Version May 2024
INTRODUCTION — The electrocardiogram (ECG) provides a graphic record of the electrical activity of the heart (see "Basic principles of electrocardiographic interpretation"). Each cardiac cell generates an action potential as it becomes depolarized and then repolarized during a normal cycle (movie 1). (See "Cardiac excitability, mechanisms of arrhythmia, and action of antiarrhythmic drugs".)
Depolarization of cardiac cells proceeds in an orderly fashion in the normal situation beginning in the sinus node, and then spreading sequentially through the atria, atrioventricular (AV) node, and ventricles (see "ECG tutorial: Physiology of the conduction system"). The electrical signal spreads through the heart as wavefronts of depolarization. These wavefronts result in a minute electrical field that can be detected at the body's surface as an ECG (see "Left bundle branch block" and "Right bundle branch block" and "ECG tutorial: Intraventricular block"). In particular, the surface ECG records the activation and recovery signals of working myocardial cells, not of the pacemaker cells of the sinoatrial and AV nodes or of the specialized conduction system.
ELECTRICAL FIELDS — All electrical fields have two important associated parameters: magnitude and direction. The standard ECG is simply a graphical representation of the direction and magnitude of the electrical field of the heart as it changes with time; each lead "looks at" this electrical field from a different angle.
One can imagine, for example, a strip of cardiac muscle that is being stimulated by an external source. Initially, the inside of the cell is negative while the outside is relatively positive. When the muscle is stimulated and sodium ions flow into the cell, the outside becomes negative with reference to the inside of the cell (which is now positive). However, the section of the myocardial membrane that has not yet become depolarized (or is in a resting state) remains positive. Thus, this sequence establishes a dipole that has direction and magnitude (figure 1).
The direction of the dipole is simply the net direction of the positive charge relative to the negative charge; this corresponds to the direction of the wavefront of depolarization for the muscle strip. The magnitude of the dipole is determined by the amount of positive sodium ions that flow into the cell (eg, how positive the inside of the cell becomes). Since the heart is a complex three-dimensional structure consisting of millions of cells, the direction of the dipole changes with time and is the result of a summation of all the instantaneous dipoles. At any point in time, the mean dipole can be determined. This changing direction of the electrical impulse is the basis of vectorcardiography.
The mean direction of the electrical field during the entire cardiac cycle is called the electrical axis. The magnitude of the electrical field as measured by two leads is proportional to the distance between the electrodes and the dipole, the orientation of the electrodes to the dipole, and the size of the dipole. On the surface ECG, the distance between the electrodes and dipole may be influenced by chest wall shape, presence of thoracic obesity, subcutaneous edema, pulmonary emphysema, pericardial effusion, or other factors. The size of the dipole is predominantly related to the mass of tissue being depolarized.
Three pairs of electrodes placed along the muscle strip will record different electrical signals:
●The first pair of electrodes are located at the site of origin of depolarization. The electrical signal moves away from the origin at the start of depolarization; thus, a strong negative charge is recorded. The electrode continually sees a negative charge throughout depolarization of the strip since the dipole is constantly moving away from it until depolarization of the muscle strip is completed, there is no further movement of an electrical charge, and an isoelectric baseline is restored.
●The second pair of electrodes records a dipole as seen from the middle of the muscle strip. During the first half of depolarization, there is a positive electrical signal moving toward it; thus, a continually growing upward deflection is recorded. When the dipole passes the electrode, however, there is a sudden change from a positive to a negative deflection since the electric charge is now moving away from the electrode. This deflection is at its most negative magnitude immediately after the dipole passes beneath it; the magnitude of the negative deflection subsequently decreases, returning to an isoelectric baseline at the end of depolarization.
●The third pair of electrodes, located at the end of the muscle strip, sees the electrical signal continuously moving towards it. Thus, it records a positive deflection that increases in magnitude as the dipole approaches it until the very end of depolarization when the potential returns to an isoelectric baseline.
These depolarization waves, with associated positive and negative signal projections, are recorded on the surface ECG; P waves are generated within the atrial myocardium, while QRS complexes are generated within the ventricular myocardium. The dipole moves in the opposite direction during repolarization, and, therefore, the deflection generated on the surface ECG is in the opposite direction. This corresponds to the T wave, and when recorded directly from the muscle strip its direction is opposite to that of the QRS complex. On the surface ECG, however, the T wave is oriented in relatively the same direction (QRS-T concordance) as the QRS complex since depolarization in the intact heart begins at the endocardium and moves towards the epicardium, while repolarization begins at the epicardium and moves towards the endocardium.
Thus, information about the direction of the moving dipole and its magnitude (dependent upon its proximity to the electrodes) are derived from the deflections recorded on the surface ECG. The theoretic basis of the ECG depends upon the assumptions that the heart is a single dipole generator, the body is a homogeneous conductor, and all electrodes are equidistant from the dipole generator. While none of the assumptions are entirely correct, they serve as the construct upon which the science of ECG is based.
CARDIAC DEPOLARIZATION — The sinus node spontaneously depolarizes, thereby initiating the electrical cycle of the heart (see "ECG tutorial: Physiology of the conduction system"). The impulse spreads through the atria via specialized conduction pathways and the interatrial bundle of Bachmann. This creates the P wave, which represents right and left atrial activation or depolarization, and results in contraction of the atria. As the electrical signal reaches the AV node, the impulse is delayed (the PR segment on the ECG), thereby allowing the ventricles to fill via atrial contraction. The wavefront of depolarization finally spreads via the His-Purkinje system to the ventricles, resulting in uniform ventricular activation and contraction. QRS deflections on the ECG correspond to depolarization of the ventricles. (See "Left bundle branch block" and "Right bundle branch block".)
At the beginning of ventricular activation, the left side of the septum is the first structure to depolarize, resulting in a small negative deflection (Q wave) in leads that look from a right-to-left direction on the ECG (ie, Leads I, aVL, and V5 to V6). The mass of the ventricles subsequently depolarizes; a large positive deflection (R wave) results because of the larger mass of the left ventricle compared to that of the right ventricle. Repolarization occurs after a delay (the ST segment on the ECG), and results in the T wave.
ECG LEADS — The ECG records the electrical field over time as it is generated by all the cells of the heart. Instead of electrodes placed next to the muscle strip, there are standard lead systems that record the potential generated by the heart at the body surface. The most widely used system was developed by Willem Einthoven, the father of modern electrocardiography, although it was modified in subsequent years. Einthoven described a system of three bipolar leads located at the right arm, left arm, and left leg (the right leg has a ground electrode) to form a triangle (Einthoven triangle). In addition to the original three bipolar leads, the standard 12-lead ECG now records three augmented limb leads and six precordial (chest) leads.
Limb leads — Limb lead electrodes are placed on the arms and legs distal to the shoulders and hips. In the Einthoven triangle, lead I records the difference between the right arm and the left arm (impulses directed toward the left generate a positive waveform), lead II between the right arm and left leg (impulses directed toward the leg or downward generate a positive waveform), and lead III between the left arm and left leg (impulses directed toward the leg or downward generate a positive waveform) (figure 2).
A "reference electrode," also called the Wilson central terminal, is created for the augmented leads by connecting the three limb electrodes (right arm [R], the left arm [L], and the left leg [F]) which, when summated, give zero potential (figure 3). The electrical signals for the augmented leads are created by disconnecting the exploring electrode from the reference electrode. Put another way, the augmented leads use a limb electrode for one electrode and the average of the other two limb electrodes as the other electrode (ie, aVF is the difference between the left leg and the average of right arm and left arm electrodes). The three augmented leads are designated aVR, aVL, and aVF. Impulses directed toward the augmented lead generate a positive waveform, while a negative waveform is generated if the impulse moves away from the lead.
The three bipolar limb leads and the three augmented limb leads can be assembled into a hexaxial reference system:
●Lead I, directed horizontally right to left, is the 0° line.
●Lead II is 60°
●Lead III is 120°
●Lead aVF is 90°
●Lead aVL is -30°
●Lead aVR is -150°
Thus, these six leads divide the heart into 30 degree sectors and view the electrical field in a frontal plane (eg, right, left, superior, and inferior).
Precordial leads — The precordial leads use the same reference electrode as the augmented limb leads (eg, the Wilson central terminal) (figure 4 and figure 5). The exploring electrodes are connected to the chest wall as follows:
●V1 – Fourth intercostal space to the right of the sternum
●V2 – Fourth intercostal space to the left of the sternum
●V3 – Midway between V2 and V4
●V4 – Fifth intercostal space at the midclavicular line
●V5 – Anterior axillary line at the level of V4, or halfway between V4 and V6 if the anterior axillary line is unclear
●V6 – Midaxillary line at the level of V4
Thus, these electrodes measure the electrical field in a horizontal plane (eg, right to left, anterior to posterior). Note that guidelines of the major cardiology societies advocate that, pending further studies, the precordial leads should be placed under rather than on the left breast in adult women [1]. However, the placement (on versus under breast) of left chest leads in women has generated controversy over the years with respect to effects on waveform morphology and amplitude.
Additional leads can be used in certain situations. As an example, right sided precordial leads (V1R to V6R) are helpful for establishing right ventricular infarction (see "Right ventricular myocardial infarction"). Leads V7 (left posterior axillary line), V8 (tip of the left scapula), and V9 (to the left of the vertebra) are useful for diagnosing a true posterior wall infarction [2].
FILTERING — An ECG is obtained analog and, in current machines, is converted into a digital signal, where it is filtered to block out some of the "noise" while keeping relevant parts of the "signal." Low frequency signals such as respiration are eliminated using a high pass filter. High frequency signals such as noncardiac muscle potentials are attenuated using a low pass filter. Specific "notch" filters that eliminate electromagnetic interference at 50 to 60 Hz may also be used.
Digital ECG data may also be compressed when sent to a database to be retrieved for later use. Therefore, there may be a small difference in appearance of an ECG printed at the bedside versus one downloaded from an electronic medical record [1].
SUMMARY
●The electrocardiogram (ECG) provides a graphic record of the electrical activity of the heart. Each cardiac cell generates an action potential as it becomes depolarized and then repolarized during a normal cycle. Depolarization of cardiac cells proceeds in an orderly fashion in the normal situation beginning in the sinus node, and then spreading sequentially through the atria, atrioventricular (AV) node, and ventricles. (See 'Introduction' above.)
●The sequential movement of electrical current with each depolarization, from the sinus node to the entire ventricular myocardium, creates specific phases of the ECG: P wave, PR segment, QRS complex, ST segment, and T wave. (See 'Cardiac depolarization' above.)
●The electrical activity of the heart is captured by recording electrodes or "leads" placed on both arms and legs (called the limb leads), and six on the chest (called the precordial leads). The 12-lead ECG is created by simultaneously comparing electrical activity from two of these in the following manner:
•Lead I – Right arm to left arm (see 'Limb leads' above)
•Lead II – Right arm to left leg
•Lead III – Left arm to left leg
•Lead aVR – The average of left arm and left leg electrodes to the right arm
•Lead aVL – The average of the right arm and left leg electrodes to the left arm
•Lead aVF – The average of the right arm and left arm electrodes to the left leg
The precordial leads, V1 to V6, compare electrical activity with one place on the chest to the reference electrode. (See 'Precordial leads' above.)
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