper right atrium of the heart within the sinus (or SA) node and spreads to the lower chambers (ventricles) via the AV node and His bundle. The electrical wave of depolarization that spreads throughout the heart during cardiac activation can be detected from surface of the body using a specialized recording device called an electrocardiograph. The records produced by an electrocardiograph are termed electrocardiograms (ECG) and they are produced by placing ECG electrodes on the body surface (over the chest and extremities). The standard ECG may include recordings from 12 different locations around the surface of the body, the so-called 12-lead ECG. The basic signal recorded during a single heart beat is shown in the figure below.

The P-wave corresponds to activation of the right and left atria. The QRS complex represents activation of the right and left ventricles. The T-wave represents recovery of the ventricles following their activation. The PR interval represents the time required for the electrical impulse to conduct from its origin in the right atria to the ventricles. The duration of the QRS complex represents the time required to activate entirely both the right and left ventricles. The QT interval is the time required for complete activation and recovery of both ventricles and approximately represents the time period during which the ventricles are refractory and cannot be re-excited by another electrical impulse. Because the heart is continuously beating, an ECG recording typically displays a consecutive string of these P-QRS-T waves as seen in figure below. An arrhythmia occurs when this normal activation of the heart (P-QRS-T sequence) is disrupted.

Figure 1. Three lead ECG rhythm strip. Recordings made using ECG leads I, aVF, and V1.

12-Lead ECG Recording

The standard 12-lead ECG printout displays recordings of the electrical activity of the heart as seen from 12 standard locations around the surface of the body. These standard recording sites are called "leads", hence the term 12-lead ECG. The advantage of using a standard "lead system" is that for each patient comparisons can be made over time. Each time the ECG technician attaches the recording electrodes, they are attached in the same locations. For example, if a patient suffers a heart attack or develops an arrhythmia, a current ECG can be compared to one recorded in the past, even years ago. Changes in a patient's cardiac electrical system will be reflected in changes in the ECG appearance over time using these standard leads. The standard ECG leads are denoted as I, II, III, aVR, aVL, aVF, V1, V2, V3, V4, V5, V6. Leads I, II, III, aVR, aVL, aVF (the "limb leads") record the heart's electrical activity in what is termed the frontal plane. The "V-leads" (or "precordial leads") record the electrical activity in the so-called "transverse plane". The schematic illustration below shows the normal appearance of the electrical waves recorded by each of the leads on a standard 12-lead ECG printout.