Frequency Of Deep Vein Thrombosis Research Articles

Low Frequency of Phlebographic Deep Vein Thrombosis After Laparoscopic Cholecystectomy—A Pilot Study

To investigate the rate of deep vein thrombosis (DVT) after laparoscopic surgery, 50 patients underwent bilateral phlebography 7-11 days after laparoscopic cholecystectomy (LC). All received thromboembolism prophylaxis, either low molecular weight heparin (LMWH) or dextran. Three patients were converted to open cholecystectomy. D-dimer was investigated preoperatively, on day 1 and on the day of phlebography. One asymptomatic DVT was found. One phlebogram was incomplete. Seven phlebograms were not optimal but of sufficient quality to rule out DVT. The frequency of DVT was thus 1 of 49 or 2.0% (95% confidence interval, 0-6.0%). No anticoagulants were prescribed after discharge. No patient developed late thromboembolic complications. D-dimer values increased significantly at day 1 and were further increased at the time of phlebography. The frequency of phlebographical DVTs thus seems to be low despite prophylaxis of questionable efficacy. The D-dimer values, however, suggest that the effects of LC on coagulation/fibrinolysis have a duration of longer than 1 week.

Doppler ultrasonography versus venography in the detection of deep vein thrombosis in patients with pulmonary embolism

In patients with acute pulmonary embolism (PE) the frequency of deep vein thrombosis (DVT) varies between 13-93%. The aim of this study was to compare Doppler ultrasonography (DUSG) and venography in the detection of DVT in patients with PE. Fifty-one patients who were clinically diagnosed as having PE from January 1st 2001 to January 31st 2005 were entered into the study and comorbid conditions and risk factors were noted. The diagnosis of PE was confirmed by ventilation-perfusion (V/Q) scintigraphy, spiral tomography and angiotomography while the diagnosis of DVT was made by DUSG and venography. DVT was confirmed by both DUSG and venography in 19 (37,3%) patients. In the remaining 32 patients DUSG was negative. Venography confirmed DVT in 6 of these patients while in 26 no DVT was found. The sensitivity and specifity of DUSG in the diagnosis of DVT were 76% and 100% respectively and the negative and positive predictive values were 81% and 100% respectively. The mean d-dimer concentration was 1,187 in patients with DVT and 641 in patients without DVT (p > 0.05). Aquired risk factors were found in 4 of 6 patients with DVT, CRP was elevated in 5 (83%) and ALT-AST were elevated in 2 (33%). Although DUSG alone is considered sufficient for the diagnosis of DVT, venography still remains the gold standard in the diagnosis of DVT. Especially in patients with PE, where the diagnosis of DVT may increase the success of treatment, venography or other diagnostic tools may be used instead of a second DUSG if the first DUSG is negative.

Unpleasant findings during transcatheter closure of patent foramen ovale: The importance of venous Doppler ultrasound and femoro-iliac venography

It is well known that one of the determinants of the paradoxical embolism through a patent foramen ovale is the presence of deep venous thrombosis. Deep venous thrombosis was detected in nearly 10% of patients with PFO and paradoxical embolism as the sole identifiable cardiac risk factor. Deep vein thrombosis is often clinically silent [ 1 Lethen H. Flachskampf F.A. Schneider R. et al. Frequency of deep vein thrombosis in patients with patent foramen ovale and ischemic stroke or transient ischemic attack. Am J Cardiol. 1997; 80: 1066-1069 Abstract Full Text PDF PubMed Scopus (176) Google Scholar , 2 Stollberger C. Slany J. Schuster I. Leitner H. Winkler W.B. Karnik R. The prevalence of deep venous thrombosis in patients with suspected paradoxical embolism. Ann Intern Med. 1993; 119: 461-465 Crossref PubMed Scopus (194) Google Scholar ]. Unfortunately, in this era in which the transcatheter closure of patent foramen ovale is gaining a worldwide acceptance and the technical procedural point of view seems to be the most attractive, the loss of attention for the main pathophysiological mechanism of paradoxical embolism may create some difficulties.

Frequency of postoperative deep vein thrombosis in high-risk surgical patients.

To find the frequency of postoperative deep vein thrombosis (DVT) in high-risk surgical patients. Cohort study. The surgical department of the Combined Military Hospital, Rawalpindi from October 1997 to January 2000. Patients of both genders were chosen from all the major branches of surgery provided they belonged to the high-risk categories. They were all over the age of 40 years and were to undergo a major surgical procedure. The selected patients were subjected to the radionuclide venography both pre- and postoperatively. This was performed using Technetium (Tc)-99 labeled RBCs blood pool venography technique. Out of 117 patients studied, postoperative deep vein thrombosis was observed in 15 patients. So the frequency of postoperative deep vein thrombosis after high-risk surgery was 12.82%. In this study the 12.82% frequency of deep vein thrombosis was lower than that in the Western literature. There is a need to provide prophylactic anticoagulant therapy to those older patients who are to undergo major orthopaedic surgery, have advanced malignancy or have more than one risk factors.

Association of clinical features with HLA in chronic pulmonary thromboembolism

The aetiology of chronic thromboembolic pulmonary hypertension (CTEPH) is largely unknown and may be heterogeneous, because there are several ethnic differences in the clinical characteristics of CTEPH. Female predominance and a higher ratio of chronic to acute pulmonary thromboembolism have been reported in Japan as compared with the USA. Because such ethnic differences may be controlled by genetic factors, the current study investigated HLA polymorphisms in Japanese patients with CTEPH. HLA typing by serological and/or DNA typing methods was performed (for HLA-A, B, DPB1, DRB1) in 80 patients and 678 controls, and the association of clinical characteristics with HLA alleles was studied. The frequencies of HLA-B*5201 (40 versus 24%) and DPB1*0202 (19 versus 6%) were significantly higher in the patients. HLA-B*5201 positive patients showed a significant female predominance. Total pulmonary vascular resistance and mixed venous oxygen tension were better in the HLA-B*5201 positive patients. In contrast, cardiac index and gas exchange parameters were worse in the HLA-DPB1*0202 positive patients. In the patients carrying HLA-B*5201 and/or -DPB1*0202, the frequency of deep vein thrombosis was significantly lower than the other patients. These observations suggested that both the susceptibility and clinical characteristics of chronic thromboembolic pulmonary hypertension were controlled in part by the HLA-B and -DPB1 loci.

Deep vein thrombosis and heterotopic ossification in spinal cord injury: a 3 year experience at the Swiss Paraplegic Centre Nottwil.

Retrospective review of patient data. (i) To determine the incidence and time of deep vein thrombosis (DVT) under low molecular weight heparin (LMWH) prophylaxis in spinal cord injury (SCI), (ii) to determine the incidence and time of heterotopic ossification (HO) and (iii) to assess a possible aetiologic relationship in the pathogenesis of DVT and HO. Swiss Paraplegic Centre, Nottwil. We analyzed the incidence of DVT and HO in 1209 SCI patients (275 first rehabilitations) at the Swiss Paraplegic Centre Nottwil from 1998 to 2000. Clinical files and laboratory data were scrutinised for particularities preceding DVT and HO. The incidence of DVT was 6.55% for first rehabilitation compared to only 1.59% in all patients hospitalised. DVT was complicated by pulmonary embolism (PE) in 1.45% and 0.47% respectively. Incidence of HO was 8% for first rehabilitation and 1.82% for all patients hospitalised. In first rehabilitation patients the peak for DVT occurred around day 30 contrary to HO with a peak around day 120. In single patients HO was identified by MRI as a rapidly progressing process. Laboratory profiles were inflammatory in both HO and DVT. Increased physical activity preceding HO was observed in four patients. In two patients acute HO was complicated by ipsilateral DVT. Prophylaxis with LMWH and elastic stockings significantly reduces the frequency of DVT during first rehabilitation in SCI. DVT and HO are both associated with laboratory parameters of non-infectious inflammation. The later onset of HO coinciding with ongoing mobilisation, argues for a different pathogenetic mechanism. Acute HO of the hip region appears to favour ipsilateral DVT by well known thrombogenic mechanisms.

Seasonal variations of deep vein thrombosis and its influence on the location of the thrombus

The role of season as a predisposing factor for the development of deep vein thrombosis is controversially discussed. We analysed retrospectively seasonal variations in the occurrence of deep vein thrombosis of the leg in 905 patients between 1996 and 2000. Additionally, we evaluated the relationship between the season and the location of the thrombus. Our study revealed that the frequency of deep vein thrombosis of the leg was significantly higher during the winter half than during the summer half ( p<0.001). Regarding the seasonal variation of the location of the thrombus, distal deep vein thrombosis was more frequent during the winter half while proximal deep vein thrombosis was diagnosed more often during the summer half of the year. In conclusion, we suggest that the season has an influence on the development of deep vein thrombosis in the leg as well as on the location of the thrombus.

Reduction in fatal pulmonary embolism and venous thrombosis by perioperative administration of low molecular weight heparin: Gynecological ward retrospective analysis

Objectives: To compare the frequency of deep vein thrombosis (DVT) and pulmonary embolism (PE) in patients undergoing gynecological operations without low molecular weight heparin (LMWH) prophylaxis and those receiving such prophylaxis. Design: Retrospective, hospital record-based study. Material and Methods: About 1785 consecutive patients without LMWH prophylaxis and 1871 ones treated with nadroparin (Fraxiparine) 7500ICU s.c. 2h before the operation and repeated daily for 5–7 days or until the patient was fully mobile. DVT was diagnosed on the basis of clinical symptoms and ultrasound examination, and PE on clinical symptoms, gasometric data, electrocardiography and chest X-ray. Results: Among the patients without prophylaxis, four cases of PE occurred (0.22%), two fatal (0.11%), 13 cases of proximal DVT (0.72%) and 41 distal DVT complications (2.3%). In LMWH, group 3 proximal DVT (0.16%) and 18 distal DVT (0.96%) developed but there were no clinically expressed pulmonary embolism. According to the Fisher’s exact test, the difference between the complications in the analyzed groups is significant P<0.05. Conclusion: The perioperative applying of LMWH to prevent DVT in the patients operated on gynecologically is effective.

Frequency and prevention of symptomless deep-vein thrombosis in long-haul flights: a randomised trial

The true frequency of deep-vein thrombosis (DVT) during long-haul air travel is unknown. We sought to determine the frequency of DVT in the lower limb during long-haul economy-class air travel and the efficacy of graduated elastic compression stockings in its prevention. We recruited 89 male and 142 female passengers over 50 years of age with no history of thromboembolic problems. Passengers were randomly allocated to one of two groups: one group wore class-I below-knee graduated elastic compression stockings, the other group did not. All the passengers made journeys lasting more than 8 h per flight (median total duration 24 h), returning to the UK within 6 weeks. Duplex ultrasonography was used to assess the deep veins before and after travel. Blood samples were analysed for two specific common gene mutations, factor V Leiden (FVL) and prothrombin G20210A (PGM), which predispose to venous thromboembolism. Asensitive D-dimer assay was used to screen for the development of recent thrombosis. 12/116 passengers (10%; 95% CI 4.8-16.0%) developed symptomless DVT in the calf (five men, seven women). None of these passengers wore elastic compression stockings, and two were heterozygous for FVL. Four further patients who wore elastic compression stockings, had varicose veins and developed superficial thrombophlebitis. One of these passengers was heterozygous for both FVL and PGM. None of the passengers who wore class-I compression stockings developed DVT (95% CI 0-3.2%). We conclude that symptomless DVT might occur in up to 10% of long-haul airline travellers. Wearing of elastic compression stockings during long-haul air travel is associated with a reduction in symptomless DVT.

Sonographic diagnosis of thrombosis of the calf muscle veins and the risk of pulmonary embolism

To determine the frequency of deep vein thrombosis in the veins of the calf muscles. The risk of embolism in relation to the localisation of thrombosis was also evaluated. 357 consecutive patients submitted for colour Doppler sonography of the lower extremities were prospectively examined for deep vein thrombosis (DVT). Both axial and muscular calf veins were investigated. 184 of these patients underwent additional investigation for pulmonary embolism. Diagnosis of DVT was made by means of colour Doppler sonography in 179 patients. Soleal veins (n = 88), peroneal veins (n = 84), the popliteal vein (n = 69), and the superficial femoral vein (n = 53) were the most common sites of thrombosis. Thrombosis of the gastrocnemial veins (n = 49) occurred less frequently. 85 patients (47% of all patients having DVT) showed isolated calf vein thrombosis, in 45 patients (25%) the gastrocnemial and/or soleal veins were the only site of thrombosis. 60% of patients with symptomatic DVT also had pulmonary embolism. The embolic frequency for isolated calf vein thrombosis and muscular calf vein thrombosis was 48% and 50%, respectively. The veins of the calf muscles are a common site of acute DVT and a source of pulmonary embolism. They should always be investigated in patients with suspected DVT of the calf, and in patients with pulmonary embolism.

Thromboprophylaxis in Hip Fracture Surgery: A Pilot Study Comparing Danaparoid, Enoxaparin and Dalteparin

A pilot study was performed to compare the thromboprophylactic effect of danaparoid, enoxaparin and dalteparin in patients with hip fracture. The study was a prospective, randomised assessor-blind, four-centre trial. Prophylaxis was given for 9–11 days, whereafter bilateral phlebography was performed. A total of 197 patients were randomised. There were no statistically significant differences in the frequency of deep vein thrombosis, in blood loss or bleeding complications between the three prophylaxis groups. In conclusion, this moderately sized study revealed no statistically significant difference in efficacy or safety between danaparoid, enoxaparin and dalteparin in patients undergoing hip fracture surgery.

Preoperative vs postoperative initiation of low-molecular-weight heparin prophylaxis against venous thromboembolism in patients undergoing elective hip replacement.

Although preoperative and postoperative initiation of prophylaxis for deep vein thrombosis (DVT) with low-molecular-weight heparin (LMWH) are effective, the relative effectiveness and safety of these approaches is unknown. In the absence of a published definitive level 1 trial addressing this question, a meta-analysis is appropriate. To report a meta-analysis comparing preoperative with postoperative initiation of prophylaxis of DVT in patients undergoing elective hip replacement. Relevant trials were identified, and potential biases in the meta-analysis were minimized by analyzing all rigorously performed randomized trials that met all of the following criteria for conduct of the trial: (1) double-blind design, (2) objective documentation of the frequencies of DVT by ascending contrast venography, (3) venography performed before or at the time of discharge from the hospital, (4) initiation of the same LMWH preoperatively or postoperatively in dosages shown to be effective, (5) compliance with the criteria for a level 1 trial, and (6) objective documentation of major and minor bleeding according to strict criteria. Treatment with LMWH initiated preoperatively was associated with a DVT frequency of 10.0% compared with a frequency of 15.3% when the LMWH was initiated postoperatively (P = .02, Fisher exact test). Major bleeding was less frequent in patients receiving preoperatively initiated LMWH than in patients receiving postoperatively initiated LMWH (0.9%, vs. 3.5%; P = .01, Fisher exact test). Our findings support the need for a randomized comparison of preoperative and postoperative initiation of pharmacological prophylaxis of DVT. Such a trial would resolve the divergent practices for DVT prophylaxis between Europe and the North American countries, the United States and Canada, and would affect the treatment for thousands of patients on both continents.

Mechanisms of Hypercoagulability

Following certain major operative procedures, large amounts of tissue factor may be released from damaged tissues to venous blood. Mechanical and chemical injury to the collecting veins exposes subendothelial procoagulant proteins. This initiates a marked local hypercoagulable process. Subsequently, venous bloodborne procoagulant debris induce a substantial thrombin generation as blood passes the lung capillaries. Thus, the lungs seem to have a central role in the mechanisms of hypercoagulability in high-risk patients. Hypercoagulable blood is squeezed out in the peripheral circulation and may favor thrombosis formation both in central and peripheral vessels. In addition, reduced venous blood flow for several days to weeks after the operation may put these patients at-risk for thromboembolic complications for a long time. Several predisposing genetic and acquired factors associated with thrombophilia have been proposed to contribute to this hypercoagulable process. However, few studies and conflicting results have been reported. The clinical penetrance of the described thrombophilic abnormalities and their contribution to the hypercoagulable process in "high-risk" patients are, at present, unclear. The post-traumatic hypercoagulability seems to be a systemic phenomenon, at least following major orthopedic surgery. Cardiorespiratory and vascular complications play a prominent role as a cause of death and morbidity during and after this kind of surgery.

Thromboprophylaxis in hip arthroplasty. New frontiers and future strategy.

Coagulation-related complications are a frequent cause of death following hip replacement surgery. Venographically-proven deep vein thrombosis (DVT) is found in a high frequency. Most cases have no symptoms. Fatal pulmonary embolism (PE) may develop from subclinical thrombi. In addition, arterial thromboses may induce serious cardiovascular events and an unknown number of patients may develop cardiorespiratory insufficiency, due to non-fatal venous PE. Finally, several patients may develop venous insufficiency. Recent prospective double-blind studies have shown that the frequency of deep vein thrombosis increased after hospital discharge in patients undergoing hip replacement surgery. Prolonged thrombo-prophylaxis with low-molecular-weight heparin (dalteparin or enoxaparin) is recommended for at least 5 weeks after the operation.

Frequency of deep vein thrombosis in patients with patent foramen ovale and ischemic stroke or transient ischemic attack

To evaluate the additional value of transesophageal (TEE) compared with transthoracic (TTE) echocardiography and the role of patent foramen ovale (PFO) and deep vein thrombosis in the work-up of embolic events, patients with presumed cardiac embolic stroke or transient ischemic attack (neurovascular etiology was excluded) were prospectively studied by transthoracic and transesophageal contrast echocardiography. If PFO was detected echocardiographically, PFO size was assessed semiquantitatively and phlebography of both legs was performed. Two hundred forty-two consecutive patients (153 men, 60 ± 15 years) were studied. In 197 patients, neuroimaging showed evidence of embolic infarction. TEE identified 138 potential cardiac sources of embolism in 111 patients, compared with 69 by TTE (p < 0.01) in 59 patients. TEE detected potential cardiac sources in 52 patients with negative TTE examination and was significantly superior compared with TTE for identifying left atrial thrombi, spontaneous echo contrast, PFO, atrial septal aneurysm, and atheroma of the ascending aorta. In patients with a positive TTE, additional diagnostic information by TEE was found in only 6 patients and did not change therapy. Phlebography was performed in 53 patients with PFO and revealed deep vein thrombosis in 5 patients (9.5%); all had medium or large PFOs. Thus, in patients with cerebral ischemia of suspected cardiogenic origin and a normal TTE examination, TEE detects potential causes of embolism in 31% of patients and is therefore of diagnostic relevance. Conversely, in the presence of a diagnostic TTE an additional TEE confers only marginal diagnostic benefit. Deep venous thrombosis was detected in nearly 10% of patients with PFO as the sole identifiable cardiac risk factor. Given that in 4 of 5 patients deep vein thrombosis was clinically silent, phlebography should be performed in patients with medium or large interatrial shunts if paradoxical embolism is suspected.

Prophylaxis of Venous Thromboembolism in Stroke Patients

Venous thromboembolism is a common complication in patients with acute thrombotic stroke. Estimates of the frequency of deep vein thrombosis (DVT) in untreated patients range from 20 to 75%. This wide range reported depends on the methods used to detect DVT and, importantly, on the degree of lower limb paralysis. Most thrombi occur in the paralyzed limbs in which the frequency ranges from 60 to 75%. Of these thrombi, 25% occur in the proximal segment and present a high risk for pulmonary embolism. Indeed, pulmonary embolism is the third most common cause of death in stroke patients and occurs in 1 to 2% of patients who do not receive prophylaxis. A number of methods of preventing DVT have been shown to be safe and effective in stroke patients. These include low-dose heparin, low-molecular-weight heparin, and a heparinoid. Of these, the data with the heparinoid danaparoid provide the most solid evidence for efficacy, and in comparative trials it has been shown to be more effective than heparin.

Coagulation Activation Markers in the Prediction of Venous Thrombosis after Elective Hip Surgery

Despite prophylaxis, deep vein thrombosis (DVT) after hip surgery continues to occur frequently. Thus it would be helpful if before surgery patients at higher risk of DVT could be identified and more adequate prophylaxis given. As part of an international study on the prevention of DVT after total hip replacement, we investigated whether preoperative levels of three coagulation activation markers, prothrombin fragment F1 + 2 (F1 + 2), thrombin-antithrombin III complexes (TAT) and D-dimer, correlate with results of postoperative venography. 159 patients undergoing total hip replacement were randomized to receive 10, 15 or 20 mg desirudin bid or 5000 IU unfractionated heparin tid immediately before surgery and then for 11 days, until bilateral venography was performed. Preoperative F1 + 2, TAT and D-dimer plasma levels were measured using ELISA procedures. As no difference among anticoagulant treatments or in the interaction between treatments and DVT was detected for any of the three variables, results are reported as pooled data. The frequency of DVT was 18.8% in the low (0.75-1.33 nM) vs 65.7% in the high third of distribution (1.77-3.47 nM) of F1 + 2 (p < .001), 27.3% in the low (2.00-2.50 micrograms/l) vs 57% in the high third (5.10-61.00 micrograms/l) of TAT (p = .042), and 29.4% in the low (39-59 micrograms/l) vs 57.1% in the high third (129-651 micrograms/l) of D-dimer (p = .051). Preoperative F1 + 2, TAT and D-dimer levels are associated with the risk of development of DVT after total hip replacement.

Prevention of deep-vein thrombosis after total hip replacement: direct thrombin inhibition with recombinant hirudin, CGP 39393

Summary Background The frequency of thromboembolism after major orthopaedic surgery continues to be high despite prophylaxis. New agents such as CGP 39393, a recombinant form of hirudin, may be more effective than existing therapies. Methods In this double-blind, multicentre, European study the efficacy of three doses of CGP 39393, in comparison with unfractionated heparin, were examined in 1119 patients undergoing elective hip surgery. Patients were randomly allocated to receive by subcutaneous injection either 10, 15, or 20 mg of CGP 39393 twice daily or 5000 IU of heparin three times daily. All treatments were started just before surgery and continued for 8-11 days, until bilateral venography was performed. Findings The occurrence of thromboembolism was significantly reduced in patients treated with CGP 39393 compared to heparin. The frequency of deep-vein thrombosis was 34·2% in the heparin group as compared to 23·9% (p=0·0113), 18·4% (p=0·0003), and 17·7% (p=0·0001) in the 10 mg, 15 mg, and 20 mg CGP 39393 groups, respectively. At all dose levels, CGP 39393 was more effective than heparin in preventing proximal deep-vein thrombosis. The frequency of proximal thrombosis was 19·6% in the heparin group as compared to 8·5% (p<0·001), 3·1% (p<0·001), and 2·4% (p<0·001) in the 10 mg, 15 mg, and 20 mg CGP 39393 groups, respectively. All treatments were well tolerated. Interpretation This study indicates that specific inhibition of thrombin by prophylactic CGP 39393 significantly reduces thromboembolic complications in patients undergoing total hip replacement.

Increased activation of coagulation and formation of late deep venous thrombosis following discontinuation of thromboprophylaxis after hip replacement surgery

Hip replacement surgery (HRS) is associated with a high frequency of deep vein thrombosis (DVT). At the same time there is a substantial systemic and local activation of coagulation. This study indicates that discontinuation of thromboprophylaxis one week after surgery may allow a second wave of coagulation and fibrinolysis activation to occur. An almost parallel increase in plasma TAT and D-dimer levels between the 6th and the 35th postoperative day may indicate late DVT formation. Repeated bilateral ascending venography is thought to be necessary to evaluate the suitability of using selected activation markers of the coagulation and fibrinolytic systems as indices of DVT formation.

Augmentation of calf blood flow by epinephrine infusion during lumbar epidural anesthesia.

Lower rates of deep vein thrombosis after hip surgery performed under epidural anesthesia were noted in patients who received intravenous epinephrine infusion. To determine whether this effect could be mediated by enhanced lower extremity blood flow, 12 patients were studied immediately preoperatively. Calf blood flow increased from 3.7 +/- 1.6 to 5.0 +/- 2.2 mL.100 mL-1.min-1 with an intravenous epinephrine infusion (1.9 +/- 0.5 microgram/min). After epidural anesthesia calf blood flow increased to 6.3 +/- 3.3 mL.100 mL-1.min-1 with epinephrine infusion (2.7 +/- 1.0 micrograms/min) but was reduced to 2.6 +/- 1.5 mL.100 mL-1.min-1 when norepinephrine (0.8 +/- 0.7 microgram/min) was infused. Calf vascular resistance decreased from 27.7 +/- 13.5 to 7.9 +/- 4.6 mmHg.mL-1.100 mL-1.min-1 after epidural anesthesia with epinephrine (2.7 +/- 1.0 micrograms/min) but was unchanged when norepinephrine (0.8 +/- 0.7 microgram/min) was infused. The augmentation of lower extremity blood flow when epinephrine is used in conjunction with epidural anesthesia may in part explain the low frequency of deep vein thrombosis with conduction anesthesia.