Toe 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.
- 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. I.IA)
- 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 (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 the epicardial surface.
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).
- Tue height of a positive deflection and the depth of a negative deflection is 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 t·he extent to which they are transmitted to the recordil1g electrode on the body surface.
Lets see 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).
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 the 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).