Narrow Complex Tachycardia

Question 1

What is narrow complex tachycardia?

Answer 1

ECG shows rate of >100 bpm and QRS complex duration of <120 ms.

Narrow complex tachycardias occur when the ventricles are depolarised via the normal conduction pathways.

Question 2

Briefly describe the normal narrow complex tachycardia

Answer 2

Initiated by the sinoatrial node (SAN), electrical activity spreads around the atria. The atrioventricular node (AVN) receives this activity, pauses, then passes it on, down the bundle of His which splits left and right bundle branches. These cause depolarisation of the ventricular myocardium from bottom (apex) to top (outflow tracts).

Question 3

Give examples of arrythmias that cause narrow complex tachycardias

Answer 3

• Sinus tachycardia
• Atrial tachycardia
• Multifocal atrial tachycardia
• Junctional rhythms
• Atrial flutter
• Atrial fibrillation
• Atrio-ventricular node re-entrant tachycardia (AVNRT)
• Atrio-ventricular re-entrant tachycardia (AVRT)

Question 4

What is sinus tachycardia?

Answer 4

Conduction occurs in ‘normal’ matter but impulses are initiated at a higher frequency.

Causes include infection, pain, exercise, anxiety, dehydration, bleed, systemic vasodilation, drugs, anaemia, pulmonary embolism and hyperthyroidism.

Question 5

What is focal atrial tachycardia?

Answer 5

A group of atrial cells act as a pacemaker, outpacing the SAN. P-wave morphologhy is different to sinus.

Often seen in patients with chronic lung disease.

Question 6

What is multifocal atrial tachycardia?

Answer 6

Multiple groups of atrial cells act as a pacemaker, outpacing the SAN. P-wave morphologhy is different to sinus.

Question 7

What is junctional rhythms?

Also known as junctional tachycardia

Answer 7

Cells in the AVN become the pacemaker, giving narrow QRS complexes as impulses reach the ventricles through the normal routes. P-waves may be inverted and late.

Question 8

What is atrial flutter?

Answer 8

Electrical activity circles the atria 300 times per minute, giving a ‘sawtooth’ baseline. The AVN passes some of these impulses through, resulting in ventricular rates that are factors of 300 (e.g. 150, 100 and 75).

Question 9

What is atrial fibrillation?

Answer 9

A chaotic and irregular atrial rhythm at 300-600 bpm; the AV node respond intermittently, hence an irregular ventricular rhythm.

Question 10

What is atrioventricular re-entry tachycardia (AVRT)?

Answer 10

An accessory pathway (e.g. Wolff-Parkinson-White) allows electrical activity from the ventricles to pass to the resting atrial myocytes, creating a circuit: atria-AVN-ventricles-accessory pathway-atria.

This direction is called ‘orthodromic’ conduction and results in narrow QRS complexes as ventricular depolarisation is triggered via the Bundle of His. Conduction in the other direction is called ‘antidromic’ and results in broad QRS complexes.

Question 11

What is atrioventricular nodal re-entry tachycardia (AVNRT)?

Answer 11

Circuits form within the AVN causing narrow complex tachycardias. This is very common.

Question 12

Briefly describe how narrow complex tachycardias are treated

Answer 12

If the patient is compromised use DC cardioversion.

Identify and treat the underlying rhythm.

If AVNRT or AVRT are suspected, consider transiently blocking the AVN. This should break the atrio-ventricular re-entry rhythm, allowing sinus rhythm to re-establish. If the underlying rhythm is not atrial in origin (e.g. flutter), AVN blockade will not treat the rhythm but the paused ventricular activity will unmask the rhythm therefore aiding diagnosis and management.

AVN blockade can be achieved by:

1. Vagal manoeuveres (e.g. carotid sinus massage or Valsalva manoeuveres)
2. IV adenosine

Question 13

Why is adenosine used in narrow complex tachycardia?

Answer 13

Leads to transient AV block. It has a short half-life (10-15s) and works by:

1. Transiently slowing ventricles to show the underlying atrial rhythm
2. Cardioverting a junctional tachycardia to sinus rhythm