metoprolol and diltiazem

Both metoprolol and diltiazem suppress the sinoatrial and atrioventricular nodes. Metoprolol is a beta-1 selective blocker that reduces sympathetic drive. Diltiazem is a non-dihydropyridine calcium channel blocker that inhibits calcium influx in cardiac conductive tissue. Their combined effects on cardiac conduction are at least additive, leading to a risk of profound bradycardia and heart block.

Patients receiving both drugs may develop symptomatic bradycardia, AV block, hypotension, and worsening of heart failure. The risk is highest in patients with pre-existing conduction abnormalities, reduced left ventricular function, or concurrent use of other negative chronotropic agents. Symptoms include dizziness, fatigue, syncope, and exercise intolerance.

Co-prescription should be approached with caution. Measure baseline heart rate, blood pressure, and ECG. Monitor these parameters regularly, especially after dose changes. Avoid the combination in patients with marked bradycardia, second or third degree AV block without a pacemaker, or significant heart failure. Consider alternative agents or lower doses with careful titration.

Well-established from pharmacological principles and clinical case reports. Pharmacokinetic interactions are not considered clinically significant because metoprolol is predominantly metabolised by CYP2D6, which is not inhibited by diltiazem. The main interaction is pharmacodynamic.