Study objective: To compare the value of ECG measurements from lead aVR with the QRS-interval duration in predicting seizures and ventricular arrhythmias due to acute tricyclic antidepressant (TCA) toxicity.

Design: Prospective cohort series of referral calls from hospitals to a regional poison control center. Participants: Seventy-nine patients (mean age, 30±15 years) who presented within 24 hours of ingestion. Seizures occurred in 16 patients (20%) and ventricular arrhythmias in 5 (6%). Interventions: The amplitude of the terminal R wave in lead aVR (R_aVR), the R-wave/S-wave ratio in lead aVR (R/S_aVR), and the maximal limb-lead QRS interval were measured on the initial ECG. Results: R_aVRwas greater in those patients who had seizures or arrhythmias than in those who did not (4.4 versus 1.8 mm, P<.001), as was R/S_aVR(1.4 versus .5, P<.001). The sensitivity of an R_aVRof 3 mm or more was 81% and that of an R/S_aVRof .7 or more was 75%, compared with 82% for QRS intervals greater than 100 milliseconds. The positive predictive value (PPV) of an R_aVRof 3 mm or more was 43% and that of the R/S_aVRof .7 or more 46%, compared with a PPV for QRS interval of 100 milliseconds or more of 35%. Multiple logistic-regression analysis demonstrated that an R_aVRof 3 mm or more was the only ECG variable that significantly predicted seizures and arrhythmias (OR, 6.9 [95% CI, 1.2 to 40], P =.03). Conclusion: R_aVRand R/S_aVRwere greater in patients in whom seizures or arrhythmias developed after an acute TCA overdose. R_aVRof 3 mm or more was the only ECG variable that significantly predicted these adverse outcomes. [Liebelt EL, Francis PD, Woolf AD: ECG lead aVR versus QRS interval in predicting seizures and arrhythmias in acute tricyclic antidepressant toxicity. Ann Emerg Med August 1995;26:195-201.]