How can I differentiate atrial flutter from sinus tachycardia on an electrocardiogram?

Differentiating Atrial Flutter from Sinus Tachycardia on ECG

Look for the presence or absence of discrete P waves with an isoelectric baseline between them—sinus tachycardia shows distinct P waves separated by a flat baseline in all leads, while atrial flutter displays a characteristic saw-tooth pattern of continuous flutter waves without any isoelectric baseline, particularly visible in leads II, III, aVF, and V1. 1

Key ECG Features to Examine

Atrial Flutter Characteristics

• Saw-tooth pattern: Atrial flutter produces regular flutter (ƒ) waves creating a continuous undulation without an isoelectric baseline between deflections, most prominently visible in leads II, III, aVF, and V1 1

• Atrial rate: The atrial rate typically ranges from 240 to 320 beats per minute in untreated atrial flutter 1

• Flutter wave morphology: In typical atrial flutter, ƒ waves are inverted (negative) in leads II, III, and aVF, and upright in lead V1 1

• Ventricular rate pattern: Atrial flutter commonly presents with 2:1 AV block, resulting in a regular ventricular rate of 120-160 beats per minute, most characteristically around 150 beats per minute 1

• Reverse typical flutter: The circuit direction can reverse, producing upright ƒ waves in leads II, III, and aVF and inverted waves in lead V1 1

Sinus Tachycardia Characteristics

• Discrete P waves: Sinus tachycardia displays clearly identifiable P waves that are separated by an isoelectric (flat) baseline in one or more ECG leads 1

• P wave morphology: P waves maintain normal sinus morphology—upright in leads I, II, and aVF, and typically biphasic in V1 1

• Rate range: Heart rate is elevated above 100 beats per minute but typically remains below 150-160 beats per minute in adults at rest 2

• Gradual rate changes: Sinus tachycardia demonstrates gradual acceleration and deceleration in response to physiological demands, unlike the abrupt onset/offset often seen with atrial flutter 2

Diagnostic Algorithm

Step 1: Identify Atrial Activity Pattern

• Examine leads II, III, aVF, and V1 specifically for the presence or absence of an isoelectric baseline between atrial deflections 1

• If continuous undulation without flat baseline is present → suspect atrial flutter 1

• If discrete P waves with flat baseline between them are present → suspect sinus tachycardia 1

Step 2: Measure Atrial Rate

• Calculate the atrial rate by measuring the interval between consecutive atrial deflections 1

• Atrial rates of 240-320 bpm strongly suggest atrial flutter 1

• Atrial rates typically <200 bpm favor sinus tachycardia, though overlap can occur 2

Step 3: Assess Ventricular Response Pattern

• Determine if the ventricular rhythm is regular or irregular 1

• A regular ventricular rate around 150 bpm (suggesting 2:1 conduction) is highly characteristic of atrial flutter 1

• Variable ventricular response with changing AV conduction ratios (2:1,3:1,4:1) also indicates atrial flutter 1

Step 4: Evaluate P Wave Morphology

• In sinus tachycardia, P waves should maintain normal sinus morphology with consistent axis 1

• In atrial flutter, the saw-tooth flutter waves have characteristic negative deflections in inferior leads (II, III, aVF) for typical counterclockwise flutter 1

Common Pitfalls and How to Avoid Them

• Misdiagnosing atrial flutter as sinus tachycardia: When atrial flutter presents with 4:1 or higher AV block, the ventricular rate may be normal and flutter waves less obvious—always carefully examine leads II, III, and aVF for subtle saw-tooth patterns 1

• Confusing atrial flutter with atrial fibrillation: When atrial activity is prominent on ECG in multiple leads, atrial fibrillation may be misdiagnosed as atrial flutter—look for the regularity of atrial deflections (regular in flutter, irregular in fibrillation) 1

• Overlooking 2:1 atrial flutter: The classic presentation of atrial flutter with 2:1 block produces a ventricular rate around 150 bpm, which can be mistaken for sinus tachycardia if flutter waves are not carefully identified 1

• QRS complexes obscuring flutter waves: Use vagal maneuvers or adenosine to temporarily slow AV conduction and unmask flutter waves hidden within QRS complexes 1

Additional Diagnostic Maneuvers

• Carotid sinus massage or adenosine administration: These interventions transiently increase AV block, slowing the ventricular rate and unmasking flutter waves that may be partially hidden by QRS complexes 1

• Examine multiple leads simultaneously: Flutter waves may be most prominent in certain leads while barely visible in others—always review all 12 leads 1

• Look for rate consistency: Sinus tachycardia typically shows some beat-to-beat variability in response to respiration and autonomic tone, while atrial flutter maintains a more constant atrial rate 2