Prophylaxis of postoperative deep vein thrombosis: selective use of low-dose heparin in high-risk patients

Defective Fibrinolysis in Venous Thrombosis

In previous studies a variety of attempts have been made to define laboratory tests which could predict patients who would develop deep vein thrombosis post-operatively. Such tests included the partial thromboplastin time, haematocrit and platelet adhesiveness. In this study patients undergoing gynaecological surgery were screened in advance with a panel of clinical measurements and laboratory tests and the development of DVT was noted using 125I-fibrinogen leg scanning. The values for the selected variables were analysed using logistic discrimination. The factors with the greatest prediction were the euglobulin lysis time, age, varicose veins, serum FR-antigen and percentage overweight for height.

When this formula was applied to a further group of patients it allowed a ‘high risk’ group to be defined and these were given low-dose heparin with a significant reduction in the incidence of post-operative DVT.

An unexpected finding was that patients who smoked cigarettes appeared to be protected from post-operative DVT.

Prediction of postoperative venous thrombosis using haemostasis tests

The prediction of patients who are at sufficiently high risk of postoperative deep venous thrombosis to indicate perioperative antithrombotic prophylaxis has traditionally used only clinical risk factors. The associations of preoperative and/or postoperative haemostatic tests with postoperative deep venous thrombosis are reviewed. In general, the results support the biological concept of a preoperative and postoperative prothrombotic tendency in patients who develop deep venous thrombosis. Increased levels of coagulation activation markers and decreased assays of fibrinolytic potential show consistent relationships to postoperative deep venous thrombosis. At present, however, the clinical utility of such tests is unproven; so that at present they cannot be advocated for routine preoperative or postoperative screening.

Anticoagulants in the prevention of venous thromboembolism

For 50 years, the key to successfully preventing venous thrombosis (VT) or pulmonary embolism (PE) among high-risk patients has been the judicious use of anticoagulants: first through full doses of oral anticoagulants and more recently through low-dose heparin prophylaxis. Low-dose heparin has become the standard of comparison for other preventive methods, since it is relatively safe and simple, its ability to prevent approximately 65% of the subclinical VT found by leg scanning after elective general surgery is well known, and recent meta-analysis of the many pertinent published clinical trials (large and small) strongly suggests a much greater benefit: a 65% reduction in the risk of postoperative death from major PE. In addition, there are trials that have also found low-dose heparin to be effective in general medical patients, although its value in this clinical setting is much less well documented. Although several effective approaches other than low-dose heparin are available, many of these tend to be either more cumbersome (intermittent external leg compression) or probably less powerful (graded pressure elastic stockings). There are situations where low-dose heparin prophylaxis fails, most obviously after orthopaedic surgery where the use of more complex regimens, including adjusted-dose heparin treatment and various schedules of warfarin prophylaxis, becomes appropriate. Recent progress has come from the intensive clinical exploration of various low molecular weight heparin fractions or fragments which appear to be effective after once daily administration to general surgical patients and show great promise of effectiveness and safety after hip surgery. The level of warfarin effect needed for VT prophylaxis has also been reinvestigated, with trials suggesting a need for less warfarin and a lower prothrombin time effect than was previously thought to be appropriate. Given that any attempts to minimize mortality from PE in hospital patients must rely on the widespread and systematic use of simple, safe, and cost-effective preventive methods, it is hoped that these advances will help move anticoagulant prophylaxis further out of the realm of clinical research and into that of common clinical practice.

Risk prediction in medicine and surgery: ethical and practical considerations

Risk prediction is a subject of increasing clinical interest, and publications in this area are likely to have an important influence on patient care in the near future. A multiplicity of risk prediction systems, many of them computer-based, will raise a number of ethical and practical questions. These questions need to be addressed by the originators of systems, the editors of journals, practising clinicians, and the lay public.

Duplex scanning of central vascular access sites in burn patients

Seventy-one burned patients requiring intensive care unit management underwent 570 central venous and 167 femoral arterial catheterizations. These patients were surveyed by repeated physical examinations and duplex scans for vascular-related complications. Catheter sites were rotated every 3 days. No arterial thrombi or occlusions were noted. Fourteen patients (19.6%) had 19 positive venous duplex scans. Five patients (7%) had symptomatic deep venous thrombosis (DVT) and nine (12.6%) had asymptomatic DVT. Mean number of venous cannulations before a positive scan was 4.3 (range 1 to 17). All five symptomatic patients had DVT that originated in the lower extremities. No patient had clinical evidence of a pulmonary embolus, or limb morbidity resulting from the DVT. Follow-up duplex scans in the five asymptomatic and three symptomatic patients showed complete resolution in each case. This study demonstrates the high incidence and natural history of central DVT in a group of critically ill burn patients.

Venous thrombosis: an overview

Venous thromboembolic disease contributes to morbidity and mortality in certain groups of hospitalized patients, particularly those who have undergone surgery. Although principles of treatment have changed relatively little during the past 20 years, significant advances have been made in the diagnosis of deep vein thrombosis (DVT). Venography, once the only reliable diagnostic technique, has been largely replaced by noninvasive tests: impedance plethysmography, venous Doppler, 125I-radiofibrinogen-uptake test, and phleborheography. Virchow's triad of stasis, vessel injury, and hypercoagulability remains a valid explanation of the pathogenesis of thrombus formation, but laboratory and clinical data have refined our knowledge of how these factors interact to result in clinically significant disease. Knowledge of the natural history of venous thrombosis, plus heightened awareness of the long-term morbidity and expense associated with the postphlebitic syndrome, have led to increased interest in preventing DVT. Clinically and economically, venous thrombosis is best managed by prevention.

Complications of low-dose heparin prophylaxis following pelvic lymphadenectomy

All patients who underwent pelvic lymphadenectomy for prostatic carcinoma over a five-year period were reviewed to evaluate the effect of minidose heparin prophylaxis on lymphocele formation and postoperative complications. The overall incidence of complications was found to be consistent with that reported in the current literature. However, when the rate of lymphocele occurrence was correlated with perioperative minidose heparin therapy, the incidence rose from 2.5 per cent in those receiving no heparin to 38 per cent in patients receiving both pre- and postoperative prophylactic heparinization. Other major postoperative complications were also significantly increased when minidose heparin was pre- or postoperatively administered. A strong recommendation is made for caution in the routine use of prophylactic low-dose heparin during pelvic surgery.

Incidence of post-operative deep vein thrombosis in gynaecological oncology

Using 125I-fibrinogen scanning the incidence of post-operative DVT amongst patients with gynaecological malignancy was shown to be 37.9% which is substantially higher than the 10% to 15% expected in a general gynaecological population. Twenty per cent of the total group studied had isotopic evidence of bilateral venous thrombosis post-operatively. The inaccuracy of clinical diagnosis was demonstrated, and also the need to investigate both limbs if thrombosis was suspected. The incidence of post-operative DVT was found to be significantly lower in smokers (11.5%) compared with non-smokers (68.4%) (p less than 0.00025).

Prediction and selective prophylaxis of venous thrombosis in elective gastrointestinal surgery

Clinical features were noted and routine and non-routine laboratory variables were measured before elective major gastrointestinal surgery in 63 patients aged 40 years or more. Deep-vein thrombosis (DVT), detected by routine 125I-fibrinogen leg scanning, developed in 21 patients. Five clinical variables but no laboratory variables were significantly associated with DVT: age; percent mean weight for age, sex, and height (%MW); presence of varicose veins; cigarette-smoking; and sex. The most useful discriminant index of these variables was age in years plus 1.3 x %MW. The index was validated prospectively in a further 41 patients, in 18 of whom DVT developed. The value of the index in selective prophylaxis was then assessed in a further 40 patients, of whom 24 (60%) with high-risk index received low-dose heparin (5000 units twice daily). DVT developed in 4 of the 40 patients, an incidence of 10% compared with the incidence of 37.5% (39 of 104) in the earlier studies with no prophylaxis.