Class 3 antiarrhythmics are also known as potassium channel blockers – they block the K channels responsible for repolarisation, delaying it. This therefore increases the duration of the action potential and increases the effective refractory period.
Sotalol also has Class 2 antiarrhythmic properties (being a beta-blocker).
Oral or intravenous administration is used depending on the circumstance. Intravenous administration should be through a central vein as Amiodarone is highly irritant. A loading dose is required due to the long half-life of Amiodarone. The lowest possible maintenance dose should be used and therapy reviewed regularly.
Amiodarone has a wide range of problematic long term side effects (which may be fatal) and should be used only after other treatments have failed. Some effects will not reverse on stopping the drug.
- Photosensitivity
- Skin discolouration
- Corneal microdeposits
- Pulmonary fibrosis
- Peripheral neuropathy
- Hyper or hypothyroidism
- Hepatotoxicity
- Blood dyscrasias
Baseline chest x-ray, ECG, thyroid and liver function tests should be undertaken. Regular monitoring of thyroid and liver function is required.
Solatol
Dronedarone
- Bradycardia
- Heart Failure
- QT-prolongation
- GI disturbance
Amiodarone interacts with numerous other drugs (including through the cytochromes P450 pathways) so the BNF should be checked before prescribing.
Amiodarone inhibits warfarin metabolism and may cause an increase in INR.
Amiodarone inhibits the excretion of Digoxin which may necessitate a dose reduction (the combination should be prescribed with care).
Amiodarone and dronedarone should be avoided along with other drugs which prolong the QT interval such as class 1 antiarrhythmics, some antibiotics, antihistamines and antidepressants.
Class 3 drugs should also be avoided alongside other drugs which reduce the heart rate (unless this is part of a deliberate therapeutic strategy).
Patients should be warned about the long term adverse effects of Amiodarone and the need for regular monitoring. They should be advised to use sun-block cream when outdoors. Any breathlessness should be immediately reported.
Amiodarone has an extremely long-half life with an average plasma half-life of 50 days (reported range 20-100 days) and has a high volume of distribution. This necessitates a loading regime, but also accounts for ongoing adverse effects after the drug is withdrawn.
