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Wednesday, 2 July 2014

#ECGclass - Summer 4 - AVNRT

Welcome to this week's #ECGclass, authored by @J_vanOppen. Please use the hashtag #ECGclass if joining in the live and interactive Twitter discussion.
If you are new to ECGclass, you may like to read this brief explanation of what it's all about: #ECGclass and how it works

A 33-year old woman, Mrs N, presents to your surgery complaining of “a wobbly chest”. She has been experiencing these most weeks over the last year and is concerned that her symptoms make it hard to concentrate while working as a primary school teacher.

1. Before you power-up the ECG machine, is there anything you’d like to ask or do?

While exploring her history, you clarify that during these episodes Mrs N feels her heart “beating hard against her chest, and far too fast.” There do not seem to be any particular triggers, with the episodes starting suddenly both on exertion and at rest and stopping after several minutes. She does not appear to have any associated symptoms – she never experiences dizziness, chest pain, or shortness of breath.

Here is her ECG trace:

2. What does this ECG show? Let’s use a sensible approach of rate – rhythm – axis (2.5 minute demo here: Anything to worry about here?

Rate: about 80: divide 300 by the number of big squares between R-wave peaks.
Rhythm: first degree AV block: p-waves precede each QRS complex, but the PR interval just exceeds 0.2sec (1 big square). This wouldn't explain our patient's palpitations so is probably an incidental finding.
Axis: normal: leads I and aVF both point up and R-wave transition occurs at V3.
Overall: reassuring ECG. Not pathological.

Typically, just as you finish unplugging the leads and tell Mrs N that her ECG was normal, she experiences another episode of palpitations. Here is her symptomatic ECG:

3. What’s going on here? What has changed?

Rate: tachycardic at 150.
Rhythm: Supraventricular tachycardia: this gets tricky. Firstly we can exclude a ventricular tachycardia as the QRS complexes are narrow. There are p-waves, but these are abnormal, are inverted, and appear after the QRS complex (look at the first half of the rhythm strip – they appear in alternate beats. In the second half, they appear every beat).
Axis: normal: R-wave transition at V2.
Overall: abnormal ECG: SVT.

4. What is the diagnosis?

This is Atrio-Ventricular Node Re-entrant Tachycardia (AVNRT).

AVNRT can occur at any age, but is more common in younger adults affecting around 1/1000 with females affected 3:1.

The defect is a division of the AV node, which is thought to be an acquired abnormality. Two pathways are present – a slow (with a short refractory period) and a fast (with a longer refractory period). Most of the time, the atrial depolarisation wave follows the fast pathway and spreads through the ventricles via the Bundles of His, producing a normal ECG.

Now, should a Premature Atrial Complex (PAC) arrive at the AV node too early while the fast pathway is still refractory, the depolarisation wave must instead follow the slow pathway, after which it can track back up the fast pathway towards the atrium, thus forming the re-entrant circuit. This retrograde depolarisation is the cause of the inverted p-wave, which is obscured or occurs just after the QRS complex.

5. How would you manage Mrs N’s palpitations?

Vagal-stimulating techniques can terminate AVNRT, and some advocate teaching patients the Valsalva manoeuvre or carotid sinus massage (one side only…).

Adenosine is first-line for pharmacological termination of AVNRT. DC cardioversion is very rarely required in AVNRT.

Beta-blockers, calcium channel blockers and digoxin have roles in preventing recurrence, but in this lady’s case care should be taken that these drugs do not potentiate her first-degree heart block (though it’s probably worth investigating whether this was transient…).

The diagnosis of AVNRT could be confirmed by a minimally-invasive electrophysiology study, and the re-entrant circuit could be permanently corrected using radiofrequency catheter ablation to destroy the slow pathway. This would typically be offered to patients with frequent episodes or for who drugs do not work or are not tolerated. The cure rate is greater than 95%.

Thanks to ECGpedia for the images.

Please use the hashtag #ECGclass if joining in the live and interactive conversation Twitter
and...If you want to ask the patient any questions, or request any further investigations this can  be done by the hashtag during the course of this evening. 

Alternatively, if referring to this blog at a later date, please leave comments below.
Thank you.

1 comment:

  1. thanks! do you think you can post one about avrt? I always get the two mixed up!