Pathophysiology and classification
Excitability disorders differ fundamentally from automatism and conduction disorders in that the impulse is generated in an atypical location. Extrasystole represents the most common form of cardiac arrhythmias. Arrhythmias of this type can manifest at any age, starting literally from the neonatal period.
Based on the topography (the site of origin of the premature impulse), extrasystoles are divided into two major groups:
- Supraventricular — include atrial and atrioventricular (junctional) extrasystoles.
- Ventricular.
ECG presentation of an atrioventricular extrasystole
In this form, the source of excitation (heterotopic focus) is located in the AV junction or adjacent areas. The main feature of conduction is that the impulse propagates to the ventricles via the normal pathway (antegrades), while to the atria it propagates in the opposite direction (retrograde).
Depending on the chronology of cardiac chamber depolarization, three variants of the ECG presentation are distinguished:
- With preceding atrial activation (formerly referred to as an "upper junctional extrasystole"). Retrograde atrial activation manages to occur before ventricular depolarization. The ECG records a negative P wave in leads II, III, and aVF, which appears before the QRS complex. The P-Q interval is pathologically shortened, measuring less than 0.12 s.
- With simultaneous activation of the atria and ventricles. The depolarization of both chambers coincides in time. The P wave is completely absent on the cardiogram as it merges with the larger QRS complex.
- With subsequent atrial activation (outdated term "lower junctional extrasystole"). In this case, the ventricles manage to depolarize first. ECG criterion: inverted (negative) P waves are located after the QRS complex, superimposed on the ST segment or T wave.
Organic and functional extrasystoles
When analyzing Holter monitoring or a standard ECG, it is important to differentiate functional extrasystoles from organic pathology. Functional extrasystole is characterized by specific criteria:
- It most commonly debuts in the prepubertal and pubertal periods, which is associated with marked hormonal and autonomic remodeling of the body. The peak increase in arrhythmia frequency is observed precisely during puberty.
- Characterized by marked variability during serial follow-up.
- Response to exertion: the number of extrasystoles decreases sharply or they disappear completely during physical activity and changes in body position.
- Topography: most often of supraventricular origin or originating from the right ventricle.
Progression to paroxysmal tachycardia
If three or more consecutive supraventricular extrasystoles are recorded on the ECG, extrasystole is described as a repetitive phenomenon. This serves as the formal ECG diagnostic criterion for paroxysmal tachycardia.
In marked tachycardia with a high heart rate, it often becomes impossible to visualize the P wave on the isoelectric line. Consequently, distinguishing atrial tachycardia from atrioventricular tachycardia is extremely difficult. Therefore, in clinical practice, these conditions are grouped under the term "supraventricular paroxysmal tachycardia."
The main pathophysiological mechanisms of such paroxysms are the re-entry of the excitation wave (re-entry) in the AV junction and Wolff-Parkinson-White (WPW) syndrome. In WPW syndrome, paroxysms of supraventricular tachycardia debut in childhood or adolescence, mediated by accessory pathways.
To easily remember the three variants of AV extrasystoles, track the "journey" of the negative P wave: it can precede the QRS complex (high), be hidden within it (mid), or appear after it (low).
Class and exam questions
A single extrasystole is an isolated premature contraction. However, if three or more extrasystoles occur consecutively, the condition is classified as a repetitive phenomenon and diagnosed as paroxysmal tachycardia.
A single extrasystole is an isolated premature contraction. However, if three or more extrasystoles occur consecutively, the condition is classified as a repetitive phenomenon and diagnosed as paroxysmal tachycardia.
This is due to retrograde conduction. The impulse originates in the AV junction and propagates to the atria from bottom to top, in the reverse direction. On the ECG, this is reflected as the inversion of the P wave in leads II, III, and aVF.
This is due to retrograde conduction. The impulse originates in the AV junction and propagates to the atria from bottom to top, in the reverse direction. On the ECG, this is reflected as the inversion of the P wave in leads II, III, and aVF.
In functional arrhythmias, physical activity and changes in body position cause extrasystoles to disappear completely or their frequency to decrease sharply.
In functional arrhythmias, physical activity and changes in body position cause extrasystoles to disappear completely or their frequency to decrease sharply.
What else to study on this topic
- Complete list of general ECG features for all types of AV extrasystoles
- Ventricular excitability disorders (extrasystole and paroxysmal tachycardia)
- Sick sinus syndrome (SSS) and tachycardia-bradycardia syndrome
- Heterotopic rhythms: wandering supraventricular pacemaker and dissociation with interference
- Pathophysiology of Wolff-Parkinson-White (WPW) syndrome and re-entry mechanisms
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