Asymptomatic abnormalities

Answer: Sinus rhythm with manifest pre-excitation indicative of an antegrade conducting accessory pathway. Also known as “WPW pattern” – not syndrome.

A small proportion (approx. 1 in 1000) of people born with an extra electrical connection between the Atria & Ventricles known as an “Accessory Pathway” or “Bypass Tract”. This extra electrical connection allows for a re-entrant circuit to form between the AV node and Accessory pathway, sustaining an SVT circuit. This accessory pathway can be targeted with ablation to “cure” SVT.

 

ECG signs of an accessory pathway are:

1. PR interval <120ms (Ventricles pre-excited by accessory pathway
2. Delta wave (slurred upstroke of initial QRS phase)
3. Abnormal repolarisation features – ST segment changes & T wave inversions

 

 

Explaining Pre-excitation ECG features:

1. Short PR interval:

• The accessory pathway allows for the ventricles to be depolarised before the atrial impulse can travel through the AV node to mediate ventricular depolarisation.
• This shortens the PR interval because ventricular depolarisation (& therefore the QRS complex) begins earlier than it normally would (hence the term “pre-excitation”).

 

2. The Delta Wave

• Initial ventricular depolarisation via the accessory pathway occurs slowly due to a lack of ventricular depolarisation via the bundle branches or His-Purkinjie system.

 

• This results in an initially slow myocyte-to-myocyte depolarisation, creating a slurred or sluggish initial QRS upstroke, known as a “delta wave.”
  • The gradient of the delta wave represents the slow initial depolarisation of the ventricles via the accessory pathway.
  • Abnormal depolarisation of the ventricles often widens the QRS complex, although a wide QRS should not be considered a necessary criterion for “pre-excitation”.

 

3. Abnormal repolarisation:

• Abnormal depolarisation, results in abnormal repolarisation, manifesting as various ST & T wave abnormalities. The extent of these repolarisation abnormalities are somewhat dependent on the accessory pathway location and the degree of pre-excitation present.

 

The dynamic nature of pre-excitation:

• Pre-excitation is a dynamic ECG feature. It is often most prominent when the patient is at rest, because the AV nodal conduction velocity slows in response to high vagal tone, allowing the accessory pathway (which is relatively less influenced by autonomic tone) to contribute to a greater proportion of ventricular depolarisation, thereby producing a larger, more obvious delta wave.

 

• Put the same patient on the treadmill, and their AV nodal conduction velocity will improve during exercise and adrenaline. This means their delta wave often becomes less and less obvious the harder and harder they exercise, since the AV node and His-Purkinjie system contributes more and more to ventricular depolarisation .

 

A few other factoids about accessory pathways:

1. Many (and perhaps the majority of) accessory pathways do not conduct in an antegrade direction – they may participate in orthodromic AVRT, but do not display signs of pre-excitation on a resting ECG.

• They only way to know for sure if the patient has a retrograde-only conducting pathway is via an EP study.

 

2. Not all people with accessory pathways will experience an SVT episode, although the presence of an accessory pathway does predispose someone to SVT (specifically AVRT).

• An asymptomatic patient such as in our scenario, therefore often does not require treatment, except for in very specific circumstances where there are occupational concerns, or concerns regarding concurrent atrial fibrillation and sudden death risks.  

 

3. ***Important Nomenclature to know***

“Pre-excitation” & “WPW-pattern” refer to a resting ECG with signs of pre-excitation.

• “WPW syndrome” refers to a patients with signs of pre-excitation on an ECG AND they have symptomatic AVRT.
  

 

My Two cents:

• Manifest Pre-excitation on an ECG is synonymous with “WPW pattern” but does NOT necessarily mean that the patient has WPW syndrome.
  • WPW syndrome = SVT + Manifest pre-excitation.

 

• Manifest pre-excitation ECG features are a short PR, the presence of a delta wave, and usually some repolarisation abnormalities.

 

• The location of the accessory pathway can be deduced based on the morphology of the delta wave – this will be explored in another Beat Box.

 

• Many accessory pathways do not conduct in an antegrade direction and therefore will not show up on a resting ECG. These retrograde only accessory pathways are called “concealed accessory pathways”

 

Thanks for tuning in :)
Cheers
Mitch & the CPiP Team

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