Back To CoursePathophysiology Textbook
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If you've ever had to cross the border between one country and another, then you were kind of lucky if you were allowed to easily pass through a checkpoint. The reality is that many times there's a block somewhere that causes a backup of cars, buses, and the like, resulting in a very slow pace to the border. Sometimes, the wait to get into or out of a country can be measured in many hours.
Your heart can also slow down - something known as bradycardia - if its natural pacemaker, like the engine of the car, runs too slowly or if the conduction of the electrical signals is blocked. We'll be focusing on the latter point in this lesson.
Normally, the pacemaker of the heart is the SA node. It fires off an electrical impulse that depolarizes the atria and then travels to the AV, or atrioventricular, node. By the way, 'depolarization' refers to the electrical excitation of the atria, resulting in their contraction.
The AV node serves as a checkpoint that delays the electrical signal a little bit before allowing the electrical impulse to pass through to the ventricles of the heart, where it will cause their depolarization and contraction during a heartbeat. All of this is done in order to pump blood out of the heart and into circulation.
If at our checkpoint, the AV node, there is a problem that causes the electrical impulse to be completely blocked or abnormally delayed, then something known as a heart block develops. Again, a heart block is a problem with the heart where the electrical impulse generated at the SA node enters the ventricles with an abnormal delay or fails to reach the ventricles entirely.
Since the AV node is located at the junction of the atria and ventricles, I liken it to a checkpoint in location and function at an actual border crossing. Just like an actual checkpoint causes a normal and slight delay in traffic flow while documents and luggage are checked to make sure nothing improper makes it through the border, so too the AV node normally provides a small delay to make sure proper electrical signals reach the ventricles. But if the checkpoint guard is working too slowly or the checkpoint is completely blocked off, then the traffic slows to a grind. In our case, that causes a slow heart rate, or bradycardia.
Heart blocks can be best understood by looking at an EKG (aka ECG). This is a test that shows us the electrical conduction of the heart with a series of waves. The P wave represents the depolarization of the atria right before their contraction. The PR interval symbolizes the time the electrical signal is held up at the AV node (our checkpoint). It's a measurement of how long it takes the border guard to do their job properly. Normally, it's less than 0.20 seconds (it's actually between 0.12-0.20 seconds to be precise). That border guard is pretty darn quick!
With that in mind, the first and least concerning type of heart block I want to discuss is known as a first degree AV heart block. This is a delay in the conduction of electrical impulses through the AV node, resulting in a prolonged PR interval.
All electrical impulses reach the ventricles in the first degree AV block, they are just delayed. That is why the QRS complex and T wave (both of which are representative of electrical activity of the ventricles) are all present after the P wave despite a delay in conduction, and there is a P wave for every QRS complex.
In this heart block, our border guard is simply slow. More technically, the PR interval is greater than 0.20 seconds. This leads to a long line to form at the border crossing and hence a longer-than-normal PR interval on our EKG as well.
There are two types of second degree AV block. The first is called a Mobitz type I AV heart block (aka Wenckebach Block), a heart block with a progressively increasing PR interval.
Again, here the PR interval increases progressively until no electrical impulse is conducted from the atria at all, meaning eventually a P wave occurs without a QRS complex or T wave at all, since the signal never made it to the ventricles through the AV node. A pause follows, a normal beat occurs thereafter, and this cycle repeats itself again. This means the PR interval is longest right before the block and shortest right after the block.
In this case, I liken it to a border guard at the checkpoint getting progressively slower and slower. He gets so slow that the station officer removes him from his post entirely. At that point, this means no one can get through to the other side at all since there's no border guard on duty (hence the lack of ventricular electrical activity). After a brief delay while the officer is finding a normal border guard, a new chap comes in. He works normally at first but begins to slow down just like the guy he replaced and is eventually fired himself, and so the cycle continues.
The other type of second degree heart block, something more serious than Mobitz type I, is known as the Mobitz type II AV heart block. This is a heart block that entails a normal PR interval with intermittently dropped ventricular beats. What I mean is, unlike Mobitz type I, the PR interval is normal in this case but the electrical impulse intermittently doesn't make it through the AV node and into the ventricles, leading to a P wave here and there that has no QRS complex or T wave thereafter, meaning the ventricles fail to contract since they never get the electrical signal to depolarize and therefore contract. A pattern or ratio of conducted-to-blocked P waves is often observed.
The final and most serious heart block I want to discuss is the third degree AV heart block (aka complete heart block), a form of heart block where there is absolutely no electrical conduction from the atria and into the ventricles.
What happens is the SA node fires off its electrical signals, but they do not get through to the ventricles. This is really bad and can lead to death, but thankfully your heart actually has three separate intrinsic pacemakers. Only the fastest one, normally the SA node, actually controls the heart rate. If the SA node is sick or its signals don't get through to the ventricles, the second-fastest pacemaker (the AV node) takes over for the rhythm of the ventricles in this emergency situation (we term this secondary pacemaker's role as a junctional escape rhythm).
Anyways, this escape rhythm produces regular but slow electrical impulses for the ventricles since the impulses generated by the SA node never make it through to there. Furthermore, since the atria and ventricles contract independently of one another, this scenario is also known as a form of AV dissociation.
Since we no longer have one dominant pacemaker controlling the entire heart, but now have two pacemakers playing a role that are kind of doing their own things, we get P waves and QRS complexes that have absolutely no relationship to one another in terms of rhythm on the EKG.
I liken this to the border crossing being completely blocked off for some reason. Therefore, the people in line rise up and nominate one of their own to serve as a sort of civil servant that directs traffic and helps people escape across the border. Since the servant (our secondary pacemaker) doesn't really know what they're doing, they're kind of slow, but at least they get into the regular rhythm of things.
As a parting note, the severity of the heart block treatment may range from nothing at all in simple cases to the implantation of an artificial pacemaker to control the heart rate in more serious instances of heart block.
Hopefully this lesson didn't give you a brain freeze, since there was a lot of important stuff.
Remember that a heart block is a problem with the heart where the electrical impulse generated at the SA node enters the ventricles with an abnormal delay or fails to reach the ventricles entirely.
In a first degree AV heart block, there is a delay in the conduction of electrical impulses through the AV node, resulting in a prolonged PR interval.
A second degree heart block known as the Mobitz type I AV heart block (aka Wenckebach Block) is a heart block with a progressively increasing PR interval.
Another second degree heart block, known as the Mobitz type II AV heart block, is a heart block that entails a normal PR interval with intermittently dropped ventricular beats.
Finally, the most serious heart block is the third degree AV heart block (aka complete heart block) and is a form of heart block where there is absolutely no electrical conduction from the atria and into the ventricles.
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Back To CoursePathophysiology Textbook
20 chapters | 274 lessons