ECG Blog #138 (SVT – AV Block – Atrial Flutter – Atrial Tachycardia – Digoxin)

How would you interpret the rhythm in Figure-1? What is your differential diagnosis? Can you be sure of your answer from looking at this lead MCL-1 rhythm strip?

• Why is it important to know if this patient is on Digoxin?

Figure-1: Lead MCL1 rhythm strip. Is this AFlutter or ATach? 

Interpretation: The rhythm in Figure-1 is regular with a ventricular rate of ~115/minute. P waves outnumber QRS complexes by two to one (Figure-2) — making the atrial rate ~230/minute. The QRS complex is narrow, implying a supraventricular mechanism — and each QRS complex is preceded by a P wave with a constant PR interval. Thus P waves are related to the QRS complexes, albeit only one of every two P waves is conducted to the ventricles. Therefore, this is an SVT (SupraVentricular Tachycardia) rhythm with 2:1 AV conduction. The differential diagnosis is between AFlutter (Atrial Flutter) vs ATach (Atrial Tachycardia).

Figure-2: We have labeled Figure-1 by adding red arrows for the regular atrial activity. There are 2 P waves for each QRS complex (See text).

Diagnostic Considerations:

• In favor of AFlutter — is regular and rapid atrial activity with a peaked upward deflection in this right-sided MCL-1 monitoring lead. That said, the atrial rate of 230/minute is a bit below the usual atrial rate range for untreated atrial flutter (of 250-350/minute) — and, the expected “sawtooth” pattern of atrial flutter is missing in this lead.
• In favor of ATach — is the atrial rate (below 250/minute) and, the isoelectric baseline (rather than sawtooth) in this lead.
• Note: We are not told if this patient is taking an antiarrhythmic agent (such as flecainide, amiodarone, sotalol, etc.) that might slow the atrial rate of flutter. We are also not told if this patient is taking Digoxin. This is important because SVT with 2:1 conduction in a patient taking Digoxin should strongly suggest the possibility of digitalis toxicity. Given greatly reduced use of this drug at the current time — atrial tachycardia with block due to digitalis toxicity is no longer commonly seen — but it remains important to inquire about this medication since you may occasionally encounter patients who are still taking Digoxin.

Bottom Line: It is impossible to be certain of the rhythm diagnosis in Figure-2 from this single rhythm strip without the benefit of additional information (ie, previous clinical history; knowing what medications the patient is taking, etc.). Seeing a full 12-lead ECG might help by revealing a typical sawtooth pattern in other leads. That said, the clinical reality is that neither rate nor baseline appearance (sawtooth vs isoelectric baseline) have been shown to reliably distinguish between ATach vs AFlutter. Fortunately, from a non-cardiologist’s perspective — both initial and long-term management of these two SVT rhythms is similar (once you have ruled out the possibility of digitalis toxicity).  Initial efforts entail slowing the ventricular response, with consideration of EP = electrophysiology referral if the arrhythmia is persistent or recurs.

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Clinical Note: Assuming this patient is not on Digoxin — the terminology used to describe the arrhythmia seen in Figure-1 is far less important than the overall clinical concepts involved. This is because what used to be classified as “atrial tachycardia” in non-Digoxin toxic patients is now often referred to as an “atypical” form of AFlutter.

• Included within the broad category of “atypical” AFlutter rhythms are various types of atrial tachycardias that may arise from anywhere within the atria or neighboring pulmonary veins.
• Some atrial tachycardias may be “focal” or automatic (often recognizable by non-sinus P wave appearance — “warm up” phenomenon until the ectopic tachycardia is established — relatively slower rate — and on occasion slightly variable P-P intervals).
• Other atrial tachycardias may be much faster, perfectly regular, lack an isoelectric baseline — and be clinically indistinguishable from AFlutter based on ECG appearance. That said, since practically speaking both persistent ATach as well as persistent AFlutter are indications for EP referral — definitive diagnosis from the initial ECG is not essential.

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For More on this Subject:

• See also ECG Blog #40 — for review of a relevant SVT case.

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