Arrhythmias - classification, etiology, main pathogenetic mechanisms.
Arrhythmias (or dysrhythmias) are abnormalities of heart rhythm, involving disturbances in impulse formation, conduction, or both.
Classification
- 1 By heart rate
- 2 By origin
- 3 By conduction disturbance
Etiology
Intrinsic cardiac causes
- Ischemic heart disease (e.g., after myocardial infarction).
- Cardiomyopathies (e.g., dilated or hypertrophic).
- Valvular heart diseases.
- Myocarditis.
- Degenerative changes in the conduction system (fibrosis, sclerosis).
Extrinsic (secondary) causes
- Electrolyte imbalances ( hypokalemia, hyperkalemia, hypomagnesemia).
- Drugs (antiarrhythmics, digoxin, beta- blockers, caffeine, alcohol).
- Hypoxia, hypercapnia.
- Thyroid dysfunction (especially hyperthyroidism → tachyarrhythmias).
- Increased sympathetic activity (stress, fever).
Main pathogenetic mechanisms
- 1 Disorders of impulse formation (automaticity)
- Enhanced automaticity:
- Increased spontaneous depolarization rate in pacemaker cells (e.g., sinus tachycardia).
- Ectopic foci in atria or ventricles (e.g., atrial premature beats).
- Reduced automaticity:
- SA node dysfunction → sinus bradycardia or sinus arrest.
- 2 Triggered activity (afterdepolarizations)
- Early afterdepolarizations (EADs):
- Occur during repolarization (phase 2 or 3).
- Can lead to torsades de pointes (especially with prolonged QT).
- Delayed afterdepolarizations (DADs):
- Occur after full repolarization (phase 4).
- Common with calcium overload (e.g., digoxin toxicity).
- 3 Disorders of impulse conduction
- Re-entry circuits (most common cause of tachyarrhythmias):
- An impulse re- excites the same area repeatedly, creating a loop.
- Example: AV nodal reentrant tachycardia (AVNRT), atrial flutter.
- Conduction blocks:
- Complete or partial interruption of impulse (e.g., AV block).
- Accessory pathways: