ECG criteria and diagnostics
Excitation processes within the sinoatrial node (SA node) itself cannot be recorded on a standard surface ECG. The mass of atypical cardiomyocytes in this area is insufficient to generate an electromagnetic field of the required strength. Therefore, the diagnosis of automaticity disorders is made exclusively based on indirect signs:
- Heart rate (HR) drops below normal — less than 60 beats per minute.
- The rhythm remains regular, which is reflected in the regularity of R-R intervals.
- The pacemaker is sinus. This is confirmed by the presence of a P wave of normal polarity before each ventricular QRS complex.
Isolated sinus bradycardia detected during monitoring without connection to the patient's complaints is considered clinically insignificant.
Pathogenesis and hemodynamic impact
At the molecular level, the frequency of impulse generation depends on the rate of spontaneous diastolic depolarization — the shift of the SA node cell membrane potential toward the threshold level. This process is driven by a specific ionic current (slow influx of Na⁺/Ca²⁺ against a background of decreased outward K⁺ current). In bradycardia, parasympathetic (vagal) influences predominate, depolarization slows down, and the threshold is reached more slowly.
At the systemic level, a slow rhythm leads to a drop in cardiac output despite a compensatory increase in stroke volume. When the heart is unable to adequately increase its rate in response to exertion (chronotropic incompetence), cardiac output fails to meet the body's demands. This clinically manifests as symptoms of systemic and cerebral hypoperfusion.
Sick sinus syndrome (SSS)
Bradycardia can be a manifestation of SSS — a persistent suppression of automaticity where the node is unable to maintain a rhythm adequate for life. The morphological basis of the syndrome is dystrophy and fibrosis in the SA node area, as well as pronounced autonomic imbalance.
The most informative ECG markers of SSS:
- Tachy-brady syndrome: persistent or intermittent sinus bradycardia alternating with tachycardia.
- Episodes of sinoatrial block (sinus arrest).
- Slow recovery of normal rhythm after termination of a tachycardia attack.
- Transient atrial fibrillation or atrial flutter.
Pathophysiologically, SSS leads to decreased automaticity and activation of subsidiary ectopic rhythms (atrial or junctional).
Clinical features in children
In children, rhythm disorders are often based on myocardial instability and depletion of sympathoadrenal regulation reserves. The predisposing background may include a complicated perinatal period, frequent acute respiratory viral infections, foci of chronic infection, and a burdened family history.
The clinical presentation has distinct specificities:
- Latent course: about 40% of arrhythmias are detected incidentally during ECG screening or after an infection.
- Scarcity of complaints: children rarely feel palpitations or heart "fluttering".
- Age-related changes: during prepuberty and puberty, psychovegetative disorders lead to cardialgia, insomnia, weather sensitivity, and increased excitability.
- Dangerous symptoms: hemodynamically significant bradycardia can lead to reduced cerebral blood flow, causing weakness, dizziness, and syncopal states (fainting).
The problem can be suspected during a physical examination: pulse palpation reveals a slowed rhythm, while auscultation and blood pressure measurement help rule out primary organic heart disease.
Criteria for sinus bradycardia — the "Three P" rule: Low rate (less than 60), Regular rhythm, Present normal P wave.
Class and exam questions
Why is the sinus node activity not directly visible on an ECG?
There are too few atypical cardiomyocytes in the sinus node. They do not create an electromagnetic field of sufficient strength to be recorded by surface electrodes, so the diagnosis is made based on the P wave and R-R intervals.
How does sinus bradycardia differ from escape rhythms?
In sinus bradycardia, the pacemaker remains in the SA node (normal P wave preserved). In escape rhythms, the impulse originates from the atria, AV junction, or ventricles, which changes the shape and polarity of the P wave (or hides it) and may deform the QRS.
Is bradycardia dangerous without symptoms?
Isolated bradycardia detected on 24-hour monitoring, in the absence of complaints and syncopal episodes, has no clinical significance.
What else to study on this topic
- Sinoatrial blocks (SAB) of various degrees and their ECG picture.
- Heterotopic (ectopic) rhythms: atrioventricular and idioventricular.
- Migration of the supraventricular pacemaker and escape beats.
- Sinus arrhythmia: criteria for R-R interval instability.
- Differential diagnosis of non-physiological sinus tachycardia.
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