What does an ECG (electrocardiogram) of atrial flutter with a 2:1 block look like?

ECG Characteristics of Atrial Flutter with 2:1 Block

Atrial flutter with 2:1 AV block presents with a characteristic saw-tooth pattern of flutter waves (particularly visible in leads II, III, aVF, and V1) at an atrial rate of 240-320 bpm, resulting in a regular ventricular rate most characteristically around 150 beats per minute. 1

Key ECG Features

Flutter Wave Morphology in Typical Atrial Flutter

• Saw-tooth pattern without an isoelectric baseline between deflections, most prominent in the inferior leads 1

• In counterclockwise (common) typical flutter:

  • Flutter waves are inverted (negative) in leads II, III, and aVF 1
  • Flutter waves are upright (positive) in lead V1 1
  • The pattern shows a slow descending/fast ascending F wave morphology 2

• In clockwise (reverse) typical flutter:

  • Flutter waves are upright in leads II, III, and aVF 1
  • Flutter waves are inverted in lead V1 1
  • Shows a balanced ascending/descending waveform 2

Rate Characteristics with 2:1 Block

• Atrial rate: 240-320 beats per minute in the untreated state 1

• Ventricular rate with 2:1 conduction: 120-160 beats per minute, most characteristically approximately 150 bpm 1

• The ventricular rhythm can be regular or irregular despite the 2:1 block 1

Clinical Recognition Pitfalls

Common Diagnostic Challenges

• The 2:1 block can obscure flutter waves, making them harder to identify because every other flutter wave may be buried in the QRS complex or T wave 1

• When you see a regular narrow-complex tachycardia at approximately 150 bpm, always suspect atrial flutter with 2:1 block until proven otherwise 1

• Variable AV conduction (alternating between 2:1,3:1, or 4:1 block) may make the flutter waves more apparent as the ventricular rate slows and unmasks the underlying atrial activity 1

Distinguishing from Other Arrhythmias

• Unlike atrial fibrillation, atrial flutter shows organized, regular atrial activity rather than chaotic fibrillatory waves 1

• Unlike other atrial tachycardias, flutter waves have no isoelectric baseline between deflections in the inferior leads 1, 2

• Atrial flutter may be misdiagnosed as AF when atrial activity is prominent on the ECG in more than one lead 1

Practical Diagnostic Approach

To unmask hidden flutter waves in suspected 2:1 block:

• Perform vagal maneuvers or give adenosine to transiently increase AV block, which will slow the ventricular rate and reveal the underlying flutter waves more clearly 1

• Examine leads II, III, aVF, and V1 most carefully, as these are where flutter waves are typically most visible 1

• Look for the characteristic ventricular rate of ~150 bpm as a clue to 2:1 conduction of flutter 1