Basic analysis principles
The electrocardiogram itself is a graphical recording of biopotential changes that occur as an electrical wave propagates through the myocardium. Assessing rhythm and conduction is the first step in ECG analysis.
At this stage, it is necessary to:
- Evaluate the regularity of contractions.
- Calculate the heart rate (HR).
- Locate the P wave and evaluate its relationship with the ventricular QRS complex.
It is precisely the detailed analysis of the atrial component (the P wave) that allows for the precise determination of the signal's origin.
Sinus rhythm and automaticity disorders
Normally, the pacemaker is the sinus node. Its stable function is confirmed by the presence of a normal P wave preceding each QRS complex. However, automaticity function can change:
- Sinus tachycardia. The heart rate exceeds 90–100 beats per minute while maintaining a normal P wave morphology. An inappropriate form is also distinguished, where the pulse persistently remains above 100 beats at rest or with minimal exertion. At the same time, the episode develops gradually.
- Sinus bradycardia. The heart rate drops below 60 beats per minute. If it is not accompanied by clinical symptoms, its isolated finding on the ECG has no clinical significance.
- Sinus arrhythmia. The origin remains sinus, but the rhythm becomes irregular. The main criterion is that the differences between R-R intervals exceed 0.16 seconds.
- Sick sinus syndrome (SSS). A severe condition in which the node, due to dystrophy or fibrosis, fails to provide an adequate rhythm. It is characterized by alternating bradycardia and tachycardia (tachy-brady syndrome), episodes of sinus arrest (sinus arrest), and slow rhythm recovery after tachycardic episodes.
Heterotopic (ectopic) rhythms
If the sinus node functions poorly or a pause occurs, escape automaticity centers («escape» impulses) are triggered. Heterotopic rhythms include:
- Slow atrial rhythm. The focus is usually located in the left atrium. Slow impulses are observed at a rate of less than 70–80 beats per minute.
- Atrioventricular (junctional) rhythm. The signal originates from the AV junction. On the ECG, the ventricular complex remains narrow and normal, but the P wave either disappears, merges with the QRS, or becomes inverted (in leads II, III, aVF). The heart rate drops to 40–60 beats per minute.
- Idioventricular (ventricular) rhythm. Impulses are generated within the ventricles themselves. The electrocardiogram records wide and deformed QRS complexes without a preceding P wave. This is an escape rhythm during marked suppression of higher centers, with a rate of 20–40 contractions per minute.
- Migration of the supraventricular pacemaker. The source constantly shifts between the sinus node, atria, and AV junction. As a result, the morphology, polarity, and amplitude of the P wave change from cycle to cycle, while the P-R and R-R intervals vary.
Tachycardias from ectopic foci
Heterotopic foci can provoke a sharp acceleration of heart activity:
- Atrial paroxysmal tachycardia. It occurs against the background of organic heart disease (CAD) or cardiac glycoside intoxication. The episode starts and terminates abruptly. The heart rate reaches 150–250 beats per minute. The P wave is present but deformed (biphasic, inverted, or flattened), while the QRS complex remains narrow. A unique feature during digitalis intoxication is potential conduction slowing with the development of AV block.
- Supraventricular tachycardias. The formal ECG criterion is the recording of three or more consecutive supraventricular premature beats. For example, in sinus nodal re-entry tachycardia, a sudden episode of palpitations occurs, which (unlike inappropriate sinus tachycardia) can be terminated by vagal maneuvers.
To quickly assess the rhythm source, look at the P wave and the width of the QRS complex. Normal P — sinus rhythm. Altered, absent, or inverted P with a narrow QRS — atrial or junctional rhythm. Wide deformed QRS without a P wave — ventricular rhythm.
Class and exam questions
How to distinguish inappropriate sinus tachycardia from nodal re-entry tachycardia?
Inappropriate sinus tachycardia develops gradually and is persistent in nature. Re-entry tachycardia starts sharply and suddenly, and can be terminated by vagal maneuvers or adenosine administration.
What is dissociation with interference?
This is independent and simultaneous activity of two pacemakers at once (e.g., the sinus node and the AV junction), in which they do not suppress each other's activity.
What is the peculiarity of atrial paroxysmal tachycardia in cardiac glycoside overdose?
Unlike other fast tachycardias, digitalis intoxication may present with slowed AV conduction accompanied by first- or second-degree atrioventricular block.
What else to study on this topic
- Determination of the electrical axis of the heart (EAH) in the frontal plane
- Detailed analysis of the ventricular QRST complex and the Q-T interval
- Sinus node dysfunction and sinoatrial blocks of varying degrees
- ECG criteria for coronary artery disease and myocardial infarction
- Atrial fibrillation and atrial flutter on the ECG
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