Can atrial fibrillation (afib) be present in certain electrocardiogram (ECG) leads while normal sinus rhythm is seen in others?

Atrial Fibrillation Cannot Exist in Some Leads While Normal Sinus Rhythm Exists in Others

No, atrial fibrillation (AF) cannot be present in certain ECG leads while normal sinus rhythm appears in others—AF is a global atrial rhythm that affects all leads simultaneously. When AF is present, the entire atrial myocardium is fibrillating, which means all ECG leads will show the characteristic absence of organized P waves and presence of fibrillatory waves, though the amplitude and visibility of these waves may vary significantly between leads 1.

Understanding the ECG Appearance of AF Across Leads

The key distinction is between AF being present versus being visible in different leads:

• AF is characterized by rapid oscillations or fibrillatory waves that replace consistent P waves across the entire 12-lead ECG, with an irregularly irregular ventricular response when AV conduction is intact 1
• Fibrillatory waves vary in amplitude, shape, and timing, which means they may be more prominent and easily recognizable in some leads (particularly V1) while appearing less obvious or nearly isoelectric in others 1
• The diagnosis requires ECG documentation showing the absence of organized atrial activity and irregular R-R intervals across the recording 1

Common Diagnostic Pitfall: Prominent Atrial Activity Can Mimic Other Rhythms

A critical caveat is that when atrial fibrillatory activity is prominent on the ECG in more than one lead, AF may be misdiagnosed as atrial flutter:

• Atrial flutter shows a characteristic saw-tooth pattern of regular atrial activation (flutter waves) particularly visible in leads II, III, aVF, and V1 1
• When fibrillatory waves in AF are prominent across multiple leads, they can create confusion with the organized flutter waves of atrial flutter 1
• The ECG pattern may actually fluctuate between atrial flutter and AF in the same patient, reflecting changing activation patterns of the atria, but this represents conversion between two distinct rhythms rather than simultaneous presence of both 1

Lead-Specific Visibility Does Not Equal Lead-Specific Presence

The varying appearance of fibrillatory waves across leads reflects differences in the electrical vector relationship to each lead, not the presence or absence of the arrhythmia:

• Lead V1 typically shows fibrillatory waves most prominently because of its anatomic relationship to the atria 1
• Some leads may show fibrillatory waves that are so low in amplitude they appear nearly flat, potentially creating the false impression of an isoelectric baseline 1
• The irregularly irregular ventricular response (R-R intervals) remains the most reliable diagnostic feature across all leads when AV conduction is intact 1

Special Circumstances That May Confuse the Diagnosis

Regular R-R intervals can occur in AF under specific conditions, but this does not indicate normal sinus rhythm in those leads:

• Complete AV block with AF results in regular ventricular escape rhythm despite ongoing atrial fibrillation 1
• Ventricular or AV junctional tachycardia occurring simultaneously with AF produces regular R-R intervals 1
• In patients with implanted pacemakers, the diagnosis may require temporary pacemaker inhibition to expose the underlying atrial fibrillatory activity 1

Diagnostic Confirmation Requirements

For definitive diagnosis, AF must be documented on at least a single-lead ECG recording or 30 seconds on a rhythm strip showing the characteristic features throughout the recording:

• The entire recording must demonstrate absence of organized P waves and presence of fibrillatory waves (even if subtle) 1
• The ventricular response must be irregularly irregular unless one of the special circumstances above is present 1
• A standard 12-lead ECG recorded at 25 mm/s for 10 seconds provides sufficient documentation across all leads simultaneously 1