What is Arrhythmia?
An arrhythmia is an abnormal heart rhythm characterized by irregular electrical activity of the heart, which can manifest as a heart rate that is too slow (bradycardia, <60 beats/min), too fast (tachycardia, >100 beats/min), or irregular in pattern. 1
Core Definition and Mechanisms
Arrhythmias arise from three primary electrophysiologic mechanisms 2:
Enhanced automaticity: Ectopic pacemaker cells in the atria, AV junction, or pulmonary veins exhibit accelerated phase 4 depolarization, firing faster than the sinus node and overriding normal pacemaker function 2
Triggered activity: Abnormal impulses generated by afterdepolarizations during repolarization that reach threshold and trigger premature action potentials 2
Re-entry (most common): Repetitive excitation occurs when an electrical impulse conducts around a fixed obstacle in a defined circuit, requiring unidirectional conduction block in one pathway and slow conduction to maintain the circuit 2
Clinical Manifestations
Arrhythmias present with a wide spectrum of symptoms or may be completely asymptomatic 2:
- Palpitations: Described as pauses, strong heartbeats, or irregularities in rhythm 2
- Hemodynamic symptoms: Fatigue, lightheadedness, chest discomfort, dyspnea 2
- Severe presentations: Presyncope, syncope (occurs in ~15% of supraventricular tachycardia patients), or sudden cardiac death 2, 3
Specific Example: Atrial Fibrillation
The ACC/AHA defines atrial fibrillation as "an arrhythmia characterized by uncoordinated atrial myocardium due to multiple reentry circuits with consequent deterioration of atrial mechanical function. Instead of intermittently contracting, the atria quiver continuously in a chaotic pattern, causing a totally irregular often tachycardic ventricular rate." 2
Pattern Recognition
The regularity or irregularity of the rhythm provides critical diagnostic clues 2:
- Irregular rhythms: Suggest premature depolarizations, atrial fibrillation, or multifocal atrial tachycardia 2
- Regular paroxysmal rhythms (sudden onset/termination): Typically indicate AV reentrant tachycardia or AV nodal reentrant tachycardia 2
- Gradual onset/termination: Characteristic of sinus tachycardia rather than true arrhythmia 2
Critical Diagnostic Principle
A 12-lead ECG during symptoms is essential for definitive diagnosis—an irregular pulse raises suspicion but cannot confirm the specific arrhythmia type without electrocardiographic documentation. 4