Dilated cardiomyopathy: Pathophysiology and diagnosis

- [Voiceover] Let's talk about the pathophysiology and diagnosis of dilated cardiomyopathy. If you break down the word cardiomyopathy you can remember that it's a disease of the heart muscle because cardio stands for heart, myo muscle, and pathy disease. So, a disease of the heart muscle. And dilated cardiomyopathy is a specific type of cardiomyopathy. Now, before we get into what causes dilated cardiomyopathy, let's first just briefly review some of the normal cardiac or heart physiology. When the heart normally beats the muscles of the ventricles, the bottom part of the heart, those muscles relax causing the ventricles to expand, and this draws blood in from the atria. Then those muscles contract and then that contraction of the muscles ejects the blood out of the aorta and the pulmonary artery to send the blood either to the body or to the lungs where it gets oxygenated. A couple of terms I want to just describe here real quick and the first one is systole. And what systole is, is it's the period of time in which the heart muscle is contracting. During systole, blood is being ejected out of the ventricles to either the body or the lungs. But then the other period of time in the cardiac cycle is known as diastole. During diastole the muscles of the heart are relaxing and so the ventricle size is getting bigger and this is what is drawing blood into the ventricles so the ventricles are filling. So just a recap of that, during systole the ventricles are contracting and ejecting blood out of the heart. And also during that time that means the atria are filling and then during diastole the atria are emptying the blood into a relaxed ventricle. So now that we've reviewed the normal cardiac cycle let's get into the pathogenesis of dilated cardiomyopathy and what I mean by that is how this disease develops. Basically, dilated cardiomyopathy is caused by a dysfunction in the heart muscle's ability to contract. And since this is when the heart is contracting, which we talked about earlier is called systole, this is known as a systolic failure. So let's see what this looks like. I'm going to adjust the drawing of the heart here, and now you can see the heart muscle's kind of thin. The walls of the ventricles have gotten a little bit thinner especially down here at the bottom it's a little bit thinner. And this is just drawn in here to demonstrate that the muscle isn't able to contract as well. So let's see what this looks like during the cardiac cycle. Now the heart is contracting but it's not able to eject all of the blood out of the ventricle. Normally the ventricles eject somewhere between 50 and 75 percent of the blood out every time they contract. But now this decreased ability of the ventricles to contract, this systolic failure, is causing more blood to remain in the ventricle. And if more blood's remaining in the ventricle less blood has left the ventricle. So there's a decreased flow to the rest of the body. This is known as a decreased ejection fraction. What happens to compensate for this ejection fraction is that the chambers dilate and this is why it's known as dilated cardiomyopathy. So how does this compensate for the decreased ejection fraction? It would make sense that the body needs a certain volume of blood with every contraction of the heart and with the decreased ejection fraction it's getting a lower volume. But if the chambers dilate the total volume of the ventricles increases so even if you have a decreased percentage of this increased volume you have an adequate volume of blood that's actually ejected from the ventricles each heart beat. This dilation gets progressively worse and worse. Eventually not only are the ventricles just dilated but the atria become dilated as well because of the backup of blood. And it gets to a point where the chambers can't dilate enough. They just become as dilated as they can get. And this is when someone with dilated cardiomyopathy starts to develop the signs and symptoms of heart failure. These signs and symptoms of heart failure are due to two processes. They're either due to a decreased ability to pump the flow forwards, which will cause things like chest pain because you're not getting enough blood to the heart or it may cause someone to faint and pass out because they're not getting enough blood to the brain. Or you can also have signs and symptoms due to the blood backing up and kind of backing up behind the heart and you get things like swelling or edema of the legs and abdomen which is known as ascites. So what actually are the causes of dilated cardiomyopathy? Well, it can be genetic and these genetic causes are due to problems with some of the proteins in the muscle cells themselves that don't allow them to contract quite as well. But it can also be due to inflammation of the heart from something like a viral infection. This is known as myocarditis. Or it can be due to toxins that affect the heart. The most common of these is alcohol. Or another unfortunate cause of this is actually pregnancy. We're not exactly sure what about pregnancy causes dilated cardiomyopathy but there is definitely an increased risk of developing cardiomyopathy during pregnancy. And then the last one I want to mention here is idiopathic, meaning we don't know what causes it. But it's also important to note what are not the causes of dilated cardiomyopathy. Specifically, dilated cardiomyopathy is not caused by ischemic heart disease, valvular heart disease so diseases of the valves of the heart, or hypertension. And it's important to note this because these diseases can cause heart failure and it has a similar presentation to dilated cardiomyopathy in that the heart dilates and results in a systolic failure. But the difference is in those conditions, the heart failure is secondary to the other condition whereas in dilated cardiomyopathy it's a primary disease of the heart muscle itself. So dilated cardiomyopathy is not caused by ischemia, valvular heart disease or hypertension. So now that we've gone over the cause of dilated cardiomyopathy how do we diagnose dilated cardiomyopathy? The first step is going over the patient's history and physical. Let me just abbreviate that: H and P. And the signs and symptoms or the history and physical are really just going to be consistent with heart failure but there's not many that are very specific to dilated cardiomyopathy. The next thing is labs and one of the most important labs in any cause of heart failure is something that's known as the brain natriuretic peptide or the BNP. And BNP is elevated in conditions that cause stretching of the heart such as dilated cardiomyopathy so it`s elevated in dilated cardiomyopathy. But that's not very specific as it's also elevated in lots of other causes of heart failure. So next we're going to go to some special tests. And the first one I want to mention is an EKG. In the EKG of dilated cardiomyopathy it's almost always abnormal but the findings are non-specific. An abnormal EKG doesn't necessarily say someone has dilated cardiomyopathy. But a normal EKG will almost rule out the disease. So almost everyone with dilated cardiomyopathy will have an abnormal but it's going to be non-specific. The next one is a chest x-ray which I'll just abbreviate CXR and this is going to show something called cardiomegaly. This just means enlarged heart. It makes sense that if in a dilated cardiomyopathy the heart gets enlarged then in the chest x-ray you'll see a big heart. The next test, and the best test, for dilated cardiomyopathy is an echocardiogram. An echocardiogram is an ultrasound of the heart. In an echocardiogram you can see the dilation of the chambers as well as the decreased ejection fraction. There's two other tests I want to mention but they're not routinely used in the diagnosis of dilated cardiomyopathy. The first one is an angiogram and this is a study that looks that the blood vessels of the heart. And since I said dilated cardiomyopathy is not caused by ischemia an angiogram is not going to show any signs of ischemia. The last thing I want to mention is a biopsy of the heart muscle itself. In a lot of types of cardiomyopathy biopsies of the heart muscle are very important and they give you an idea of what the cause of the cardiomyopathy is. And kind of like the EKG the biopsy is usually going to be abnormal. But it's not very specific so if a provider thinks a patient has dilated cardiomyopathy these are the three major tests to diagnose it. These tests may be helpful but are only really used if the provider is trying to work up the patient for another condition and then they end up realizing that the patient has dilated cardiomyopathy. So, just remember that dilated cardiomyopathy is a dysfunction in the heart's ability to contract. Which means it is a systolic heart failure that manifests as a decreased ejection fraction and so the heart will compensate by dilating it's chambers and that is why it's called dilated cardiomyopathy. And that it's diagnosed primarily with an EKG, a chest x-ray and the best test for it is an echocardiogram.