Atrial Fibrillation
Atrial fibrillation is an irregularly irregular rhythm characterized by chaotic atrial electrical activity and absent organized P waves.
Also known as: AFib, AF, A-Fib
ECG Characteristics
| Heart Rate | Variable; uncontrolled 100–180 bpm, controlled 60–100 bpm |
| Rhythm | Irregularly irregular |
| P Wave | Absent; replaced by fibrillatory (f) waves — rapid, irregular, low-amplitude oscillations |
| PR Interval | Not measurable (no organized P waves) |
| QRS Duration | < 0.12 seconds (unless aberrant conduction) |
Mechanism
Multiple chaotic re-entrant wavelets circulate through the atria, creating disorganized atrial depolarization at 350–600 impulses/minute. The AV node acts as a gatekeeper, allowing only some impulses through.
Key Features on ECG
- Irregularly irregular R-R intervals
- Absence of distinct P waves
- Fibrillatory (f) waves — irregular baseline undulations
- Narrow QRS unless pre-existing bundle branch block
- Variable ventricular rate
Causes
- Hypertension
- Valvular heart disease (especially mitral)
- Heart failure
- Hyperthyroidism
- Alcohol use (holiday heart syndrome)
- Pulmonary embolism
- Post-cardiac surgery
- Lone AFib (no identifiable cause)
Clinical Significance
AFib is the most common sustained arrhythmia. It increases stroke risk 5-fold due to atrial stasis and thrombus formation. Management focuses on rate or rhythm control and anticoagulation for stroke prevention.
Frequently Asked Questions
Why does atrial fibrillation increase stroke risk?
In AFib, the atria quiver rather than contract effectively. This causes blood to pool, particularly in the left atrial appendage, where clots can form. If a clot dislodges and travels to the brain, it causes an ischemic stroke. Anticoagulation therapy reduces this risk by approximately 60–70%.
What is the difference between rate control and rhythm control in AFib?
Rate control aims to keep the ventricular rate in a normal range (typically < 110 bpm) while allowing AFib to continue, using medications like beta-blockers or calcium channel blockers. Rhythm control aims to restore and maintain normal sinus rhythm using antiarrhythmic drugs or catheter ablation.
Is atrial fibrillation common in dogs?
AFib is the most common clinically significant arrhythmia in large and giant breed dogs. It is often associated with dilated cardiomyopathy or advanced valvular disease. In giant breeds, lone AFib without structural heart disease can occur. It is rare in cats.
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