Quetiapine Toxicity

Brisk sinus tachycardia (HR = 120 bpm)
Prolonged QTc interval (QTc = 560ms; the absolute QT interval is more than half the R-R interval)

Quetiapine (a second-generation atypical antipsychotic) is a leading cause of toxic coma in Australia.
Main toxic effects in overdose include coma, anticholinergic delirium, prolonged QTc and a brisk sinus tachycardia.
Doses > 3g are associated with coma.
Despite the prolonged QTc, Torsades de Pointes does not occur (see below).
A similar pattern of clinical and ECG features is seen with other atypical antipsychotics, such as olanzapine or clozapine.

In the context of acute poisoning with QT-prolonging agents, the risk of TdP is better described by the absolute rather than corrected QT.
More precisely, the risk of TdP is determined by considering both the absolute QT interval and the simultaneous heart rate (i.e. on the same ECG tracing).
These values are then plotted on the QT nomogram (below) to determine whether the patient is at risk of TdP.
A QT interval-heart rate pair that plots above the line indicates that the patient is at risk of TdP.
From the nomogram, you can see that QTc-prolonging drugs that are associated with a relative tachycardia (e.g. quetiapine) are much less likely to cause TdP than those that are associated with a relative bradycardia (e.g. amisulpride).