Let’s talk about ventricular fibrillation, also known as V-fib. This is an abnormal, deadly heart rhythm, where the ventricles, which I’m outlining right here, these lower chambers of the heart, the ventricles lose the ability to contract and circulate blood to the rest of the body, and that’s why V-fib is so deadly.
So, over here we have a heart diagram, looking at the inside of the heart, just like we did on the left, but this diagram on the right highlights some of the important structures in the electrical conduction system. And I’ve taken out the big vessels, so that way you can get a better view of the inside of the heart.
So, in a normal heart, the electrical conduction starts at the sinoatrial node, also known as the SA node.
So, sinoatrial, SA node. The signal travels from the SA node down to another structure called the atrioventricular or AV node. So, that stands for atrioventricular or AV node. From the AV node, the signal goes down to the ventricles and signals to the ventricles and makes the ventricular cells and the walls contract.
When these walls contract, that’s when blood is pushed out to the rest of the body.
Now in V-fib, the signal in the ventricles is no longer coordinated. This might be because you have some sort of severe oxygen deprivation or because you have some sort of abnormality in conduction, but for whatever reason, you lose coordinated signalling in the ventricles. Instead, you have this rapid, random, and chaotic signalling.
This chaotic signalling leads to the ventricular walls spasming, and when the ventricular walls spasm, blood can’t be circulated through the body. It just stays here in the heart.
If you’re not circulating blood to the rest of the body, you’re depriving all of the other organs of oxygen. Another way to think about this is, to think about if you were squeezing a tube of toothpaste. So, here you have a hand squeezing a tube of toothpaste, and you can imagine that if you squeeze with all your fingers at the same time, you’re going to get the most toothpaste out.
So, if you coordinate all your fingers to squeeze at the same time, you’re going to squeeze out the most toothpaste, as opposed to if you squeezed your pinky and then your index finger and then your thumb, you’re not going to squeeze out a bunch of toothpaste.
This is the same idea as ventricular fibrillation.
You lose this coordinated contraction and this coordinated squeeze to distribute blood to the rest of the body, and instead, you have random firing of muscle cells, leading to this spasm, and you’re not going to push out a lot of toothpaste or blood to the rest of the body. If V-fib isn’t reversed immediately through electric shock, you’re going to have permanent brain damage and death, because the brain and the body aren’t getting enough oxygen.
A person whose heart is in V-fib is not going to be conscious, since they have no blood circulating, and you won’t be able to feel a pulse. Pulses are made by forwarding the movement of blood to the vessels, so if you have no forward movement of blood to the vessels, you’re not going to have a pulse. Sometimes, right before someone collapses from ventricular fibrillation, they might complain of signs or symptoms of a heart attack, like chest pain or numbness or tingling in their left arm, and that’s because the heart is getting deprived of oxygen.
Since the heart’s not circulating blood to the rest of the body, it can’t circulate blood to the heart itself, and since the skin isn’t getting oxygenated blood, you might see the skin turn pale or even blue colour.
This is also known as cyanosis. That’s the blue-colour skin when you’re not circulating oxygen through the body. An EKG in someone in V-fib looks like this. You’re going to see course squiggly lines.
This is because you lose this smooth signalling from the SA node to the AV node to the ventricles.
Instead, in the ventricles, you have this chaotic, erratic signalling, which looks like course squiggly lines on EKG. The ventricles are just spasming. Now, what are the big risk factors of V-fib? Well, I like to separate the major risk factors into two categories.
The first one is anything that causes general irritability to the ventricular cells. Now, an irritable ventricular cell means that it’s likely to just over-fire or fire abnormally.
The other risk factor for V-fib is anything that causes scar tissue formation. Now, let’s go back to the irritable ventricular cells. What would cause ventricular cells to be irritated?
Well, the most classic example is coronary artery disease. Coronary arteries are the blood vessels that supply the heart with oxygenated blood, and over time these blood vessels can get clogged, and this might be because of years of smoking or a bad diet, but over time these vessels get clogged. When the coronary arteries are clogged, you’ve got less blood flow to the ventricular cells. These cells get less oxygen than they’re used to having, so they become irritated, and they’re more likely to over-fire or fire abnormally.
Also, certain electrolyte abnormalities, like high potassium or low calcium or low magnesium might cause ventricular cells to become irritable.
And why is that? Well, electrolytes play an important role in normal electric conduction in the heart. And you can imagine if the electrolytes don’t have the right levels, this could disrupt normal electric conduction through the heart. Okay, and now for scar tissue. So, we said scar tissue can also set up the heart for ventricular fibrillation.
So, what might cause scar tissue? Well, the most classic example is a heart attack. As we said, we have coronary arteries that supply the heart with oxygenated blood. When you have a heart attack, there is a coronary artery that gets clogged, totally, so no blood is getting to this part of the heart.
So, whereas in coronary artery disease you had less blood flow, in heart attack you have no blood flow to this part of the heart, and this tissue ends up dying, and it’s replaced by protein, kind of like a scab, and it creates this scar tissue.
Now, scar tissue doesn’t function like regular heart tissue. It’s not going to conduct signal like regular heart tissue would. Instead, it sets up for abnormal firing or abnormal circuits in the heart. And this disruption in electrical conduction can set up the heart for ventricular fibrillation. Other things that can lead to scar tissue formation is a cardiomyopathy.
So, cardiomyopathy just means disease of heart tissue, in which the heart tissue doesn’t function as well. So, it loses function. It doesn’t lose all function, but it doesn’t function as well as a normal heart would. Cardiomyopathies have a lot of causes. So, the infection can be a cause.
So, there are some viruses that affect the heart and make the heart tissue actually decrease function. There are certain genetic disorders that can cause cardiomyopathy, and these disorders will actually change the structure of the tissue of the heart, making them lose function, and also years of coronary artery disease, which I’m going to write as CAD, so, years of coronary artery disease, where the heart is constantly getting less oxygen than it optimally wants over years.
Eventually, that heart tissue is going to lose function. Again, all of these things from the cardiomyopathies predispose the heart to have scar formation, and scar disrupts normal electrical conduction and can set the heart up for V-fib. And one final thing that might cause ventricular fibrillation that’s not as common is electrocution.
In electrocution, you have some sort of outside source of electricity entering the body. So, where the heart would have a normal electrical conduction system, electrocution is going to disrupt this normal signaling, and that can lead to ventricular fibrillation..
