A ‘foolproof’ way to diagnose narrow complex tachycardias on EKGs
A hospitalist looking at an EKG showing a narrow complex tachycardia needs to be able to come up with an accurate diagnosis of the rhythm pronto. And hospitalist Meghan Mary Walsh, MD, MPH, has developed a simple and efficient method for doing so within a minute or two that she’s used with great success on the wards and in teaching medical students and residents for nearly a decade.
she promised at HM20 Virtual, hosted by the Society of Hospital Medicine.
Her method involves asking three questions about the 12-lead EKG:
1) What’s the rate?
A narrow complex tachycardia by definition needs to be both narrow and fast, with a QRS complex of less than 0.12 seconds and a heart rate above 100 bpm. Knowing how far above 100 bpm the rate is will help with the differential diagnosis.
2) Is the rhythm regular or irregular?
“If I put the EKG 10 feet away from you, you should still be able to look at it and say the QRS is either systematically marching out – boom, boom, boom – or there is an irregular sea of QRS complexes where the RR intervals are variable and inconsistent,” said Dr. Walsh, a hospitalist at the University of Minnesota, Minneapolis, and chief academic officer at Hennepin Healthcare, where she oversees all medical students and residents training in the health system.
This distinction between a regular and irregular rhythm immediately narrows the differential by dividing the diagnostic possibilities into two columns (See chart). She urged her audience to commit the list to memory or keep it handy on their cell phone or in a notebook.
“If it’s irregular I’m going down the right column; if it’s regular I’m going down the left. And then I’m systematically running the drill,” she explained.
3) Are upright p waves present before each QRS complex in leads II and V1?
This information rules out some of the eight items in the differential diagnosis and rules in others.
Narrow complex tachycardias with an irregular rhythm
There are only three:
Atrial fibrillation: The heart rate is typically 110-160 bpm, although it can occasionally go higher. The rhythm is irregularly irregular: No two RR intervals on the EKG are exactly the same. And there are no p waves.
“If it’s faster than 100 bpm, irregularly irregular, and no p waves, the conclusion is very simple: It’s AFib,” Dr. Walsh said.
Multifocal atrial tachycardia (MAT): The heart rate is generally 100-150 bpm but can sometimes climb to about 180 bpm. The PP, PR, and RR intervals are varied, inconsistent, and don’t repeat. Most importantly, there are three or more different p wave morphologies in the same lead. One p wave might look like a tall mountain peak, another could be short and flat, and perhaps the next is big and broad.
MAT often occurs in patients with a structurally abnormal atrium – for example, in the setting of pulmonary hypertension leading to right atrial enlargement, with resultant depolarization occurring all over the atrium.
“Don’t confuse MAT with AFib: One has p waves, one does not. Otherwise they can look very similar,” she said.
Atrial flutter with variable conduction: A hallmark of this reentrant tachycardia is the atrial flutter waves occurring at about 300 bpm between each QRS complex.
“On board renewal exams, the question is often asked, ‘Which leads are the best identifiers of atrial flutter?’ And the answer is the inferior leads II, III, and aVF,” she said.
Another classic feature of atrial flutter with variable conduction is cluster beating attributable to a varied ventricular response. This results in a repeated pattern of irregular RR intervals: There might be a 2:1 block in AV conduction for several beats, then maybe a 4:1 block for several more, with resultant lengthening of the RR interval, then 3:1, with shortening of RR. This regularly irregular sequence is repeated throughout the EKG.
“Look for a pattern amidst the chaos,” the hospitalist advised.
The heart rate might be roughly 150 bpm with a 2:1 block, or 100 bpm with a 3:1 block. The p waves in atrial flutter with variable conduction can be either negatively or positively deflected.
