Beta-adrenoceptor antagonists bradycardia

Q10 Beta-adrenoceptor antagonists are contraindicated in patients with asthma or respiratory obstructive diseases, bradycardia, heart block or heart failure. Adrenergic agonists are contraindicated in patients with closed-angle glaucoma and should be used cautiously in patients with hypertension or heart disease. Parasympathomimetics cause poor night vision and dimming of vision, because of development of miosis, headache and brow ache. Carbonic anhydrase inhibitors have a weak diuretic action and can induce depression, drowsiness, paraesthesia, electrolyte disturbance such as hypokalaemia, acidosis and lack of appetite. 

Beta-blockade can result in sinus bradycardia, because blockade of sympathetic tone allows unopposed parasympathetic activity. Drugs with partial agonist activity may prevent bradycardia (52). However, heart rates under 60/ minute often worry the physician more than the patient in a retrospective study of nearly 7000 patients taking beta-adrenoceptor antagonists, apart from dizziness in patients with heart rates under 40/minute (0.4% of the total group), slow heart rates were well tolerated (53). 

The bradycardia produced by digoxin can be enhanced by beta-adrenoceptor antagonists. Neostigmine enhances vagal activity and can aggravate bradycardia (410). An apparent interaction between sotalol and thiazide-induced hypokalemia, resulting in torsade de pointes (411), has prompted the withdrawal of the combination formulation Sotazide. 

Cardiac dysrhythmias in digitalis overdose should be treated only if they are life-threatening. Phenytoin is probably the treatment of choice for ventricular tachydysrhyth-mias, but lidocaine or a beta-adrenoceptor antagonist, such as propranolol, are options. After an overdose of 300 tablets of digoxin (plasma digoxin concentration 50 ng/ml), recurrent ventricular fibrillation was successfully treated with bretylium tosylate (191). Sinus bradycardia may respond to atropine. 

Clomethiazole, like the benzodiazepines, has an additive effect with other CNS depressants, and can cause profound bradycardia when combined with beta-adrenoceptor antagonists. 

Hypertension (monoamine oxidase inhibitors) clotting (tranexamic acid) Bradycardia (beta-adrenoceptor antagonists) QT interval prolongation (antiarrhythmic drugs)... 

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