What is a Pulmonary Embolism?
A pulmonary embolism is the term for a clot trapped in the lung. It is often referred to simply as PE, which of course is short for pulmonary embolism. The clot that is trapped in the lung leads to blockage of blood flow and can be extremely dangerous if not treated appropriately.
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Pulmonary Embolism – A Big Deal
Pulmonary embolism is a big deal, being both common and dangerous. It’s common with almost a million cases a year in the US alone. It is dangerous because almost a quarter of pulmonary embolism cases present as sudden death and up to a third of pulmonary embolism will ultimately be fatal. For those that survive the pulmonary embolism there is the possibility of decreased life expectancy and the development of complications that can impair quality of life. For this reason it’s critical to make sure that pulmonary embolism is correctly diagnosed and properly managed as that can make all the difference.
How Does Pulmonary Embolism Start? – It All Starts With A DVT, a Clot That Forms In the Veins of the Leg
Really where it all starts is that clots form in the veins of the leg known as a DVT, short for deep venous thrombosis. Let’s take a little time to explain this in a simple manner so we can understand pulmonary embolism better.
Every time the heart beats it pumps blood to the body through arteries. The blood is pumped with force and the flow in these arteries is fast. For that reason if you were to cut your artery, blood would spurt out fast and furiously because of the high pressures in arteries. Once the blood has reached the target organ, it must return to the heart, and it does this through the veins. Flow in the veins is much slower with a much lower pressure. If you were to cut one of your veins then blood would certainly pour out, but since the pressure is low it would be nowhere near as fast or forceful as an arterial bleed. In fact with a venous bleed you could easily stem the flow of blood with gentle pressure over the skin. With an arterial bleed some serious pressure would need to be applied.
So why do clots occur? Three things are thought to be critical in clot formation. Firstly is something called hypercoagulability, second is flow pattern and third is injury to the vessel. These three things are known as Virchow’s triad. Don’t worry, its not that complicated, I’ll explain below.
First in the triad was hypercoagulability, this basically means that for some reason the blood is more likely to form clots because of the changes in the chemicals within it. These chemicals known as clotting factors influence the whole process of clot formation. Simply put, these can make the blood super thick or super thin. Of course if the blood is super thick then clots will be more likely to form. Certain medicines, diseases or genetic abnormalities, can influence the clotting factors and make the blood more likely to be thick. A good example of this is birth control pills. These pills are therefore well-recognized risk factors for clot formation and pulmonary embolism.
Secondly in the triad was flow pattern. If you were to take some blood and pour it into a cup and wait a minute, the blood would form a clot because it’s not flowing. If you were to take the same blood and pour it down a steep hill it wouldn’t clot because it’s moving fast. The clotting factors in the blood need time to work and if the blood is flowing fast they don’t really have time. For this reason clot is much less likely to form in the high pressure arteries and much more likely to form in the low pressure veins. Every time you stand up or walk, you pump blood through the veins. If you sit still for a long time, blood flow is much slower and of course clots are more likely to form. Good examples are really long plane flights, or patients that have had surgery and are unable to be active for a while. These patients are at risk of developing blood clots in their veins and are therefore at risk of pulmonary embolism.
Third and finally is vessel injury. The arteries and veins of the body are not just lifeless tubes for blood to flow through; they are made of cells that are smart and always active known as the endothelium. These cells can sense everything from patterns of blood flow to the composition of the blood. These cells can secrete chemicals and send signals if they want certain things to happen. A good example is if there is injury to the vessel that causes bleeding. The cells will release chemicals that attract cells that can form clot in an attempt to stop the bleeding. When the cells are injured they are more likely to do that. For this reason, if there has been damage to the vessels through surgery, or if someone has fallen or been hit on the leg, a clot would be more likely to form.
You should now have a good understanding of what causes clots to form in the veins known as DVT’s. Pulmonary embolism usually starts with a DVT. We just talked about the triad that consists of blood more likely to clot, slow flow, and injury to the vein. So as you can imagine, the perfect candidate for developing a clot would be a person that is in bed after a leg injury, unable to do much activity, with a disease such as cancer leading to thicker blood, also taking medicine that leads to a higher likelihood of clot forming. That would be almost a perfect storm.
The Pulmonary Embolism Occurs – The Blood Clot Travels From the Leg Veins to the Lung Where It Is Trapped
We have now talked about a clot that forms in the veins of the leg. That clot is called a DVT, or deep venous thrombosis. The veins are like the highway used by the blood to get back to the lungs. In order to get to the lungs the blood has to travel through the right side of the heart. Now lets imagine the nice big blood clot that has formed in the leg gets loose. It slowly makes its way back up through the veins of the body, through the right side of the heart, and in to the lungs. Usually when blood reaches the lungs it is a thin fluid that can pass through the lung vessels easily. When a clot reaches the lungs however it is a large structure that basically just gets stuck, and is known as a pulmonary embolism. The larger the clot the bigger the amount of blockage caused. Sometimes the clot can be so large that it gets trapped in the main artery that leaves the right side of the heart known as the main pulmonary artery. If that happens no blood can leave the heart at all due to the obstruction, and this will almost inevitably result in sudden death. This is known as a massive pulmonary embolism.
The Seriousness of a Pulmonary Embolism Depends on the Strain Placed on the Right Side of the Heart
The pulmonary artery leaves the right side of the heart and supplies the lungs with blood. The main pulmonary artery is a large fast flowing vessel that leaves the heart. This branches into a left and right artery that that are medium sized, then these in turn branch into smaller and smaller pulmonary arteries. So basically the pulmonary arteries get smaller and smaller the further away from the heart they are. The larger the artery blocked by the pulmonary embolism, the more strain placed on the right side of the heart. The more strain placed on the right side of the heart the worse the outcome.
Why is this? The right side of the heart usually gets to pump in to a large artery, the pulmonary artery. Imagine a large pulmonary embolism where a clot was blocking that artery. The heart would have to pump the same amount of blood, but through a smaller hole. This means the heart would have to generate a higher pressure. If the blockage were large enough then the pressure generated would be so high that it overwhelms the right side of the heart. This leads to failure of the right-sided heart-pumping chamber, the right ventricle. This is known as right ventricular dysfunction. The consequences of right ventricular dysfunction can include fatal heart rhythms and development of shock.
It’s also important to note that the blood supply to the lung is also shut off. This can lead to less oxygen content of the blood and also to areas of the lung dying due to lack of blood supply known as pulmonary infarction.
Signs and Symptoms of a Pulmonary Embolism
The most common presenting symptom of a pulmonary embolism is shortness of breath, that occurs fairly rapidly, either at rest, or when performing activity. Other symptoms may include chest pain, dizziness, or passing out. Patients may have had recent leg swelling or leg pain from the clot that started in the leg.
Diagnosis of Pulmonary Embolism
Once a patient comes in with signs and symptoms suggestive of a pulmonary embolism, a number of tests can be performed to confirm the diagnosis.
- Blood tests – A test called a D-dimer may be performed, levels of which will be elevated in the presence of a clot in the body. Blood tests suggestive of heart strain, troponin levels and BNP levels may be elevated in pulmonary embolism
- CT Scan – A CT scan known as a CT angiogram is really the best way to diagnose pulmonary embolism. It can provide information on the size, extent and location of the clot. It can also provide information regarding heart strain.
- Ultrasound Scan – An ultrasound scan of the legs may be performed to look for evidence of clot in the veins of the leg.
- Echocardiogram – This is an ultrasound scan of the heart that may demonstrate evidence of heart strain. An enlarged or dysfunctional right side of the heart is suggestive of pulmonary embolism.
- EKG heart tracing – An EKG tracing shows the electrical activity of the heart and may demonstrate a fast heart rate and can also show evidence of right-sided heart strain. There are also some specific patterns on the EKG that may suggest pulmonary embolism.
- V/Q scan – This is a nuclear medicine scan that looks for areas of the lung that aren’t receiving appropriate blood supply and may therefore be affected by pulmonary embolism.
- Pulmonary Angiogram – This is the gold standard test, where contrast dye is injected directly in to the pulmonary arteries. This may be done at the time of a therapeutic procedure, however it is not typically done as an initial test because CT scanning is now so accurate.
How Do We Determine The Seriousness of a Pulmonary Embolism?
Surprisingly it’s not necessarily the amount of clot that it is important. Determining the seriousness of a pulmonary embolism really comes down to one thing, how much strain is placed on the right side of the heart. The terminology we use for pulmonary embolisms is Massive, Submassive, or lower risk.
Massive Pulmonary Embolism
Massive Pulmonary Embolisms are extremely high-risk events and as serious as you can get. Massive pulmonary embolism patients are unstable and may often present with sudden cardiac death. The right side of the heart will be severely dysfunctional and it will be difficult to maintain a blood pressure. Patients will be in shock. Patients in this category are often so unstable that they will require medicines to support their heart and may need insertion of a breathing tube. These massive pulmonary embolism patients are at high risk for sudden death. The official definition of massive pulmonary embolism is a pulmonary embolism directly causing cardiogenic shock with blood pressure <90mmHg or drop in blood pressure of over 40 points. Patients with massive pulmonary embolism are at high risk of death. Treatment of massive pulmonary embolism needs to be aggressive and emergent.
Submassive Pulmonary Embolism
Submassive pulmonary embolism means that while criteria for massive pulmonary embolism is not met, and the patient may appear to be relatively stable; there is still evidence for right-sided heart dysfunction. Although not as high risk as those with a massive pulmonary embolism, this is still considered a high-risk group at risk of poor outcomes. To be designated a submassive pulmonary embolism, there has to be evidence of right-sided heart dysfunction. This can include, blood tests suggesting right-sided heart strain (BNP, troponin), heart tracing changes suggestive of right-sided heart strain (EKG) and imaging suggesting enlarged and weak right heart (CT scan, echocardiogram). Typically these patients will have a large amount of clot in the lungs. Although submassive pulmonary embolism needs to be taken very seriously, there is a wide range of severity with some submassive pulmonary embolisms more serious than others. Not everyone with a submassive pulmonary embolism will require aggressive treatment although its important to identify those that are at high risk and treat them appropriately.
Other Pulmonary Embolism
Most pulmonary embolisms will fall in this group. Although a diagnosis of pulmonary embolism is made, there is no evidence of right-sided heart dysfunction and the patients are considered stable. For those treated appropriately the likelihood of a good outcome is high. There is generally no role for overly aggressive treatment in these patients and the mainstay of treatment is to start blood-thinning medications.
Treatment of Pulmonary Embolism
Treatment of Massive Pulmonary or High Risk Submassive Pulmonary Embolism
These are a medical emergency and immediate treatment is needed there are a few different treatment options. Often the treatment chosen will depend on what is available at that facility.
- Clot Busting Medications – The most traditional option is the use of clot busting medications known as thrombolytics, or lytics for short. These medications are typically given through an IV and they travel to the clot where they can try to dissolve the clot. The major drawback of these medications is that there is a significant risk of bleeding that can be life threatening such as a bleed in the brain. In general however the risk of treatment is seen to be less than the risk of doing nothing. Unfortunately there are often situations where the risk of these clot-busting medications are seen to be too high, such as in those patients with known bleeding issues or recent surgery.
- Open Heart Surgery – Sometimes, particularly when there is a very large clot burden or clot known to be floating inside the heart, open-heart surgery may be the preferred treatment option. The chest is opened and the clot is extracted. The downside to this approach is of course the risks of open-heart surgery. In most situations where it is considered, however, the risks of surgery are less than the risk of not treating the pulmonary embolism. The advantage of open-heart surgery is that it can allow for immediate stabilization of the situation in cases where patients are very unstable.
- Catheter Based Treatment – This is the most cutting edge of the treatment options. The pulmonary embolism is treated through small tubes that are typically inserted through the veins of the leg. Different techniques can be used to treat the clot and often injections of clot busting medicines are given directly in to the lungs. An exciting recent development is the use of ultrasound tubes that are inserted into the lung through which micro-doses of clot busting medicines are used. The advantage of this being a much lower risk of bleeding as compared to the traditional doses of the clot busting medications.
- ECMO – This is short for extra-corporeal membrane oxygenation. This is like a heart lung machine that can be used in the treatment of unstable pulmonary embolism. This allows restoration of the circulation if the heart stops or is severely dysfunctional and ensures the organs receive the blood they need. Also the machine allows the blood to be oxygenated in those cases where the lungs aren’t able to function properly. In short ECMO can allow stabilization of an unstable situation and also give time for the situation to recover.
Treatment of Pulmonary Embolism That is not High Risk – Blood Thinning Medication
Most pulmonary embolisms that present to hospital are not massive or submassive and fall in to a category that is generally lower risk. The mainstay of treatment for these is blood-thinning medications. Blood thinning medications prevent the clot from enlarging and allow the body to naturally break down the clots. Patients will have to stay on blood thinning medication from 6 months to life long depending on the circumstance.
- Heparin – Initially heparin used. This is a commonly used blood thinner and typically given in the form of a drip. The dose of heparin can be adjusted as necessary to ensure adequately thin blood. This may be transitioned to an injectable form of heparin such as lovenox.
- Warfarin – This is the longest standing medication used for blood thinning. This is taken as a tablet and it takes a few days to get the blood thin. Its levels are monitored with a blood test. The downside is that the blood needs periodic monitoring to ensure levels are in the desired range, and there are some dietary restrictions.
- NOACs – Short for novel anticoagulant agents. These include Rivaroxaban, Eliquis, Pradaxa, Savasya and others. These are given in tablet forms and the advantage is that monitoring of blood levels is not required.
Risk Factors for Developing Vein Clots
Age is a well-known risk factor. The older someone is, the higher the chance of developing a clot. Simply being in hospital is a well-established risk factor. Other risk factors include use of medicines such as oral contraceptive pills, periods of immobilization, pregnancy, surgery and cancer.
Inherited Clotting Disorders
Inherited clotting disorders are a common cause of development of vein clot (venous thrombosis) that can lead to pulmonary embolism. In patients with a family history of clot development, almost half of patients will be found to have an inherited clotting disorder. These are often due to malfunction or mutations of certain clotting factors. These include, deficiency of antithrombin, protein C, and its cofactor protein S. Another relatively common one is resistance against activated protein C caused by factor V Leiden. The presence of these disorders, in combination with other risk factors such as immobility or injury is a particularly dangerous combination.
Development of certain conditions can also predispose to clot development and therefore development of a pulmonary embolism. These can include antiphospholipid syndrome. Other conditions include polycythemia, paroxysmal nocturnal hemoglobinuria and essential thrombocytosis amongst others. The presence of these disorders, in combination with other risk factors such as immobility or injury is a particularly dangerous combination for causing vein clot and a pulmonary embolism.