Mechanism

Calcium channel blockers decrease calcium entry through L-type cellular calcium channels, acting on smooth muscle and heart.

• Dihydropyridines (-dipine): preferentially block channels in vascular smooth muscle causing significant vasodilation and reflex tachycardia. Also antagonizes calcium-mediated insulin release in the pancreas. 
• Non-dihydropyridines (verapamil, diltiazem): selectively block myocardial Ca channels, resulting in decreased cardiac contractility, slowed AV/SA node activity, and peripheral vasodilation. Can also antagonize Ca-mediated insulin release in the pancreas, leading to relative hyperglycemia.

Evaluation

• Vitals: hypotension with reflexive tachycardia (mostly in dihydropyridines), hypotension with bradycardia (in non-dihydropyridines or high-dose dihydropyridines).
• ECG: may see sinus arrest, AV block, without QRS prolongation.
• Labs: hyperglycemia (distinguishing feature from other sympatholytic agents).
• Drug interactions: do thorough review of other medications, as drug interactions may result in toxicity (e.g., hypotension may be more common in patients taking beta-blockers or nitrates).

Management

• Decontamination: activated charcoal 50g PO or via NGT, even if asymptomatic, unless poor airway protection. Strongly consider whole bowel irrigation for any extended-release ingestion or for verapamil or diltiazem, even if asymptomatic.
• Initial therapy: give IV fluids for hypotension. If symptomatically bradycardic, give atropine (0.5-1mg q2-3mins, maximum 3mg).
• IV calcium: if still symptomatic despite initial resuscitation, give Ca gluconate 10% 30-60ml IV (0.4-0.8ml/kg). May also use calcium chloride 10% 10mL (0.1-0.2 mL/kg) IV, but only if via a central line or very secure large peripheral vein, due to risk of tissue necrosis in case of infiltration. Repeat every 5-10 minutes as needed. Monitor serial serum calcium (aim for 15 mg/dL) or ionized calcium (aim for double baseline level) and ECGs.
• Insulin: high-dose insulin can have positive inotropic effects. Start with regular insulin 1 U/kg IV and infusion 0.5-1 U/kg/h titrated upward as needed. Maintain euglycemia with D10 or D50 as needed. Be mindful of hypokalemia.
• Vasopressors: may be effective; often require high doses.
• Glucagon: uncertain benefit, but may increase the heart rate. Give 5mg IV q10 mins, up to 15mg total. Caution: glucagon is highly emetic when given IV.
• Monitor: monitor vital signs and ECG for at least 6 hours after ingestion for immediate release compounds (up to 24hrs for sustained release formulations).

Kerns W 2nd. Management of beta-adrenergic blocker and calcium channel antagonist toxicity. Emerg Med Clin North Am 2007;25:309-331. 

Poisoning and Drug Overdose, 6th ed. 2012.