ECG: Normal Intervals and Waves

Basic principles of instrumental cardiac diagnostics

A comprehensive approach is used for a full assessment of the cardiovascular system. ECG serves as the main tool for analyzing rhythm and conduction. Concurrently, echocardiography (EchoCG), three-view chest radiography, magnetic resonance and computed tomography (MRI and CT), as well as angiography, are used to assess morphology, specifically the dimensions of chambers and vessels. Evaluating the functional reserve of the myocardium requires exercise stress tests with metered physical activity. Against the background of these methods, ECG stands out for its ability to finely record electrical processes through the evaluation of waves and intervals.

Age-related evolution of waves and intervals

Interpretation of the electrocardiogram requires taking into account age-related evolutionary changes. Significant physiological shifts occur during maturation, which directly affect the electrical curve:

• The mass ratio of the left and right ventricles changes.
• The anatomical position of the heart in the thoracic cavity changes.
• Age-related dynamics of the baseline heart rate (HR) occur.

Key differences between a pediatric ECG and an adult curve lie in the shortening of time intervals and wave durations. This is explained by the higher conduction velocity of the electrical impulse both through the myocardium and the specialized conduction system. A healthy child is also characterized by high parameter lability — wave widths and interval durations constantly fluctuate depending on the current heart rate. The presence of sinus respiratory arrhythmia is considered a variant of physiological norm.

ECG criteria for sinus rhythm and its disorders

The primary sign of a regular rhythm originating from the sinus node is the registration of a P wave of normal polarity before each ventricular QRS complex. Evaluating intervals allows for diagnosing the following conditions:

1. Sinus tachycardia: heart rate exceeds normal values (e.g., greater than 120 bpm in children or over 90–100 bpm in resting adults) while maintaining normal sinus P-wave morphology. Inappropriate sinus tachycardia is distinguished separately as a persistent resting HR elevation >100 bpm without obvious causes. Unlike sinoatrial reentrant tachycardia, it is characterized by a gradual rather than sudden onset.
2. Sinus bradycardia: a regular rhythm with a heart rate below 60 bpm and a preserved sinus P wave. Without a connection to clinical symptoms, isolated bradycardia has no significance.
3. Sinus arrhythmia: an irregular rhythm in which the pacemaker remains sinus, but the differences in R-R interval duration exceed 0.16 seconds.

Sick Sinus Syndrome (SSS)

With persistent suppression of automaticity, the node loses its ability to generate a rhythm adequate to the metabolic demands of the body. The morphological basis is fibrosis and dystrophy in the nodal area, as well as autonomic imbalance with a predominance of parasympathetic influences. On the ECG, this manifests as marked distortion of intervals:

• Tachy-brady syndrome (alternating periods of bradycardia and tachycardia).
• Episodes of sinus arrest (sinus arrest).
• Slow rhythm recovery after termination of tachycardia.
• Transient atrial flutter or fibrillation.

The pathophysiological consequence is chronotropic incompetence — the inability to adequately increase the heart rate during exercise. Sick sinus syndrome (SSS) is part of a broader category of sinus node dysfunction (SND), which additionally includes sinoatrial blocks of varying degrees (ranging from prolonged conduction time to complete impulse blockade).

Effect of ectopic rhythms on ECG morphology

When SA node activity decreases, subsidiary (heterotopic) pacemakers are activated. Their localization radically changes the appearance of normal waves and intervals:

• Ectopic atrial rhythm: a focus in the left atrium generates slow impulses (less than 70–80 per minute).
• Atrioventricular (junctional) rhythm: the pacemaker is located in the AV junction. The QRS complex remains narrow and unchanged, but the P wave is either absent, fused with the QRS, or retrograde (negative in II, III, aVF). Rate is 40–60 per minute.
• Idioventricular (ventricular) rhythm: characterized by wide, deformed QRS complexes without a preceding P wave and a low rate (20–40 per minute).
• Wandering supraventricular pacemaker: the source shifts, which manifests as changes in the shape, polarity, and amplitude of the P wave from cycle to cycle against the background of variable P-R and R-R intervals.

Additionally, escape beats (escape beats) can be registered on the ECG after pauses, as well as interference-dissociation — simultaneous independent functioning of two pacemakers without mutual suppression.

Class and exam questions

What else to study on this topic

• Hemodynamic and musculo-tonic functional heart murmurs
• Murmurs of growth heterochrony and minor cardiac structural anomalies (MCSA)
• Differential diagnosis of extracardiac murmurs (pericardial friction rub)
• Auscultation of major peripheral vessels in health and disease
• Evaluation of myocardial functional reserve using exercise tests

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