A thrombus is a clot, and a pulmonary embolism is a thrombus that breaks loose and travels through the veins, eventually reaching the lungs where it can cause a serious blockage of the blood flow going through the lungs and back to the heart. Thromboembolic disease refers to both of these related problems.
A pulmonary embolus is the most serious result of thromboembolic disease, but other problems include painful and persistent swelling and circulation problems if the valves in the veins are damaged from blood build-up behind the clot, leading to chronic venous hypertension.
Although thromboembolic disease is uncommon in the approximately 800,000 patients undergoing total joint replacements in the U.S. each year, it is one of the major risks and complications that can occur with major orthopaedic surgery of any sort to the lower extremities. Not surprisingly, there has been much research and effort to reduce or eliminate thromboembolic disease in the 40+ year history of joint replacement surgery. Today the risk of getting a symptomatic deep vein clot while on prophylaxis is around 1% to 5% (although silent, asymptomatic clots are probably somewhat more prevalent).
Deep Vein Thrombosis (DVT)
A deep vein thrombosis is the formation of a thrombus (or blood clot) within the deep veins, usually in the calf or thigh. There is a difference between superficial phlebitis and deep clots; the clot that forms within the very large veins in the lower extremity tends to be the one that causes concern. This clot can cause problems by itself, or it can break free and cause a pulmonary embolism.
The thrombus (clot) can partially or completely occlude the blood flow. This usually leads to significant swelling below the level of the clot, similar to a back-up in the plumbing system, because blood flow back to the heart is impeded by the clot. Nearly all patients have some degree of lower extremity swelling for months after major joint surgery, but severe swelling and calf or thigh pain are reasons for potential concern.
Some DVTs are not symptomatic. In fact, some studies have suggested that there are many more silent DVTs than are diagnosed because many do not cause any problems and resolve on their own. However, it is better to prevent their formation in the first place than to try to treat them once they occur.
Factors Contributing To DVT
Deep vein thromboses (clots) can occur even without surgery. This can happen after a prolonged trip (such as a transatlantic airline flight) or in anyone who is bedridden for a prolonged period of time.
Stagnant blood flow through the veins increases the possibility of a clot forming. This happens with prolonged bed rest or immobility. Not surprisingly, the best prevention for blood clots is early mobility and getting up as quickly as possible after surgery.
Length of surgery appears to also play a role. Although it is most important to do a good job with the surgery itself, this is one of the reasons it is better to have a surgeon whose operating times are shorter and quicker. (Incidentally, there is probably also an increased risk of infection with longer operating times.)
Coagulation, or clot formation, is a complex process which the body normally uses to prevent us from bleeding to death from cuts or injuries. In normal situations and even surgery, this normal coagulation cascade or process is beneficial. However, some patients may be prone to forming clots too easily by having underlying medical or genetic conditions that lead to hypercoaguability. Diseases like antiphospholipid syndrome, protein S or C deficiency, or a host of other genetic diseases can increase the likelihood of forming clots.
Obesity, diabetes, varicose veins, and cancer all have been shown to increase the risk of forming clots. Smoking and use of oral contraceptives also increase the likelihood of forming clots.
Injury to veins can cause clots. In particular, "kinking" of the vessels during surgery from either retractors or by awkward positioning of the leg may increase the likelihood of clot formation. There is significant evidence that surgical approaches that do not necessitate twisting the leg during surgery have less risk of clot formation; this is one of the reasons we favor the anterior approach for hip surgery in which the patient is laying flat (supine) without the need for twisting the leg during surgery (as is the case for most posterior approach surgeries).
Incidence
There is a wide variance between published reports regarding the incidence of DVT in orthopaedic surgery patients after joint replacement. One principal reason for this is the large number of asymptomatic clots that are only detected if screening tests are performed, as in research studies. Most hospitals do not routinely perform tests to check for clots (e.g., ultrasound or venograms) unless there are symptoms to suggest such a test is needed.
Some studies suggest that the incidence of DVT in patients without any prophylaxis after total joint replacement may be in the range of 40% to 80%. With prophylactic treatment, most studies report the incidence of clinically symptomatic thromboembolic disease to be less than 5%.
The most important figure is probably the number of fatal pulmonary embolisms, since this is the outcome that we are most concerned with. Most studies report that incidence to be well under 1% in patients treated with some form of prophylaxis. In our practice, we have published studies looking at the incidence of DVT and PE after hip replacement. In a series of over 2000 total hip replacements, we found about 1% incidence of clinically significant and diagnosed DVT and there was only one death from a pulmonary embolus out of several thousand patients.
Tests To Detect DVT or PE
Most surgeons and hospitals do not screen for DVT with routine diagnostic tests, but instead order these tests if a patient demonstrates clinical symptoms. The most common symptoms of a DVT are swelling in the lower extremity, edema, increasing discomfort (instead of decreasing discomfort in the time after surgery), and certain physical examination signs such as palpable cords in the back of the knee or a positive Homan's test.
The most common signs of a pulmonary embolus are sudden chest pain and shortness of breath. A patient may require supplemental oxygen to keep blood oxygen at adequate levels.
Both pulmonary emboli and deep venous thromboses often cause a mild elevation in temperature. However, it is common for patients to have a low grade fever (less than 101.5º F) for a few days after major joint replacement surgery from other factors as the body adjusts after surgery, and this is not a specific finding.
A venogram is probably the most accurate test to diagnose a deep blood clot, but it is not often used because the test itself may increase the likelihood of a clot and requires injection of contrast into the vein. Therefore, it is not routinely used in most institutions to diagnose blood clots.
An ultrasound (doppler venous ultrasound) is probably the most common test for detecting blood clots. This is a painless test in which an ultrasonic probe is placed against the back of the leg and used to view the deep blood vessels and visualize any clots that may be present.
Less commonly employed tests include magnetic resonance imaging (MRI) or plethysmography. MRI is helpful for diagnosing clots deep within the pelvis, where it is difficult to detect clots with an ultrasound, but it is a very expensive test and is not always readily available in smaller hospitals. In addition, not all patients can have an MRI (such as pacemaker patients) because of the magnetic field. Plethysmography is an older test that uses blood pressure cuffs at different locations along the leg to detect blockages, but few centers use it today because it is not as accurate as more modern methods.
A pulmonary embolus may be diagnosed with a VQ scan (also known as a ventilation-perfusion scan) or a special CT scan with contrast. A VQ scan is a nuclear imaging test in which very slightly radioactive substances are taken in to see if ventilation (breathing) matches the perfusion (or blood flow in and out of the lungs). If not, there may be a pulmonary embolus. A more accurate test is to perform a spiral computed tomography (CT) scan with a contrast agent in the blood to see if there is a clot situated within the pulmonary artery that leads away from the heart to the lungs. However, patients with kidney disease cannot have the contrast dye, and a VQ scan is used instead for these patients. An arterial blood gas sample may often be obtained (in contrast to most blood draws, which are from veins) to analyze the blood for oxygen and carbon dioxide levels, which can also be helpful in diagnosing a pulmonary embolus.
Prevention
The best treatment for thromboembolic disease is to prevent it in the first place. As mentioned before, the best prevention occurs from getting up and moving as quickly as possible. In addition to walking, patients are advised to move the ankles up and down frequently throughout the day to keep the circulation moving.
The form of anesthesia used during surgery can have some bearing on the risk for DVT formation. The rate of DVT formation can be decreased by up to 50% in some studies by using regional or spinal anesthesia instead of general anesthesia.
There are mechanical forms of prophylaxis, such as stockings, pneumatic pumps, and intravenous filters, as well as pharmacologic forms such as aspirin, heparin, or warfarin. Most surgeons use a combination of both, but there is no clear evidence that any particular method of pharmacologic prophylaxis is significantly better than others and as a result, there is wide variation in what type of prophylaxis is used. The highest risk period is probably the first 5 days, although there is evidence that there is a second peak in incidence about 2 weeks after surgery. It is also not clear how long prophylaxis needs to be used, although most studies suggest it is probably best employed for four to six weeks after surgery (possibly longer if a patient is not getting up and mobilizing well).
Mechanical Prophylaxis
Some patients may be too sedentary or forgetful to remember to keep moving their legs and ankles, which is why most hospitals also use some form of mechanical prophylaxis for most joint replacement patients. This usually is in the form of a pneumatic sleeve that fits around either the calf or the foot and periodically inflates and deflates, moving the circulation throughout the leg.
Another method of mechanical prophylaxis is the use of compression elastic stockings, which encourage venous blood flow back to the heart. These are more effective in preventing blood clots in the calf than the thigh.
The most aggressive mechanical prophylaxis is to place a small filter in the vena cava, the large vein that returns blood flow to the heart. These filters resemble a small umbrella and are usually made out of wire. These are placed in the vena cava by a vascular surgeon or radiologist via a catheterization, and while some types of filter are designed to be temporary, the majority are designed to be left in place permanently. The idea is that the filter will catch any large clots before they reach the heart, trapping them in the wire arms and holding the clot in the circulation until it breaks down. On the downside, these filters can sometimes migrate and travel to the heart or other locations. For this reason, filters are not usually used unless a patient has a high risk based on risk factors or a history of previous thromboembolic disease or blood clots.
Aspirin
Aspirin is probably the simplest and least expensive pharmacologic method of DVT prophylaxis. It is easy to take (unlike heparins that require injection), does not require monitoring (as with warfarin, which requires periodic blood tests), and does not usually lead to bleeding complications (e.g., hematoma). However, some patients cannot tolerate aspirin because of gastrointestinal issues or ulcers, interaction with other medications, or kidney disease.
We routinely use aspirin for most straight-forward, single-sided, first time total hip replacement patients in our practice. It is not the strongest blood thinner, however, and is not used for high risk patients, most knee replacement patients (who have a higher risk of blood clots in the calf), or patients having extensive or bilateral surgery. The usual dosage for prophylaxis after surgery is 325 mg of enteric coated aspirin twice a day for a total of 650 mg of aspirin daily (many patients may already take a "baby aspirin" each day for their heart, but this is a smaller dosage at 81 mg once a day).
Heparin and Low Molecular Weight Heparins
Heparin is a naturally occurring substance that inhibits clotting. It is useful because it has a very short period of effectiveness (hours), which allows it to be used for patients who may need the blood thinner "turned off" to allow a return to the operating room for additional procedures. It is also used for patients who have a condition in which the blood thinner can only be discontinued for a very short period of time to allow surgery (such as patients with mechanical heart valves who have to be on blood thinners every day of their lives).
Heparin only comes in injectable forms, commonly used as a subcutaneous injection in the soft fat of the abdomen for prophylaxis after surgery. Not all patients are able to give themselves the injections.
Heparin has been used for many years in its high (unfractionated) molecular weight form, but in recent years low (fractionated heparin) molecular weight heparin injections have become available and are in widespread use (such as enoxaparin). These low molecular weight heparins only need to be injected once or twice a day (instead of every few hours) and work faster than aspirin or warfarin, but are very expensive (often costing hundreds of dollars, as compared to the cost of aspirin each day). In addition, there is some strong evidence that these blood thinners are more likely than aspirin or warfarin to cause postoperative bleeding complications (such as a hematoma), which can sometimes be serious.
Warfarin
Warfarin (which is often known under the brand name of coumadin) has been used for decades as a blood thinner. It was originally developed as a rat poison (named after the Wisconsin Alumni Research Foundation where it was developed over 50 years ago), and works by interfering with vitamin K metabolism and the blood clotting cascade. It is taken orally and does not require injection. It also does not usually cause gastrointestinal or kidney problems, but it does have some downsides of its own.
Warfarin works slowly, often taking days to reach its desired effect. It also requires frequent monitoring with blood tests to make sure the blood is not too little or too greatly thinned (problems can arise from both). Although the medication is inexpensive, the laboratory testing and monitoring each week is not.
Because the natural antidote to warfarin is vitamin K, eating foods rich in vitamin K can decrease the effectiveness of the medication. Many medications can interact with warfarin, such as cholesterol medications, some antibiotics, and some herbal medications.
Warfarin can result in hemorrhage, excessive bruising, bleeding from the nose or gums, and blood in urine or stool. Although rare, a specific complication known as warfarin necrosis can occur that can be devastating and fatal (leading to massive thrombosis in the skin and limbs, with problems of necrosis and gangrene in the limbs). We very rarely use it in our practice, but across the country many surgeons still commonly use it as an accepted practice.
Alternative Blood Thinners
There is currently a great deal of interest in developing alternative pharmacologic agents for DVT prophylaxis because of the hundreds of thousands of patients each year who need it and the various downsides of the major methods currently used (aspirin, heparins, or warfarin).
There is interest in newer antiplatelet drugs such as clopidegrel, but thus far there is not sufficient evidence that this is adequate for DVT prophylaxis. Several promising anticoagulation drugs in development in recent years were subsequently shown to have unexpected side effects and development has slowed or stopped with these medications. Some alternative anticoagulants are in development, but these will not likely be ready for widespread use for a number of years at the time of this writing.
Treatment of DVT or PE
When a thromboembolic complication occurs (DVT or PE), treatment depends on several factors. Minor, superficial clots do not typically require any treatment beyond continuing the prophylactic dosage of whatever regimen the patient is already on.
Deep venous thromboses in the calf veins usually do not require inpatient heparin treatment and can often be treated on an outpatient basis with either warfarin or low molecular weight heparins for 6 to 12 weeks. Thromboses in the thigh may require bed rest and a week or so of heparin therapy followed by several months of warfarin. Pulmonary emboli of significant size usually require heparin, close observation and oxygen therapy, and subsequent treatment with warfarin for usually at least six months.
Please remember the information on this site is for educational purposes only and should not be used to make a decision on a condition or a procedure. All decisions should be made in conjunction with your surgeon and your primary care provider.
