Review Question - QID 205620

Amitriptyline is a tricyclic antidepressant (TCA). The most dangerous complication of TCA overdose is arrhythmia. The degree of TCA intoxication can be assessed using the QRS duration.

TCAs function by inhibiting the reuptake of serotonin, norepinephrine, or both. Because of their side effect profile and toxicities in the setting of overdose, TCAs have largely been replaced by selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs).

Adams et al. review the treatment of depression in adults. They note that while TCAs are more effective at treating depression than SSRIs, they are also more difficult to tolerate. The number needed to treat (NNT) to achieve one responder is 4 for TCAs compared to 6 for SSRIs. They contrast this to the number need to harm (cause severe side effects that necessitate treatment termination) ranging from 5-11 for TCAs, and 21-94 for SSRIs.

Thanacoody and Thomas discuss the cardiovascular toxicities of TCA poisoning. They note that the most specific electrocardiogram signs of TCA toxicity are QRS prolongation and right axis deviation of the terminal 40ms vector of the QRS complex. QTc prolongation > 430 milliseconds is associated with a high risk of developing complications.

Illustration A is an example of an electrocardiogram taken in a patient with a TCA overdose. Note the interventricular conduction delay.

Incorrect Answers:
Answer 1: While TCAs may cause respiratory depression, a severely decreased respiratory rate is more indicative of opiate overdose.
Answer 2: The mini-mental status exam can be affected by TCAs as they cause drowsiness, altered mental status, and seizure, but this is not as strong of an indicator of the severity of intoxication as QRS prolongation.
Answer 3: While TCAs can cause urinary retention secondary to their anticholinergic properties, this is not as strong of an indicator of the severity of intoxication as QRS prolongation.
Answer 4: The major effect of TCAs on the electrocardiogram is QRS prolongation, not PR prolongation.