In the 1970s, calcium channel antagonists, also known as calcium channel blockers, were widely used for many indications. This cardiovascular drug class is one of the leading causes of drug-related fatalities. They are often classified into 2 major categories, either non-dihydropyridines or dihydropyridines. The non-dihydropyridines include verapamil, a phenylalkylamine, and diltiazem, a benzothiazepine. The dihydropyridines include many other drugs, most of which end in "pine" (i.e., amlodipine and nicardipine).
Cardiovascular indications include hypertension, coronary spasm, angina pectoris, supraventricular dysrhythmias, hypertrophic cardiomyopathy, and pulmonary hypertension. In addition to these, they are also prescribed for Raynaud’s phenomenon, subarachnoid hemorrhage, and migraine headaches.
Calcium channel antagonists block the inward movement of calcium by binding to the L-type “long-acting” voltage-gated calcium channels in the heart, vascular smooth muscle, and pancreas. There are two major categories of calcium channel antagonists based on their primary physiologic effects. The non-dihydropyridines have inhibitory effects on the sinoatrial (SA), and atrioventricular (AV) nodes are resulting in a slowing of cardiac conduction and contractility. This allows for the treatment of hypertension, reduces oxygen demand, and helps to control the rate in tachydysrhythmias. The dihydropyridines, in therapeutic dosing, have a little direct effect on the myocardium, and instead are more often peripheral vasodilators which is why they are used for hypertension, post-intracranial hemorrhage associated vasospasm, and migraines.
Absorption: Calcium channel antagonists are absorbed well orally, however many have low bioavailability due to hepatic first pass metabolism, primarily by CYP3A4.
Distribution: Calcium channel antagonists are highly protein bound, and many have high volumes of distribution.
Metabolism: In repeated doses, or in overdose, the hepatic enzymes responsible for metabolism become saturated and reduce first pass effects which therefore increases absorption of the active drug. Modified release formulations and saturation of metabolism of these drugs increase the half-life of various calcium channel antagonists.
Excretion: Calcium channel antagonists are primarily excreted renally after metabolism.
There is the potential for drug-drug interactions because calcium channel antagonists are metabolized by CYP3A4, which is responsible for the metabolism of many other xenobiotics.
Calcium channel antagonists can be administered intravenously or orally.
Non-dihydropyridines may cause constipation, worsening cardiac output, and bradycardia.
Dihydropyridines may lead to lightheadedness, flushing, headaches, and peripheral edema. The peripheral edema is likely related to the redistribution of fluid from the intravascular space to the interstitium.
Gingival hyperplasia has also been reported.
Non-dihydropyridines are contraindicated in those with heart failure with reduced ejection fraction, second or third-degree AV blockade, and sick sinus syndrome because of the possibility of causing bradycardia and worsening cardiac output.
Calcium channel antagonists are also contraindicated in patients with known hypersensitivity to the drug or its components. Other contraindications include sick sinus syndrome (except in patients with an artificial pacemaker), severe hypotension, acute myocardial infarction, and pulmonary congestion. Calcium channel antagonists may cause AV blockade or sinus bradycardia especially if taken with agents known to slow cardiac conduction. Dermatologic reactions and hypotension with or without syncope have been reported with calcium channel antagonist use. Peripheral edema may occur within 2 to 3 weeks of initiating calcium channel blocker therapy. Use with caution in renal and hepatic impairment. Consider initiating therapy at a lower dose.
Patients should be monitored closely. The improvement of their symptoms of angina or maintenance of their blood pressure is an indication of efficacy. A patient should be assessed regularly if these medications are being titrated quickly.
Hypotension and bradycardia are the primary features seen in calcium channel antagonist poisoning. These findings are due to peripheral vasodilatation and reduced cardiac contractility.
Hypotension may be profound and life-threatening, it is caused by peripheral vasodilation, bradycardia, and decreased ionotropy. The cardiac conduction may also be impaired with AV conduction abnormalities, complete heart block, and idioventricular rhythms.
Patients may present asymptomatic initially and progress rapidly to severe hypoperfusion and cardiovascular collapse. Symptoms may include lightheadedness, fatigue, change in mentation, syncope, coma, and sudden death. Non-cardiac symptoms may include nausea and vomiting, metabolic acidosis secondary to hypoperfusion, and hyperglycemia from the blockade of insulin release in the pancreas. The insulin blockade also impairs the uptake of glucose by myocardial cells which further contributes to the reduction of cardiac contractility and worsens hypotension. Severe poisoning can lead to pulmonary edema, presumably as a result of precapillary vasodilation and increased transcapillary pressure.
Dihydropyridines in mild to moderate overdose may cause reflex tachycardia; however, in severe overdose, there may be a loss of receptor selectivity leading to bradycardia.
Many factors may affect the severity of overdose including the calcium-channel antagonist dose, the formulation, ingestion with other cardioactive medications such as beta-blockers, the patient’s age, and comorbidities. These medications may also be life-threatening with as little as one tablet in small pediatric patients.
Hyperglycemia has been considered a prognostic indicator of the severity of calcium channel antagonist toxicity. Beta-islet cells in the pancreas depend on the influx of calcium through the L-type calcium channels to release insulin. In the case of calcium channel antagonist overdose, there is a reduction in the release of insulin and subsequent hyperglycemia.
As in any other overdose, it is important to maintain a patent airway. Obtain an electrocardiogram and place the patient on continuous monitoring including pulse oximetry. Obtain a chest x-ray and basic labs (including acetaminophen and salicylate levels if warranted). Initiate gastrointestinal (GI) decontamination early especially in the cases of large ingestions or those with sustained-release formulations in the appropriate settings (i.e., normal mental status, recent ingestion, among others.) Administer activated charcoal if the patient has presented early and is awake, alert, oriented, and protecting their airway. Whole bowel irrigation is an important option for those patients with massive overdoses or overdoses of sustained or extended release formulations which do not already have an ileus.
In the case of hypotension, initial treatment with intravenous fluids should be given with caution in those who have congestive heart failure, pulmonary edema, or kidney disease. Intravenous calcium administration may reverse the decreased cardiac contractility. Calcium chloride 10% (10 ml for 0.1 to 0.2 ml/kg) or calcium gluconate 10% (20 to 30 ml 0.3 to 0.4 ml/kg) may be administered intravenously and may be repeated every 5 to 10 minutes. Caution must be used with calcium chloride as it may cause skin necrosis when given through a peripheral line. Atropine is a reasonable initial treatment option, but it typically does not reverse the effects of calcium channel antagonist poisoning. Give glucagon as a bolus of 5 to 10 mg intravenously with caution for nausea and vomiting, and patients may be pre-medicated with antiemetics to help avoid this. If the patient is refractory to these interventions, initiate vasopressor therapy using intravenous norepinephrine or push-dose phenylephrine while preparing hyperinsulinemia/euglycemia (HIE) therapy. HIE increases cardiac contractility by enhancing the transport of glucose into the myocardial cells which corrects they hypo-insulinemia. Administer a bolus of insulin 1 unit/kg, followed by an infusion of 1 to 10 units/kg per hour. Monitor the patient’s glucose for hypoglycemia initially every 10 minutes and then every 30 to 60 minutes to maintain glucose between 100 to 200 mg/dL. Use a concomitant dextrose infusion to maintain these levels. If the initial glucose is less than 200 mg/dL, administer a bolus dose of glucose. Monitor glucose levels and potassium levels closely. Intravenous lipid emulsion therapy lacks clear evidence for efficacy but is a consideration if all else fails. Administer a bolus of intravenous lipid emulsion 20% 1.5 ml/kg, repeat if needed, and then start an infusion of 0.25 to 0.5 ml/kg per minute for an hour. Reports suggest that the use of methylene blue, especially in those with amlodipine overdose resulting in vasodilatory shock, may be effective. Phosphodiesterase inhibitors have also been used in calcium channel antagonist therapy. They increase cardiac output by inhibiting the breakdown of cAMP. Extracorporeal membrane oxygenation (ECMO) has proven successful in cases refractory to all of the above interventions since it maintains perfusion to vital organs and continues hepatic metabolism.