Cardiac Conduction: Diltiazem prolongs AV node refractory period without significantly prolonging sinus node recovery time, except in patients with sick sinus syndrome. This effect may rarely result in abnormal slow heart rates (particularly in patients with sick sinus syndrome) or 2nd- or 3rd-degree AV block (13 of 3290 patients or 0.4%). Concomitant use of diltiazem with β-blockers or digitalis may result in additive effects on cardiac conduction. A patient with Prinzmetal's angina developed periods asystole (2-5 sec) after a single dose of 60 mg of diltiazem. (See Adverse Reactions.)
Congestive Heart Failure: Although diltiazem has a negative inotropic effect in isolated animal tissue preparations, hemodynamic studies in humans with normal ventricular function have not shown a reduction in cardiac index nor consistent negative effects on contractility (dp/dt). An acute study of oral diltiazem in patients with impaired ventricular function (ejection fraction 24±6%) showed improvement in indices of ventricular function without significant decrease in contractile function (dp/dt). Worsening of congestive heart failure has been reported in patients with preexisting impairment of ventricular function.
Hypotension: Decreases in blood pressure associated with diltiazem therapy may occasionally result in symptomatic hypotension.