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 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.
The normal P wave is ≤mV in amplitude in lead V1, ≤ 2.5mV in amplitude in lead II, and ≤0.12 seconds in duration in lead II. Increases in these parameters occur with atrial enlargement.
The PR Interval
The PR interval is measured hom the onset of the P wave to the onset of the QRS complex and is a measure of the time it rakes for the sinus impulse to traverse the atria, AV node, and His-Purkinje system before depolarizing the ventricles. The normal PR interval can vary between 0.12-0.20 seconds; this variability is mainly the result of autonomic tone. A prolonged PR segment reflects slowing of the impulse through the conduction system. An abnormally short PR segmem may represent accelerated conduction through the AV node or a “short circuit” between the atria and the ventricles (a bypass tract).
The QRS Complex
The QRS complex is measured from the onset of the Q or R wave to the end of the S wave. The initial 0.04 seconds of this complex represents depolarization of the septum, which occurs horn left to right. This results in a small initial negative deflection (“septal q wave”) in leads I and V6 because the impulse is directed away from the positive poles of these leads. The remainder of the QRS complex reflects depolarization of the right and left ventricles. Because of its greater mass, the electrical forces from the left ventricle predominate, resulting in a QRS complex that is mainly positive in leads I and V6 and negative in aVR and V1. Thus, the appearance of the QRS complex in normal sinus rhythm is characterized by:
- initial “septal Q waves” in leads I and V6
- predominantly positive QRS complex in leads I and V6
- predominantly negative QRS complex in leads aVR and V1
- incremental increase in the amplitude of R waves from V2 to V5
The normal QRS duration (the total time for ventricular depolarization) is less than 100 milliseconds. Longer QRS durations suggest conduction block or delay.
The ST Segment
The ST segment corresponds to the time during which the ventricles have completely depolarized but not yet begun to repolarize. During this time there is no net electrical activity in the heart and the ECG records a flat segment at the electrical baseline. This segment becomes abnormal during myocardial ischemia (ST segment depression) and infarction (ST segment elevation).
The T Wave
The onset of the T wave denotes the onset of ventricular repolarization. The T wave in most circumstances follows the same direction (polarity) as the predominant portion of the QRS complex, and, thus, is normally upright in leads I and V6 and inverted in lead aVR. However, the normal T wave can be either upright or inverted in leads V1, a VL, and III. Changes in T wave morphology may reflect myocardial ischemia or infarction, and can also occur as a result of metabolic abnormalities.
The QT interval is measured from the onset of the QRS complex to the end of the T wave. The normal duration of the QT interval depends on many factors, including a person’s age, gender, and heart rate.
Nonetheless, a QT interval greater than 0.46 seconds or greater than 50% of the associated R wave to R wave interval is abnormally long. A prolonged QT interval may reflect a primary congenital abnormality of myocardial repolarization or be secondary to medications or metabolic abnormalities, and predisposes to malignant ventricular arrhythmias.
QRS Axis (Figure 5-2)
The electrical axis is the overall direction of the electrical depolarization of the heart. If this direction is horizontal and to the right, the axis is assigned a value of zero. Axes directed more clockwise are assigned positive values, whereas axes directed more counterclockwise are assigned negative values. The axis can be estimated by identifying the limb lead in which the QRS complex is most isoelectric (positive and negative deflections are of equal size); the axis is perpendicular to this lead. Because the ventricles depolarize predominantly from superior to inferior and from right to left, the QRS axis normally falls between -30° and +90°.
Abnormal ECG Patterns
A variety of cardiac disorders are associated with specific patterns on ECG. Although it is beyond the scope of this text to provide a comprehensive discussion of this topic, the reader should be familiar with the ECG patterns associated with some common abnormalities, as outlined in Table 5-1.
1. The electrocardiogram is a visual representation of the electrical activity of the heart.
2. The normal electrocardiographic waveform consists of a P wave, QRS complex, and T wave representing atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively.
3. The P wave in sinus rhythm has a characteristic appearance and is upright in lead II and downward in lead aVR.
4. The QRS is normally a predominantly negative complex in lead V1 and transitions to a predominantly positive complex by V4.
5. The normal electrical axis of the heart is -30° to +90°.
6. Certain cardiac abnormalities are associated with specific patterns on ECG.
Revision date: June 22, 2011
Last revised: by Sebastian Scheller, MD, ScD