Atrial Flutter
The most likely diagnosis for an ECG showing a sawtooth pattern of P waves with tachycardia is atrial flutter. This characteristic ECG finding is pathognomonic for this arrhythmia and distinguishes it from other supraventricular tachycardias.
Diagnostic ECG Features
Atrial flutter in its typical form is definitively characterized by a sawtooth pattern of regular atrial activation called flutter (ƒ) waves on the ECG, particularly visible in leads II, III, aVF, and V1 1. This distinctive pattern represents organized macroreentrant atrial activity, unlike the irregular fibrillatory waves seen in atrial fibrillation 1.
Key ECG Characteristics:
Atrial rate: Typically ranges from 240 to 320 beats per minute in the untreated state 1
Flutter wave morphology in typical (counterclockwise) atrial flutter:
Ventricular rate: Most characteristically about 150 beats per minute due to 2:1 AV block, though can range from 120-160 bpm with regular or irregular ventricular response 1
Mechanism and Classification
Typical atrial flutter results from macroreentrant tachycardia propagating around the tricuspid annulus in the right atrium 1. The circuit proceeds superiorly along the atrial septum, inferiorly along the right atrial wall, and through the cavotricuspid isthmus between the tricuspid valve annulus and the Eustachian valve 1.
Reverse Typical Flutter:
The direction of activation in the right atrium may be reversed (clockwise), resulting in flutter waves that are upright in leads II, III, and aVF and inverted in lead V1 1.
Differential Diagnosis Considerations
A critical pitfall is misdiagnosing atrial fibrillation as atrial flutter when atrial activity is prominent on the ECG in more than one lead 1. However, atrial fibrillation is characterized by irregular fibrillatory waves that vary in amplitude, shape, and timing, with a completely irregular ventricular response—not the organized sawtooth pattern seen in flutter 1.
Distinguishing Features:
- Atrial flutter: Regular sawtooth flutter waves without isoelectric baseline, organized atrial activity 1
- Atrial fibrillation: Irregular fibrillatory waves, completely irregular R-R intervals 1
- Focal atrial tachycardia: Discrete P waves separated by isoelectric baseline in one or more leads 1
The ECG pattern may fluctuate between atrial flutter and atrial fibrillation, as atrial flutter may degenerate into AF and vice versa 1. This reflects changing patterns of atrial activation 1.
Clinical Significance
Atrial flutter commonly occurs with 2:1 AV block, which is why the ventricular rate is characteristically around 150 bpm (half of the typical 300 bpm atrial rate) 1. This regular ventricular response at approximately 150 bpm in the presence of sawtooth flutter waves is highly suggestive of the diagnosis 1.
Atrial flutter may arise during treatment with antiarrhythmic agents prescribed to prevent recurrent atrial fibrillation 1, making it important to recognize this pattern in patients on antiarrhythmic therapy.