The Role of Impedance Plethysmography (IPG) in Diagnostic Imaging for DVT in Trauma

I.   Statement of the Problem

Impedance plethysmography (IPG) is a non-invasive technique based on the principle that volume changes of the leg lead to changes in electrical resistance (impedance). Temporary venous occlusion is produced by an inflatable thigh cuff, and the venous volume response in the calf is measured during inflation of the cuff and for a few seconds following its release. Normally, there is a progressive increase in blood volume of the calf following inflation of the thigh cuff, followed by a rapid runoff when the cuff is released. IPG can be falsely normal if thrombi are non-occlusive or if proximal DVT is associated with well-developed collaterals. In addition, an elevated central venous pressure or venous compression by mass or hematoma can produce a false positive result. When a DVT is present, the runoff is notably impaired. In a critical review of IPGís effectiveness in diagnosing DVT, a clear demarcation exists between symptomatic patients and asymptomatic high-risk patients who are screened for DVT.

II.   Process

IPG was first introduced by Wheeler in 1970.¹ A Medline search since that time revealed 4,319 articles related to IPG and only four articles specifically related to trauma patients.

III.   Recommendations

A.   Level I

There are insufficient data to suggest a standard for this topic.

B.   Level II

There are insufficient data to suggest a guideline for this topic.

C.   Level III

1.   IPG testing may be used in symptomatic patients to diagnose DVT. In those patients in whom the clinical suspicion for DVT is high, and who test negative for DVT on IPG, confirmatory ultrasound or venogram is recommended.

2.   The reported low sensitivity of IPG makes it unsuitable for screening of asymptomatic high risk trauma patients.

IV.   Scientific Foundation

A.   Studies of IPG in Non-Trauma Patients

In most studies, IPG has a high sensitivity and specificity for those patients who present with signs and symptoms of DVT. A collective review by Wheeler et al.² of 18 clinical studies on the accuracy of IPG to detect proximal DVT with venography correlation, demonstrated an overall sensitivity of 90% (847/939) and specificity of 94% (2309/2175). One study by Ginsberg et al.³ of 132 patients referred to a tertiary care institution for symptoms of DVT showed only a 65% sensitivity for IPG to detect proximal DVT. This study stands in contradistinction to the numerous other studies which show sensitivities and specificities which exceed 90% for symptomatic DVT. The authors of this study hypothesized that because of a change in referral practice, an increased proportion of patients with less severe symptoms were subsequently referred to their center than in the past. These patients had thrombi that were smaller and less likely to be occlusive. According to three prospective clinical trials, anticoagulant treatment may be withheld in symptomatic patients with normal IPG. Hull et al.⁴ studied 311 symptomatic patients with negative IPG who were followed for 12 months. Only 2% developed DVT and none died of PE. Huisman et al.^5,6 in two prospective studies of patients with repeated negative IPGs found a very low rate of DVT in six-month follow-up and no deaths due to PE.

The use of IPG for asymptomatic screening has a much lower reported sensitivity and specificity. In Wheelerís ² collective review of six studies mostly in patients undergoing elective hip surgery, IPG had a reported sensitivity of 29% (74/255) and specificity of 97% (2078/2150) when compared to venography. In a similar comparison, Cruickshank et al.⁷ assessed 833 asymptomatic limbs by both IPG and venography. There were 139 DVTs detected by venography but only 18 of these were picked up by IPG (sensitivity 12.9%; CI 7.3 -18.5). In addition, IPG was positive in 13 patients with a negative venogram (specificity 98.1%, 97.1 - 99.1).

Hull et al.⁸ examined several strategies for the diagnosis of PE from a cost standpoint. In a retrospective review of 662 patients, the authors found that the strategy that required pulmonary angiography in the fewest number of patients (i.e. most cost effective) was a combination ventilation-perfusion scanning and serial IPG.

B.   Studies of IPG in trauma patients

To date, no studies have demonstrated the effectiveness of IPG to diagnose DVT in symptomatic trauma patients. Several studies have used IPG to screen for DVT in asymptomatic high risk trauma patients. Shackford et al.⁹ used IPG to screen for DVT in 177 patients admitted with at least one risk factor for venous thromboembolism. All patients with positive studies or repeatedly equivocal studies underwent venography. There were 12 DVTs (7%) and 4 PEs. Of the four PEs, two had negative IPGs prior to the PE and one was equivocal. Of the 177 patients, 35% could not be adequately screened because of complex wounds of the lower extremity. Since venography was not performed on all patients, no statement can be made as to the sensitivity and specificity of IPG as a mode of screening for DVT.

In a prevalence study of DVT in trauma patients who received no prophylaxis, Geerts et al.¹⁰ used both venography and IPG to diagnose DVT. An IPG was performed every other day and on days 10 and 21 after admission with venography done if the IPG was positive. A DVT was found in 201/349 patients (58%) of which 63 (18%) were proximal. Three patients died of massive PE prior to venography, and in only three patients was DVT clinically manifested. Unfortunately the authors of this large prospective study failed to mention how many DVTs were diagnosed by IPG and how many were diagnosed on venogram. Therefore, accurate sensitivity and specificity for IPG cannot be determined in this study population.

Rogers et al.¹¹ in a study of high risk trauma patients who received prophylactic vena cava filters to prevent PE found a 17.6% DVT rate as diagnosed by IPG surveillance. If IPG was positive, confirmatory ultrasound was performed for an overall sensitivity of 100% in the study population. Unfortunately, no confirmatory study was performed in those who tested negative for DVT on IPG so that the specificity of IPG in this study population cannot be determined.

In a population of 15 spinal cord injured patients, Becker et al.¹² used both IPG and ¹²⁵I fibrinogen scanning to determine the incidence of DVT and the effects of rotating treatment tables. When compared to ¹²⁵I fibrinogen scanning in this study, IPG had a diagnostic sensitivity of 62% and a specificity of 100%.

V.   Summary

Most studies demonstrate that IPG has a high sensitivity and specificity in the detection of proximal DVT in symptomatic patients. Its low sensitivity in detecting DVT in asymptomatic patients precludes its use as a surveillance technique in trauma patients at high risk for DVT. There are few specific studies that specifically address the role of IPG in the trauma patient.

VI.   Future Investigation

With improved reported accuracy of duplex ultrasound over IPG, we believe future investigational efforts would be better directed at the role of duplex ultrasound in screening for DVT in the trauma patient.

VI.   References

Reference Conclusions