Anti-arrhythmics: Uses, Complications, Action, Dosage by Nursesnote

 Anti-arrhythmics

Anti-arrhyhmics drugs frequently used in the critical care areas

Common drugs for supraventricular tachyarrhythmias (SVT)

• Adenosine
• Amiodarone
• Digoxin
• ß-blockers
• Verapamil
• Magnesium

Common drugs for ventricular tachyarrhythmias (VT)

• Lidocaine
• Amiodarone
• Flecainide
• ß-blockers
• Magnesium

Uses of anti -arrhythmics

• Ventricular tachyarrhythmias and supraventricular tachyarrhythmias
• Differentiation between supraventricular and ventricular arrhythmias during administration of adenosine.

Complications and Side effects

• Anti-arrhythmic are negatively inotropic, may leads to hypotension (ß-blockers, verapamil) or bradycardia (ß-blockers, digoxin, amiodarone )
• All A-V blockers are contraindicated in re-entry tachycardia (Wolff-Parkinson-White syndrome)

Nurses note

• Amiodarone: Useful against all types of tachyarrhythmia. Usually given by initial loading dose then IV infusion. When converting from IV to oral dosing, initial high oral dosing (200 mg tds) is still required. Contraindicated in patients with thyroid dysfunction. Has low acute toxicity through may cause severe bradycardia and both chronic and acute pulmonary fibrosis. Avoid with other class lll agents (e.g. sotalol). Must be given via central vein as causes peripheral phlebitis. Can also cause pulmonary fibrosis and abnormal thyroid function.
• Adenosine: Very short-acting; may revert paroxysmal DVT ( other than atrial flutter and fibrillation) to sinus rhythm. Ineffective for VT. Contraindicated in 2° and 3° heart block,sick sinus syndrome, asthma. May cause flushing, bronchospasm, and occasional severe bradycardia, especially in patients taking dipyridamole.
• ß-blockers: for SVT. Caution when using ß-blockers with verapamil.
• Digoxin: Slow-acting , requires loading (1-1.5 mg) to achieve therapeutic plasma levels over 12-24h ( 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. Contraindicated in 2° and 3° heart block. May cause bradycardia and CNS side effects, e.g drowsiness, seizures.
• Verapamil: Should generally not be given with ß-blockers are profound hypotension and bradyarrhythmias may result. Pre-treatment with 3-mL 10% calcium gluconate by slow IV bolus prevents the hypotensive effects of verapamil without affecting it's anti-arrhythmic properties.

Mode of action anti-arrhythmic drugs

Class l : 

Action : Reduce rate of rise of action potential: 

• la: Increase action potential duration e.g. disopyramide.
• lb: Shortens duration. e.g. lidocaine
• lc: Little effect. eg. flecainide 

Class ll 

Action: Reduce rate of pacemaker discharge. e.g.  ß-blockers.

Class lll: 

Action: Prolongs duration of action potential and hence length of refractory period. e.g. Amiodarone, sotalol.

Class lV: 

Action: Antagonises transport of calcium across cell membrane e.g. Verapamil, Diltiazem.

Dosages

Adenosine

6mg rapid IV bolus. If no response after 1 min, give 12 mg, If no response after 1 min, repeat 12 mg.

Amiodarone

5mg/kg over 20 min (or 150-300mg over 3min in emergency), the IV infusion of 15mg/kg/24h in 5% glucose via central vein. Reduce thereafter to 10 mg/kg/24h (approx, 600mg/day) for 3-7d, then maintain at 5mg/kg/24h (300-400mg/d).

ß-blockers

Esmolol: A titrated loading dose regimen is commenced followed by an infusion rate of 50-200mcg/kg/min.

Propranolol:

 Initially given as slow IV boluses of 1 mg, repeated at 2min intervals to a maximum of 5 mg.

Labetalol: 0.25-2mg/min.

Sotalol:

Dosage range is 20-120mg IV (0.5-1.5mg/kg) administered over 10min. Repeat 6-hourly, if necessary.

Metoprolol:

 2.5-5 mg injected IV at rate of 1-2 mg/min, repeated at 5 min intervals until satisfactory response is seen or total dose of 10-15mg given.

Atenolol: 

2.5mg IV over 2.5min, repeated at 5min intervals until response is seen or maximum of 10mg.

Digoxin:

0.5mg given IV over 10-20min. Repeat at 4-8h intervals until loading achieved (assessed by clinical response). Maintenance dose thereafter is 0.0625-0.25mg/d, depending on plasma levels and clinical response.

Lidocaine: 

1mg/kg slow IV bolus for loading, then 2-4mg/min infusion. Should be weaned slowly over 24h.

Verapamil:

2.5mg slow IV. If no response, repeat to maximum of 20mg. An IV infusion of 1-10mg/h may be tried. 10% calcium gluconate solution should be readily available.