After staing your patient on imipramine, his hea rate rises to 120/min and he has blurred vision. These effects can be explained by the fact imipramine:
Looking at the options, the correct answer would relate to anticholinergic effects. The other options might be about other mechanisms. For example, if an option says it's a beta-agonist, that's wrong because imipramine isn't one. Another might mention sodium channel blockade, which is more about arrhythmias, not tachycardia. Another could be a direct effect on the heart, but the tachycardia here is due to reduced vagal tone. So the key is understanding the anticholinergic properties of TCAs.
**Core Concept**
Imipramine, a tricyclic antidepressant (TCA), causes anticholinergic side effects due to blockade of muscarinic acetylcholine receptors. This leads to decreased parasympathetic tone, resulting in tachycardia and cycloplegia (blurred vision).
**Why the Correct Answer is Right**
Imipramine competitively inhibits muscarinic M1/M2 receptors in the heart and eyes. In the heart, this reduces vagal-mediated bradycardia, causing tachycardia. In the eye, it blocks M3 receptors on ciliary muscles, preventing accommodation (blurred vision). These effects are hallmark anticholinergic adverse effects of TCAs.
**Why Each Wrong Option is Incorrect**
**Option A:** *Beta-adrenergic agonism* is incorrect; TCAs do not stimulate Ξ²-adrenergic receptors.
**Option B:** *Sodium channel blockade* causes arrhythmias or sedation, not tachycardia.
**Option D:** *Direct myocardial stimulation* is incorrect; TCAs primarily act centrally or via receptor blockade.
**Clinical Pearl / High-Yield Fact**
All TCAs (e.g., amitriptyline, nortriptyline) have anticholinergic activity. Monitor for dry mouth, constipation, confusion, and urinary retention in elderly patients. Use caution in patients with glaucoma or cardiac conduction abnormalities.
**Correct Answer: C. Antagonizes muscarinic acetylcholine receptors**