5.3 Atrial fibrillation

Atrial Fibrillation — the irregularly irregular rhythm

Atrial fibrillation (Afib, AF) is the most common sustained arrhythmia in clinical practice, and recognizing it is a core CRAT skill. In Afib, the organized atrial contraction driven by a single SA-node impulse is replaced by multiple chaotic re-entrant wavelets firing continuously from hundreds of sites throughout the atria. The atria no longer contract as a unit — they quiver (fibrillate). Electrically, this produces the two defining features of the rhythm.

The two hallmarks

1. No discernible P waves. Because there is no single organized atrial depolarization, there are no true P waves. Instead the baseline shows chaotic, low-amplitude fibrillatory waves (f waves) — irregular, wavy undulations of continuously changing shape, size, and timing. The atrial rate is 350–600/min, far too fast and disorganized to count. Sometimes the f waves are coarse and easy to see; sometimes they are so fine the baseline looks nearly flat.

2. An irregularly irregular ventricular rhythm. The AV node is bombarded by these chaotic impulses and conducts them at random, whenever it happens not to be refractory. The result is R–R intervals that follow no pattern at all — the "irregularly irregular" rhythm that is the single most useful bedside clue to Afib. There is no repeating cycle, no group beating, and no way to predict the next beat.

Conduction below the AV node is normal, so the QRS is narrow (≤ 0.12 s) unless there is a pre-existing bundle branch block or aberrant conduction.

Ventricular response: controlled vs. rapid

Because the ventricular rate depends on how many impulses the AV node lets through, Afib is classified by that rate:

• Controlled ventricular response — ventricular rate < 100/min (roughly 60–100). The AV node is limiting conduction adequately, often with the help of rate-control medication.
• Rapid ventricular response (RVR) — ventricular rate > 100/min, often 110–180. Many impulses are conducting through, and the fast, inefficient rhythm can compromise cardiac output.
• A persistently slow ventricular response (< 60/min) may indicate AV-nodal disease or medication effect.

The CRAT technician should always report Afib together with its ventricular rate — "atrial fibrillation with RVR at 140" communicates urgency that "atrial fibrillation" alone does not.

Causes

Atrial fibrillation is associated with:

• Hypertension and coronary artery disease (the most common substrates)
• Heart failure and cardiomyopathy
• Valvular disease, especially mitral valve disease, and atrial enlargement
• Hyperthyroidism
• Pulmonary disease and pulmonary embolism
• Alcohol excess ("holiday heart"), sepsis, and post-cardiac-surgery states
• Advancing age

Why the rhythm — and the rate — matter clinically

Afib carries two major hazards that make its recognition important.

Thromboembolism and stroke. Because the atria only quiver instead of contracting, blood stagnates — particularly in the left atrial appendage — and can form a thrombus. If a clot dislodges, it can travel to the brain and cause an embolic stroke. Afib is one of the leading cardiac causes of stroke, which is why patients are risk-stratified (for example with CHA₂DS₂-VASc) and are frequently placed on anticoagulation. Loss of the atrial "kick" also reduces ventricular filling and can drop cardiac output by 10–30%, worsening heart-failure symptoms.

Rate. A sustained rapid ventricular response shortens diastolic filling time, increases myocardial oxygen demand, and can cause palpitations, hypotension, angina, or, over time, a tachycardia-induced cardiomyopathy. That is why rate (and rhythm) control are therapeutic priorities.

Clinical classification and a caution

Clinically, Afib is described by duration: paroxysmal (self-terminating, usually within 7 days), persistent (lasting beyond 7 days or requiring cardioversion to stop), and permanent (accepted as the long-term rhythm). These labels do not change the strip's appearance but often appear as exam distractors. One important caution: when a short R–R interval follows a longer one, a beat may conduct with aberrancy and look wide — the Ashman phenomenon — so a lone wide complex in Afib is more often aberrant conduction than a true PVC. Conversely, if the ventricular rhythm in a known Afib patient becomes regular, suspect complete AV block with a junctional or ventricular escape rhythm taking over.

Distinguishing Afib from other irregular rhythms

Several rhythms look irregular, and the CRAT exam frequently tests telling them apart:

The decision tree is straightforward: if the rhythm is irregular, look at the baseline. Uniform sawtooth waves at ~300 = flutter; three or more different P shapes with a P before every QRS = MAT; normal identical P before every QRS that speeds up and slows with breathing = sinus arrhythmia; and a chaotic, wavy baseline with no organized P waves and truly random R–R intervals = atrial fibrillation. The absence of any organized atrial activity combined with an irregularly irregular ventricular response is the fingerprint of Afib.