The differential for narrow complex tachycardia is extremely important as it is the most commonly seen abnormal EKG in the emergency department. It includes rhythms such as sinus tachycardia, AVnRT, AVRT, atrial flutter, ectopic atrial tachycardia (EAT), atrial fibrillation, atrial flutter, and multifocal atrial tachycardia (MAT).
The goal of this blog is to run through this differential and give some methods to differentiate the rhythms. Although we will not delve too deep into antiarrhythmics, it is important to note that electricity is safe in all unstable rhythms no matter the etiology.
There are several different ways to break down the differential for narrow complex tachycardia. The one that I find the most useful is breaking them into regular and irregular rhythms. Furthermore, the regular rhythms can be broken down into “AV Nodal dependent” and “AV nodal independent” rhythms, which can help us remember the treatments as well.
Regular AV Nodal Independent Rhythms
Just a quick review that sinus is defined as a QRS complex preceded by a P wave. Sinus is the default rhythm of the heart and originates from the SA node. Sinus tachycardia is often caused by a systemic issue (e.g. fever, hypovolemia, PE, stress/anxiety, exercise), and not due to a cardiac issue . Therefore, the treatment revolves around the treatment of the underlying cause.
Atrial flutter can come in a few different flavors (e.g. 2:1, 3:1, 4:1, and variable block). These refer to number of P waves preceding each QRS waves. For instance atrial flutter with 2:1 AV block (most common type) will have two P waves before QRS wave (seen below). The big thing to look for is that the P waves are “marching out” meaning that the P waves are consistently occurring without much variation.
In 2:1 block, it can be difficult to differentiate AFlutter as occasionally the P waves will become buried in the QRS, and it may resemble AVnRT (SVT). It is not harmful to give these patients adenosine as it will simply block the conduction of P waves (though they will feel awful during this time). The EKG below shows a patient where AVnRT and AFlutter could not be differentiated and was given adenosine. We can see that the P waves continue to march out (confirming AFlutter as diagnosis) while the QRS complexes stop. Luckily, adenosine only lasts 10-15 seconds, and ventricular contractions will start shortly after this.
Ectopic Atrial Tachycardia (EAT)
This occurs when there is a single ectopic focus in the atria (not the SA node) conducting the pathway. Since it originates outside the SA node and within the atria, it will have an accelerated rate compared to the SA node (often 110-130 bpm). Therefore, it will often resemble sinus tachycardia. However, as the P wave does not originate from the SA node, it will have an abnormal appearance often seen as an inverted P wave in the inferior leads (II, III, aVF) and upright in V1 (seen below)
Regular AV Node Dependent Tachycardias
This is what we think of when we say SVT. Although we commonly say SVT (and are unlikely to change), SVT can refer to any tachyarrhythmia arising above the AV Node. AVnRT can be distinguished from other tachyarrhythmias as often appears as regular tachycardia with HR ~160 bpm (though can range ~140-280 bpm). P waves can be difficult to see and often are buried in the QRS complex. Often if P waves are visible, they appear in retrograde fashion as inverted P waves following QRS waves in II, III, and aVF. It is also not uncommon to see ST segment depression with AVnRT which is rate-related and not necessarily a sign of ischemia. Since the reentry tachycardia resides in the AV node, treatment consists of vagal maneuvers, adenosine, or BB/CCB to block the AV node.
AVRT is a much less commonly seen compared to AVnRT. It contains rhythms such as WPW. It occurs from a reentry circuit formed by the normal conduction system (AV Node) and an accessory pathway (bundle of Kent).
AVRT comes in two flavors: orthodromic and antidromic. Orthodromic is the more common consisting of anterograde conduction thru the AV node and return of the circuit via the accessory pathway. Its rate is typically 200-300 bpm, and since the circuit conducts through the AV node, it presents as a narrow complex tachycardia. Although the rate is often faster than SVT, it can be difficult to differentiate from AVnRT if prior EKG is not present (prior EKG may show delta wave). Diagnosis can also be made difficult by buried P waves as will not be able to visualize the short PR interval (<200 msec). Below EKG shows AVRT and difficult of distinguishing from AVnRT as rate buries P waves and no prior EKG is available. Luckily, the treatment is the same for AVRT and AVnRT with vagal maneuvers, adenosine, and BB/CCB as tachycardia travels through the AV node.
Antidromic conduction is much less common (~5% of cases) and anterograde conduction occurs via the accessory pathway. Since anterograde conduction falls outside the AV node, it presents as a wide complex tachycardia (separate blog).
Irregular Narrow Complex Tachycardias
This is the most common sustained arrhythmia. It can be distinguished by lack of P waves, absence of isoelectric baseline, and variable ventricular rate. It is caused by disorganized atrial automaticity leading to fibrillation of the upper heart. Although most EKGs will show the above characteristics, some EKGs in Afib may show “fibrillatory waves,” which can be fluctuations in the baseline >0.5mm and are occasionally mistaken as P waves, however, it is important to note that they will not “march out” and will have changing morphology. Treatment consists of rate control, anti-arrhythmics, and anticoagulation depending on comorbidities.
Atrial Flutter with Variable Block
This is a specialized type of Atrial flutter where instead of there being a consistent 2:1 block or 3:1 block that would present as a regular narrow complex tachycardia, the block can be variable fluctuating between a 2:1 block, 3:1 block, etc., and therefore, presents as an irregular narrow complex tachycardia. Although the rhythm is irregular, one way to help differentiate AFib from AFlutter with variable block is that P waves are still present and march out (seen below).
Multifocal Atrial Tachycardia (MAT)
This is a rhythm caused by multiple foci of conduction within the atria. While ectopic atrial tachycardia (EAT) has a single ectopic focus (therefore, a regular rhythm), multiple atrial tachycardia (MAT) has multiple foci conducting, and therefore, has an irregular rhythm. Much like EAT, MAT will have a HR typically 100-130 bpm as foci exist outside the SA node. Another helpful property to help diagnose MAT is that the P waves will have different morphologies as they conduct from different foci. In fact, you must have at least 3 distinct P-wave morphologies in same lead to make diagnosis.
MAT is most often seen in seriously ill elderly patients with severe respiratory disease (e.g. COPD, CHF). It is often a poor prognostic sign. Treatment relies on treatment of underlying disorder.