TCA Overdose

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This is a fun one. The EKG is consistent with TCA overdose and a pretty nasty one at that. So this patient likely presented with:

So how do we know that 1) this is a TCA overdose and 2) that it is a bad one.

Na Blockade Shifting the DEpolarization to the right

Na Blockade Shifting the DEpolarization to the right

When you look at the EKG you notice that this person is tachycardic, the QRS is wide, and AVR does not look quite normal - that is because there is a Terminal R-Wave - the deflection of the QRS complex is positive at the end. While this is not specific to TCAs and is caused by sodium channel blockade, TCAs are the most common cause. Other examples of Na-blockers include quinidine, mexilitine, phenytoin, lidocaine, and procainamide.

Look to the chart on the right to see where these anti-arrhythmic drugs work. The sodium blockers are going to slow the influx of Na into cells, slowing the action potential - what would you expect to happen to the QRS if the slope of the line at point 0 is not as acute? At the chart on the left you can see that Na-blockade shifts the whole depolarization to the right - prolonging the QRS.

Can you use the QRS to see what people are at risk for? Yes! (You knew that by virtue of me asking…) In TCA overdoses, the QRS width is correlated with toxicity.

  • QRS > 100 ms is predictive of seizures

  • QRS > 160 ms is predictive of ventricular arrhythmias (e.g. VT)

How do you treat this? If you don’t know…maybe time to read that UpToDate Article again.