Normal ECG: sinus rhythm and electrical axis of the heart

Key elements of a normal ECG

Each wave and interval on the electrocardiograph tracing corresponds to a specific stage of the cardiac cycle:

• P wave: captures the atrial component, i.e., the electrical activity of the atria.
• PQ (or PR) interval: shows the time it takes for the impulse to travel from the atria to the ventricles. This includes the obligatory delay in the AV node, which is necessary for the proper redistribution of blood.
• QRS complex: demonstrates overall ventricular depolarization (where the initial Q wave is sometimes poorly pronounced).
• S wave: reflects impulse conduction delay. In the first two standard leads, it should not be too deep, unlike in the chest leads.
• T wave: accounts for the active phase of ventricular repolarization.

Signs of pathologies on a standard tracing

Deviations from the norm on the graph indicate disruptions in the heart's conduction system:

1. Bundle branch block: in case of damage to the right or left bundle branch, the normal shape of the R wave is distorted, deformed, or completely absent.
2. Atrial re-entry: a pathological condition in which impulses continuously circulate in the atria without the ability to stabilize. On the graph, this looks like a continuous chaotic stream.
3. Extrasystole: sudden appearance of premature ventricular depolarization, resulting in an untimely QRS-T complex being registered out of turn.

Instrumental methods of advanced diagnostics

When a standard study is insufficient for a complete assessment of the patient's condition, specialized hardware techniques are used:

• Holter monitoring: 24-hour or continuous recording necessary for counting arrhythmias and correlating them with the patient's complaints or physical exertion.
• Transesophageal ECG: performed via an esophageal probe at a depth of about 35–40 cm for detailed visualization of the P wave, which is located close to the left atrium. The method also allows for pacing to terminate tachycardias.
• Signal-averaged ECG (SAECG): amplifies low-amplitude late potentials immediately following the QRS complex, indicating zones of delayed conduction in the ventricles.
• Intracardiac EPS: an invasive study using catheters for topical diagnosis of pathways and identification of the re-entry mechanism.

Comprehensive approach and multidisciplinary evaluation

Since rhythm disorders are often associated with extracardiac factors, diagnostics may extend far beyond cardiology:

• Conducting neurological and psychophysiological examinations (brain MRI, EEG).
• Consultations with subspecialists: endocrinologist, neurologist, ENT specialist, ophthalmologist.
• Specific laboratory tests (immunological, virological, and molecular genetic tests) to rule out myocarditis and channelopathies.

Class and exam questions

What else to study on this topic

• Technical nuances of heart rate calculation at a paper speed of 50 mm/s
• Indications and methodology for intracardiac EPS
• Radical treatment of arrhythmias using ablation and electrocoagulation
• Criteria and diagnosis of mitral valve prolapse
• Principles of surface ECG mapping

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.