Several considerations in the differential diagnosis for narrow complex regular tachycardias can be excluded. There appears to be some sort of atrial activity, so junctional tachycardia, narrow-complex ventricular tachycardia, and tachydysrhythmias originating from the atrioventricular (AV) node are eliminated. This leaves sinus tachycardia, atrial tachycardia, and atrial flutter. Arriving at the correct interpretation paradoxically calls for you to scrutinize the tracing, but step back and view it as well.
Scrutiny of the two leads that classically best show P waves (leads V1 and II) is helpful here. In lead V1, there are regularly spaced small amplitude deflections appearing just before the QRS complex (straight arrows), but the PR interval appears to be too short for sinus tachycardia (i.e., it is less than 0.12 sec). Also, there are unexplained small amplitude deflections (curved arrows) that occur with regularity a short distance before those marked with the straight arrows. The interval between each of these deflections (asterisk) corresponds to a rate of 300 bpm.
Remembering this adage enables you to arrive at the diagnosis: narrow complex tachycardias at a rate of approximately 150 bpm are atrial flutter with 2:1 conduction until proven otherwise. Indeed, although the computer tells us the ventricular rate is 162 bpm, use of an ECG caliper tells us that the ventricular rate is very close to 150 bpm. Because the most common conduction ratio in atrial flutter is two atrial flutter beats per one ventricular QRS complex, the diagnosis is clinched (because the atrial rate is roughly 300 bpm, or twice that of the ventricular rate, and atrial rates in flutter are generally approximately 250 bpm to 350 bpm). Atrial tachycardias generally feature a slower atrial rate (120 bpm to 250 bpm), although they too may demonstrate AV block.
Stepping back and looking at the tracing also is helpful. Scanning the ECG, you can notice the regularity of the QRS complexes, but your eye is also called to the “saw tooth” pattern, particularly in the inferior leads (II, III, and aVF) and in lead aVL. This pattern is characteristic of atrial flutter. The axis of these saw teeth (triangles) is negative, which is distinctly unusual in sinus tachycardia. A normal P wave axis is 45 degrees to 60 degrees, or roughly parallel to the lead II vector. We have come to expect an upright P wave in lead II when the rhythm originates from the sinus node.
Figure 3 is an ECG demonstrating atrial flutter in a different patient. In this tracing, the computer once again originally interpreted the rhythm as sinus tachycardia, but there is evidence of atrial activity at 300 bpm (dots), and the ventricular response is approximately 150 bpm. The correct interpretation is atrial flutter with 2:1 conduction.
There are others characteristics that facilitate differentiation of atrial flutter from sinus tachycardia. Atrial flutter will usually remain at a constant rate of approximately 150 bpm (if 2:1 conduction prevails), while sinus tachycardia will vary somewhat over time (or with fluids, pain relief, or antipyretics, depending on the cause). Beware the telemetry monitor, however; atrial flutter will not stay at exactly 150 bpm, and patient movement, artifact, etc., will affect the telemetric computation and display of rate. Vagal maneuvers will not affect the ventricular rate in sinus tachycardia, but may well increase the degree of AV block with atrial flutter.© 2005 Lippincott Williams & Wilkins, Inc.