A computerised system of screening for deep venous thrombosis

The Effect of Graduated Compression Stockings on Lower Limb Venous Haemodynamics

Objectives:

To compare the effect of four types of thigh-length graduated compression stockings (GCS) on lower limb venous blood flow using strain-gauge Plethysmography (SGP).

Design:

A randomized controlled study.

Setting:

Inpatient Orthopaedic Centre.

Patients:

200 preoperative patients admitted for elective lower limb orthopaedic surgery.

Interventions:

Patients were randomized to five groups to wear one of four types of GCS or no stocking (control). Resting venous flow parameters were measured prior to application of GCS and after 20 min bed rest with the stockings in situ.

Main outcome measures:

Effects on resting venous capacitance (Vc) and venous outflow (Vo).

Results:

In the control group 20 min bed rest had no effect on the parameters studied. Both Vc and Vo were significantly increased ( p<0.001) in all the groups wearing stockings. There was a significant variation in the changes in outflow produced by the different stocking types ( p<0.05).

Conclusions:

GCS have a beneficial effect on lower limb venous outflow, preventing venous stasis, an important factor in the development of deep venous thrombosis. This effect varies depending on which type of stocking is used.

Meta-analysis of plethysmography and rheography in the diagnosis of deep vein thrombosis

BACKGROUND

Plethysmography and rheography techniques have been widely studied as diagnostic tests for deep vein thrombosis (DVT). This study aimed to systematically review the accuracy of these tests for diagnosing clinically suspected DVT.

METHODS

The following databases were searched: Medline, EMBASE, CINAHL, Web of Science, Cochrane Database of Systematic Reviews, Cochrane Controlled Trials Register, Database of Reviews of Effectiveness, the ACP Journal Club (1966 to 2004), and citation lists of retrieved articles. Studies that compared plethysmography or rheography to a reference standard of ultrasound or contrast venography were selected. Standardised data were extracted and study quality determined against validated criteria. Data were analysed by random effects meta-analysis and meta-regression.

RESULTS

The meta-analysis included 78 studies, reporting 82 patient cohorts. Sensitivity and specificity (95% CI) were: 75% (73% to 77%) and 90% (89% to 91%) for impedance plethysmography, 83% (81% to 85%) and 81% (79% to 82%) for strain-gauge plethysmography, 85% (79% to 90%) and 91% (81% to 95%) for air plethysmography, 91% (87% to 94%) and 71% (66% to 75%) for light-reflex rheography, and 86% (83% to 89%) and 93% (91% to 95%) for phleborheography. Meta-regression was limited by poor reporting of studies. There was some evidence that diagnostic performance depended on the proportion of males in the cohort and reporting of study setting.

CONCLUSIONS

Although plethysmography and rheography techniques add diagnostic value, they have inadequate diagnostic performance to act as a stand-alone test in DVT diagnosis. Evaluation of their role in combination with other tests, or standardised clinical assessment, is required.

Altered venous function and deep venous thrombosis following proximal femoral fracture

The effect of surgery for femoral neck fracture on lower limb venous blood flow and its relationship to deep vein thrombosis was investigated in 179 patients. Blood flow was measured using strain gauge plethysmography before surgery, in the 1st week after surgery, and at 6 week review. There was a significant reduction in both venous outflow and venous capacitance, affecting both fractured and non-fractured legs but significantly greater in the fractured leg. Venous function remained significantly impaired in both lower limbs 6 weeks after surgery. There was a significant correlation between the reduction in venous function and the development of deep vein thrombosis.

Computer assisted strain-gauge plethysmography is a practical method of excluding deep venous thrombosis

AIM

To evaluate a computed strain-gauge plethysmograph (CSGP) as a screening tool to exclude above knee deep venous thrombosis (DVT).

METHODS

The first phase took place in the Radiology department. One hundred and forty-nine patients had both Doppler ultrasound and CSGP performed. Discordant results were resolved by venography where possible. The second phase took place in an acute medical admissions ward using a modified protocol. A further 173 patients had both studies performed. The results were collated and analysed.

RESULTS

Phase 1. The predictive value of a negative CSGP study was 98%. There were two false-negative CSGP results (false-negative rate 5%), including one equivocal CSGP study which had deep venous thrombosis on ultrasound examination. Two patients thought to have thrombus on ultrasound proved not to have acute thrombus on venography. Phase 2. The negative predictive value of CSGP using a modified protocol was 97%. There were two definite and one possible false-negative studies (false-negative rate 4-7%).

CONCLUSION

Computer strain-gauge plethysmograph can provide a simple, cheap and effective method of excluding lower limb DVT. However, its use should be rigorously assessed in each hospital in which it is used. Goddard, A. J. P., Chakraverty, S. & Wright, J. (2001). Clinical Radiology56, 30-34.

Numerical model of deep venous thrombosis detection using venous occlusion strain gauge plethysmography

Strain gauge plethysmography (SGP) is a non-invasive method used in the detection of deep venous thrombosis (DVT). The technique is based on the measurement of calf volume changes in response to venous occlusion by a thigh cuff, the volume changes reflecting the rates of arterial inflow and venous outflow. A numerical model of the blood circulation within the limb and the response of this to a SGP test has been derived, based on treating the different parts of the circulatory system in the leg as resistance and capacitance elements. The simulation results were compared with clinical studies and support the ability of SGP to detect non-occlusive clots of more than 50-60% of the lumen, as well detecting calf vein occlusion. The non-linear behaviour of the venous compliance with intra-luminal pressure appears to be a particularly important factor within the model. In addition, increases in venous tone due to post-operative venospasm were shown to be a potential source of false positive results.

Computerized strain-gauge plethysmography - An alternative method for the detection of lower limb deep venous thrombosis?

AIM

To test the ability of computerized strain-gauge plethysmography to act as a screening test for lower limb deep venous thrombosis (DVT).

MATERIALS AND METHODS

Over an 8-month period, all patients referred to our Medical Assessment Unit with suspected lower limb DVT were considered for inclusion in the study. Each patient underwent both plethysmography and ascending venography within 24 h, and the presence or absence of thrombus in the popliteal, superficial femoral or iliac veins was noted. The results of the two tests were then used to determine the accuracy of computerized strain-gauge plethysmography in detecting above knee DVT.

RESULTS

The screening tests and venograms of 239 patients referred with clinically suspected lower limb DVT were compared. The false negative rate of plethysmography was 15.4%, which is significantly different from the 4.8% claimed by the manufacturers of this device (P = 0.00003).

CONCLUSIONS

In a population of acute admissions with suspected lower limb DVT, computerized strain-gauge plethysmography is not suitable for use as a screening test due to an unacceptably high proportion of false negative screens.

Computerized strain-gauge plethysmography in the diagnosis of symptomatic and asymptomatic venous thrombosis

This study compares the results of a computerized strain-gauge plethysmograph with ascending lower limb venography in 94 patients with clinical deep venous thrombosis, and in 121 patients with asymptomatic legs being screened after total hip replacement. In the symptomatic patients, strain-gauge plethysmography had a sensitivity of 100%, an accuracy of 73%, a specificity of 64% and a negative predictive value of 100% for thrombosis above the popliteal confluence. In the screened patients, the figures were 38.1%, 55.4%, 60.0% and 81.1% respectively. Computerized strain-gauge plethysmography is a safe, non-invasive, reliable and portable method of excluding proximal thrombosis in a symptomatic patient. It avoids the need for urgent venography when anticoagulation therapy is reserved for those with proximal thrombosis. The device was not valuable as a screening tool after total hip replacement, since it had a low specificity and did not reliably detect the non-occlusive mural femoral thrombi which typically follow this procedure.