Value of initial ECG findings and plasma drug levels in cyclic antidepressant overdose

Electrocardiographic changes and quantitative plasma drug levels associated with cyclic antidepressant (TCA) overdose have been the subject of many reports in the medical literature. Heart rate, QRS duration, QT interval, and most recently, the terminal 40 ms QRS (T40) axis have been reported to be valuable indicators in TCA overdose. The value of plasma drug levels and their ability to predict complications has been debated. To evaluate the discriminant and predictive abilities of ECG parameters and quantitative drug levels in a large series of TCA overdoses, we retrospectively reviewed the charts of all overdose patients admitted to ICU in our facility during a 30-month study period. The TCA-positive group (187) had statistically significant differences (P less than .001) from the TCA-negative group (171) in heart rate, QRS duration, QTc, and T40 axis. However, none of these parameters could correctly classify more than 66% of cases. The sensitivity and specificity of the T40 axis were found to be only 29% and 83%, respectively. Correlation coefficients for plasma quantitative TCA levels by ECG parameter yielded an R of less than .33 in all cases. QRS duration of 100 ms or more and heart rate of 100 or more, or TCA level of 1,000 ng/mL or more, were present in all of a few patients with complications. We conclude that ECG parameters cannot be relied on to include or exclude the diagnosis of TCA overdose and that TCA levels do not correlate with ECG parameters.