Components of a normal ECG
A normal heart rhythm is composed of the sequential activity of all its chambers. On the electrocardiogram, this activity manifests as the alternation of specific elements—waves and intervals.
- P wave reflects the electrical activity of the atria.
- PQ interval (in some leads, PR) demonstrates impulse conduction from the atria to the ventricles. Normally, it includes the physiological signal delay in the AV node, which is necessary for the proper transition of the impulse to the ventricular myocardium.
- QRS complex corresponds to the period of general ventricular depolarization. In some cases, the very first wave of the complex—the Q wave—is weakly expressed.
- S wave reflects the delay of impulse conduction in the myocardium. Normally, in standard leads (first and second), it should not be too deep. At the same time, a deep S wave is a characteristic feature specifically of the chest leads.
- T wave records the active stage of ventricular repolarization.
Pathological changes on the cardiogram
Disorders in impulse generation and conduction lead to alterations in the normal ECG pattern.
- In bundle branch block (right or left), the normal morphology of the R wave is distorted: it may be completely absent or take an altered shape.
- In extrasystole, a sudden extra ventricular depolarization occurs. On the cardiogram, this manifests as the QRS-T complex arising prematurely, out of turn.
- In atrial “overdrive” (the re-entry mechanism), the conduction system operates continuously. The atria can neither stabilize for effective impulse transmission nor stop. Visually on the ECG, this looks like a continuous chaotic stream of waves.
ECG during intrauterine development and in newborns
The fetal electrocardiogram begins to be recorded as early as the 15th–17th week of pregnancy. At early stages, it consists exclusively of ventricular complex waves, while their amplitude gradually increases in parallel with the gestational age.
In newborns, the ECG is characterized by significant fluctuations in parameters due to rapid anatomical and functional changes in the cardiovascular system. A specific feature of this age is the “right” ECG type. In this type, the alpha angle ranges from 90 to 180° (on average about 150°). This deviation occurs due to the relatively large mass of the right ventricular myocardium.
The ratio of R and S waves clearly defines the right ECG type: a tall R wave is recorded in lead III and a deep S wave in lead I. No rhythm disturbances are normally noted, except for mild sinus arrhythmia.
Wave and interval standards in infants of the first year of life
Normally, the P wave in infants of the first year of life is tall and often peaked, which is associated with the dominance of the right atrium over the left and the high excitability of the sympathetic nervous system. Notching of the P wave in lead III (distance between notches no more than 0.02 sec) reflects physiological asynchrony. The T wave is relatively low and inconstant (may be biphasic or negative), which is due to the incomplete closure of the ductus arteriosus (Botallo's duct).
| Parameter | Normal values in newborns |
|---|---|
| P wave | Duration 0.05–0.06 sec |
| P to R ratio | 1:3 (up to 3 years; in adults — 1:8) |
| P-R (PQ) interval | No more than 0.11 sec |
| QRS complex | 0.03–0.09 sec (average 0.05 sec), often notched |
| Q-T interval | 0.22–0.32 sec |
Influence of birth weight on ECG parameters
Birth weight significantly affects conduction and rhythm parameters. In newborns with borderline low birth weight (2500–2950 g) or, conversely, high birth weight (4000–5500 g), compared to children with normal birth weight (3000–3950 g), pronounced differences are detected:
- Relatively more pronounced bradycardia.
- Significant heart rate lability.
- Prolonged ventricular repolarization processes.
- Slowed intra-atrial, atrioventricular, and intraventricular conduction.
Over the course of 6–10 days of life, such infants show the most pronounced signs of physiological right atrial load and T wave variability, indicating the instability of metabolic processes in the myocardium. This reflects the overall strain of cardiovascular system development in the early neonatal period.
To understand the right ECG type in infants, remember the rule: a tall R wave in the third lead is combined with a deep S wave in the first lead.
Class and exam questions
What causes the impulse delay in the PQ interval?
This pause is necessary for the transition of excitation to the ventricles and is associated with the physiological delay of the electrical signal in the AV node.
What does the ECG look like in atrial re-entry syndrome?
Due to the continuous circulation of the impulse, the atria cannot stabilize, and a continuous chaotic stream without distinct waves is recorded on the cardiogram.
Why do newborns exhibit a tall P wave?
This is due to the physiological predominance of the right atrial mass over the left, as well as the high excitability of the sympathetic nervous system in infants.
At what gestational age can a fetal ECG be recorded?
Electrical activity is recorded from the 15th–17th week of pregnancy, with initially only ventricular complex waves present on the tracing.
What else to study on this topic
- Mechanism of formation and deformation of the QRS complex in bundle branch blocks
- Causes and significance of QRS complex notching in lead I in infants
- Association of incomplete closure of the ductus arteriosus with T wave inversion
- Assessment of physiological atrial asynchrony in children under 3 years of age
- Clinical significance of T wave variability on days 6–10 of a newborn's life
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