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© 1998 The Journal of Bone and Joint Surgery, Inc. Comparison of the Use of a Foot Pump with the Use of Low-Molecular-Weight Heparin for the Prevention of Deep-Vein Thrombosis after Total Hip Replacement. A Prospective, Randomized Trial*   Investigation performed at the University Department of Orthopaedics, Avon Orthopaedic Centre; the Department of Radiology, Southmead Hospital; and the Department of Social Medicine, University of Bristol, Bristol
We conducted a prospective, randomized trial to compare the safety and effectiveness of the A-V Impulse System foot pump with that of low-molecular-weight heparin for reducing the prevalence of deep-vein thrombosis after total hip replacement. Of 290 patients who were to have a primary total hip replacement, 143 were randomized to receive enoxaparin (forty milligrams daily) for seven days after the operation and 147, to use the foot pump for seven days. The primary outcome measure was the prevalence of deep-vein thrombosis, as determined by venography on the sixth, seventh, or eighth postoperative day. Secondary outcome measures included transfusion requirements, intraoperative blood loss, postoperative drainage, blood-loss index, appearance of the site of the wound according to a subjective visual-analog scale, and swelling of the thigh. The patients' compliance with the regimen for use of the foot pump was monitored with an internal timing device, and their acceptance of the device was assessed with a questionnaire. Symptoms consistent with pulmonary embolism were investigated with ventilation-perfusion scanning. The patients were contacted later for detection of symptoms of venous thromboembolism that may have occurred during the first three months after discharge from the hospital. Venography was performed on 274 patients: 136 who used the foot pump and 138 who received enoxaparin. Deep-vein thrombosis was detected in twenty-four (18 per cent) of the patients who used the foot pump compared with eighteen patients (13 per cent) who received enoxaparin (95 per cent confidence interval for the difference in proportions, -3.9 to +13.0 per cent). Thrombosis in the calf was found in seven patients (5 per cent) in the former group compared with six patients (4 per cent) in the latter (95 per cent confidence interval for the difference, -4.2 to +5.8 per cent), and proximal thrombosis was observed in seventeen patients (13 per cent) in the former group compared with twelve patients (9 per cent) in the latter (95 per cent confidence interval for the difference, -3.5 to +11.1 per cent). None of these differences was significant. No patient in either group had major proximal deep-vein thrombosis; all proximal thrombi were isolated entities involving the femoral valve cusp and were of unknown importance. One patient who used the foot pump had a non-fatal pulmonary embolism. One patient who received enoxaparin had a symptomatic deep-vein thrombosis during hospitalization. Two patients (one from each group [0.7 per cent]) were readmitted to the hospital because of a symptomatic deep-vein thrombosis despite normal venographic findings at the time of discharge. There was no difference in the transfusion requirements or the intraoperative blood loss between the two groups. There were more soft-tissue side effects in the patients who received enoxaparin than in those who used the foot pump: there was more bruising of the thigh and oozing of the wound (p < 0.001 for each), postoperative drainage (578 compared with 492 milliliters; p = 0.014), and swelling of the thigh (twenty compared with ten millimeters; p = 0.03). Of 124 patients who used the foot pump and were asked about the acceptability of the device, fourteen (11 per cent) said that it was uncomfortable, twenty-one (17 per cent) reported sleep disturbance, and four (3 per cent) stated that they had stopped using the device. Conversely, ten (8 per cent) found it relaxing. We concluded that the foot pump is a suitable alternative to low-molecular-weight heparin for prophylaxis against thromboembolism after total hip replacement and that it produces fewer soft-tissue side effects. Tolerance of the device is a problem for some patients.
Routine prophylaxis against thromboembolism after total hip replacement is recommended14. Prophylaxis can be either mechanical or chemical. Although chemical prophylaxis, particularly with use of low-molecular-weight heparin, effectively reduces the frequency of deep-vein thrombosis as diagnosed with venography after total hip replacement, many orthopaedic surgeons are concerned about the potential for soft-tissue side effects, especially hemorrhagic complications, and therefore are attracted to mechanical methods22. Total hip replacement is associated with venous stasis, which is an important etiological factor in the development of deep-vein thrombosis. The operative maneuvers that are needed to implant prosthetic components obstruct femoral vein flow32, the patient is relatively immobile for several days after the operation, and the physiology of the venous system appears to be altered for some weeks postoperatively19. A fairly recent anatomical discovery has led to the development of a new concept for overcoming the venous stasis associated with total hip replacement. Gardner and Fox, with use of video venography, demonstrated a large venous plexus in association with the lateral plantar arteries in the sole of the foot9,10. When the metatarsal arch is flattened on weight-bearing, this plexus is stretched, expressing about thirty milliliters of blood into the deep venous system of the lower limb. This bolus flushes the valve cusps where thrombi form, and it may enhance fibrinolysis9,10. The A-V Impulse System foot pump (Novamedix, Andover, United Kingdom) was developed to reproduce this physiological mechanism in patients who are unable to bear weight. A cuff is held around the foot by a soft, non-expandable slipper with a hard sole between the heel and the metatarsal heads. The cuff is inflated at twenty-second intervals by an electronic controller16. This flattens the metatarsal arch, emptying the venous plexus and thus reproducing the effect of normal weight-bearing. Several randomized trials have shown that the foot pump is an effective device for prophylaxis against thromboembolism in orthopaedic patients4,8,28,30,36,37. Fordyce and Ling reported a relative risk of 0.32 for deep-vein thrombosis, as detected venographically, in a study comparing use of the A-V Impulse System foot pump with no prophylaxis after seventy-nine total hip replacements8. Bradley et al. reported a relative risk of 0.24 for deep-vein thrombosis, also as detected venographically, in a study comparing treatment with and without the foot pump after seventy-four total hip replacements4. Santori et al. documented a relative risk of 0.15 for proximal deep-vein thrombosis, as seen on ultrasound, in a comparison of use of the foot pump with use of unfractionated heparin after 132 total hip replacements28. However, each of these studies was relatively small or involved the use of ultrasound, which is less accurate than venography35, for screening. Furthermore, we know of no previous study in which the outcomes obtained with use of the foot pump have been compared with those obtained with use of low-molecular-weight heparin, which is currently one of the most effective and widely used measures for prophylaxis against thromboembolism. The acceptability of the foot pump to patients and the degree to which they comply with the regimen for its use, factors that have limited the employment of mechanical devices that compress the calf and thigh, have not been assessed previously, to our knowledge. We believe that such information would aid in the decision regarding which prophylactic method to use. The aim of the current study was to compare the frequency of thromboembolism after total hip replacement in patients who were randomized to be managed either with the foot pump or with low-molecular-weight heparin. In addition, we sought to evaluate the side effects, acceptance, and compliance associated with use of the foot pump.
Study Subjects All patients were fitted with graduated compression stockings and were advised to wear them for six weeks after the operation. The use of non-steroidal anti-inflammatory medications was discontinued whenever possible two weeks before the operation because of their potential interaction with low-molecular-weight heparin. Regional anesthesia (spinal or epidural) was used at the discretion of the anesthetist; the operative approach and the method of fixation of the implant were chosen by the surgeon. The study was approved by the Avon Health Authority Southmead Medical Research and Ethics Committee. All patients gave written informed consent.
Randomization
Primary Outcome Clinical symptoms consistent with pulmonary embolism were investigated with ventilation-perfusion scanning. All patients were contacted by telephone or letter to establish the prevalence of clinically evident thromboembolic events during the first three months after discharge from the hospital.
Secondary Outcomes Perioperative blood loss was calculated by adding the amount of intraoperative blood loss (the weight of the swabs and the volume of the contents of the suction bottle, corrected for the amount of saline solution used for lavage of the wound) to the amount of postoperative drainage at thirty-six hours. (Two 12-gauge drains were used; one was placed deep to the fascia lata, and the other was placed in the subcutaneous tissues.) The hemoglobin level and hematocrit were recorded preoperatively, on the second postoperative day, and before discharge. The transfusion requirements also were recorded. The blood-loss index, a validated measure used in clinical studies to determine blood loss in patients being managed with therapeutic anticoagulation17, was calculated with the equation: blood-loss index = preoperative level of hemoglobin - level of hemoglobin before discharge + number of units transfused. Bruising of the thigh and oozing of the wound were assessed with use of a subjective visual-analog scale on the fourth and seventh postoperative days. With regard to bruising, a score of 0 points indicated no discoloration; 1 point, light-yellow discoloration; 2 points, dark-yellow discoloration; 3 points, yellow discoloration greater than the area of three palms; 4 points, yellow-and-black discoloration; and 5 points, yellow-and-black discoloration greater than the area of three palms. With regard to oozing, 0 points indicated none; 1 point, one small strip of oozing from the operative wound; 2 points, two or three small strips; 3 points, four strips or more, or a continuous strip at least three centimeters in length; 4 points, hematoma; and 5 points, dehiscence. Swelling was assessed by measuring the circumference of the thigh on both the involved and the uninvolved side on the fourth and seventh days after the operation. The precise level (twenty centimeters proximal to the knee) was marked with indelible ink to standardize each measurement. Soft-tissue side effects (oozing, bruising, and swelling) were assessed with knowledge of the randomization category as physical examination was necessary and the use of a placebo foot pump was not practical. One hundred and twenty-four of the patients who used the foot pump and 122 of those who received enoxaparin completed a simple questionnaire on which they assessed the acceptability of the treatment. The questionnaire was completed before venography to prevent bias due to perceived effectiveness. The three-page questionnaire focused on postoperative pain, sleep disturbance, and aspects of the prophylactic intervention. Patients were asked if they found the treatment painful, comfortable, relaxing, or annoying, and the responses were recorded on a 5-point ordinal descriptive scale. (The proportion in the top three categories of each scale is presented in the Results section.) Compliance with the foot-pump regimen also was measured, with an internal meter that recorded the total number of hours that the device was activated, for the 124 patients.
Statistical Methods The first stage of the present analysis was to consider the baseline distributions of the characteristics of the patients (for example, age, gender, weight, and type of osteoarthrosis) and the operations in each group to confirm that the randomization was adequate. The second stage was to compare the prevalences of venographically evident deep-vein thrombosis (the primary outcome) with use of a chi-square test (at a two-sided 5 per cent level of significance) and a 95 per cent confidence interval for the difference in proportions. The confidence interval rather than a p value is presented as the former is a more informative statistic. The primary outcomes for all patients who had venography on an intention-to-treat basis were compared. Similarly, 95 per cent confidence intervals were calculated for differences in the patterns of thrombosis. The secondary outcomes were compared with use of the t test, the chi-square test, or the Mann-Whitney U test as appropriate. Because of the number of tests, only a p value of less than 0.005 (a Bonferroni correction of 0.05 divided by ten secondary outcomes) was regarded as significant. The secondary outcomes were compared on an intention-to-treat basis, regardless of whether venography was performed.
Study Subjects
Thromboembolism (Table II)
One patient who was managed with the foot pump had symptoms that were consistent with a pulmonary embolism on the fifth postoperative day; the diagnosis was confirmed by a ventilation-perfusion scan showing a high probability of pulmonary embolism. Venography performed on the seventh day showed multiple small isolated proximal thrombi. The internal compliance meter showed that the patient had used the foot pump for twelve hours each day until the fifth day. In one patient who received enoxaparin, symptoms that were consistent with deep-vein thrombosis led to venography being performed before the scheduled time. The venogram demonstrated a thrombus in the calf veins. Two patients were readmitted to the hospital because of symptomatic deep-vein thrombosis, which was confirmed venographically. One of these patients, who had been managed with enoxaparin, was readmitted to the hospital on the fifteenth day after the operation because of symptomatic calf-vein thrombosis. The venographic findings on the seventh day had been normal. The other patient, who had been managed with the foot pump, was readmitted on the seventeenth day because of calf-vein thrombosis; again, the venographic findings had been normal on the seventh day. The rate of readmission was therefore 0.7 per cent (95 per cent confidence interval, 0.07 to 2.2 per cent) in each group. No other patient reported symptoms consistent with deep-vein thrombosis or pulmonary embolism after discharge.
Side Effects (Table III)
Acceptability of the Foot Pump and Compliance with the Regimen for Its Use The internal compliance meter indicated that the foot pump had been used for a mean of 101 hours (range, twenty-three to 133 hours) during the seven postoperative days (for a median of fifteen hours each day) by the 124 patients for whom these data were available. There was no significant difference with regard to the use of the pump between the patients who did and did not have deep-vein thrombosis. Deep-vein thrombosis developed in eight (13 per cent) of the sixty-two patients who used the pump for fifteen hours a day or more compared with thirteen (21 per cent) of the sixty-two who used it for less than fifteen hours (p = 0.72, Mann-Whitney U test; p = 0.023, chi-square test).
Our primary outcome measure was the venographic prevalence of deep-vein thrombosis. This is a surrogate outcome measure; the widely assumed relationship between the prevalence of venographic events and that of clinical events remains speculative7. As in other investigations11,13,26,39, deep-vein thrombosis was assessed unilaterally rather than bilaterally. The protocol for bilateral venography has drawbacks; the trial is notably more expensive, and some patients are excluded because they withdraw consent between the time of the two venograms. Thrombi are uncommon in the unaffected lower limb unless there is a larger thrombus in the involved lower limb. Any underestimation of the prevalence of deep-vein thrombosis based on the use of unilateral rather than bilateral venography would probably be small and of similar magnitude in each randomized group. As mentioned earlier, the present study was designed as an equivalence trial15. Both the foot pump and enoxaparin produced a clinically important and statistically significant reduction in the risk of deep-vein thrombosis compared with the approximately 50 per cent risk that is expected in the absence of either type of prophylaxis14. In a recent study, which was similar to the current study with regard to the selection of patients and the use of graduated compression stockings and regional anesthesia, the venographic prevalence of deep-vein thrombosis in the control group of seventy-eight patients was 47 per cent34. In the current study, the 18 per cent prevalence in the patients who used the foot pump and the 13 per cent prevalence in those who received low-molecular-weight heparin suggest that the latter form of prophylaxis is possibly more beneficial; however, the observed difference was attributable to isolated thrombi involving the femoral valve cusp, which, as will be discussed later, are of unknown clinical importance. The patterns of thrombosis that were identified in the current study warrant additional discussion. According to traditional methodology, we have presented the prevalences of thrombosis overall, in the calf, and in the proximal veins. No patient had major thrombi involving both the calf and the proximal veins. A continuous thrombus within the calf, popliteal, and proximal veins is generally believed to be associated with a risk of pulmonary embolism21 and chronic venous insufficiency. The isolated femoral thrombi in the present study involved the femoral valve cusps, were usually less than two centimeters long, and were non-occlusive (and therefore asymptomatic); one larger, occlusive isolated femoral thrombus was associated with a pulmonary embolism. Isolated femoral thrombi are common after total hip replacement and are almost unique to that procedure. They tend to form in the valve cusps and perhaps are attributable to disruption of the blood flow distal to the venous obstruction caused by intraoperative maneuvers. In most reported clinical trials, these thrombi have been thought to have the same ominous importance as major thrombi involving both the calf and the proximal veins; however, the former may resolve spontaneously without treatment38. There is no evidence in the literature that smaller isolated femoral cusp thrombi are associated with pulmonary embolism, with propagation of major thrombosis involving both the calf and the proximal veins, or with the later development of chronic venous insufficiency20,33. The greater prevalence of the isolated femoral thrombi in the patients who used the foot pump suggests that disruption of blood flow is not the primary etiological factor. The rates of thrombi in the calf were similar in the two groups. Thrombi in the calf are of unknown importance. Although a few may propagate into major thrombosis involving both the calf and the proximal veins23, it was decided, in most instances, not to treat them in the present study, and no patient had a symptomatic thromboembolic event within the three months after the operation. We chose to compare the results obtained with the foot pump with those obtained with low-molecular-weight heparin because the latter is probably one of the most effective and widely used types of chemical prophylaxis. Studies of animals have suggested that low-molecular-weight heparin may be associated with less risk of bleeding than unfractionated heparin, but this has not been confirmed in clinical studies. Low-molecular-weight heparin may cause more postoperative bruising and oozing of the wound than are seen in controls34, but otherwise the risk of major bleeding complications, such as increased transfusion requirements or perioperative bleeding, seems no greater18. Any prophylactic measure must be used with proper consideration of side effects. The frequency of symptomatic deep-vein thrombosis and pulmonary embolism after total hip replacement is probably less than 5 per cent31, so even a small number of important side effects can offset the potential benefit of prophylaxis. Our results confirm the previous finding that low-molecular-weight heparin is not associated with a risk of major bleeding complications such as perioperative hemorrhage or increased transfusion requirements. However, the advantage of the foot pump was that there was significantly less bruising and oozing. There was also less swelling of the thigh and postoperative drainage, and although these advantages were not significant with use of the Bonferroni correction for multiple-significance testing, these findings are consistent with the contention that the foot pump is associated with fewer soft-tissue side effects than is low-molecular-weight heparin. The clinical importance of these differences is unclear as it is not known how they are related to the cost of the dressings, the duration of hospitalization, the formation of hematomas, or the rate of infection. Furthermore, the data necessarily were collected with knowledge of the randomization category, which introduces the risk of observer bias, and the scales for the measurement of bruising and oozing were subjective. Nevertheless, the association between low-molecular-weight heparin and soft-tissue side effects is perceived by many orthopaedic surgeons to be a disadvantage22, and it would be viewed as particularly disadvantageous if these side effects were to be associated with the disastrous complication of infection. Infection is rare, and it was beyond the scope of this study to demonstrate such a relationship. We used traditional methodology by considering the result of venography performed on only one occasion as the primary outcome. This methodology has an important limitation. Venography reveals the prevalence (the frequency at the moment in time that the test was done) rather than the incidence (the total number of events during a specific period of time after the operation). In recent randomized, controlled studies in which venography was performed approximately five weeks after the operation, the risk of late-onset deep-vein thrombosis was reduced by prolonging the administration of low-molecular-weight heparin for at least five weeks3,6,24. However, these conclusions were based on venographic evidence; the prevalence of symptomatic events was very low. In the current study, only two patients (0.7 per cent) had a symptomatic deep-vein thrombosis after they were discharged from the hospital; in the orthopaedic literature on thromboembolism, the frequency of symptomatic events after discharge has consistently been approximately 1 per cent27,29. The concept of prolonged prophylaxis should be viewed with circumspection against the appropriate background of symptomatic rather than venographic evidence. Prolonged prophylaxis with use of the foot pump would be expensive and impractical. Patients who are managed with low-molecular-weight heparin may have pain or discomfort from the injection, bruising, and discharge from the wound. Little extra work is demanded from the nursing staff. The foot pump requires compliance and tolerance not only by the patient but also by the nursing staff, physiotherapists, and other patients. Almost all of our patients found the foot pump to be acceptable, but some cited sleep disturbance or discomfort. Approximately the same proportion of patients who considered the device to be uncomfortable found it to be relaxing. Repeated explanation of the purpose of prophylaxis during the hospital stay improved compliance and tolerance. There appeared to be no relationship between compliance and the prevalence of deep-vein thrombosis. The device was used for a median of fifteen hours each day; this suggests that it does not need to be used constantly, particularly at night. Confirmation of this hypothesis in appropriately designed studies may improve tolerance by the minority of patients who report discomfort or sleep disturbance. All patients wore graduated compression stockings, and almost all had regional anesthesia. The role of graduated compression stockings in prophylaxis against thromboembolism after orthopaedic operations is unclear12, although it is recommended that they be worn by patients who use the foot pump, in order to control venous capacitance and thus maximize the effect of the pump. Regional anesthesia also serves as prophylaxis against thromboembolism, although its effect is probably not additive with that of other means of prophylaxis25. In summary, we found the A-V Impulse System foot pump to be a suitable alternative for orthopaedic surgeons and patients who are concerned about the soft-tissue side effects that are associated with chemical prophylaxis. The prevalence of deep-vein thrombosis was significantly less than the rate that would have been expected had no prophylaxis been used; also, there were fewer soft-tissue side effects than in the patients who were managed with enoxaparin. There was no significant difference in the prevalence of deep-vein thrombosis between the two groups. Some patients cannot tolerate the foot pump.
NOTE: The authors thank the staff of the Avon Orthopaedic Centre and the Radiology Department at Southmead Hospital for their support of this project.
*One or more of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund or foundation, educational institution, or other non-profit organization with which one or more of the authors is associated. Funds were received in total or partial support of the research or clinical study presented in this article. The funding source was the South and West Research and Development Directorate. The foot-pump controllers and slippers were provided without cost by Novamedix Services, Andover, United Kingdom.
#Department of Social Medicine, University of Bristol, Bristol BS8 2PR, United Kingdom.
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