The ECG in the poisoned patient

Most patients presenting to the ED with either accidental or intentional drug ingestion will get an ECG. In most departments I’ve worked in, the senior doctor looks at all the ECGs, primarily for STEMI, but for other findings too. The juniors will come to me later discussing the case and when I ask about the ECG, they frequently say it’s normal. This always starts me on a bit of a rant as when I ask them, it turns out they have no idea what they’re actually meant to be looking for in the ECG of a poisoned patient. [The same goes for syncope patients…]

So, after considering what I look for on the ECG, here’s my list of things to check in the poisoned patient.

What to look for on the ECG

• long QT

  • all kinds of drugs

  • results from prolonged K efflux

• wide QRS

  • Na Channel blockade

    • TCA

    • propanolol

    • cocaine

    • lots of other unexpected drugs too

• dominant R aVR

  • Na channel blockade

• scooped ST segments

  • digoxin

• bradycardia

  • digoxin

  • beta blockers

• AV block

  • beta blockers

  • digoxin

Or can be expressed as 5 main cardiac toxicities if you’re into the pathophysiology of it

1. Na channel blockade [depolarisation]

   • wide QRS (>100ms is the cut off here)

   • prolongation of the last 40ms of the QRS which produces a right axis on ECG

   • dominant R aVR [PMID 7618783]

2. potassium efflux blockade [repolarisation]

   • primarily prolongs the QT

3. Na/K/ATPase pump blockade

   • all about digoxin here

   • can produce almost any rhythm

   • tachys with AV blocks are a big clue

4. beta blockade

   • brady

   • AV blocks

5. calcium channel blockade

   • brady

   • AV blocks

If you know of any other interesting ECG patterns in tox patients please let me know in the comments.

Perhaps the best comment came from Domhnall:

Succinctly summarised as the “horizontal” ECG as opposed to the “vertical” ECG of IHD….

References:

Critical Decisions in Emergency and Acute Care Electrocardiography. Brady and Truwit 2009 Wiley

LITFL ECG Library

Liebelt, E L, P D Francis, and A D Woolf. “ECG Lead aVR Versus QRS Interval in Predicting Seizures and Arrhythmias in Acute Tricyclic Antidepressant Toxicity..” Annals of Emergency Medicine 26, no. 2 (August 1995): 195–201. PMID 7618783