In a 57-year-old man with a 24‑hour history of palpitations, an electrocardiogram showing a heart rate of 140 beats per minute, irregular RR intervals, and absent P waves, what is the most likely diagnosis?

Diagnosis: Atrial Fibrillation

The most likely diagnosis is A) Atrial Fibrillation. The clinical presentation of irregular RR intervals, heart rate of 140 bpm, and absent P waves on ECG is pathognomonic for atrial fibrillation 1.

Diagnostic Reasoning

Classic ECG Criteria for Atrial Fibrillation

The patient's ECG findings meet all three defining characteristics of AF as established by the European Society of Cardiology 1:

1. Absolutely irregular RR intervals - RR intervals that do not follow any repetitive pattern (sometimes called "arrhythmia absoluta")
2. Absence of distinct P waves on the surface ECG
3. Rapid ventricular response - heart rate of 140 bpm is consistent with AF when AV conduction is intact

The ACC/AHA/ESC guidelines further confirm that AF is characterized by "rapid oscillations, or fibrillatory waves that vary in amplitude, shape, and timing, replace consistent P waves, and there is an irregular ventricular response that is rapid when conduction is intact" 2, 3.

Why Other Diagnoses Are Excluded

B) Paroxysmal Supraventricular Tachycardia (PSVT)

• PSVT typically presents with regular RR intervals, not irregular
• P waves are usually present (though may be buried in QRS or T waves)
• The irregularity of RR intervals rules this out

C) Polymorphic Ventricular Tachycardia

• Would show wide QRS complexes with varying morphology
• The question doesn't mention wide QRS or changing QRS morphology
• Ventricular rates >200 bpm would suggest VT or accessory pathway 2

D) Myocardial Infarction

• MI is not a rhythm diagnosis
• While MI can precipitate AF, the ECG findings described are rhythm-specific, not ischemic changes

E) Acute Pericarditis

• Pericarditis typically shows diffuse ST elevation and PR depression
• Does not cause the specific rhythm pattern described

Clinical Context

The 24-hour duration of symptoms suggests this could be classified as first-detected AF or paroxysmal AF (if it terminates within 7 days) 4, 5. At 57 years old, this patient requires:

• Immediate rate control if hemodynamically stable (beta-blockers or non-dihydropyridine calcium channel antagonists preferred) 2
• Stroke risk assessment using CHA₂DS₂-VASc score to determine anticoagulation needs
• Consideration of rhythm control vs rate control strategy
• Evaluation for underlying causes (thyroid disease, structural heart disease, pulmonary disease)

Important Caveat

Pre-excitation syndrome (Wolff-Parkinson-White) can present with AF and irregular wide QRS tachycardia 6. If the QRS is wide and irregular with rates >200 bpm, this suggests AF with accessory pathway conduction, which is a medical emergency requiring different management (avoid AV nodal blocking agents like diltiazem, verapamil, or digoxin as they can accelerate conduction through the accessory pathway) 1.

However, based on the information provided (heart rate 140, irregular RR intervals, absent P waves), standard atrial fibrillation remains the most likely diagnosis 1, 4.