Overdosage of a tricyclic antidepressant can result in serious cardiotoxicity related to sodium channel blockade. This can result in hypotension, conduction abnormalities and cardiac arrhythmias which may be fatal.


Findings in the 12 lead electrocardiogram:

(1) prolonged QRS duration > 0.10 seconds

(1a) ventricular arrhythmias may be seen with QRS durations > 0.16 seconds

(1b) seizures occur with QRS duration > 0.10 seconds

(2) may have a prolongation in the corrected QT interval (QTc)

(3) rightward axis (130° to 270°) for the terminal 40 milliseconds of the frontal plane QRS complex

(4) presence of an R wave in aVR with an S wave in I and aVL

(4a) R wave in aVR >= 3 mm (sensitivity 81%; in multivariate analysis significantly predicts seizures and arrhythmias)

(4b) R/S ratio in aVR >= 0.7 (sensitivity 75%)



• In aVR, the S wave is taken as the depth of the initial downward deflection and the R wave is taken as the height of the terminal upward deflection of the QRS complex.

• The capability to determine the axis of the terminal frontal plane QRS axis may not be readily available since it requires specialized computer analysis.


Serum drug levels:

(1) do not accurately correlate with toxicity

(2) may not be readily available

(3) do confirm that a patient has taken a tricyclic antidepressant.


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