Introduction
Supraventricular tachycardias (SVT) arise from the atrium and atrioventricular junction. They are characterised by fast rhythms with narrow QRS complexes. The majority of sustained regular tachycardias are due to re-entry mechanisms or increased automacity.
Atrial ectopic
Premature depolarisation from an atrial source.
ECG findings
Premature, abnormally shaped P wave.
Atrial tachycardia
Atrial rate of 120-250 beats per minute, usually from an ectopic source. An uncommon arrhythmia that accounts for 10% of all SVTs.
ECG findings
Regular rhythm with atrial rate >120 beats per minute. P wave morphology is abnormal.
Atrial flutter
Organised atrial rhythm with an atrial rate of 250-300 bpm. It is usually associated with atrial fibrillation and, thus, should be managed similar to it.
ECG findings
P waves have a
sawtooth pattern with and atrial rate of 300 beats per minute. There is usually
2:1 atrioventricular block, so the heart rate is 150 beats per minute.
Atrial fibrillation
Ineffective, chaotic, irregular and rapid (300-600 beats per minute) atrial activity. This results in deterioration of mechanical function. It can be classified as paroxysmal (intermittent, recurrent and self-terminating episodes), persistent (lasts longer than 7 days and does not spontaneously terminate) or permanent.
ECG findings
No P waves,
irregular QRS complexes. There is normal, narrow QRS complex morphology and T wave shape
Atrioventricular nodal re-entry tachycardia (AVNRT)
Re-entrant SVTs occur due to an additional, abnormal electrical connection between the atria and ventricles. It is the commonest cause of SVTs. The abnormal connection is found within the atrioventricular node (AVN). In AVNRT, there are
2 distinct electrical pathways in the AVN:
- Fast conduction with long refractory period pathway(normal pathway)
- Slow conduction with short refractory period pathway (abnormal pathway)
Normally, impulses are conducted along the fast pathway. If an atrial impulse occurs early (e.g. ectopic beat), the slow pathway can take over. This allows the impulse to go down the slow pathway and then travel back up to the atrium via the fast pathway, introducing a re-entry circuit between the fast and slow pathways.
ECG findings
Similar to atrial tachycardia. Regular rhythm with narrow complex QRS at a rate of 120-250 beats per minute.
Atrioventricular re-entry tachycardia (AVRT)
Re-entrant SVTs that occur due to an additional, abnormal electrical connection between the atria and ventricles. The
abnormal connection is found outside the AVN. Wolff-Parkinson-White syndrome (WPW) is the commonest cause of AVRT. There is an abnormal band of conducting tissue (bundle of Kent) connecting the atria and ventricles. It conducts very rapidly with a short refractory period, producing pre-excitation. Since the AVN and accessory pathway have different conduction velocities and refractory periods, a re-entry circuit can develop to produce tachycardias.
ECG findings in pre-excitation and WPW
Short PR interval,
wide QRS and
delta waves (slurred upstroke in QRS complex). During tachycardias, P waves and delta waves are lost and QRS width is normal.
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