THE ELECTROCARDIOGRAM
The electrocardiogram (ECG) provides a graphic depiction of the electrical forces generated by the heart. The ECG graph appears as a series of deflections and waves produced by each cardiac cycle.
Before going on to the genesis of individual deflections and their terminology, it would be worthwhile mentioning certain important facts about the direction and magnitude of ECG waves and the activation pattern of myocardium.
Direction
- By convention, a deflection above the baseline or isoelectric (neutral) line is a positive deflection while one below the isoelectric line is a negative deflection (Fig. 1.1A).The direction of a deflection depends upon two factors namely, the direction of spread of the electrical force and the location of the recording electrode.In other words, an electrical impulse moving towards an electrode creates a positive deflection while an impulse moving away from an electrode creates a negative deflection o(Fig. 1.1B). Let us see this example.We know that the sequence of electrical activation is such that the interventricular septum is first activated from left to right followed by activation of the left ventricular free wall from the endocardial to epicardial surface.Fig. 1.1A: Direction of the deflection on ECG:
- Above the baseline: positive deflection
- Below the baseline: negative deflection
If an electrode is placed over the right ventricle, it records an initial positive deflection representing septal activation towards it, followed by a major negative deflection that denotes free wall activation away from it (Fig. 1.2).If, however, the electrode is placed over the left ventricle, it records an initial negative deflection representing septal activation away from it, followed by a major positive deflection that denotes free wall activation towards it (Fig. 1.2).
Magnitude
- The height of a positive deflection and the depth of a negative deflection are measured vertically from the baseline. This vertical amplitude of the deflection is a measure of its voltage in millimeters (Fig. 1.3A).
- The magnitude of a deflection depends upon the quantum of the electrical forces generated by the heart and the extent to which they are transmitted to the recording electrode on the body surface. Let us see these examples:
- Since the ventricle has a far greater muscle mass than the atrium, ventricular complexes are larger than atrial complexes.
- When the ventricular wall undergoes thickening (hypertrophy), the ventricular complexes are larger than normal.
- If the chest wall is thick, the ventricular complexes are smaller than normal since the fat or muscle intervenes between the myocardium and the recording electrode (Fig. 1.3B).
Fig. 1.3A: Magnitude of the deflection on ECG:
- Positive deflection: height
- Negative deflection: depth
Fig. 1.3B: Effect of chest wall on magnitude of deflection:
- Thin chest—tall deflection
- Thick chest—small deflection
Activation
- Activation of the atria occurs longitudinally by contiguous spread of electrical forces from one myocyte to the other. On the other hand, activation of the ventricles occurs transversely by spread of electrical forces from the endocardial surface (surface facing ventricular cavity) to the epicardial surface (outer surface) (Fig. 1.4).Therefore, atrial activation can reflect atrial enlargement (and not atrial hypertrophy) while ventricular activation can reflect ventricular hypertrophy (and not ventricular enlargement).
THE ELECTROPHYSIOLOGY
The ECG graph consists of a series of deflections or waves. The distances between sequential waves on the time axis are termed as intervals. Portions of the isoelectric line (base-line) between successive waves are termed as segments.
In order to understand the genesis of deflections and the significance of intervals and segments, it would be worthwhile understanding certain basic electrophysiological principles.
- Anatomically speaking, the heart is a four-chambered organ. But in the electrophysiological sense, it is actually two-chambered. As per the “dual-chamber” concept, the chambers of the heart are the bi-atrial chamber and the bi-ventricular chamber (Fig. 1.5).This is because the atria are activated together and the ventricles too contact synchronously.Therefore, on the ECG, atrial activation is represented by a single wave and ventricular activation by a single wave-complex.
- In the resting state, the myocyte membrane bears a negative charge on the inner side. When stimulated by an electrical impulse, the charge is altered by an influx of calcium ions across the cell membrane.This results in coupling of actin and myosin filaments and muscle contraction. The spread of electrical impulse through the myocardium is known as depolarization (Fig. 1.6).Once the muscle contraction is completed, there is efflux of potassium ions, in order to restore the resting state of the cell membrane. This results in uncoupling of actin and myosin filaments and muscle relaxation. The return of the myocardium to its resting electrical state is known as repolarization (Fig. 1.6).Depolarization and repolarization occur in the atrial muscle as well as in the ventricular myocardium. The wave of excitation is synchronized so that the atria and the ventricles contract and relax in a rhythmic sequence.Atrial depolarization is followed by atrial repolarization which is nearly synchronous with ventricular depolarization and finally ventricular repolarization occurs.We must appreciate that depolarization and repolarization of the heart muscle are electrical events, while cardiac contraction (systole) and relaxation (diastole) constitute mechanical events.However, it is true that depolarization just precedes systole and repolarization is immediately followed by diastole.
- The electrical impulse that initiates myocardial depolarization and contraction originates from a group of cells that comprise the pacemaker of the heart.The normal pacemaker is the sinoatrial (SA) node, situated in the upper portion of the right atrium (Fig. 1.7).From the SA node, the electrical impulse spreads to the right atrium through three intra-atrial pathways while the Bachmann's bundle carries the impulse to the left atrium.Having activated the atria, the impulse enters the atrioventricular (AV) node situated at the AV junction, on the lower part of the inter-atrial septum. The brief delay of the impulse at the AV node allows time for the atria to empty the blood they contain into their respective ventricles.After the AV nodal delay, the impulse travels to the ventricles through a specialized conduction system called the bundle of His. The His bundle primarily divides into two bundle branches, a right bundle branch (RBB) which traverses the right ventricle and a left bundle branch (LBB) that traverses the left ventricle (Fig. 1.7).A small septal branch originates from the left bundle branch to activate the interventricular septum from left to right. The left bundle branch further divides into a left posterior fascicle and a left anterior fascicle.The posterior fascicle is a broad band of fibers which spreads over the posterior and inferior surfaces of the left ventricle. The anterior fascicle is a narrow band of fibers which spreads over the anterior and superior surfaces of the left ventricle (Fig. 1.7).Having traversed the bundle branches, the impulse finally passes into their terminal ramifications called Purkinje fibers. These Purkinje fibres traverse the thickness of the myocardium to activate the entire myocardial mass from the endocardial surface to the epicardial surface.
THE DEFLECTIONS
The ECG graph consists of a series of deflections or waves. Each electrocardiographic deflection has been arbitrarily assigned a letter of the alphabet. Accordingly, a sequence of wave that represents a single cardiac cycle is sequentially termed as P Q R S T and U (Fig. 1.8A).
By convention, P, T and U waves are always denoted by capital letters while the Q, R and S waves can be represented by either a capital letter or a small letter depending upon their relative or absolute magnitude. Large waves (over 5 mm) are assigned capital letters Q, R and S while small waves (under 5 mm) are assigned small letters q, r and s.
The entire QRS complex is viewed as one unit, since it represents ventricular depolarization. The positive deflection is always called the R wave. The negative deflection before the R wave is the Q wave while the negative deflection after the R wave is the S wave (Fig. 1.8B).
Relatively speaking, a small q followed by a tall R is labelled as qR complex while a large Q followed by a small r is labelled as Qr complex.
Similarly, a small r followed by a deep S is termed as rS complex while a tall R followed by a small s is termed as Rs complex (Fig. 1.9).
Two other situations are worth mentioning. If a QRS deflection is totally negative without an ensuing positivity, it is termed as a QS complex.
Secondly, if the QRS complex reflects two positive waves, the second positive wave is termed as R' and accordingly, the complex is termed as rSR' or RsR' depending upon magnitude of the positive (r or R) wave and the negative (s or S) wave (Fig. 1.9).
Significance of ECG Deflections
P wave | : Produced by atrial depolarization. |
QRS complex | : Produced by ventricular depolarization. It consists of: |
| : First negative deflection before R wave. |
| : First positive deflection after Q wave. |
| : First negative deflection after R wave. |
T wave | : Produced by ventricular repolarization. |
U wave | : Produced by Purkinje repolarization (Fig. 1.10). |
Within ventricular repolarization, the S-T segment is the plateau phase and the T wave is the rapid phase.
Fig. 1.10: Depolarization and repolarization depicted as deflections.
(Note: Atrial repolarization is buried in the QRS complex)
12You would be wondering where is atrial repolarization. Well, it is represented by the Ta wave which occurs just after the P wave. The Ta wave is generally not seen on the ECG as it coincides with (lies buried in) the larger QRS complex.
THE INTERVALS
During analysis of an ECG graph, the distances between certain waves are relevant in order to establish a temporal relationship between sequential events during a cardiac cycle. Since the distance between waves is expressed on a time axis, these distances are termed as ECG intervals. The following ECG intervals are clinically important.
P-R Interval
The P-R interval is measured from the onset of the P wave to the beginning of the QRS complex (Fig. 1.11). Although the term P-R interval is in vogue, actually, P-Q interval would be more appropriate. Note that the duration of the P wave is included in the measurement.
13We know that the P wave represents atrial depolarization while the QRS complex represents ventricular depolarization. Therefore, it is easy to comprehend that the P-R interval is an expression of atrioventricular conduction time.
This includes the time for atrial depolarization, conduction delay in the AV node and the time required for the impulse to traverse the ventricular conduction system before ventricular depolarization ensues.
Q-T Interval
The Q-T interval is measured from the onset of the Q wave to the end of the T wave (Fig. 1.11). If it is measured to the end of the U wave, it is termed Q-U interval. Note that the duration of the QRS complex, the length of the ST segment and the duration of the T wave are included in the measurement.
We know that the QRS complex represents ventricular depolarization while the T wave represents ventricular repolarization. Therefore, it is easy to comprehend that the Q-T interval is an expression of total duration of ventricular systole.
Since the U wave represents Purkinje system repolarization, the Q-U interval in addition, takes into account the time taken for the ventricular Purkinje system to repolarize.
THE SEGMENTS
The magnitude and direction of an ECG deflection is expressed in relation to a base-line which is referred to as the isoelectric line. The main isoelectric line is the period of electrical inactivity that intervenes between successive cardiac cycles during which no deflections are observed.
It lies between the termination of the T wave (or U wave if seen) of one cardiac cycle and onset of the P wave of the next cardiac cycle. However, two other segments of the isoelectric line, that occur between the waves of a single cardiac cycle, are clinically important.14
P-R Segment
The P-R segment is that portion of the isoelectric line which intervenes between the termination of the P wave and the onset of the QRS complex (Fig. 1.12). It represents conduction delay in the atrioventricular node. Note carefully that the length of the P-R segment does not include the width of the P wave while the duration of the P-R interval does include the P wave width.
S-T Segment
The S-T segment is that portion of the isoelectric line which intervenes between the termination of the S wave and the onset of the T wave (Fig. 1.12). It represents the plateau phase of ventricular repolarization. The point at which the QRS complex ends and the ST segment begins is termed the junction point or J point.