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Sodium channel blocker toxicity can be caused from a wide variety of medications and produces reliable EKG findings with toxicity, though the time to EKG changes is variable by medication. Sodium channel toxicity increases the QRS (usually >100ms) and causes right axis deviation of the terminal QRS. QRS complexes that are at least 200ms are almost always from a metabolic or toxicologic etiology and this is a common one (with hyperkalemia being the most common). QRS duration is predictive of worsened clinical findings: a QRS>100ms is predictive of seizures and QRS>160ms is predictive of ventricular arrhythmias. Right axis deviation of the terminal QRS is defined by either dominant R' wave >3mm in aVR or R/S>0.7 in aVR. Of note, hyperkalemia causes part of its toxicity by causing sodium channel blockade.
Examples:
Worsened toxicity
Note the QRS complexes ~200ms consistent with a metabolic or toxicologic etiology (not VT)
Source: LITFL
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