All antiarrhythmic agents have side-effects; other than digoxin they are negatively inotropic to greater or lesser degrees and may induce profound hypotension (e.g. verapamil, p-blockers) or bradycardia (e.g. p-blockers, amiodarone, digoxin, lidocaine). p-blockers in particular should be used with caution because of these effects.

All A-V blockers are contraindicated in re-entry tachycardia (e.g. Wolff-Parkinson-White syndrome).

• Adenosine: very short-acting; may revert paroxysmal SVT to sinus rhythm. Ineffective for atrial flutter and fibrillation, VT. Contraindicated in 2° and 3° heart block, sick sinus syndrome, asthma. May cause flushing, bronchospasm and occasional severe bradycardia.

• Amiodarone: effective against all types of tachyarrhythmia. Usually given by IV infusion for rapid effect but requires initial loading dose. When converting from IV to oral dosing, initial high oral dosing (200mg tds) is still required. Contraindicated in patients with thyroid dysfunction. Has low acute toxicity, though may cause severe bradycardia and both chronic and acute pulmonary fibrosis. Avoid with other Class III agents (e.g. sotalol). Must be given via central vein as causes peripheral phlebitis.

• p-blockers: for SVT, esmolol is preferred due to its short half-life though may cause vasodilatation. Initially, increasing loading doses required; an infusion may be needed thereafter. Propranolol can be given by slow IV boluses of 1mg repeated at 2min intervals to a maximum of 5mg). Do not give p-blockers with verapamil.

• Bretylium: may take 15-20min to take effect; now used predominantly for resistant VF/VT. CPR should be continued for at least 20min.

• Digoxin: slow-acting, requires loading (1-1.5g) to achieve therapeutic plasma levels which can be monitored. Loading ideally given over 12-24h but can be done over 4-6h. Contraindicated in 2° and 3° heart block. May cause severe bradycardia. Low K+ and Mg2+ and markedly raised Ca2+ increase myocardial sensitivity to digoxin. Amiodarone raises digoxin levels.

• Lidocaine: 10ml of 1% solution contains 100mg. No effect on SVT. Loading achieved by 1mg/kg slow IV bolus followed by infusion. Contraindicated in 2° and 3° heart block. May cause bradycardia and CNS side-effects, e.g. drowsiness, seizures.

• Verapamil: should not be given with p-blockers as profound hypotension and bradyarrhythmias may result. Pretreatment with 3-5ml 10% calcium gluconate by slow IV bolus prevents the hypotensive effects of verapamil without affecting its antiarrhythmic properties.


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