Normal and pathological automatism
Normally, the primary source of a regular, correct rhythm is the sinus node. On the ECG, this is confirmed by the presence of a P wave of normal polarity preceding each QRS ventricular complex. Depending on the heart rate, the following are distinguished:
- Sinus tachycardia: Resting heart rate exceeds 90–100 bpm. A non-physiological (inappropriate) form is distinguished separately, which develops gradually, is persistent in nature, and has no obvious causes.
- Sinus bradycardia: Heart rate below 60 bpm. If it is isolated and not associated with clinical symptoms, it has no pathological significance.
- Sinus arrhythmia: The pacemaker remains the same (P wave is preserved), but the difference in R-R interval duration exceeds 0.16 seconds.
Ectopic centers are activated when normal control is lost.
Causes of heterotopic rhythms
The appearance of escape ectopic rhythms is closely associated with sick sinus syndrome (SSS). This syndrome features persistent suppression of automatism function: the node is incapable of providing a rhythm adequate to the body's metabolic demands (chronotropic incompetence develops).
The morphological basis of SSS is fibrosis and dystrophy in the region of the node, while causes include congenital or acquired heart defects, as well as autonomic imbalance with a predominance of parasympathetic influences.
SSS is part of a broader pathology — sinus node dysfunction (SND), which encompasses various types of sinoatrial blocks ( grados I to III SA block) and sinus node arrest.
Classification of ectopic rhythms
Depending on the localization of the pathological automatism focus, the following types of heterotopic rhythms are distinguished on the ECG:
- Slow atrial rhythm. The ectopic focus is most frequently located in the left atrium. Impulses are infrequent (less than 70–80 per minute). Observed in neuroses, cardiomyopathies, and heart defects.
- Atrioventricular (junctional) rhythm. The source is located in the AV junction. The rate is 40–60 contractions per minute. The QRS complex remains narrow and unchanged. The P wave may be absent, superimposed on the QRS, or retrograde (negative in leads II, III, aVF).
- Idioventricular (ventricular) rhythm. Considered a replacement rhythm during pronounced depression of superior centers. The source is located in the ventricles. Characterized by a very low rate (20–40 per minute) and wide, deformed QRS complexes without a preceding P wave.
Complex rhythm disorders
In addition to stable ectopic rhythms, transitional and combined phenomena may be recorded on the ECG:
- Migration of the supraventricular pacemaker: The source gradually shifts between the SA node, atria, and AV junction. On the ECG, from cycle to cycle, the shape, amplitude, and polarity of the P wave change with varying P-R and R-R intervals.
- Escape beats: Single replacement discharges from a lower-level center occurring after a pause if the primary pacemaker is delayed.
- Atrioventricular dissociation with interference: Simultaneous independent operation of two pacemakers (e.g., sinus and AV junctional) without mutual suppression.
To memorize the rate of heterotopic rhythms: Atria — up to 80, AV junction — up to 60, Ventricles — up to 40. The lower the focus, the slower the pulse.
Class and exam questions
What serves as the formal ECG criterion for supraventricular tachycardia?
The formal diagnostic criterion is the recording on the cardiogram of a "run" of three or more consecutive supraventricular extrasystoles.
How to differentiate inappropriate sinus tachycardia from sinus reentrant tachycardia?
Sinus node reentrant tachycardia begins suddenly and abruptly. Inappropriate sinus tachycardia, by contrast, is characterized by a gradual increase in heart rate.
What is "tachy-brady" syndrome?
This is one of the most informative signs of SSS, in which periodic or constant sinus bradycardia alternates with episodes of sinus tachycardia.
What else to study on this topic
- Re-entry mechanism in supraventricular tachycardias and management methods (vagal maneuvers, adenosine)
- ECG signs of sinoatrial blocks of various degrees
- Sinus node arrest
- Transient atrial flutter and fibrillation in the setting of SSS
- Chronotropic incompetence of the heart
Master the entire topic in the "Propaedeutics" course
This is a summary. The course covers everything tested in classes and exams.
- Comprehensive study guide on "Ectopic rhythm on ECG" and 3,152 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 113 video lessons on the subject
Other topics in the "Additional Topics" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.