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Part 7: The PR (interval)
As any UK EM doctor will tell you, the only strict definition we know about, given it is beaten into us every hour of every shift is the "Four hour target".
Nevertheless, although a surgeon can disengage brain and rely upon the donut of truth as a substitute for actually examining the patient, at the moment no such equivalent exists for ECG interpretation (yet...)
It is important to distinguish that the PR segment is different to the PR interval.
These are two distinct entities.
In other words:
The faster the heart rate, the shorter the PR duration, and vice versa.
The reason for this is it reflects the "slow impulse" between the AV node to the Purkinje system.
What does the PR interval's duration let us know?
This can be affected by a multitude of things, such as sympathetic and parasympathetic influences.
This is also why the notion of a first degree heart block (PR >0.2s) is an electrographical definition, not a pathological one.
It is also why the author has the following reaction when a colleagues tries to tell him a trifascicular block involves a first degree HB within its definition:
(If you are interested in a deep dive into the literature, pathophysiology and rationale behind the quagmire that is the "trifascicular block" - this is covered in part 2 of my ECG lecture series!)
The PR interval reflects the time between initiation of an atrial contraction vs subsequent ventricular response
This allows us infer the origin of the beat based upon it's duration as well.
If the PR interval is >0.12s, it implies the origin is within the atria. After all, the wave needs time to travel to the AV node.
If instead the PR interval is instead <0.12s, it implies no such travel time is needed.
To demonstrate this point, here is an ECG:
Hopefully you will notice the PR interval is shortened.
This is an example of Wolff-Parkinson-White.
Due to the accessory pathway, the AV node can be bypassed with respect to time from initiation of the wave form to propagation and depolarisation of the ventricles.
The author will also apologise to the community as it has taken an embarassingly long time to recognise "Wolff" is spelt with two F's.
There is unfortunately documentation lying around in the world where instead, a wild animal was referred to.
Click for part 8!
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