Describe the physiological effects and principles of management of a tricyclic antidepressant overdose.

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College Answer

Good answers to this question were those that gave an accurate account of the physiological effects, e.g. inhibition of the fast sodium channels in the His-Purkinje system as well as the atrial and ventricular myocardium, decreasing conduction velocity (differential conduction inhibition of RBB being more susceptible) and increasing duration of repolarization, and the absolute refractory periods. Once having done that the rest of the answer would have flowed more easily, e.g. ECG changes and conduction disturbances. The effects of tricyclic antidepressants on Na channels and as a consequence the cardiovascular conduction abnormalities were often omitted. Anti cholinergic (e.g. slowing GIT motility) and antihistamine effects (e.g. obtundation) were often overlooked. Also frequently overlooked were basic pharmacology relevant to treatment, e.g. lipophilic, large volume of distribution, systemic acidosis reduces the extent of protein binding and increases unbound (active) drug. Additional points were available to those who not only mentioned sodium bicarbonate, but also mentioned the principles behind its use for this circumstance