Cardiology: Cardiac Arrhythmias - SVTs

Question 1

Describe the difference between Atrio-ventricular nodal re-entrant tachycardia (AVNRT)
and Atrio-ventricular re-entrant tachycardia (AVRT)

Answer 1

Atrio-ventricular nodal re-entrant tachycardia (AVNRT):
- Originates from a re-entrant retrograde electrical circuit involving the AV node, resulting in initiation and propagation of a cardiac tachyarrhythmia

Atrio-ventricular re-entrant tachycardia (AVRT):
- originates via a re-entrant retrograde electrical circuit
- involves an accessory pathway between the atria and the ventricles, rather than the AV node
- Some forms of AVRT may exhibit a Wolff-Parkinson-White pattern

https://www.osmosis.org/learn/Atrioventricular_nodal_reentrant_tachycardia_%28AVNRT%29

Question 2

What common ECG changes do you see in AVRT? [2]

Answer 2

Pre-excitation:
- Initial slurring of QRS complex: the first part of QRS is less steep due to activation of an accessory pathway
- Reduced PR interval

ADD PHOTO to this from lecture

Question 3

Describe what is meant by orthodrominc and antidromic conduction? [2]

Describe how these differ on an ECG [1]

Answer 3

Orthodromic and antidromic refer to the direction of accessory pathways

Orthodromic (common):
- Antegrade conduction via AVN, retrograde conduction via accessory pathway ventricle is activated down the Purkinje-His system; and circiut completed via accessory pathway
- Appearance similar to AVNRT. Rapid, narrow complex (QRS < 120 ms) with absent P waves (or abnormal if retrograde conduction seen).

Antidromic (uncommon):
- antegrade conduction via accessory pathway, retrograde conduction via AVN venticle is activated via accessory pathway and circuit is completed up the Purkinje-HIs system
- Appearance similar to VT. Rapid, broad-complex (QRS >120 ms) with absent P waves.

Question 4

The most common type of AVRT is [], where the accessory pathway is called the []

Answer 4

The most common type of AVRT is Wolff-Parkinson-White syndrome, where the accessory pathway is called the Bundle of Kent.

Question 5

AVRTs lead to what specific waves on ECGs? [1]

Answer 5

This accessory pathway is a bypass tract, therefore avoiding the AV node and therefore the normal delay which occurs at this point
Leads to the characteristic ‘delta wave’ on ECG, whereby there is up-sloping at the start of the QRS complex due to the early ventricular excitation

Question 6

Describe what is meant by atrial tachycardia [1]

Answer 6

Atrial tachycardia is where the electrical signal originates in the atria somewhere other than the sinoatrial node.

This is not caused by a signal re-entering from the ventricles but from abnormally generated electrical activity in the atria.

May be either focal or multi-focal

• Focal: due to a single focus of atrial tissue generating more rapid action potentials, leading to a rapid tachyarrhythmia
• Multi-focal: this is synonymous with atrial flutter, whereby there is a re-entrant electrical circuit in the atria (usually around the tri-cuspid annulus and cavo-tricuspid isthmus) leading to rapid and recurrent de-polarisation without normal SA node functioning and conduction

Question 7

Describe the symptoms experienced when SVT is occurring [5]

Answer 7

• Recurrent episodes of palpitations (95%)
• Dizziness or light-headedness (75%)
• Dyspnoea (45%)
• Chest pain or tightness
• Progressive fatigue
• Pounding in the head and/or neck

Question 8

Describe the time course of SVT [1]

Answer 8

Average duration of episode 10-15 minutes, but can last any period of time from seconds to hours with abrupt onset and offset

SVTs become more frequent and severe with time

Question 9

Describe the ECG changes seen in SVTs [2]

Answer 9

Regularity of rhythm:
- Typically regular rate
- (atrial fibrillation, and multi-focal atrial tachycardia are only irregular ones)

QRS complex:
- narrow-complex tachycardia (QRS < 120ms)
- delta waves

ECG showing AVNRT

Question 10

What changes are seen specific to WPWs? [3]

Answer 10

Short PR interval, less than 0.12 seconds
Wide QRS complex, greater than 0.12 seconds
Delta wave

Question 11

What does the delta wave in WPWs actually mean? [1]

Answer 11

It is caused by the electricity prematurely entering the ventricles through the accessory pathway.

Answer 12

drominate

Question 13

Describe ECHO changes seen in SVT patients [1]

Answer 13

In majority of patients with SVT, an echocardiogram will be normal

Question 14

A polymorphic wide complex tachycardia would indicate which pathologies are ongoing? [3]

Describe the clinical significance of this finding [2]

Answer 14

Combination of atrial fibrillation or atrial flutter AND WPW ocurs
.
This is because the normal rate-limiting effects of the atrioventricular (AV) node are bypassed, and the resultant excessive ventricular rates

Life-threatening medical emergency
- HR can be above 200, or even 300 bpm
- VF and cardiac arrest can follow

Question 15

What is the definitive treatment for WPWs? [1]

Answer 15

radiofrequency ablation of the accessory pathway

Question 16

Which drugs are contra-indicated in a patient with known atrial fibrillation (or atrial flutter) and Wolff-Parkinson-White? [4]

Why? [1]

Answer 16

• Beta blockers
• CCBs
• digoxin
• adenosine

These medications may trigger ventricular fibrillation.

Question 17

Describe the stepwise acute managment of SVT of patients without life-threatening features

Answer 17

Continuous ECG monitoring during management

Step 1: Vagal manoeuvres
Step 2: Adenosine (6,12,18 mg)
Step 3: Verapamil or a beta blocker
Step 4: Synchronised DC cardioversion

Question 18

Describe the MoA of adenosine [1]

Answer 18

Interrupts the AV node or accessory pathway during SVT and “resets” it to sinus rhythm.

Question 19

Why does adenosine need to be given in rapid bolus to have an effect? [1]

Answer 19

The half-life of adenosine is less than 10 seconds.

Therefore needs to be rapid bolus ensure it reaches the heart with enough impact to interrupt the pathway for a short period

Question 20

Describe how adenosine is administered [1]

What doses are given? [3]

Answer 20

Adenosine must be given as a rapid IV bolus into a large proximal cannula (e.g., grey cannula in the antecubital fossa). The patient should be warned about the scary feeling of dying or impending doom when it is injected. This feeling quickly passes.

Three doses are attempted until sinus rhythm returns:

Initially 6mg
Then 12mg
Then 18mg

Question 21

Describe what happens if you give adenosine to someone with atrial tachycardia (e.g. atrial flutter)

Answer 21

Increases AV block & create more flutter waves

C.f. AVRT & AVNRT which have excitation in the AVN, so adenosine works to treat them

Question 22

Describe three different vagal manoeuvres used in the acute management of regular narrow-complex tachycardia (haemodynamically stable) [3]

Answer 22

Forceful exhalation against a closed airway for approximately 15-20 seconds / blowing into an occluded straw / or adopting a head-down position for approximately 15-20 seconds (approximately 25% success rate for terminating SVT episodes)

Cold stimulus:
- usually application of a bag filled with ice and cold water over the face, for approximately 15 to 30 seconds

Carotid sinus massage:
- contraindicated if any history of carotid artery disease, and usually only performed in a monitored healthcare setting

Question 23

You are treating a patient of AF, they have been in AF for + 48hrs. How does this change how you approach their treatment? [1]

Answer 23

They should NOT be treated initially with cardioversion until they have been anti-coagulated for at least three weeks, to reduce the risk of dislodging an atrial thrombus

Question 24

What drug is given after cardioversion in AF patients? [2]

Answer 24

Heparin treatment AND oral anti-coagulation should be commenced after cardioversion (whether successful or not)

Question 25

Usually, long-term management of SVT is only indicated if the frequency and severity of SVT episodes significantly impacts on the patients quality of life and functioning.

What is the stepwise treatment options for long term management? [3]

Answer 25

1st line:
- radio-frequency ablation

2nd line (if decline RAB)
- BB or CCB

3rd line:
- flecainide and sotalol

Question 26

Describe how Synchronised DC Cardioversion works [1]

Answer 26

Synchronised DC Cardioversion:

• electric shock applied to the heart to restore normal sinus rhythm
• A defibrillator machine monitors the electrical signal, particularly identifying the R waves.
• An electric shock is synchronised with a ventricular contraction, at the R wave on the ECG. If successful, the shock will be followed by sinus rhythm.

Question 27

What is the aim to avoid when performing synchronised DC cardioversion? [1]

Answer 27

Avoid shocking the patient during a T wave

Delivering a shock during a T wave can result in ventricular fibrillation and, subsequently, cardiac arrest.

Question 28

Describe why pre-excitation & afib is bad [1]

How do you tx this? [2]

Answer 28

Atrial fibrillation is conducted down to the ventricle via the AVN AND the accessory pathway

Can lead to VF

Treat with DC cardioversion & ablation of accessory pathway