by Karl Loren
The Role of Ultrasound in Diagnostic Imaging for DVT in Trauma
I. Statement of the Problem
Early identification of DVT in trauma patients would allow for treatment to be initiated, thus decreasing the frequency of complications. Ultrasound scanning has the advantage as a diagnostic tool in detecting DVT because it is non-invasive, requires no contrast medium, can be performed at the bedside, and is able to detect nonocclusive thrombus. Two types of ultrasound scanning will be discussed. Doppler ultrasound involves a hand-held probe placed over the skin of the vein being studied. Duplex ultrasonography employs real time B-mode sonography which produces a two-dimensional image using high frequency sound waves and Doppler ultrasound. It is important for the reader to distinguish between these two technologies in the accuracy of ultrasound to detect DVT. Further, in the critical review of ultrasound technology to detect DVT, a dichotomy exists in the sensitivity of ultrasound to detect DVT in symptomatic vs. asymptomatic patients.
II. Process
A Medline search from 1966 to present revealed several thousand articles related to the ultrasound diagnosis of DVT. Several of the more seminal articles and review articles related to the ultrasound diagnosis of DVT in the non-trauma patient are included to provide a perspective on the current state of the technology. A total of 16 articles related to the ultrasound diagnosis of DVT in the trauma patient are discussed in this review.
III Recommendations
A. Level I
Duplex ultrasound may be used to assess symptomatic trauma patients with suspected DVT without confirmatory venography.
B. Level II
There are insufficient data to suggest Level II recommendations for this topic.
C. Level III
Hand-held Doppler ultrasound may be used to assess symptomatic trauma patients with suspected DVT. Confirmatory venography may be needed in patients who screen positive for DVT with Doppler ultrasound.
Serial duplex ultrasound imaging of high-risk asymptomatic trauma patients to screen for DVT may be cost-effective and decrease the incidence of PE. However, the use of ultrasound in screening asymptomatic patients is burdened by a low sensitivity when compared to venography.
IV. Scientific Foundation
A. Ultrasound Diagnosis of DVT in the Non-Trauma Patient.
1. Doppler Ultrasound
The use of a Doppler flowmeter for the diagnosis of DVT has some appeal because of its relatively low cost and the additional benefit of being able to be performed at the bedside or as an outpatient. The accuracy is very much dependent on the experience of the user.1 Comerota et al.2 compiled a meta- analysis of 23 studies examining the accuracy of Doppler ultrasound compared to venography. Overall, in symptomatic patients Doppler ultrasound had a sensitivity of 85% (722/847) and a specificity of 88% (1,415/1,615) to detect proximal DVT.
2. Duplex Ultrasound
Duplex ultrasound employing both real-time B-mode scanning and Doppler ultrasound allows for non-invasive visualization of the veins of the leg. In most patients, it is easy to visualize the common femoral, proximal superficial femoral, and popliteal veins. It can be difficult to visualize the superficial femoral vein in Hunterís canal and also to detect calf DVTs. An acute DVT is identified by the presence of a dilated vein, lack of compressibility, and absence of Doppler flow sounds. Again, the technical quality of the study is very much user-dependent. In patients who present with symptoms of DVT (symptomatic), ultrasound has a high sensitivity and specificity. Comerotaís collective review of 25 studies in which duplex was used to diagnose proximal DVT in symptomatic patients had a sensitivity of 96% (1,132/1,178) and a specificity of 96% (1,384/1,450).2 In the 10 series in which duplex was used to diagnose calf DVT in symptomatic patients, it had a sensitivity of 80% (122/153).
In asymptomatic high risk patients duplex, ultrasound does not appear as accurate as a screening technique for DVT, however, the reports are quite variable in their success rates. Most of these studies have been performed in orthopedic patients undergoing elective surgery. Agnelli et al.3 attempted to shed some light on the diagnostic accuracy of duplex ultrasonography in patients with asymptomatic DVT by performing an overview on the studies taking into account their study methodology. A study was classified as Level I if consecutive patients were admitted, bilateral venography was performed in every patient, and ultrasonography was performed and judged before venography. Studies not fulfilling these criteria were considered Level II. Overall, there were four Level I studies and eight Level II studies.
Table 1 summarizes these results:
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The differences in sensitivity and specificity between Level I and Level II studies were statistically significant (p < 0.001).
B. Ultrasound Diagnosis of DVT in the Trauma Patient
There are several studies on the use of ultrasound to screen for DVT in asymptomatic patients at high risk for DVT. Unfortunately, most of these studies had significant methodological flaws, and few, if any, employed a confirmatory venogram to check the accuracy of their techniques. Of additional concern was the fact that several of these series reported on a number of PEs that occurred in the absence of documented DVT, leading to speculation on the possibility that ultrasound screening missed a clinically significant DVT. Nevertheless, these ultrasound studies do offer a glimpse of the incidence of the occult DVTs that occur in high risk trauma patient, and they provide additional data as to their location and origins as well as the role that prophylaxis plays in decreasing the incidence of DVT.
Burns et al.4 did a comprehensive color Doppler ultrasound exam twice weekly of all major venous structures in 57 patients classified as high risk during an eight-month period. Both upper and lower extremities were examined as well as the internal jugular, subclavian and axillary veins, the inferior vena cava, and common iliac, internal and external veins. Twelve high-risk trauma patients (21%) were identified as having occult DVT. In 23% of patients, they were unable to get a complete ultrasound exam. No confirmatory study was performed in those who tested positive on ultrasound. Of note, there were 2 PEs in this group of high risk patients (confirmed by pulmonary angiogram), and both patients at the time had screened negative for DVT. Napolitano et al. 5 retrospectively reviewed the results of biweekly duplex screening in 458 trauma patients admitted to their ICU over a five-year period. The incidence of DVT was 10%, and all were asymptomatic. Multiple logistic regression revealed age, length of stay, spinal cord injury, ISS and TRISS scores as being significant risk factors for the development of DVT. There was no confirmatory study employed in those patients who tested positive for DVT, and a PE occurred in this population. In a commentary which accompanied the article, Dr. M. Knudson pointed out several methodological flaws with the study. At issue were the timing of the scans performed, the retrospective nature of the study, and the use of only ICU patients in the screening protocol which introduces a bias eliminating other high risk patients such as those with pelvic or lower extremity fractures which may not need ICU admission.
Meythaler et al.6 performed a cost analysis of routine screening for proximal DVT using color-Doppler ultrasound in 116 head-injured patients being admitted to a rehab unit over a 21-month period. Fourteen (8.5%) patients were found to have DVT on initial screening. No confirmatory studies were performed, and all were asymptomatic. The authors conducted a complicated cost-benefit analysis of ultrasound screening for DVT in this population and found that the cost per year of life saved was $2,977.65 ($129,527.83/43.5years). This compared favorably to the $8,280 per year of life saved for biennial mammograms for women age 50-59 and the $35,054 per year of life saved for annual fecal occult blood tests beginning at age 65. As is indicative of such an analysis, there are a number of underlying assumptions which may not reflect reality, nevertheless it does lend perspective on the cost issues relative to other screening programs.
In a study of 60 patients with major fractures of the pelvis, White et al.7 performed serial duplex ultrasound to determine the incidence of DVT. In this study, confirmatory contrast venography was used in those who tested positive for DVT on ultrasound. Eight (15%) patients developed DVT of which 6 were proximal and two were distal (calf). All were asymptomatic for DVT. There was one PE in this population in a patient who subsequently tested positive for a proximal DVT. One weakness of the study was that the screening ultrasound was first performed 7 days after admission. The authors stated that they checked the accuracy of duplex ultrasound as a screening test in 32 high risk orthopedic patients (including those with lower extremity and pelvic fractures) by comparing it to ascending venography. Eleven patients had positive duplex ultrasounds, and all had positive venograms. There was one patient who had a negative duplex but a positive venogram. Overall, the predictive value of a positive duplex in this study population was 100% (11 out of 11) and that of a negative duplex ultrasound, 95% (21 of 22).
Chu et al. 8 looked at the 21 spinal cord injured patients admitted to a rehab unit over an 11-month period who were screened with Doppler ultrasound and IPG on alternate weeks. Only two patients developed DVT during an 8-week period, and both were detected clinically prior to diagnostic testing. It should be noted that this study somewhat contradicts other studies of DVT in spinal cord injured patients in which the incidence of DVT approached 100%. 9,10 It also should be noted that the authors of this study used Doppler ultrasound with an unknown sensitivity and specificity as a screening procedure in the asymptomatic patient.
Meredith et al.11 looked at the incidence of DVT with femoral vein catheterization using 8.5F Swan-Ganz introducer catheters. Patients were followed with serial duplex ultrasonography. Not surprisingly, these large bore catheters were associated with a 14% iliofemoral DVT rate on the side of the catheter. All were clinically occult.
In a different study of 96 elderly patients with isolated hip fractures Dorfman et al.12 used compression sonography (i.e. Doppler ultrasound) perioperatively and every 3 days until discharge. Positive ultrasounds were confirmed with venography, and venography was performed on all patients at discharge. There were 18 patients (19%) who had a diagnosed DVT. Of these, 5 were diagnosed on the pre-discharge venogram. This gives ultrasound a sensitivity of 73% and specificity of 100% in this series of patients.
Jongbloets et al.13 in a series of 100 patients undergoing craniotomy compared serial Doppler ultrasound and contrast venography. Venography demonstrated proximal DVT in 13 patients (13%). Doppler ultrasound also identified DVT in 5 of these patients (sensitivity 38%, 95% CI 8-69%). In the 87 patients without proximal DVT on venography, Doppler ultrasound gave four false positive results (specificity 95%, 88-99%).
In a study examining modes of prophylaxis in 281 high risk trauma patients, Dennis et al.14 scanned for DVT on admission and every 5 days thereafter with a duplex scanner or Doppler ultrasound. Approximately 25% were scanned using duplex and 75% employed Doppler. The authors did not indicate the reason for different modes. No confirmatory study was performed in patients who tested positive on ultrasound, and in 20% of exams there was an incomplete study. There were 18 cases of DVT (4.6%) and 4 cases of PE (1%) in the absence of DVT, three of which were fatal. Of concern in all 3 fatal PEs, none had shown evidence of DVT on routine surveillance with ultrasound before their deaths. In a similar prospective study examining prophylaxis of DVT in trauma patients, Knudson et al.15 used serial duplex ultrasound to detect thigh vein thrombus. In a few cases, the authors used venography to confirm a positive duplex result which was 100% accurate. Overall, there was approximately a 10% DVT rate, but again there were 4 cases of PE in the absence of detectable DVT, leading to speculation on the sensitivity of duplex to detect clinically significant DVT. In a larger study, again examining modes of DVT prophylaxis in trauma patients, Knudson et al.16 used weekly serial duplex ultrasound as the diagnostic modality to detect DVT. Of 251 patients in this randomized prospective study, 15(6%) developed DVT as detected by duplex. Only 20% had clinical symptoms of DVT, the rest were occult. Again, two patients developed PE, one of which was fatal, following repeated negative ultrasound exams.
V. Summary
