Assessment of QRS amplitude and morphology
The QRS ventricular complex reflects the spread of excitation through the ventricular myocardium. Its normal duration is 0.06 to 0.1 seconds.
Morphological analysis follows a strict rule: waves must be completely clean, without any notches or pathological slurring. The only acceptable exception is the presence of small notches in standard lead III, as well as in the augmented left leg lead (aVF).
Voltage, or total amplitude, is calculated mathematically by summing the absolute values of the tallest positive R wave and the largest negative wave (which may be Q or S).
- The average normal voltage value is 5–20 mm.
- If tracing analysis reveals that the complex amplitude does not exceed 5 mm in any recorded lead, a conclusion of "low-voltage ECG" is made.
Vector analysis of the atrial P wave
Analysis of the atrial complex involves two mandatory stages: measuring specific parameters (amplitude, duration, polarity, shape) and determining the direction of the excitation wave vector.
Vector analysis helps identify the source of the rhythm:
- Norm (Sinus Rhythm): The impulse originates in the sinus node, and the excitation wave naturally propagates from top to bottom. The electrocardiographic sign of this is a positive P wave in standard lead I.
- Pathology (Retrograde Conduction): The impulse is generated in the atrioventricular junction or the ventricles themselves. In this case, the excitation wave travels from bottom to top toward the atria. On the ECG, this appears as negative P waves.
Age-related dynamics of wave voltage in children
Pediatric electrocardiography has significant differences from adult ECG. Wave height is directly influenced by age-related factors such as heart rate, cardiac position in the chest cavity, the mass ratio of the right and left ventricles, and their excitation pattern.
Wave amplitude in children is unstable and depends on the electrical conductivity of surrounding tissues, chest wall thickness, and the relative size of the heart itself.
Dynamics of voltage changes with age:
- First 5–10 days of life: Low voltage of QRS waves is recorded. This is a physiological phenomenon associated with temporarily reduced myocardial electrical activity.
- Post-neonatal period: Amplitude begins to gradually increase.
- From infancy to 8 years: Higher wave amplitude is observed (especially noticeable in chest leads). The reasons lie in anatomy: the child's chest is thinner, the heart is larger relative to thoracic volume, it undergoes rotation around axes, and the myocardium lies closer to the anterior chest wall. It is important to remember that in pediatrics, the ratio of R and S waves holds diagnostic significance, not just their absolute value.
Effect of the electrical axis of the heart on the ECG
Shift of the electrical axis of the heart (EAH) is another distinctive feature of pediatric ECGs. While adults and school-age children predominantly show a normal position (though horizontal and vertical occur), infants present a different picture.
| Age | EAH Position |
|---|---|
| Neonates and first months | Significant right deviation (90–180°, average 150°) |
| From 3 months to 1 year | Predominantly vertical (75–90°), with variations of 30–120° |
| Around 2 years | 2/3 vertical, 1/3 normal (30–70°) |
The reasons for these changes are related to the evolving mass ratio of the ventricles. In infants during their first months of life, the right ventricle predominates anatomically and electrophysiologically. As the child grows, left ventricular mass increases at a faster rate. The heart rotates, contact between the right ventricle and the chest wall decreases, and the EAH shifts from rightogram to normogram. These spatial shifts can be judged by the ratio of R and S wave amplitudes in standard and chest leads, as well as the shift of the so-called transition zone.
To remember the sinus rhythm vector: "A normal impulse falls from heaven to earth" — from the sinus node from top to bottom, so the P wave in lead I "points" upward (positive). If the impulse comes "from underground" (retrograde), the P wave will be negative.
Class and exam questions
How to correctly calculate ventricular complex voltage on an ECG?
To calculate, sum the absolute values of the positive R wave and the deepest negative wave (Q or S). Normally, this sum is 5–20 mm.
What does the term "low-voltage ECG" mean?
This term is used when the QRS complex amplitude does not exceed 5 mm in any recorded lead. In adults, this may indicate pathology, whereas in neonates during the first 5–10 days of life, it is a variant of normal.
Why is wave amplitude higher in infants than in adults?
This is due to a thinner chest wall, larger heart size relative to thoracic volume, and closer apposition of the heart to the chest wall.
Can notches be present on the QRS complex in normal conditions?
Normally, waves should be clean. Minor notches are permitted only as an exception in standard lead III and lead aVF.
What else to study on this topic
- Changes in left atrial electrical activity (P-mitrale)
- Generalized inversion or flattening of the T wave
- Atrial fibrillation and other rhythm disorders
- Atrioventricular blocks
- Intraventricular conduction disturbances
- Pseudo-infarction changes on the electrocardiogram
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