Case of the Month #35 Calcium Channel Blocker Overdose

There are three classes of calcium channel blockers: 

1. Phenyalkylamines (verapamil) 

2. Benzothiazepines (diltiazem) 

3. Dihydropyridines (nifedipine, amlodipine, nicardipine, nimodipine) 

All three classes exert their effect on L-type voltage-gated calcium channels. These channels are functionally important in cardiac myocytes, vascular smooth muscle cells and beta islet cells.  

Toxic effects include¹: 

• Cardiac: Verapamil and diltiazem suppress cardiac contractility, sino-atrial node automaticity and atrioventricular node conduction. Dihydropyridines exert less of an effect on cardiac pacemaker cells and myocardial contractility.  

• Vascular smooth muscle: Verapamil and dihydropyridines cause profound vasodilation and consequently systemic hypotension. Diltiazem causes less vasodilation.  

• Metabolic: hypoinsulinaemia and insulin resistance.  

  • There is impairment of the myocardial adaptive response of moving from free fatty acids as a preferred substrate to carbohydrates. This is due to impaired uptake of glucose and an impairment of calcium-dependent mitochondrial activity required for glucose catabolism.