11/14/24, 10\:54 AM Narrow Complex Tachycardia (Supraventricular Tachycardia)

Narrow Complex Tachycardia (Supraventricular Tachycardia)

Table of contents
Key points ⚡
Succinct notes to superpower your revision
Supraventricular tachycardia (SVT)\: rapid heart rate originating above/within the atrioventricular (AV) node; not a
diagnosis, but a starting point for diagnosis.
Focal Tachycardias\:
Sinus tachycardia\: focal tachycardia from the sinoatrial node, usually regular.
Atrial tachycardia\: di
Multifocal atrial tachycardia\: P waves with di
Junctional rhythms\: AV node takes over when sinoatrial node is non-functional; P waves often hidden in QRS complex.
Management of Focal Tachycardias\:
Inappropriate sinus tachycardia\: beta-blockers or ivabradine.
Appropriate sinus tachycardia\: treat underlying cause (e.g., sepsis, anaemia).
Atrial tachycardias\: rate control with beta-blockers or calcium channel blockers.
Re-entry Tachycardias\:
Atrial
Atrial
AVNRT\: AV node re-entrant tachycardia, with a circuit within the AV node; induces AV node blockade to terminate.
AVRT\: atrio-ventricular re-entrant tachycardia involving normal AV conduction and accessory pathway (AP)\:
Orthodromic\: down AV node, up AP; narrow QRS complex.
Antidromic\: down AP, up AV node; broad QRS complex with delta waves.
Management of Re-entry Tachycardias\:
Haemodynamically unstable\: DC cardioversion.
Atrial
Rate control\: beta-blockers, calcium channel blockers, digoxin.
Rhythm control\: amiodarone,
Ablation for
Anticoagulation based on CHADS2 VASc score.
AVNRT\: induce AV node blockade (adenosine, beta-blockers, verapamil).
AVRT\:
Avoid AV node blockade; electrical cardioversion or
Ablation of the accessory pathway to prevent VF and sudden death.
Adenosine\: causes transient complete AV node blockade; diagnostic tool for narrow complex tachycardias.
Administer fast with saline
Observe for AV block; if not seen, likely ventricular tachycardia.
Terminates AVNRT or orthodromic AVRT; reveals atrial activity in other SVTs.
Summary\: SVTs can be challenging to diagnose; systematic ECG analysis and adenosine can aid in diagnosis and
management.
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Everyone loves the term 'supraventricular tachycardia'
. It describes any fast heart rate that you aren't sure what it is and
usually triggers an immediate cardiology referral after a certain amount of panic. However, with a bit of thought (and
possibly some adenosine), the underlying diagnosis can be revealed and appropriate treatment can be started.
Firstly, a statement...SUPRAVENTRICULAR TACHYCARDIA IS NOT A DIAGNOSIS
A 'supraventricular tachycardia' is a rapid heart rate that originates from above or within the atrioventricular (AV) node. It is a
blanket term that includes a lot of di
myriad of possibilities, a bit of basic electrophysiology terminology needs to be applied.

Focal Tachycardias

The tachycardia originates from a single point (or points) in the atrium or AV node. Also knows as 'enhanced automaticity'
–
the sinus node is meant to be the most autonomic part of the heart and as such takes charge. If another part of the heart
becomes MORE autonomic (or the sinus node becomes LESS autonomic), it takes over and a focal tachycardia results. This
means there will be an organised atrial contraction and a wave similar to a P wave will appear before the QRS complex.

Sinus tachycardia

Yes, this is a focal tachycardia
The 'focus' is the sinoatrial node!
Usually regular
Sinus tachycardia

Atrial tachycardia

A di
complexes
Often seen in patients with chronic lung disease
Normally > 100 bpm and regular
'Normal' P waves should LOOK like a regular P wave and should be POSITIVE in the inferior leads (as they originate from
the top of the atrium and propagate towards the inferior lead)
Atrial tachycardia is common amongst patients with concomitant lung disease (e.g. COPD)
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Atrial tachycardia

Multifocal atrial tachycardia

The name says it all!
The P waves will have di
Similar to atrial tachycardia in all other respects.

Junctional rhythms

The atrioventricular node is the next most autonomic piece of tissue in the heart after the sino-atrial node. If the sinoatrial
node is not functioning (e.g. high vagal tone, sick sinus syndrome), the AV node takes over.
The impulse originates from the AV node and propagates to the atrium and the ventricles simultaneously.
P waves are often hidden in the QRS complex.
Junctional rhythm
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Management

INAPPROPRIATE sinus tachycardia can be slowed using beta-blockers or ivabradine (selective sinus node blocker),
however it is usually best left alone.
APPROPRIATE sinus tachycardia (due to concurrent sepsis/anaemia/thyrotoxicosis/pain) should be left alone and the
underlying trigger treated.
Atrial tachycardias can usually be rate controlled with beta-blockers or calcium channel blockers.

Re-entry tachycardias

These account for more than 75% of all tachyarrhythmias, both atrial and ventricular.

Atrial

Well known for its sawtooth baseline (see below), atrial
This means there is a single large re-entry circuit around the atrium which stimulates the AV node every time it passes.
TYPICAL atrial
Atrial
ATYPICAL
di
tachycardia at or around 150bpm.
The sawtooth appearance is best seen in the inferior leads and is caused by the circuit alternately heading towards the
inferior leads and away as it speeds around the atrium.
As the circuit is
chamber,
The ventricular rate then depends on the degree of AV block\:
2\:1 = 150bpm
3\: 1 = 100bpm
Variable – can produce an irregularly irregular rhythm

Atrial

The mechanism of AF is complex and not entirely clear. The easiest way to think of it is as a MICRO-RE-ENTRANT
tachycardia with lots of tiny circuits contributing to the chaotic and random
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As such, you can sometimes see the very rapid F (
AF is irregularly irregular because of this disorganised activity – the rate, again, depends on the degree of AV block

AVNRT (atrio-ventricular node re-entrant tachycardia)

This is the rhythm that always gets labelled as an 'SVT'
.
AVNRT
Because it is technically WITHIN the node, the resulting circuit activates both the ventricles and atria almost
simultaneously. Two anatomical pathways next to the AV node (the s l o w and f a s t pathways) form a circuit with very rapid
conduction that produces a rapid regular tachycardia. This is seen as a 'pseudo R wave' (see the RED arrows) which is
actually the retrograde P wave superimposed on the QRS complex. Treatment involves inducing complete transient AV
node blockade.

AVRT (atrio-ventricular re-entrant tachycardia)

This requires two pathways – the normal AV conduction system and an accessory pathway (AP).
Accessory pathways can conduct\:
ANTEGRADE (atria to ventricles) which is shown as pre-excitation on the resting ECG (the mythical DELTA wave)
RETROGRADE (ventricle to atria) which is not seen on a resting ECG
Not all AP's can form AVRT circuits – this is because they have a long 'refractory period' which is the time it takes the
pathway to become active again after it has conducted an impulse. Therefore the impulse has travelled the rest of the
circuit BEFORE the AP is ready to receive it again and the circuit is terminated. These are 'safe' pathways and do not need
treatment.
Types of AVRT's
ORTHODROMIC – down the AV node and up the accessory pathway (in green)
As the impulse goes down the normal conduction pathway, the QRS complex is narrow.
The impulse then propagates slowly across the ventricular myocardium before reaching the accessory pathway and
heading back to the atrium resulting in a delay in atrial activation. This results in a LONG RP interval (marked on the
above diagram) before the P wave (arrow) which marks the atrial contraction.
COMPARE this to the AVNRT section above where the P wave is almost superimposed on the QRS complex (a
pathognomonic feature).
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Orthodromic AVRT
ANTIDROMIC – down the accessory pathway and up the normal AV conduction
As the impulse goes down the AP, the ventricular contraction starts in an abnormal way (pre-excitation).
Therefore, the ECG will have a broad QRS complex caused by the presence of delta waves.
DO NOT USE AV NODAL BLOCKING AGENTS. This promotes transmission down the accessory pathway and if there is
AF, the
Antidromic AVRT
Management of AVRT's
therapy later!
sudden death.
The safest way to deal with AVRT's is to electrically cardiovert the patient back to sinus rhythm and worry about medical
Flecainide can be used to prevent AVRT's but ablation is usually the mainstay of treatment given the risk of VF and
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Treating re-entrant 'SVT's'

As you can see, the varying mechanisms of the di
IF THE PATIENT IS HAEMODYNAMICALLY UNSTABLE D U E T O T H E U N D E R L Y I N G R H Y T H M , DC CARDIOVERSION IS THE
TREATMENT OF CHOICE (as per ALS algorithms).

Atrial Fibrillation and Atrial Flutter

RATE control\:
Beta-blockers
Calcium channel blockers
Digoxin
RHYTHM control\:
Chemical cardioversion (amiodarone,
Electrical cardioversion (DC shock)
Ablation\:
More likely to succeed in atrial
More likely to succeed in paroxysmal AF than persistent AF
1 in 3000 risk of death during the procedures
Anticoagulation\:
According to stroke risk vs bleeding risk (CHADS2 VASc score)
Choosing a treatment strategy
Which strategy you adopt depends on the patient and a couple of key facts\:
AF is NOT life-threatening
A patient with well-controlled AF and normal LV function has the same life expectancy as someone in sinus rhythm.
Therefore, symptomatic patients with well rate controlled AF will not feel better or live longer by returning them to sinus
rhythm!
Patients who should be considered for cardioversion\:
New-onset AF
Symptomatic AF
LV dysfunction secondary to AF
Patients with a reversible cause of AF (e.g. post-surgical, post-infection)
Patients who have a reasonable chance of maintaining sinus rhythm\:
Previous successful cardioversion – a l t h o u g h s u b s e q u e n t c a r d i o v e r s i o n s a r e o f t e n l e s s a n d l e s s s u c c e s s f u l i n t e r m s
o f t i m e s p e n t i n s i n u s r h y t h m
Minimal dilatation of the left atrium
No signi

AVNRT

Induce AV node blockade and the tachycardia is terminated\:
Mechanical (vasovagal manoeuvres, carotid sinus massage)
Chemical (adenosine, beta-blockers, verapamil)

AVRT

AVOID AV node blockade as this can promote conduction down the AP and AF can transmit to VF.
Electrical cardioversion is the best initial option although
Ablation of the pathway is a curative option. Should be considered in all antidromic AVRT's because of risks of VF and
death if AF were to occur, symptomatic retrograde AVRT's and is MANDATORY in airline pilots and deep-sea divers!
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Using Adenosine

Adenosine is a purine nucleoside that, administered intravenously, causes transient complete AV node blockade. It is a
useful diagnostic tool as well as a treatment in the setting of a narrow complex tachycardia.
NOTE\:
Adenosine is safe in pregnancy
It can also usually be used safely in asthmatics unless they have brittle severe asthma

Administering adenosine

1. Place a LARGE (green or grey) cannula in the antecubital fossa.
2. Ensure the patient is connected to 12 lead monitoring which can produce a rhythm strip.
3. Draw up the adenosine in the smallest syringe possible (2-5ml depending on the dose).
4. Draw up a 20ml normal saline
5. Connect the adenosine syringe to the top port of the cannula and the
medication will mean that it will not reach the heart in time!
6. Warn the patient that they will about the upcoming 'sense of impending doom' that will pass after a few seconds.
7. Start the 12 lead rhythm strip recording.
8. Give the adenosine AS FAST AS POSSIBLE followed IMMEDIATELY by the full saline
9. Observe the rhythm strip for AV block.

E

If you have administered the adenosine correctly, you will see a few seconds of total AV block (a complete pause in
ventricular contraction on the rhythm strip). DO NOT PANIC!!!
If you do NOT see AV block, one of two things have occurred\:
1. You haven't given it properly and the adenosine has not reached the heart in time given its very short half-life. Try again
with a bigger dose and faster
2. The rhythm is not involving the AV node – it is, therefore, VENTRICULAR TACHYCARDIA.
If you terminate the tachycardia, the rhythm is an AVNRT or an orthodromic AVRT (as you have cut the AV node out of the
circuit).
If you pause the tachycardia but it then restarts as the e
SUPRAVENTRICULAR but you have not terminated it. However, you have proved that the re-entry circuit does NOT involve
the AV node (or it would have been terminated) – such as atrial
reveal what the atria are up to and give you the diagnosis.
Example of atrial
AV block attained con
The atrial activity is shown to be
The rhythm continues, demonstrating the circuit does not involve the AV node.
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Related notes

Acute Coronary Syndrome (ACS)
Acute Heart Failure
Atrial Fibrillation (AF)
Atrioventricular Block
AF treated with adenosine
Brugada Syndrome

Summary

Test yourself

Supraventricular tachycardias can be a challenge to diagnose.
However, by systematically working through the ECG (rhythm, presence of P waves, P wave morphology) and applying a

Contents

little adenosine if needed, the diagnosis can be made and management put in place.
Focal Tachycardias
Re-entry tachycardias
Treating re-entrant 'SVT's'
Using Adenosine
Summary
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