Practice Management Guidelines for the Prevention of Venous Thromboembolism in Trauma Patients: The EAST Practice Management Guidelines Work Group

The Eastern Association for the Surgery of Trauma (EAST) has taken a leadership role in the development of evidenced-based practice guidelines for trauma. 1 These original guidelines were developed by interested trauma surgeons in 1997 for the EAST Web site (http://www.east.org), where a brief summary of four guidelines was published. A revised, complete, and significantly edited practice management guidelines for the prevention of venous thromboembolism in trauma patients is presented herein. The step-by-step process of practice management guideline development, as outlined by the Agency for Health Care Policy and Research (AHCPR), has been used as the methodology for the development of these guidelines. 2 Briefly, the first step in guideline development is a classification of scientific evidence. A Class I study is a prospective, randomized controlled trial. A Class II study is a clinical study with prospectively collected data or large retrospective analyses with reliable data. A Class III study is retrospective data, expert opinion, or a case report. Once the evidence is classified, it can be used to make recommendations. A Level I recommendation is convincingly justifiable on the basis of the scientific information alone. Usually, such a recommendation is made on the basis of a preponderance of Class I data, but some strong Class II data can be used. A Level II recommendation means the recommendation is reasonably justifiable, usually on the basis of a preponderance of Class II data. If there are not enough Class I data to support a Level I recommendation, they may be used to support a Level II recommendation. A Level III recommendation is generally only supported by Class III data. These practice guidelines address eight different areas of practice management as they relate to the prevention and diagnosis of venous thromboembolism in trauma patients. There are few Level I recommendations because there is a paucity of Class I data in the area of trauma literature. We believe it is important to highlight areas where future investigation may bring about definitive Level I recommendations. RISK FACTORS FOR VENOUS THROMBOEMBOLISM AFTER INJURY I. Statement of the Problem A number of factors have been reported to increase the risk of venous thromboembolism (VTE) after injury. Because VTE prophylaxis is associated with complications, it is essential to identify subgroups of trauma patients in whom the benefit of VTE prophylaxis will outweigh the risk of its administration. This is important because the benefits from the different methods of prophylaxis are still unclear when compared with no prophylaxis. Because the literature is inconsistent, a systematic review is needed to produce the best available evidence. Below, we describe the results of a meta-analysis of the existing literature. The reader needs to remember the limitations of meta-analysis. In addition, the fact that a risk factor was not identified as significant in meta-analysis does not mean that this factor must be ignored. Absence of proof does not equal proof of absence. It only means that enough evidence does not exist and that further studies of high quality are needed. II. Process Three literature databases were searched (MEDLINE, EMBASE, and Cochrane Controlled Trials Register) for articles reporting risk factors of VTE. All articles were reviewed by two independent reviewers and a third reviewer in cases of disagreement. The review was prepared against predetermined screening criteria, and the articles were given a numerical quality score. From an initial broad research that identified 4,093 relevant titles, 73 articles met all the inclusion criteria and were finally accepted for meta-analysis. Pooled effect sizes (odds ratio [OR] and their 95% confidence intervals [CIs]) were estimated by the DerSimonian and Laird random-effects model. Shrinkage graphs were produced to display the effect size of each study and to compare with the overall model estimate. The heterogeneity among studies was tested by the Q statistic and p value for the χ2 test of heterogeneity. A level of significance at p < 0.05 was used for all comparisons. To include a risk factor for meta-analysis, three or more studies reported on the risk factor. Risk factors identified only in one or two studies were not included. The risk factors identified were treated as either dichotomous or continuous variables as appropriate. For instance, if three or more studies provided data on the incidence of VTE in patients who were older or younger than 55 years old, then the risk factor was "age > 55," a dichotomous variable. On the other hand, if three or more studies provided data on the age of patients with or without VTE by using only a mean and SD, the risk factor was simply "age," a continuous variable (Table 1).Table 1: Studies Reporting on Risk Factors of Venous Thromboembolism in Trauma PatientsIII. Recommendations Level I: Patients with spinal cord injuries or spinal fractures are at high-risk for venous thromboembolism after trauma. 2–12 Level II: 1. Older age is an increased factor for venous thromboembolism, but it is not clear at what exact age the risk increases substantially. 4,5,9,11,13,14 2. Increasing Injury Severity Score (ISS) and blood transfusion appear to increase the risk of venous thromboembolism, but this association is still unclear. 3,5,8,9,14,15 3. Traditional risk factors such as long bone fractures, 3–6,9–13,15–17 pelvic fractures, 3–5,9–12,15,18 or head injuries, 3–9,15 although significantly associated with a high risk of venous thromboembolisms in single-institution studies, were not found to be powerful risk factors on meta-analysis. IV. Scientific Foundation Risk factors As Dichotomous Variables The following variables were reported in three or more studies and were included in the meta-analysis: gender, 3,13,18,19 head injury, 3–9,15 long bone fracture, 3–6,9–13,16,17,19 pelvic fracture, 3–5,9–12,15 spinal fracture, 3–12 and spinal cord injury. 4,9–12 A number of studies included age as a risk factor, but the different cut-off points used in each study (age > 30, 40, 50, 55, etc.) did not allow an analysis of this variable. The only risk factors found to place the patient at higher risk for development of deep venous thrombosis (DVT) were spinal fractures (OR, 2.260; 95%; CI, 1.415–3.610) and, even greater, spinal cord injury (OR, 3.017; 95% CI, 1.794–5.381). No significant heterogeneity was reported among studies on the different risk factors. Although long bone fractures were not found to bear statistical significance on meta-analysis, at least one high-quality study 17 with a valid regression model and an adequate sample size found long bone fractures to be a significant risk factor for venous thromboembolism. Risk Factors As Continuous Variables Three continuous variables (i.e., age, 5,9,11,13,14 ISS, 3,5,9,11,14,15 and units of blood transfused 3,14,15) were reported in more than three studies and were included in the meta-analysis. Compared with patients without DVT, patients with DVT were significantly older (8.133 ± 1.504 [95% CI, 5.115–11.141]) years and had a significantly higher ISS (1.430 ± 0.747 [95% CI, 0.000–2.924]). The statistical difference in ISS was marginal, as shown by the lower limit of the 95% CI, and had minimal clinical significance. The difference of blood transfused between patients with and without DVT was not statistically significant (1.882 ± 2.815; 95% CI, −3.637–7.401), and no heterogeneity was reported among these studies. V. Summary The existing evidence supports the presence of two risk factors of posttraumatic VTE: spinal fractures and spinal cord injuries. Older age was an additional risk factor, but it was not clear at what exact age the risk increases substantially. Inadequate literature evidence exists to support that other frequently reported risk factors, such as long bone fractures, pelvic fractures, or head injuries, really increase the risk for VTE. However, a need exists for additional research in this area. In particular, adequate sized prospective studies should reevaluate the role of long bone fracture, pelvic fractures, head injuries, as well as specific age, blood transfusion, and ISS thresholds. V. Future Investigation Adequately sized studies should reevaluate the role of long bone fracture, pelvic fractures, and head injuries, as well as age, blood transfusion, and ISS thresholds and their association with the development of VTE after trauma. Large databases could be used to quantify risk using logistic regression profiles and could be the basis of specific prevention strategies. THE USE OF LOW-DOSE HEPARIN FOR DVT/PE PROPHYLAXIS I. Statement of the Problem The fact that DVT and pulmonary embolism (PE) occur after trauma is incontrovertible. The optimal mode of prophylaxis has yet to be determined. Low-dose heparin (LDH), given in doses of 5,000 units subcutaneously two or three times daily, represents one pharmacologic treatment modality for prophylaxis against DVT/PE. In contrast, LDH has not been shown to be particularly effective in preventing VTE in trauma patients. Three recent prospective trials demonstrated that LDH was no better in preventing DVT than no prophylaxis at all in patients with an ISS > 9. Sample sizes in these studies were small, and hence a type II statistical error cannot be excluded. The results of LDH use in trauma, with regard to PE, are even more vague. II. Process A MEDLINE review from 1966 to the present revealed several hundred articles related to the use of LDH in medical and general surgical patients. Only the nine articles related to the use of LDH in trauma patients were used for the following recommendations (Table 2).Table 2: Lose-Dose HeparinIII. Recommendations Level I: A Level I recommendation on this topic cannot be supported because of insufficient data. Level II: Little evidence exist to support the benefit of LDH as a sole agent for prophylaxis in the trauma patient at high-risk for VTE. 3,7,10,14,20–22 Level III: For patients in whom bleeding could exacerbate injuries (such as those with intracranial hemorrhage, incomplete spinal cord injuries, intraocular injuries, severe pelvic or lower extremity injuries with traumatic hemorrhage, and intra-abdominal solid organ injuries being managed nonoperatively), the safety of LDH has not been established, and an individual decision should be made when considering anticoagulant prophylaxis. IV. Scientific Foundation Heparin is a naturally occurring polysaccharide varying in molecular weight from 2,000 to 40,000. LDH augments the activity of antithrombin III, a potent, naturally occurring inhibitor of activated factor X (Xa) and thrombin, which produces interruption of both the intrinsic and extrinsic pathways. Low-dose heparin causes only minimal or no change in conventional clotting tests, such as the partial thromboplastin time. Studies on the use of LDH in trauma patients are inconclusive. In addition, many of these studies are single-institution studies with small sample sizes and lack randomization. These studies are summarized in Table 2. 7,20,21 Studies with larger sample sizes and randomization will be discussed herein. 3,5,10,14,17,22 Knudson et al. 3 reported on 251 patients in a cohort study who received LDH, a pneumatic compression device (PCD), or no prophylaxis. These authors failed to show any effectiveness with prophylaxis in most trauma patients, except in the subgroup of patients with neurotrauma in which PCD was more effective in preventing DVT than control. Upchurch et al. 14 compared 66 intensive care unit (ICU)-dependent trauma patients who received either LDH or no VTE prophylaxis. No significance difference was seen in VTE rates between the two groups. In this same study, the authors performed a meta-analysis of the current literature concerning the use of LDH in 1,102 trauma patients. This meta-analysis demonstrated no benefit of LDH as prophylaxis compared with no prophylaxis (10% vs. 7%;p = 0.771). Geerts et al. 17 randomized 344 trauma patients to receive low-molecular-weight heparin (LMWH) or LDH and found significantly fewer DVTs with LMWH than with LDH (31% vs. 44%, p = 0.014 for all DVT; and 15% vs. 6%, p = 0.012 for proximal DVT). This study had no control group. However, when compared with the predicted DVT rate if the study patients had not received prophylaxis, the risk reduction for LDH was only 19% for DVT and only 12% for proximal DVT, whereas the comparative risk reductions for LMWH were 43% and 65%, respectively. Napolitano et al. 10 used a serial ultrasound screening protocol for DVT in 437 patients who were given four types of prophylaxis (LDH, PCD, LDH and PCD, and no prophylaxis) according to their attending surgeon's preference. No significant difference was seen in DVT rates between groups (8.6%, 11.6%, 8.0%, and 11.9%, respectively). Velmahos et al. 5 looked at the use of LDH and PCD or PCD alone in 200 critically injured patients who were then followed with biweekly Doppler examinations to detect proximal lower extremity DVT. The incidence of DVT was 13% overall, and no difference was seen between the two groups. The majority (58%) of DVT developed in the first 2 weeks. In a meta-analysis conducted under the auspices of the Agency for Healthcare Research and Quality, Velmahos and colleagues 22 looked at all randomized controlled and nonrandomized studies on the use of LDH in trauma patients. The four randomized controlled studies on the use of LDH in trauma patients showed no difference in the incidence of DVT between those receiving LDH versus no prophylaxis (OR, 0.965; 95% CI, 0.360–2.965; vs. OR, 1.33; 95% CI, 0.360–2.965). V. Summary In summary, to date, LDH has very little proven efficacy in the prevention of VTE after trauma. Most studies on the use of LDH in trauma patients suffer from severe methodologic errors, poor study design, and small sample size, suggesting the possibility of a type II statistical error. VI. Future Investigation Enough accumulated data do not exist to support the use of LDH in a trial in high-risk trauma patients. Future studies should focus on the potential benefit of more efficacious agents such as low-molecular-weight heparin. THE ROLE OF ARTERIOVENOUS FOOT PUMPS IN THE PROPHYLAXIS OF DVT/PE IN THE TRAUMA PATIENT I. Statement of the Problem In 1983, Gardner and Fox 23 discovered a venous pump on the sole of the foot that consists of a plexus of veins that fills by gravity and empties on weightbearing, thus increasing femoral blood flow without muscular assistance. A mechanical device, the arteriovenous (A-V) foot pump, has been developed to mimic this effect of weightbearing. The major advantage of this system is that it only requires access to the foot, which enables its use in patients with Jones dressings, casts, or externally fixed limbs that previously were unsuitable for a PCD. One study has shown that the pulsatile action of the A-V foot pump increased venous blood flow velocity in the popliteal vein by 250%. 24 II. Process With the recent clinical introduction of the A-V foot pump, there is a paucity of relevant literature related to this subject. A MEDLINE review dating back to 1980 revealed 12 articles on A-V foot pumps, with 8 articles specifically related to the use of A-V foot pumps in the trauma patient. These eight studies were the basis for the recommendations below (Table 3).Table 3: A-V Foot PumpsIII. Recommendations Level I: A Level I recommendation for this topic cannot be supported because of insufficient data. Level II: A Level II recommendation for this topic cannot be supported because of insufficient data. Level III: A-V foot pumps may be used as a substitute for pneumatic compression devices in those high-risk trauma patients who cannot wear PCDs because of external fixators or casts and cannot be anticoagulated because of their injuries. It should be noted that in trauma patients, A-V foot pumps have not been shown to be as efficacious as PCDs and are associated with some significant complications. 12,25,26 IV. Scientific Foundation Most of the studies involving the use of A-V foot pumps are found in the orthopedic literature, and many of these series involve small numbers of patients. Although little has been documented on the effects of A-V footpumps on DVT in trauma patients, other beneficial effects have been observed. In 71 patients who had operations or casts for traumatic lower extremity injuries, Gardner and Fox 27 showed a significant decrease in pain, swelling, and measurement of compartment pressures in the affected extremities with the use of the A-V foot pumps. In the discussion to this article, the authors hypothesized that the increased blood flow seen with the pumps was because of hyperemia mediated by endothelial-derived relaxing factor (now thought to be nitric oxide) that was liberated by the endothelium secondary to sudden pressure changes, which could have been caused by the A-V pumps. This endothelial-derived relaxing factor release could encourage the opening of critically closed capillaries, enabling reabsorption of fluid, hence the decrease in compartment pressures. In addition, reports have been documented of A-V foot pumps improving arterial blood flow with the relief of ischemic rest pain. 28,29 In addition to preventing VTE, all of these proposed foot pump mechanisms of action may be potentially beneficial in healing extremity injuries. In a recent prospective randomized study by Knudson et al., 12 A-V foot pumps were one arm of a number of prophylactic measures (LMWH and PCDs were the other arms) used to prevent DVT in high-risk trauma patients. Of 372 patients enrolled in the study, the DVT rate was 5.7% for the A-V foot pumps, 2.5% for the PCDs, and 0.8% for the low-molecular-weight heparin as determined on follow-up serial duplex ultrasound. Of note, in 8 of 53 patients who wore foot pumps, severe skin changes, including blistering and wound problems, occurred. This required three patients to be removed early from the study. Spain et al. 25 compared the use of A-V foot pumps to PCDs in 184 consecutively injured patients. In this nonrandomized study, patients who could not receive a PCD because of lower extremity injuries were placed in A-V foot pumps. Overall, no significant difference was seen in DVT rates between the two groups, with PCDs at 7% and A-V foot pumps at 3%. The authors of this study concluded that A-V foot pumps were a reasonable alternative to PCDs when lower extremity fractures preclude the use of PCDs. Anglen et al. 26 performed a randomized prospective trial comparing A-V foot pumps with PCDs in high-risk orthopedic patients and followed them with serial ultrasound. In 124 patients, the overall incidence of DVT was 4% in those with A-V foot pumps and 0% in those with PCDs. Unfortunately, meaningful analysis of such a study was confounded by the heterogeneity of the two groups and the fact that a sizable number of patients received either aspirin or warfarin postoperatively. In another study by Anglen et al. 30 in a trauma population of ICU and ward patients, the A-V foot pumps were found to be applied properly and functioning correctly only 59% of the time, a problem similar to that reported by Comerota et al. 31 for PCDs. V. Summary Only one clinical series in trauma patients compares A-V foot pumps with other standard techniques of DVT prophylaxis. The results from this series were not definitive in terms of benefits of A-V foot pumps preventing DVT. However, a use of A-V foot pumps may exist in the high-risk trauma patient who has a contraindication to heparin because of injuries or who cannot have PCDs placed on lower extremities secondary to external fixators or large bulky dressings. VI. Future Investigations Prospective randomized studies are needed comparing A-V foot pumps to standard prophylactic measures in trauma patients at high risk for the development of DVT. THE USE OF PNEUMATIC COMPRESSION DEVICES IN THE PREVENTION OF DVT/PE I. Statement of the Problem The role of intermittent PCDs for prophylaxis against DVT has been studied and increasingly used in general surgery patients, 32 orthopedic patients, 33,34 and trauma patients. 4,5,7,22,35,36 Attacking the long-recognized risk factor of stasis, PCDs have been shown to increase mean and peak femoral venous blood velocities in the lower extremity. 37 In addition, PCDs have been shown to have a direct effect on the fibrinolytic pathway that acts to shorten the euglobulin lysis time, increases levels of coagulation cascade inhibitor molecules, and affects the balance of plasminogen activation. 38,39 In a number of prospective randomized studies, PCDs have been shown to reduce the incidence of both DVT and PE. 7,36,40 Unanswered questions regarding the use of PCDs include the mechanism by which PCDs act, the efficacy of PCDs worn on the upper extremities or a single lower extremity compared with both lower extremities, the nature of risk involved in discontinuing PCDs periodically during use, and the duration of PCD use. Reports suggest that PCDs should be worn with thromboembolism-deterrent stockings (TEDS); however, this practice has not been widely used. Complications of PCDs have been noted in case reports and have been associated with improper positioning of the lower extremity during surgery, which should be avoided. II. Process A MEDLINE search from 1986 to the present produced a large number of articles on this topic. Those articles pertinent to trauma-related thromboembolism prevention were reviewed. Twenty-three of these trauma-related articles were evaluated to formulate the following guidelines (Table 4).Table 4: Pneumatic Compression DevicesIII. Recommendations Level I: A Level I recommendation on this topic cannot be supported because of insufficient data. Level II: A Level II recommendation on this topic cannot be supported because of insufficient data. Level III: In a meta-analysis of pooled studies on the benefit of PCDs in trauma patients, no benefit of the use of PCDs over no prophylaxis was reported. 22 In the subset of head-injured patients, 3,41 PCDs may have some benefit in isolated studies. IV. Scientific Foundation The factors that are felt to form the basis of the pathophysiology of venous thromboembolic disease are stasis (reduction of blood flow in the veins), injury (to the intimal surface of the vessel), and hypercoagulability. Scientific and clinical evaluations of PCDs strongly suggest that the nature of the effect on DVT prophylaxis derives from their ability to increase mean and peak femoral vein velocity and possibly affect the systemic coagulation and fibrinolytic mechanisms. Keith et al. 37 measured peak venous velocity (PVV) at the common femoral vein using Doppler ultrasound in postoperative nontrauma patients and in healthy control subjects. In the control subjects, PVV was increased from a mean velocity of 23.8 cm/s at rest to 45.5 cm/s with knee-high PCDs and 53.2 cm/s with thigh-high PCDs. In postoperative patients, the PVV was similarly raised from a resting velocity of 21.8 cm/s to 55.1 cm/s. In both of these evaluations, the differences were statistically significant when compared with controls and were not further augmented by the concomitant use of TEDS. Spectral recording of blood flow velocity during inflation and deflation of the PCDs revealed a temporal association with inflation and increased PVV that suggested a mechanical effect derived from inflation of the PCDs. Studies 38,39 have evaluated in vivo fibrinolytic effects of PCDs. In a well-designed study, Jacobs et al. 39 showed that euglobulin lysis times were not reproducible as a marker for fibrinolytic activation. Their study focused on measured changes in tissue plasminogen activator (tPA), plasminogen activator inhibitor (PAI-1), and tPA-PAI-1 complex. They demonstrated a significant increase in tPA–PAI-1 (hence an obligatory decrease in PAI) in patients undergoing pneumatic compression and postulated a (complex and incompletely proven) role of PCDs in the systemic balance of plasminogen activation and inhibition. They found that fibrinolytic activity began to decay within minutes of discontinuing PCDs. This observation proved to have important clinical implications in that PCDs must be worn continuously to avoid rapid decay in fibrinolytic activity. A recent study documented patients in whom PCDs have been ordered, but who spent less than 50% of the time actually wearing the devices, which possibly decreased their effectiveness. 31 Another important finding in the study by Jacobs et al. was that there appeared to be an incremental decrease in fibrinolytic activity when blood was sampled in sites remote from the area of PCD placement. This difference in local and systemic effects has important implications on the ability of PCDs worn on the arms to prevent DVT in the legs. A paucity of studies exists specifically regarding the use of PCDs in trauma patients with multiple injuries. In a prospective study by Knudson et al., 15 113 trauma patients received either PCDs and TEDS or LDH. This study showed a 12% rate of VTE in the PCD group versus 8% in the LDH group, which was not significantly different. This study did not demonstrate that either method of attempted prevention (LDH or PCD) was better than no prophylaxis. Dennis et al. 7 conducted a prospective, nonrandomized study of 395 trauma patients admitted with an ISS > 9 who received either PCDs, LDH, or no prophylaxis, and who underwent serial ultrasound screening for DVT at 48 hours, 5 days, and 10 days after admission. They demonstrated a VTE rate of 8.8% in the no-prophylaxis group, 2.7% in the PCD group, and 3.2% in the LDH group. No statistically significant difference was noted in VTE rates in the prophylaxis groups, but a significant difference was seen in those who received prophylaxis versus no prophylaxis (p < 0.02). Head- and spinal cord–injured patients, two very-high-risk groups, seemed to benefit greatly from prophylaxis. Overall, risk reduction of VTE with prophylaxis was from 16.7% to 1.4% in head-injured patients and 27.3% to 10.3% in spinal cord–injured patients. However, problems occurred during the course of this study in that 67 patients (37%) originally assigned to receive no prophylaxis were switched to receive some sort of prophylaxis at the discretion of the attending surgeon. This may have confounded the DVT rates for each prophylactic modality assignment. In a prospective trial, Knudson et al. 3 compared PCD, LDH, and no prophylaxis. Neither LDH nor PCD appeared to offer any protection to trauma patients with multiple injuries, except in the specific subgroup of patients with neurotrauma in which PCD was more effective in preventing DVT than control (p = 0.057). In contrast to the study by Knudson et al., Gersin et al., 35 in a nonrandomized prospective study, looked at the incidence of VTE in a group of 32 severely head-injured patients with Glasgow Coma Scale (GCS) scores < 8. Fourteen patients received PCDs and 18 did not because of concomitant lower extremity fractures. Within the group receiving PCDs, four (28%) developed PE and none developed DVT. In the group not receiving prophylaxis, two developed PE and two developed DVT. Although the study population was small, the findings in this study questioned the efficacy of PCD even in severe head-injured patients. In a group of 304 orthopedic trauma patients with hip and pelvic fractures, PCDs were found to reduce thromboembolic events significantly over those who had no prophylaxis (11% vs. 4%;p = 0.02). In subgroup analysis, PCDs were only effective in the hip fracture group, not in those with pelvic fractures. Compression devices appear to be well-tolerated, with minimal side effects. Isolated cases of pressure necrosis from a too tightly fitted PCD have been reported. 42 Also, peroneal palsy and compartment syndromes have been reported with PCDs. 43 A potential complication of PCDs is elevated intracranial pressure (ICP) in patients with severe head injury. This was addressed by Davidson et al. 41 in 24 severely brain-injured patients (mean GCS score of 6) who had ICP and cerebral perfusion pressure (CPP) calculated after 0, 10, 20, and 30 minutes of intermittent pneumatic leg compression. The authors found no significant increase in ICP or CPP with the use of PCDs at any time points, and concluded that PCDs can be used safely in stable head-injured patients. In an evidenced-based meta-analysis sponsored by the Agency of Healthcare Research and Quality on the incidence of DVT after trauma, Velmahos et al. 22 found that PCDs offered no benefit over no prophylaxis in both pooled randomized control studies (OR, 0.769; 95% CI, 0.265–2.236) and in pooled nonrandomized controlled studies (OR, 0.527; 95% CI, 0.190–1.460). In another study, Velmahos et al. 5 compared PCD, LDH, and a combination of PCD and LDH in a prospective study of 200 critically injured patients followed by weekly Doppler ultrasound to detect proximal DVT. In all three groups, the proximal DVT rate was 13%, leading the authors to question whether any of the three prophylactic regimens were sufficient in the high-risk patient. V. Summary Clinical studies demonstrating the effectiveness of PCDs in trauma patients are few. Although the exact mechanism of action of PCDs is unknown, their effect is believed to be based on a combination of factors addressing stasis (which is well accepted) and the fibrinolytic system (which is less clear). Until these mechanisms are better studied and understood, answers to specific questions regarding the appropriate use of PCDs are forthcoming. VI. Future Investigation More studies need to be performed specifically relating to the use of PCDs in trauma patients at risk for VTE. Questions regarding the efficacy of using the device on one lower extremity ve