What is the characteristic rhythm of atrial fibrillation on an electrocardiogram (ECG) tracing?

ECG Characteristics of Atrial Fibrillation

Atrial fibrillation is characterized on ECG by absolutely irregular RR intervals, absence of distinct P waves (replaced by fibrillatory waves), and when visible, atrial cycle length <200 ms (≥300 bpm). 1

Defining ECG Features

The three cardinal ECG characteristics that define atrial fibrillation are:

• Absolutely irregular RR intervals that do not follow any repetitive pattern, sometimes called "arrhythmia absoluta" 1
• Absence of distinct P waves on the surface ECG, replaced by rapid oscillations or fibrillatory waves that vary in amplitude, shape, and timing 1
• Variable atrial cycle length (when visible) that is usually <200 ms, corresponding to atrial rates ≥300 bpm 1

Fibrillatory Wave Characteristics

• The fibrillatory waves represent chaotic, uncoordinated atrial electrical activity rather than organized atrial depolarization 2
• Some apparently regular atrial electrical activity may be visible in certain ECG leads, most often in lead V1, but this does not represent true organized P waves 1
• The irregular ventricular response occurs when AV conduction is intact and depends on AV nodal properties, vagal and sympathetic tone, presence of accessory pathways, and drug effects 1

Differential Diagnosis Considerations

Atrial flutter must be distinguished from atrial fibrillation, as flutter shows:

• Saw-tooth pattern of regular atrial activation (flutter waves) particularly visible in leads II, III, aVF, and V1 1
• Atrial cycle length ≥200 ms (atrial rate 240-320 bpm) 1
• No isoelectric baseline between deflections in typical flutter 1

Common pitfall: When atrial activity is prominent on ECG in more than one lead, AF may be misdiagnosed as atrial flutter 1

Diagnostic Requirements

• A 12-lead ECG of sufficient duration and quality is necessary to properly evaluate atrial activity and confirm the diagnosis 1, 2
• Any arrhythmia with ECG characteristics of AF lasting ≥30 seconds on a rhythm strip should be considered AF 1
• When ventricular rate is fast, AV nodal blockade during Valsalva maneuver, carotid massage, or intravenous adenosine can help unmask atrial activity 1

Clinical Context Answers

Question 1 (Rhythm identification): The correct answer is C. Atrial fibrillation based on the characteristic features described above 1

Question 2 (ECG tracing): Without seeing the actual tracings, the correct ECG would show irregular RR intervals with absent P waves replaced by fibrillatory waves 1, 2

Question 3 (True statement about AF): The correct answer is c. It can be precipitated by a high sympathetic drive 1. The incidence increases (not decreases) with age 3, hyperthyroidism (not hypothyroidism) is a common cause, and elderly patients rarely have spontaneous resolution without treatment.

Question 4 (Complication): The correct answer is b. Cardio-embolic stroke 2. The absence of P waves and loss of coordinated atrial contraction leads to blood stasis and thrombus formation, primarily in the left atrial appendage, significantly increasing stroke risk 2

Question 5 (ARISTOTLE trial): The correct answer is d. Apixaban was superior to warfarin in preventing stroke or systemic embolism, caused less bleeding, and resulted in lower mortality - this represents the actual findings of the ARISTOTLE trial demonstrating apixaban's superiority across multiple outcomes.

Question 6 (2024 ESC Guidelines): The correct answer is d. Direct oral anticoagulants are recommended in preference to VKAs in eligible patients with AF undergoing cardioversion for thromboembolic risk reduction - this aligns with current guideline recommendations favoring DOACs over vitamin K antagonists.