Been busy in the real world, so I'm late to the new ITE keyword subforum ... I don't really do keywords, just test questions that I turn into keyword-like exercises. I'm looking for a better reference than http://www.drugs.com/mmx/verapamil.html for explaining the answer to this question. I didn't find anything really compelling in Miller, Barash, or M&M. Old question from the 1995 ITE: In a patient taking a beta-adrenergic blocker, the drug most likely to produce atrioventricular junctional block (A) diltiazem (B) fentanyl (C) halothane (D) nifedipine (E) verapamil Fentanyl and halothane are clearly wrong answers here, so the question comes down to which calcium channel blocker is worst when it comes to beta-blocker interactions and AV block. Not D. Dihydropyridines such as nifedipine, felodipine, nicardipine, and nimodipine cause the most vasodilation of all calcium channel blockers, with a reflex increase in HR, but SA and AV node depression don't occur. According to a chart on that web page, diltiazem, a benzothiazepine, maybe has less negative inotropic effect than verapamil, an L-type calcium channel blocker ... but it was a small difference between a + and a +/- on the chart. My take was that verapamil probably carries the most risk for AV block and the answer key does say verapamil is the correct answer.