Sequential Compression Devices (SCD)
in the Prevention of DVT/PE
Deep vein thrombosis
Blood clotting in the veins of the inner thigh or leg.
The formation of a thrombus (blood clot) in the lower leg.
Blood clots can break off (as an embolism) and makes its way to the lung where it has the potential of causing respiratory distress and respiratory failure.
The role of intermittent sequential compression devices (SCDs) for prophylaxis
[prophylaxis : The prevention of disease, preventive treatment. Origin: Gr. Prophylassein = to keep guard before against Deep Vein Thrombosis (DVT) has been studied and increasingly utilised in general surgery patients.]
The factors that are felt to form the basis of the pathophysiology of venous blood clotting disease are:
- Stasis (reduction of blood flow in the veins),
- Injury (to the Inner layer of blood vessel), and
- Hypercoagulability (increased risk of clotting).
Scientific and clinical evaluations of SCDs strongly suggest that the nature of their effect on DVT prophylaxis derives from their ability to increase mean and peak femoral vein (in the leg) velocity as well as their effect on the systemic coagulation and fibrinolytic mechanisms.
(Characterised by causing the dissolution of Fibrin by enzymatic action. Fibrin is the insoluble protein formed during normal clotting of blood; Fibrin forms the essential portion of the blood clot).
The sequential pattern of compression has been well described: chambers of the extremity garment are sequentially inflated from ankle to knee (or mid thigh) to a maximum pressure of 45-50mm Hg at the ankle, 35 mm Hg at the calf, and 30mm at the thigh (hence the term "gradient" compression). The duration of compression is 11 seconds with a 60-second relaxation period between compressions.
Attacking the long-recognised risk factor of stasis (reduction of blood flow in the veins), SCDs have been shown to increase mean and peak femoral venous blood velocities on the lower extremity.
The use of SCDs worn on the lower extremity in patients at high risk for DVT and to reduce the rate of DVT is widely accepted, however, clinical studies demonstrating their effectiveness in trauma patients are few. While the exact mechanism of action of SCDs is not known, their effect is felt to be based on a combination of factors addressing stasis (reduction of blood flow in the veins), and hypercoagulability. Until these mechanisms are better studied and understood, answers to specific questions regarding the appropriate use of SCDs are still to be forthcoming.
SCDs have been shown to reduce the incidence of both DVT and PE.
(Pulmonary embolism - The lodgement of a blood clot in the lumen of a pulmonary artery, causing a severe dysfunction in respiratory function. Pulmonary emboli often have origin in the veins of the lower extremities where clots form in the deep leg veins and then travel to the lungs via the venous circulation).
Unanswered questions regarding the use of SCDs include the mechanism by which SCDs act, the efficacy of SCDs worn on the upper extremities or a single lower extremity compared to both lower extremities, the nature of risk involved in discontinuing SCDs periodically during use, and the duration of SCD use. Reports suggest that SCDs should be worn with thromboembolism-deterrent stockings (i.e. TEDS), however, this practice has not been widely studied and is not standard. Complications of SCDs have been noted in case reports and have been associated with improper positioning of the lower extremity during surgery which should be avoided.
More studies need to be done specifically related to the use of SCDs in trauma patients at risk for VTE. Questions regarding the efficacy of using the device on one lower extremity vs. two, and whether an arm vs. a leg provides equal protection, all need to be addressed. There are a number of commercial vendors of compression devices. Whether they all provide equal protection or one vendor is superior needs to be determined. Finally, the role of multimodality therapy, mechanical and and anti-clotting pharmacological, to provide any additional protection from VTE needs to be ascertained.
Compression devices appear to be well tolerated with minimal side effects. Isolated case reports of pressure necrosis* from a too tightly fitted SCD have been reported. SCDs can be used safely in stable head-injured patients.
*Necrosis : The sum of the morphological changes indicative of cell death and caused by the progressive degradative action of enzymes, it may affect groups of cells or part of a structure or an organ.