Supraventricular tachycardias

These arise from the atrium or the atrioventricular junction. Conduction is via the His-Purkinje system and the QRS shape during tachycardia is usually similar to that seen in the same patient during baseline rhythm

Sinus tachycardia

• Sinus tachycardia is a physiological response during exercise and excitement. It also occurs in fever, anaemia, heart failure, and thyrotoxicosis, acute pulmonary embolism, hypovolaemia and drugs (e.g. catecholamines and atropine). Treatment is aimed at correction of the underlying cause. If necessary, Beta-blockers may be used to slow the sinus rate, e.g. in hyperthyroidism

Atrioventricular junctional tachycardias

• Tachycardia arises as a result of re-entry circuits in which there are two separate pathways for impulse conduction. They are usually referred to as paroxysmal SVTs and are often seen in young patients with no evidence of structural heart disease.
• Atrioventricular nodal re-entry tachycardia (AVNRT) is the commonest type of SVT. It is due to the presence of a 'ring' of conducting pathway in the atrioventricular (AV) node of which the 'limbs' have differing conduction times and refractory periods. This allows a re-entry circuit and an impulse to produce a circus movement tachycardia. On the ECG, the P waves are either not visible or are seen immediately before or after the QRS complex. The QRS complex is usually of normal shape because the ventricles are activated in the normal way, down the bundle of His. Occasionally the QRS complex is wide, because of the rate-related bundle branch block, and it may be difficult to distinguish from ventricular tachycardia.
• Atrioventricular reciprocating tachycardia (AVRT) is due to the presence of an accessory pathway that connects the atria and ventricles and is capable of anterograde or retrograde conduction, or in some cases both. Wolff-Parkinson-White syndrome is the best known type of AVRT in which there is an accessory pathway (bundle of Kent) between atria and ventricles. The resting ECG in Wolff-Parkinson-White syndrome shows evidence of the pathway's existence if the path allows some of the atrial depolarisation to pass quickly tot he ventricle before it gets through the AV node. The early depolarisation of part of the ventricle leads to a shortened PR interval and a slurred start to the QRS (delta wave). The QRS is narrow. These patients are also prone to atrial and occasionally ventricular fibrillation.
• Symptoms
• The usual history is of rapid regular palpitations usually with abrupt onset and sudden termination. Other symptoms are dizziness, dyspnoea, central chest pain and syncope. Exertion, coffee, tea or alcohol may aggravate the arrhythmia
• Acute management
• The aim of treatment is to restore and maintain sinus rhythm:
• Unstable patient - emergency cardioversion is required in patients whose arrhythmia is accompanied by adverse symptoms and signs
• Haemodynamically stable patient - increase vagal stimulation of the sinus node by the Valsalva manoeuvre (ask the paitent to blow into a 20-mL syringe with enough force to push back the plunger) or right carotid sinus massage (contraindicated in the presence of a carotic bruit)
• Adenosine is a very short acting AV nodal blocking drug that will terminate most junctional tachycardias. Other treatments are intravenous verapamil or beta blockers, e.g. metoprolol, verapamil is contraindicated with beta lockers, if the QRS is wide and therefore differentiation from VT difficult or there is AF and an accessory pathway

Long term management

• Radiofrequency ablation of the accessory pathway via a cardiac catheter is successful in about 95% of cases. Flecainide, verapamil, sotalol and amiodarone are the drugs most commonly used.