Take Home Points:

In the context of poisoning, a “wide QRS” is anything greater than 100 milliseconds.
A newly “wide QRS”, especially with hemodynamic instability, should prompt consideration of sodium channel blockade and not ventricular tachycardia. Treatment is guided by administration of sodium-bicarbonate.
Recall that the resultant alkalemia driven by sodium-bicarbonate will shift potassium intracellularly. As a result, if a bicarbonate infusion is started, potassium should
simultaneously be given as to avoid life-threatening hypokalemia.

REBEL Core Cast 109.0 – Na Channel Blocker Poisoning

Definition and Physiology

Standard definition of a wide QRS is anything > 120 msec (3 small boxes on the ECG)
In the context of poisoning, a “wide QRS” is anything greater than 100 milliseconds. (Boehnert 1985).
- Authors evaluated the relationship between QRS duration and negative clinical events in patients with confirmed tricyclic anti-depressant (TCA) poisoning.
- If QRS >100 msec = 33% chance of seizures
- If QRS >160 msec = 50% of ventricular dysrhythmias
- Often extrapolated to other sodium channel blocking agents: diphenhydramine, loperamide, cocaine, lamotrigine, Type 1A/1C Anti-Dysrhythmics.

Clinical Manifestations

The right bundle is more susceptible to sodium-channel blockade than the left bundle and as a result, rightward manifestations will appear on the ECG: right axis deviation, terminal R wave in aVR, and a widened QRS complex.
With severe toxicity, the ECG can mimic ventricular tachycardia and clinically, the patient may decompensate hemodynamically (ie. tachycardia and hypotension)

Management

Critically ill patients will be hemodynamically unstable and present with a “wide complex tachycardia.” While ACLS will recommend shocking these patients, as with everything else in medicine, clinical context is essential.
If pre-test probability is high for poisoning, this is sodium channel blockade until proven otherwise. These patients need sodium-bicarbonate and not electricity.
- Dosing: 1-2 mEq/kg bolus
- If there is a response, initiate an infusion: 150 mEq in 1L of D5W at maintenance
Severely poisoned patients, may require multiple boluses of sodium-bicarbonate until the QRS narrows. (Mohan 2021)
Recall that the subsequently alkalemia will shift potassium intracellular. As a result, it is essential to replete potassium simultaneously.

References

Boehnert MT, Lovejoy FH Jr. Value of the QRS duration versus the serum drug level in predicting seizures and ventricular arrhythmias after an acute overdose of tricyclic antidepressants. N Engl J Med. 1985 Aug 22;313(8):474-9. doi: 10.1056/NEJM198508223130804. PMID: 4022081.
Mohan S, Backus T, Furlano E, Howland MA, Smith SW, Su MK. A Case of Massive Diphenhydramine and Naproxen Overdose. J Emerg Med. 2021 Sep;61(3):259-264. doi: 10.1016/j.jemermed.2021.04.020. Epub 2021 Jun 17. PMID: 34148773.

Post Created By: Sanjay Mohan MD

Post Peer Reviewed By: Salim Rezaie MD (Twitter @SRRezaie)

Introduction