At the moment I’m doing my SSC, although it’s usually at a premium, there seems to be an abundance of free time at the moment (though this time tomorrow I expect I’ll have regretted saying this). While me and colleague and friend Lucy are waiting for people to arrive back on the ward from surgery to assail them with questionnaires we’ve been having a little bit of time to get down and dirty with some independent study. So a shout-out to Lucy for today’s topic. Well done you.
I hope you’re all impressed with this graphic. It took me longer to make than it should have.
So heart block. I’m going to briefly go over AV blocks here, but don’t forget about other types of heart block, like right and left bundle branch blocks.
Heart block basically means that the normal electrical conduction that occurs in the heart with every beat has gone a bit dodgy. Usually, the the electrical impulse that causes the wave of depolarisation and contraction of the myocardium starts at the sino-atrial node (SAN) up in the right atrium. The wave of depolarisation spreads through the atrial walls to the atrio-ventricular node (AVN), bundle of His then down the interventricular septum and out to the ventricle walls. An “AV” block means that there is a conduction problem at the AV node, and so the electrical impulse that starts out at the SAN doesn’t always manage to pass beyond the AVN and down into the ventricles. Heart blocks of this type can be classified as first, second or third degree block, depending on how much electrical impulse makes it down into the ventricles.
FIRST DEGREE HEART BLOCK
In first degree heart block, the electrical impulse that starts at the SAN always makes it down to the ventricles via the AVN, but because there is an impairment of conduction at the AVN it just takes a bit longer for the impulse to reach the ventricles. You can see this on an ECG as an abnormally long PR interval (remember, the P wave is atrial depolarisation, and the QRS complex is ventricular depolarisation). A normal PR interval is 0.2 seconds.
So, with first degree heart block you get a QRS complex with every P wave, but the time between the two traces on the ECG is unusually long.
I’ve just stumbled on a (much fancier) wordpress blog post about first degree heart block that talks about it in a bit more detail. You can find it HERE.
Important causes of first degree heart block include coronary artery disease and electrolyte disturbances.
SECOND DEGREE HEART BLOCK
Unlike in first degree heart block, when every impulse starting at the SAN made it past the AVN to the ventricles, in second degree heart block this doesn’t always happen. Some impulses make it through, but others don’t. There are a few different types of second degree heart block, that I’ll cover briefly:
MOBITZ TYPE 1, aka WENCKEBACH
The PR interval on the ECG gets longer and longer, until finally an impulse doesn’t get past the AVN and you see a P wave but no associated QRS. The ventricles have “missed a beat”.
MOBITZ TYPE 2
With this type of second degree heart block, most impulses are conducted past the AVN and in to the ventricles but every now and again an impulse fails to make it to the ventricles, and on the ECG you see a P wave but no associated QRS. Unlike in wenckebach, the PR interval remains constant. This type of heart block can also lead to third degree heart block.
2:1 and 3:1 CONDUCTION
This refers to the ratio of P waves to the number of QRS complexes seen on the ECG- that is, for every impulse starting at the SAN how many make it past the AVN to depolarise the ventricles? So a 2:1 conduction means that you get alternating conducted and non conducted impulses (P wave + QRS complex, then P wave + no QRS, P wave + QRS and so on). It’s the same idea for 3:1 conduction, but for every 3 P waves you only see 1 conducted impulse and QRS complex. In this type of AV conduction defect, the PR interval remains constant. This type of heart block can also lead to third degree heart block.
THIRD DEGREE HEART BLOCK
This is complete heart block- no impulses manage to make it past the AVN to the ventricles, and so when the ventricles depolarise they’re controlled by the intrinsic pacing capacity of the ventricles themselves, which is much slower (around 30bpm) than you’d get with pacing from the SAN (which is normally around 80bpm). Because of this, on the ECG you get complete dissociation of the QRS complexes from the P waves. The P waves look all over the place compared to the QRS complexes, and the PR intervals seem to increase and decrease. But if you look closer you’ll notice the P waves are regular, and the QRS complexes are regular but they just aren’t happening at the same rate- remember the ventricles are pacing themselves at a slower rate because no impulses from the SAN are making it past the AVN.
Here’s a 12 lead ECG to demonstrate:
So there you have it. AV heart block for dummies.
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