Pre-excitation: Heart's Speedy Connections
What is pre-excitation, and why you should know about it
It’s all communication
The cornerstone of collaboration is communication. This is a law of nature that you can see not only in humans but in our physiology as well.
The atria and the ventricles must communicate and collaborate perfectly to optimise cardiac output for each situation (rest, exercise, stress etc.).
Pre-excitation is a way where communication between the atria and ventricles has been compromised, leading to serious problems.
What is pre-excitation?
Think of pre-excitation when there is a short PR interval (< 120 msec).
Two different pathological entities are associated with pre-excitation syndrome:
Wolff-Parkinson-White (WPW) syndrome
Lown-Ganong-Levine (LGL) syndrome
These syndromes have the following things in common:
a short PR interval
can cause symptoms: specifically, they can cause palpitations due to arrhythmias
This is the overview of pre-excitation. Now let’s dive deeper into each separate entity in more detail.
Wolff-Parkinson-White (WPW) syndrome
In this case, there are two pathways connecting the atria with the ventricles (so the syndrome is congenital):
The normal pathway (through the AV junction1)
The accessory pathway (Kent bundle) which bypasses the normal pathway (either from the left or the right).
The significant difference between these two pathways that you need to remember is that the accessory pathway has a shorter refractory period than the normal pathway. As discussed in a previous post about the PR interval, the AV junction causes an important delay in the electrical transmission from the atria to the ventricles. This serves the purpose of allowing the atria to empty before the ventricles contract, optimising cardiac output.
The problem with the accessory pathway is that it conducts faster than the AV junction. This leads to a premature stimulation (pre-excitation) of the ventricular myocardium.
![Accessory pathway in WPW Accessory pathway in WPW](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F4550508b-732a-43bc-b65b-858e49bc170a_1802x1426.png)
As you can see in the image above, the premature stimulation of the ventricles by the accessory pathway will produce an initial positive upstroke of the QRS complex, called the delta wave. This causes prolongation of the QRS complex, although there is no problem with the intraventricular conduction.
The QRS in WPW represents a fusion beat; the initial portion arises from slow ventricular activation via the accessory pathway, while the terminal part of ventricular activation is via the normal conduction system.
Overall, the WPW pattern (not syndrome) has the following characteristics:
Short PR interval
Delta wave
Prolonged QRS complex
![The Delta wave The Delta wave](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F34f8e720-82c0-43a8-b851-5ef6c18be0e7_800x224.png)
![The WPW pattern The WPW pattern](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Ff0df8b2f-891b-4e41-a0a6-8ab9eed61308_3004x1599.jpeg)
To say that a patient has WPW syndrome, they need to have the WPW pattern present with the coexistence of a tachyarrhythmia and clinical symptoms of tachycardia such as palpitations, presyncope, syncope, or even cardiac arrest.
Why know about WPW syndrome?
In my opinion, the most persuasive phrase to learn about this has been said already, and that is “cardiac arrest”. It’s also quite common in exams…
Two major tachyarrhythmias can occur as a result of WPW syndrome:
AVRT (Atrio-Ventricular Reentrant Tachycardia)
Pre-excited atrial fibrillation or flutter
The reason these arrhythmias occur is the accessory pathway and its unique electrophysiological properties (the shorter refractory period).
This is what they look like:
AVRT (Atrio-Ventricular Reentrant Tachycardia)
![AVRT AVRT](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F0cafa268-992b-4e90-ae84-8996eb1a3844_3000x1907.jpeg)
Pre-excited atrial fibrillation
![Pre-excited AF Pre-excited AF](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fb67f66b9-d618-41b4-807f-7a746489556e_1786x1264.jpeg)
The latter can be so fast that it can cause haemodynamic compromise even if there’s no underlying structural heart problem! Note: you should NOT give any medications that can cause AV block, like adenosine, bisoprolol, or verapamil (they will make the tachycardia even faster, likely leading to cardiac arrest).
*** Please let me know if you’d like me to get into more detail about these arrhythmias in future posts by kindly leaving a comment:
Lown-Ganong-Levine (LGL) syndrome
Short PR interval
Normal QRS duration (no delta wave)
Palpitations or supraventricular tachycardia (SVT)
The above three combined have been termed the Lown-Ganong-Levine (LGL) syndrome.
The exact reason for the short PR interval in LGL syndrome isn't entirely clear, but it could be due to:
Faster conduction within the AV node.
Special connections, like the Brechenmacher or James fibres, bypassing the usual route.
![LGL syndrome ECG LGL syndrome ECG](https://substackcdn.com/image/fetch/w_1456,c_limit,f_auto,q_auto:good,fl_progressive:steep/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2F1d20591c-7ef0-4525-9455-bf711fa6de9c_780x324.jpeg)
Conclusion
Pre-excitation syndromes, including WPW and LGL, underscore the heart's intricate electrical dance.
The common findings of pre-excitation syndromes are:
a short PR interval
they cause symptoms: specifically, they can cause palpitations due to arrhythmias
It’s important to know about them, especially WPW syndrome, as they are associated with significant arrhythmias and even cardiac arrest.
References
Prutkin, Jordan. “ECG tutorial: Preexcitation syndromes”. UpToDate, Jan 19, 2022.
Chhabra L, Goyal A, Benham MD. Wolff-Parkinson-White Syndrome. [Updated 2023 Jan 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK554437/
Just a reminder: the AV junction consists of the AV node and the His Bundle.