Blood thinners are a group of medicines that help keep blood flowing smoothly by preventing blood clot formation and growth of clots. Blood thinners belong to two main classes of drugs. One is the anticoagulants which refers to drugs that prevent clotting of blood. This includes heparin and drugs like warfarin. Anti-platelet agents like aspirin and clopidogrel are the other type. Clotting of blood is an important mechanism to prevent loss of blood after damage to blood vessels as may occur from cuts and bruises. In the absence of such a protection mechanism we would be at risk of bleeding even to death from relatively minor wounds. Bleeding is normally prevented by three separate mechanisms acting in a synergistic manner. The initial mechanism is contraction of blood vessel to restrict blood from flowing out. This is adequate on its own to stop bleeding from minor cuts. The second mechanism is formation of a platelet plug by them clumping together to breach the defect in the blood vessel. Platelets, along with red blood cells and white blood cells, are one of the cellular components of blood. They not only clump together in areas of damage to plug the leak but also activate clotting. Formation of a clot is an elaborate process involving sequential activation of various clotting factors circulating in blood in an inactive form. A clot is usually able to stop bleeding even in case of quite significant damage to blood vessels. The need for clotting factors to circulate in an inactive form is evident since inappropriate activation would lead to calamitous clotting within blood vessels.
It is apparent that what is needed is to prevent blood loss in case of an injury but at the same time avoid clots from forming within blood vessels. Clots do sometimes form inside blood vessels, usually when their internal lining has become rough or irregular from disease or when smooth flow of blood through the vessel is interfered with from any mechanism. This has the potential to cause major problems.
Blood thinners are generally indicated to prevent blood clots from forming within blood vessels and to prevent such clots from becoming larger
The three types of blood vessels are the arteries conveying oxygenated blood to tissues, veins carrying deoxygenated blood back to the heart and capillaries which form a network between arteries and veins. A blood clot inside a critical artery often compromises blood supply to tissues beyond the clot. The technical term for a blood clot forming inside a blood vessel is thrombus and this process of clot formation is called thrombosis. Examples of such problems are coronary thrombosis causing a heart attack and cerebral thrombosis causing a stroke. Also, the clot may get displaced from where it is formed and be carried in the blood stream till it gets impacted, usually in an artery narrower than the clot. This process is called embolism and the clot impacting at a distant site is called an embolus. Inappropriate clotting within veins interferes with return of blood from the site and causes localized swelling because of pooling of venous blood. The clot inside the vein may also get dislodged from the site where it forms and is then carried in the blood stream till it gets lodged in a blood vessel with a narrower diameter. Veins from all parts of the body (except the lungs) return blood to the right side of the heart which pumps blood into the lungs to oxygenate it. Embolizing blood clots from veins in the leg thus get impacted in arteries in the lung which is known as pulmonary embolism, a potentially life-threatening problem. Another important cause of embolism is atrial fibrillation in which the muscle in the atrial chambers of the heart contract in an unsynchronized, irregular and ineffective way. Because of the lack of effective contraction of the chamber a clot is liable to form as there is pooling of blood within the chamber. Should atrial fibrillation revert to normal heart rhythm, as happens spontaneously or due to treatment, the clot within this heart chamber is pushed into circulation with a significant possibility of disastrous embolism to the brain or other vital organs.
Blood thinners are generally indicated to prevent blood clots from forming within blood vessels and to prevent such clots from becoming larger. Prevention of ischaemic heart disease which includes myocardial infarction or coronary thrombosis and angina is the commonest indication. Blood thinners are also indicated in the prevention and treatment of stroke. Deep vein thrombosis (DVT) which is the formation of blood clots in the deep veins of the legs may occur in a number of circumstances such as with prolonged immobilization for medical illness, after surgery, during lengthy journeys especially when sitting cramped in economy class, etc. Blood thinners are prescribed to prevent and treat pulmonary embolism which may occur because of DVT. People with atrial fibrillation too often require blood thinners to prevent emboli. Any foreign tissue in contact with circulating blood tends to result in clot formation at this site and thus people with stents placed in their arteries by angioplasty and those with heart valve replacement too require these drugs.
The anti-coagulant blood thinners interfere with clotting of blood while anti-platelet blood thinners act against the actions of platelets. In general, anti-coagulants have more powerful effects and are used in acute cases as well as in more serious cases. Thus, in conditions like evolving stroke, acute coronary syndrome, pulmonary embolism, etc anti-coagulants are preferred. They are often replaced with anti-platelet agents once the position stabilises or when the risk perception is considered to have reduced. Anti-coagulants are also regularly used to reduce embolisation in atrial fibrillation and in people with heart valve replacement. In occasional patients both anti-coagulants and anti-platelets are prescribed on a long-term basis. This is done only when the risk for a thrombotic or embolic episode is considered to be unusually high. What is desired from the ideal blood thinner is to prevent clot formation within the blood vessel without interfering with control of external bleeding. Since no such ideal blood thinner exists it is aimed to use these medicines at a dose that will reduce clot formation without constituting a significant hazard for external bleeding from the blood vessel. In most patients this is possible to achieve to a practically acceptable level. In some patients this safe level of blood thinner drug where an internal clot does not form but external bleeding risk is not too high is not achieved. With too low a dose of the blood thinner there is a risk for a medical event due to the original problem such as stroke or heart attack while with too high a dose the patient develops the complication of bleeding. Such bleeding may be evident as bleeding into the skin, during dental or other surgery and from trauma or an internal bleed, most often into the gastrointestinal tract. Bleeding is likelier with higher doses of blood thinners or when they are given in combination. Significant bleeding complications are also higher with anti-coagulants and especially so when combined with an anti-platelet agent.
Heparin and warfarin are the prototype anti-coagulants. Heparin has to be administered by injection while warfarin is taken orally. Patients requiring short-term anti-coagulation are generally advised heparin. Those requiring long-term anti-coagulation may be started on heparin to get an immediate response and switched to warfarin for continued effect. Heparin is generally considered safe for use in pregnancy in contrast to warfarin. Low molecular weight heparin is a heparin derivative with more predictable action and its use does not require close monitoring with coagulation tests and is generally preferred. Warfarin use has to be monitored regularly by a clotting test called prothrombin time / INR and the dose adjusted according to the test result. Blood levels of the drug are influenced to a great degree by use of many other medicines and also by some food items and hence a good deal of care has to be taken to avoid swings in blood levels which could cause major complications. A number of alternative oral anti-coagulants to warfarin are also available and they offer some advantages such as easier reversal of anti-coagulant action in case of bleeding complications.
Aspirin and clopidogrel (original brand name Plavix) are the prototype anti-platelet drugs. Low dose aspirin in particular is very widely used. It is often recommended in older individuals without underlying problems to reduce their risk for heart attack and stroke. This is open to debate because in lower cardiac risk individuals the reduction in heart attacks is offset by an increase in significant bleeding episodes from the gastrointestinal tract. It is also recommended for use in most diabetics to prevent heart disease, the commonest cause of death in them. Clopidogrel and a number of newer antiplatelet agents are alternatives to aspirin and in higher risk individuals both are given together, albeit with a somewhat higher risk of complications. These medicines are best taken after food. Use of anti-platelet agents generally does not warrant special monitoring tests. Blood thinners are thus very useful drugs but the potential complication of bleeding necessitates due care in their use.
Print
Send to Friend
Blood thinners are a group of medicines that help keep blood flowing smoothly by preventing blood clot formation and growth of clots. Blood thinners belong to two main classes of drugs. One is the anticoagulants which refers to drugs that prevent clotting of blood. This includes heparin and drugs like warfarin. Anti-platelet agents like aspirin and clopidogrel are the other type. Clotting of blood is an important mechanism to prevent loss of blood after damage to blood vessels as may occur from cuts and bruises. In the absence of such a protection mechanism we would be at risk of bleeding even to death from relatively minor wounds. Bleeding is normally prevented by three separate mechanisms acting in a synergistic manner. The initial mechanism is contraction of blood vessel to restrict blood from flowing out. This is adequate on its own to stop bleeding from minor cuts. The second mechanism is formation of a platelet plug by them clumping together to breach the defect in the blood vessel. Platelets, along with red blood cells and white blood cells, are one of the cellular components of blood. They not only clump together in areas of damage to plug the leak but also activate clotting. Formation of a clot is an elaborate process involving sequential activation of various clotting factors circulating in blood in an inactive form. A clot is usually able to stop bleeding even in case of quite significant damage to blood vessels. The need for clotting factors to circulate in an inactive form is evident since inappropriate activation would lead to calamitous clotting within blood vessels.