Narrow complex tachyarrhythmias have a QRS duration <0.12 seconds. They arise above the bundle of His.

NARROW COMPLEX TACHYCARDIA

As narrow complex tachycardias involve ventricular activation through the normal His-Purkinje system, they must originate within the atria and are therefore often referred to as supraventricular tachycardia (SVT). There are five common types of SVT. They are: Atrial tachycardia, Atrial fibrillation, Atrial flutter, Atrioventricular nodal
re-entry tachycardia, Atrioventricular re-entry tachycardia. When faced with an ECG of narrow complex tachycardia, (i) we should examine the P-wave and (ii) check the QRS regularity

SINUS ARRHYTHMIA/TACHYCARDIA/BRADYCARDIA (from SA Node)

ATRIAL FIBRILLATION 

There is completely disorganised atrial activity, with P-waves replaced by an irregular baseline due to fibrillation waves, and QRS complexes occur in an irregularly irregular fashion (Please the post on AF)

ATRIAL FLUTTER 

There is a self-perpetuating wave of atrial depolarisation usually circulating within the right atrium, causing regular, saw-toothed flutter waves at 300 bpm and QRS complexes every second, third, or fourth flutter wave. We can see classical sawtooth flutter waves.Drug control of the ventricular rate is not often successful.

ATRIAL TACHYCARDIA 

There is an abnormal atrial focus driving the ventricular rate. This rhythm can be difficult to distinguish from sinus tachycardia, but P-wave morphology and axis is usually abnormal. If the atrial focus is close to the AV node, a junctional tachycardia may occur and P-waves may be absent.

In case of Atrial tachycardia with AV block after halting glycoside therapy (and ensuring normokalaemia), lidocaine 1 mg kg−1 IV is the drug
of choice. Alternatively DC cardioversion or atrial
pacing may be effective.

ATRIO VENTRICULAR NODAL REENTRY TACHYCARDIA (AVNRT)

This is the commonest type of paroxysmal supraventricular tachycardia (PSVT). It is often seen in people without any heart disease, and is usually benign. There is a rapid reentry circuit within the AV node resulting in simultaneous atrial and ventricular depolarisation. The P-wave is usually buried within the QRS or ST-segment. There will be fast regular narrow complex tachycardia, and P-waves can be seen buried in the terminal portion of the QRS complex which may easily be mistaken for a second, small R-wave. The very close proximity of the QRS and P-waves implies near simultaneous depolarisation of atria and ventricles.

ATRIO VENTRICULAR REENTRY TACHYCARDIA (AVRT)

This occurs in patients with WPW, and is usually benign unless there is coexisting structural heart disease. There is an accessory pathway bridging the atria and ventricles allowing antegrade conduction down the AV node (causing a narrow QRS) and retrograde conduction back to the atria via the accessory pathway. Since the depolarisation wave takes time to complete this circuit, the P-wave occurs after the QRS complex and is often buried within the T-wave. AVRT can occur with antegrade conduction to the ventricles via the accessory pathway, but this will result in ventricular depolarisation via an abnormal route and consequently a broad QRS. In sinus rhythm, antegrade conduction via the accessory pathway produces a short PR interval (as the normal delay in the AV node is avoided) and the abnormal activation of the ventricles produces a slurred upstroke
in the QRS called a delta wave. The QRS complex is said to be pre-excited and can be associated with repolarisation abnormalities. There are seven sinus beats followed by a ventricular ectopic beat that conducts to the atria retrogradely through the atrioventricular node and then returns to the ventricles via the accessory pathway. This cycle repeats and triggers a broad complex tachycardia. (Please see post on ‘WPW Syndrome’ also).

An unstable patient presenting with a regular narrow complex tachycardia should be treated with electrical cardioversion. If this is not immediately available, adenosine should be given as a first-line treatment. A stable patient presenting with a regular narrow complex tachycardia should initially be treated by vagal
manoeuvres such as carotid sinus massage or the Valsalva manoeuvre, as these will terminate up to a quarter of episodes of PSVT. Carotid sinus massage should be avoided
in the elderly, especially if a carotid bruit is present, as it may dislodge an atheromatous plaque and cause a stroke

Management

A stable patient presenting with a regular narrow complex tachycardia should initially be treated by vagal manoeuvres such as carotid sinus massage or the Valsalva manoeuvre, as these will terminate most episodes of PSVT. Carotid sinus massage should be avoided in the elderly, especially if a carotid bruit is present, as it may dislodge an atheromatous plaque and cause a stroke. If the tachycardia persists and is not atrial flutter, 6 mg of adenosine should be given as an IV bolus, followed by a 12 mg bolus if no response. A further 12 mg bolus of adenosine may be given if the tachycardia persists. Vagal manoeuvres or adenosine will terminate almost all AVNRTs or AVRTs within seconds, and therefore failure to convert suggests an atrial tachycardia such as atrial flutter. If adenosine is contraindicated, or fails to terminate a narrow complex tachycardia, without first demonstrating it as atrial flutter, give a calcium-channel blocker, e.g. verapamil 2.5–5 mg IV over two minutes. Atrial flutter should be treated by rate control with a beta-blocker.

An irregular narrow complex tachycardia is most likely to be atrial fibrillation (AF) with an uncontrolled ventricular response, but may also be atrial flutter with variable block. If the patient is unstable, synchronised electrical cardioversion should be used to treat the arrhythmia