Wide Complex Tachycardia? Check aVR!

Resident Clinical Pearl – June 2016

Robin Clouston, PGY3 iFMEM, Dalhousie University, Saint John, New Brunswick

A patient presents with an altered level of consciousness and a wide complex tachycardia on cardiac monitor. The differential is broad, including a primary cardiac dysrhythmia, electrolyte abnormality, intracranial hemorrhage and a variety of toxicities.

Here is their ECG:

How can this ECG help narrow your differential?

When presented with an undifferentiated patient with wide complex tachycardia, remember to check aVR lead to rule in sodium channel blocker toxicity.

A variety of toxins exhibit sodium channel blockade, including:

Tricyclic antidepressants (TCAs) (most common)
Carbamazepine
Quinidine
Procainamide
Lidocaine
Amantidine
Cocaine
Diphenhydramine

In sodium channel blocker toxicity, including TCA overdose, there is:

a terminal R’ wave in aVR, of >3mm
an R/S ratio of >0.7

Practically speaking, there is an abnormally tall R’ wave in aVR. This finding is specific to sodium channel blockade and occurs due to blockade of myocardium fast sodium channels.

Other findings in TCA / sodium channel blocker toxicity:

Wide QRS > 100ms à increased risk of seizure
Wide QRS > 160ms à increased risk of VT/VF
Prolonged QTc
Rightward deviation of the terminal 40 msec of QRS (difficult to measure)

Treatment:

Give sodium bicarbonate 100mEq amp every few minutes until QRS narrows. The sodium is the active ingredient (not the bicarb) and is used to overwhelm the sodium channels and reverse the sodium channel blockade.

Conclusion:

So the next time you have an undifferentiated patient with a wide complex tachycardia, be sure to check aVR and consider TCA / sodium channel blocker toxicity!

References:

http://lifeinthefastlane.com/ecg-library/basics/tca-overdose/
http://foamcast.org/2014/09/03/episode-13-tricyclic-antidepressants-and-sodium-channel-blockade/
BK Blok et al. First Aid for the Emergency Medicine Boards 3^rd 2016. McGraw-Hill Education. p375-377.