Anti-arrhythmic drugs

Arrhythmias are due to a disturbance of the electrical impulses which regulate the heart. The heart may beat too slowly (bradycardia), too quickly (tachycardia) or in an irregular way. The normal heart rate is between 60-100 beats per minute for adults.

• Management of an arrhythmia requires precise diagnosis of the type of arrhythmia. Electrocardiography is essential.¹

• Underlying causes (eg, myocardial infarction, heart failure, thyrotoxicosis, alcohol abuse) require appropriate treatment.

Classifying anti-arrhythmic drugs

Anti-arrhythmic drugs can be classified clinically into:

• Those that act on supraventricular arrhythmias - eg, adenosine, verapamil, cardiac glycosides and beta-blockers.

• Those that act on both supraventricular and ventricular arrhythmias - eg, amiodarone, beta-blockers, disopyramide, flecainide, and propafenone.

• Those that act on ventricular arrhythmias - eg, lidocaine and moracizine.

Anti-arrhythmic drugs can also be classified according to their effects on the electrical behaviour of myocardial cells during activity but this classification is of less clinical use. The Vaughan Williams classification is based on the movement of ions (sodium, potassium, calcium) into heart cells:

• Class I: membrane-stabilising (Ia) procainamide, disopyramide, (Ib) lidocaine, (Ic) flecainide.

• Class II: reduce adrenergic input - beta-blockers.

• Class III: potassium blockers - eg, amiodarone, sotalol.

• Class IV: calcium influx blockers - eg, verapamil (but not dihydropyridines).

Sotalol has both Class II and Class III actions. Digoxin does not fit into this classification.

Presentation

• Arrhythmias can present as palpitations or with the symptoms of reduced cardiac outflow: chest pain, dyspnoea, dizziness or blackouts (typically with a rapid recovery).

• The history from an observer can be invaluable in distinguishing an arrhythmia from a cerebral event or convulsion.

• Arrhythmias range in severity from a minor inconvenience to a potentially fatal problem. They are common, particularly in the elderly. They can have a profound effect on quality of life. Appropriate information and support can relieve psychological as well as physical problems.

Main types of arrhythmias

• Bradycardia.

• Extrasystoles.

• Supraventricular tachycardia.

• Wolff-Parkinson-White syndrome.

• Atrial fibrillation.

• Atrial flutter.

• Ventricular tachycardia.

• Torsades de pointes.

• Ventricular fibrillation.

Diagnosis^{1 2}

• Accurate diagnosis of a suspected arrhythmia requires a prompt recording and archiving of a 12-lead ECG. Even if symptoms have subsided, this improves the chance of accurate diagnosis and treatment:

• High-resolution ECG averages signals to reduce background noise and can reveal areas of slow conduction.

• Supraventricular (atrial and atrioventricular (AV) node initiated) fast rhythms which are transmitted by the normal conducting pathway are generally narrow complex tachycardias.

• Tachyarrhythmias which either originate from the ventricle, or are atrial rhythms but are aberrantly propagated through the ventricular muscle rather than completely through the conducting pathway, have wider QRS complexes and are called broad complex tachycardias.

• Ambulatory ECG monitoring over 24-48 hours allows analysis of heart rate variability and matching of arrhythmia to symptoms, but is only useful if the patient has symptoms frequently over a two-day period. If the frequency is less than that, a 5-7 day ECG will be more appropriate. More detailed electrophysiological studies require cardiac catheterisation.

• All causes and effects of any arrhythmia must be thoroughly evaluated. This will depend on the particular arrhythmia and clinical context but may include:
  

  • Blood tests - eg, renal function, electrolytes, TFTs.

  • Echocardiogram - structural and functional heart abnormalities or detection of intracardiac thrombus.

  • Exercise tolerance testing.

Principles of pharmacological treatment

• Deciding on appropriate therapy depends on distinguishing supraventricular from ventricular rhythms. This is not always easy and expert advice should be sought if there is any doubt.

• All anti-arrhythmic drugs have potentially serious side-effects. They may worsen or provoke arrhythmias in certain circumstances, such as hypokalaemia. Close monitoring is therefore essential.

• The negative inotropic effects of anti-arrhythmic drugs tend to be additive when more than one drug is required. Therefore special care should be taken if two or more are used, especially if myocardial function is impaired.

Other management options available

Implantable cardioverter defibrillators can be used. A combination of one drug plus an implantable device is also appropriate in some situations. Ablation therapy provides an alternative for treating certain cardiac arrhythmias.³