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%)
where:
• 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.
