Extrasystole and its ECG variants
Disorders of excitability are associated with the occurrence of premature depolarization impulses. The most common form of such disorders is extrasystole. It manifests as a premature contraction of the entire heart or individual chambers, usually followed by a compensatory pause.
Based on the localization of the heterotopic focus, supraventricular extrasystoles are divided into atrial and atrioventricular (AV). If extrasystoles originate from the AV junction, the impulse propagates in two directions: antegrade (downward to the ventricles) and retrograde (upward to the atria).
Depending on the conduction velocity of excitation to the atria and ventricles, three ECG patterns of AV extrasystoles are distinguished:
- With preceding atrial excitation. Retrograde atrial depolarization occurs before ventricular activation. On ECG, a negative P wave is recorded in leads II, III, aVF strictly before the QRS complex. The P-Q interval is shortened to less than 0.12 s. Previously, this was called an "upper nodal" extrasystole.
- With simultaneous excitation. Depolarization of both chambers coincides. Because of this, the P wave merges with the large ventricular QRS complex and is not visualized on the isoelectric line.
- With subsequent atrial excitation. The ventricles are activated faster than the atria. On ECG, inverted P waves appear after the QRS complex, overlaying the ST segment or T wave (obsolete term: "lower nodal" form).
Paroxysmal supraventricular tachycardia
With a sharp increase in heart rate, a diagnostic problem arises: the P wave is often difficult to discern on the ECG. Consequently, it is extremely difficult to clinically distinguish atrial tachycardia from atrioventricular tachycardia. Therefore, in practice, all such conditions are grouped under the term "paroxysmal supraventricular tachycardia".
The formal ECG criterion for making this diagnosis is the recording of a series of three or more consecutive supraventricular extrasystoles.
The pathogenesis of supraventricular tachycardias is based on the following mechanisms:
- Circulation of the excitation wave via the re-entry mechanism in the AV junction.
- Wolff-Parkinson-White (WPW) syndrome. When an accessory conduction pathway is present, paroxysms of tachycardia occur (often debuting in childhood or adolescence), and less frequently, episodes of atrial fibrillation. At the same time, the syndrome itself can be an incidental ECG finding in healthy individuals.
- Presence of other accessory impulse conduction pathways.
Functional and organic causes of arrhythmias
Extrasystole can have both a benign functional and a severe organic nature. In children, extrasystoles dominate among all rhythm disorders (about 40–45% of cases). A pronounced manifestation peak occurs in prepuberty and puberty, which is associated with hormonal and autonomic restructuring of the body.
Criteria for functional extrasystole:
- Inconstancy of manifestations.
- Localization: usually supraventricular or right ventricular.
- Response to exertion: the number of extrasystoles sharply decreases or they completely disappear during physical activity.
Nevertheless, supraventricular arrhythmias are often a concomitant syndrome in organic myocardial damage:
- Mitral valve prolapse and hypertrophic cardiomyopathy (associated with a high risk of sudden death).
- Right ventricular cardiomyopathy.
- Myocarditis (sometimes ECG changes are the only proof of active inflammation).
- Alcoholic cardiomyopathy (accumulation of arrhythmogenic substances and myocardial lipomatosis can convert paroxysms into ventricular fibrillation).
- Toxic drug effects (e.g., glycosides or intravenous administration of vinca alkaloids).
Indications for ECG monitoring
A routine ECG is indicated for all patients at the slightest suspicion of supraventricular tachycardia—this is the primary basis for differential diagnosis. However, excitability disorders are often transient and are not captured on a standard ECG tracing.
Symptoms requiring prolonged Holter monitoring:
- Paroxysmal palpitations.
- Episodes of dizziness.
- Presyncope and syncope (fainting).
There is an important rule in differential diagnosis: if the patient develops syncope or palpitations while wearing the monitor, but no arrhythmia is recorded on the ECG, the cardiac origin of the symptoms is excluded. The physician must look for other etiological factors.
Class and exam questions
How to distinguish atrial tachycardia from atrioventricular tachycardia on an ECG?
In pronounced tachycardia, this is often impossible because the P wave merges with the ventricular complex. This is why clinicians use the umbrella term—paroxysmal supraventricular tachycardia.
Is having WPW syndrome considered a disease?
The phenomenon of premature ventricular excitation itself can be an incidental finding on an ECG in healthy people. However, its main clinical manifestation is precisely paroxysms of tachycardia via the re-entry mechanism.
Do extrasystoles always require stopping physical activity?
No, not always. In functional extrasystoles characteristic of puberty, the arrhythmia is inconstant and disappears during physical activity.
What else to study on this topic
- ECG signs of ventricular extrasystole and ventricular paroxysmal tachycardia
- Diagnosis of paroxysms of atrial fibrillation
- Effect of cardiac glycosides on rhythm and ventricular automaticity
- Atrial flutter: mechanisms and ECG patterns
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