A deep vein thrombosis (DVT) is a blood clot that develops in the veins deep within the body, partially or totally blocking blood flow. In human patients this is most likely to occur in the veins within the muscles of the calf and thigh although it will occasionally form in other veins. We presume that the situation is similar in our pet patients.
Deep vein thrombosis forms part of a complex called venous thromboembolism. Venous relates to those vessels that return blood to the heart. A thrombus is a blood clot and an embolus is a part of a clot that breaks off and lodges in another vessel. When a clot lodges in a large vein supplying the lungs this is called a pulmonary thromboembolus (PTE).
Thrombi form in veins for many reasons; however these fall broadly into three categories: changes in blood flow, changes in blood composition, or changes in the vessel wall. Therefore situations which alter any of these three things will predispose to the formation of blood clots and the potential for PTE. Blood usually flows rapidly through veins aided by movement of the legs. A patient that is immobile for a long period of time eg: during a surgical operation or due to illness is at risk of clot formation. Long distance air travel in people is a well known risk factor. High blood pressure will also alter blood flow increasing the risk of clots forming. Changes to the vessel wall happen when the lining becomes inflamed. This can happen during surgery or in serious illness such as sepsis when a vasculitis may develop. Other damaging events include the need for repeated catheterisation, eg: if a patient requires intravenous fluid therapy or from certain drugs.
Changes to the blood itself that result in clotting are usually due to alterations in blood proteins that prevent inappropriate clots developing. With all body systems there is a delicate balance between providing a protective defence but avoiding an inappropriate reaction that may itself cause damage. Diseases that cause excessive loss of protein such as some kidney diseases and some gastrointestinal disease are termed prothrombotic. Some hormonal conditions such as hyperadrenocorticism (Cushing’s disease) can also alter components of the clotting system and combined with their hypertensive effects can result in increased probability of blood clot formation. Any patient with hypertension as part of their illness such as kidney patients, heart patients and diabetics may need observation for clot formation when hospitilised and checking of blood pressure.
DVT can go undetected if the thrombus does not result in complete blockage of the vessel. However, if a piece of thrombus breaks off and forms an embolus which passes through the veins and heart to the lungs symptoms may be serious. These range from mild to severe breathing problems and in a few cases sudden death. It is estimated that 1 in 10 people with an undiagnosed DVT will develop a clot large enough to be at risk of serious PTE. Fortunately our understanding of the reasons for clot formation allows us to take certain measures to help prevent it in our veterinary patients. These include making sure animals get up and about as soon as possible after surgery. This is done by providing appropriate pain relief, but in those animals whose operations make walking difficult our nurses ensure the patient is turned frequently, receives appropriate cage side physiotherapy and when ready receives assistance in exercising such as supported walking and use of an exercise ball.
In sick medical patients with known predisposing factors for clotting are identified we can provide anticoagulant medication such as heparin and low molecular weight heparin to ‘thin the blood’ and stop a clot developing. Patients with hypertension can have this treated with various medications. If a pulmonary embolus does develop patients are given oxygen therapy. Unfortunately we do not yet have veterinary emergency anti-clot drugs like those used in human medicine eg streptolysin because of both expense and poor patient outcomes where they have been used historically. We therefore put lots of effort into identifying pet patients at risk of clotting and thinking of the best patient plan to minimise the risk of this.
Feature, General
