Class I anti-arrhythmic

The class I antiarrythmics are mostly used for atrial arrhythmias. They are used on a healthy heart to prevent recurrence of atrial fibrillation - or atrial flutter. These problems involve disorganised (fibrillation) or accelerated (flutter) contractions of the atria resulting in similar episodes in the ventricles.

How do these class I antiarrythmics work

The class I antiarrythmics interfere with the sodium channel. They reduce the rate of sodium entry into cells during the first phase of the action potential. They act as "membrane stabilisers", in other words they bind to the cell membranes and prevent sodium (Na+) and potassium (K+) ions passing through. They are divided into three families depending on their mechanism of action:

• class Ia is represented by disopyramide and the quinidine derivatives. This prolongs the action potential and the rest potential. This class is used particularly to prevent recurrence of atrial disorders;
• class Ib is represented by lidocaine, mexiletine and phenytoin. This reduces the length of the action potential and rest potential. It is normally used first line to reduce a ventricular tachycardia; class
• Ic is represented by flecainide, propafenone and cibenzoline. This does not change the action potential but prolongs the rest potential. They is used for atrial dysrhythmias.

Do they have contraindications or precautions?

Class 1 antiarrythmics are contraindicated in heart failure, ischaemic heart disease or in left ventricular dysfunction. In the latter of these situations they can have pro-arrythmogenic effects on the ventricle with a risk of triggering ventricular tachycardia, or even ventricular fibrillation.

Sources:

• Interview with Dr Joël Kerjean, cardiologist in Angers, 20 June 2011
• Principles of Internal Medicine, ed TR Harrison. Flammarion