Tachycardias (4.6). Extrasystoles

Extrasystoles (also called premature atrial, junctional or ventricular beats) may originate from any site of the heart. The distinction between ventricular and supraventricular extrasystoles is conventional. Supraventricular extrasystoles have a narrow QRS complex in the absence of bundle branch block or pre-excitation. The P-wave morphology in atrial extrasystole is different from that in the sinus beats.
Sometimes there is only a slight difference in the morphology of the P wave.
In junctional extrasystoles, the P wave is absent, or is located just before the QRS complex, or after the QRS complex. The PR interval in the extrasystolic complex is less than 120 milliseconds. A retrograde P wave is often superimposed on the ST segment. On this ECG, P wave in premature junctional complex is superimposed on the ST segment.
Sometimes the P wave of an atrial extrasystole may merge with the previous T wave and may be poorly seen.
Nevertheless, the P wave in atrial extrasystole may appear at the apex of the T wave. Variations in the T wave amplitude and morphology before the premature QRS complex suggest superimposition of the P wave on the T wave.
Supraventricular extrasystoles may be frequent.
Supraventricular extrasystoles may produce bigeminy (when alternating complex is an extrasystole), trigeminy (when every third complex is an extrasystole), and quadrigeminy (when every fourth complex is an extrasystole).
Two consecutive premature beats are called coupled extrasystoles. Three or more consecutive supraventricular extrasystoles are defined as supraventricular tachycardia. However, some authors define three consecutive premature supraventricular beats as supraventricular triplets.
An atrial extrasystole non-conducted to the ventricles is called a blocked atrial extrasystole. In such a case, an abnormal P wave is not followed by a QRS complex. There is a compensatory pause after the abnormal P wave as the sinus node resets. It may be misinterpreted as a sinus exit block or atrioventricular block.
Sometimes a premature P wave may be followed by a wide QRS complex with bundle branch block pattern. This is atrial extrasystole with aberrant conduction. The right bundle branch block pattern is more common.
Sometimes it’s very difficult to find the P wave merged with the T wave of the previous beat. Aberrantly conducted atrial extrasystoles are difficult to distinguish from ventricular extrasystoles.
Premature ventricular complexes (ventricular extrasystole) arise from an ectopic focus within the ventricles. The QRS complex is wide with abnormal morphology and is usually not preceded by the P wave.
A ventricular extrasystole is often followed by a full compensatory pause. In most cases an impulse from the ventricles doesn’t spread to the atria. Thus, it doesn’t reset the sinus node. The next beat after extrasystole appears after an interval that is equal to the double preceding RR interval.
The cardiac axis in the extrasystole may not be the same as in sinus beats.
The RR interval between sinus beats around the ventricular extrasystole may be equal to the preceding RR interval. This is a feature of ventricular extrasystole with interpolation. Such an extrasystole must appear early enough to allow the atrioventricular node to go out of the refractory period and conduct the next impulse originating from the sinus node. Thus, each sinus impulse is conducted to the ventricles.
The retrograde P wave is another important feature of a ventricular extrasystole. It results from retrograde impulse conduction from the ventricles to the atria. Retrograde P-wave morphology is different from that of the sinus origin. It usually appears on the ST segment or the initial portion of the T wave.
Also, the P wave may be non-conducted due to the ‘concealed conduction’ phenomenon.
Ventricular extrasystoles may arise from two or more ectopic foci resulting in a several different QRS complex morphologies. They are called multifocal.
Ventricular extrasystoles may produce bigeminy (alternating complex is extrasystole). Frequent ventricular extrasystoles affect cardiac output.
In ventricular trigeminy, every third complex is an extrasystole.
In ventricular quadrigeminy, every fourth complex is ventricular extrasystole.
Two consecutive ventricular extrasystoles are called a ventricular couplet. Three or more consecutive ventricular ectopic beats are called ventricular tachycardia if the rate exceeds 100 bpm. Ventricular tachycardia is classified into sustained or non-sustained according to the fact whether it lasts more than 30 seconds or less.