Antiarrhythmics
From WardWiki - Foundation Doctor Helper
Antiarrhythmics are a very important class of drugs for foundation doctors as they are life saving in cardiac arrest and peri arrest situations. They are very commonly used for other indications, increasing junior exposure to their use, but their toxic effects can cause circulatory collapse in minutes, requiring vigilance by all involved in their use. They are some of the most useful and dangerous medications used by hospital doctors.
This article aims to give brief descriptions of these drugs systematically and safely, giving examples of their use and problems commonly encountered with them. This does not substitute in any way for sound, formal teaching in their use and wardwiki strongly recommends they only be used by doctors with specific training in them such as an ALS course or more specialised teaching by cardiologists in hospital.
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Vaughan Williams Class 1
These drugs all regulate fast Na channels to slow the heart down, they do have pro-arythmic side effects however. This is the easiest class of all for foundation doctors - DON'T INITIATE ANY OF THEM although lidocaine is a commonly used local anaesthetic agent that does find occaisional use in pulsed VT. They really are for cardiology cconsultant lead use only
Vaughan Williams Class 2
These are the beta blockers, used also for hypertension and heart failure. They reduced automaticity during Phase 4 of the cardiac cycle and slow AV conduction.
Vaughan Williams Class 3
These work by potassium channel blockade during repolarisation. This can arrythmias at low heart rates and their use is certainly contraindicated in bradycardia. Pure Class 3 drugs are used in the US but in the UK the drugs available are Class 3 plus at least one other class in their action.
- Amiodarone is a multiclass antiarrythmic agent
- Sotalol is a beta blocker with considerable Class 3 action, overall it LENGTHENS the QT interval.
Vaughan Williams Class 4
These are non-dihydropyridine calcium channel antagonists, verapamil and diltiazem. Their action is broadly similar to beta blockers though bradycardia and hypotension can be significant problems with verapamil whose use continues to decline as the safety profile of modern beta blockers has become established. They remain a useful alternative, especially in asthmatics.
Adenosine
A powerful negative dromotrope that is of use in SVT termination but its very short half life (measured in seconds) must be noted. It will also slow down AF and flutter and make those rhythms easier to interpret. It can in theory cause bronchospasm but again consideration should be given to the half life. It must be flushed through rapidly with saline to harness its effects. Start with 6mg and then up to two doses at 12mg before considering amiodarone.
Atropine
This is a powerful vagolytic that is given in increments of 500mcg as in the ALS bradycardia algorithm. The maximum dose is 3mg; it is not effective beyond this dose. It is now no longer recommended for use in asystole or PEA arrest. It causes dry eyes, tachycardia and rarely closed angle glaucoma due to its antimuscarinic action. It is also used in overdose management.
Cardiac glycosides
Digoxin, which is renally excreted, and the much less used digitoxin which is hepatically excreted are important cardiac glycosides. Their main use in controlling the ventricular response in atrial fibrillation and too improve cardiac output in NYHA III and IV heart failure. Bolusing these drugs and giving them in hypokalaemia is patients is very dangerous and shouldn't not be attempted.
