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PERIPHERAL VASCULAR ULTRASOUND
the interaction of fibrin, contained in thrombus, and plasmin. Increased levels of D-dimer are asso- ciated with the presence of DVT. Unfortunately, increased levels of D-dimer are also found in other conditions, such as malignancy, infection and trauma. Therefore, the D-dimer test has a high sensitivity but low specificity for the presence of DVT. Despite low specificity, negative predictive values as high as 98% have been reported (Bradley et al 2000). A negative predictive value indicates the probability that the patient will not have the disease in those who have a negative test outcome. It has been suggested that a combination of a low risk probability score and negative D-dimer test may be useful pre-selection tools to avoid unnecessary duplex examinations (Aschwanden et al 1999). Most ultrasound examinations use compression of the vein to confirm patency. This normally involves full examination of the deep veins from the groin to the calf. However, there is evidence that a limited com- pression test involving two- or three-point compres- sion at the common femoral vein, popliteal vein and distal popliteal vein (third point) is a safe and rapid method of excluding DVT (Cogo et al 1998, Khaw 2002).
Magnetic resonance imaging and CT scanning are used for imaging the iliac veins and vena cava when other imaging tests are inadequate or impossible. However, because of their cost, they are unlikely to be used for routine DVT screening in the outpa- tient clinic in the immediate future.
PRACTICAL CONSIDERATIONS FOR
DUPLEX ASSESSMENT OF DVT
The objective of the scan is to assess the deep venous system for patency and exclude the presence of a DVT. It is also important to locate the proximal position of a thrombosis, as this can influence sub- sequent treatment. Other conditions that mimic DVT can be identified with ultrasound. The main diagnostic criterion used to exclude DVT is complete collapse of the vein under transducer pressure. Color flow imaging and spectral Doppler can also be used during the assessment. At least 30 min should be allocated for a full scan, including the calf veins.
The legs should be accessible and the patient made as comfortable as possible. In very rare situ- ations, the patient may require some sedation or
analgesia before the examination if the limb is extremely painful. It is helpful to ask the patient to point to any areas of discomfort or tenderness, espe- cially in the calf, as this can often be located over the site of the thrombosis. This region should be carefully examined by duplex scanning. The exam- ination room should be at a comfortable ambient temperature to prevent vasoconstriction (20 °C). Wherever possible, the legs should be examined in a dependent position in order to fill and distend the veins. Ideally, the patient should be examined with the legs tilted downward from the head by at least 30° (reverse Trendelenburg position). Alter- natively, the patient can be examined in a standing position, with the leg to be examined not bearing weight and the patient holding a hand rail or equiv- alent for support. The calf veins and popliteal fossa are easier to scan with the legs extended, hanging over the side of the examination table, and the feet resting on a stool. It is important not to overex- tend the knee when examining the popliteal vein, as this can lead to collapse or occlusion of the vein. Wherever possible, immobile or sick patients should be tilted into a reverse Trendelenburg position, although there may be situations in which the patient cannot be moved, such as in the intensive care unit.
DEEP VEIN EXAMINATION FOR ACUTE DVT
A 5MHz, or broad-band equivalent, flat linear array transducer should be used for examining the femoral, popliteal and calf veins. The iliac veins are examined using a 3.5 MHz curved linear array trans- ducer. The scanner should be configured for a venous examination. The color PRF should be low, typically 1000 Hz, to detect low-velocity flow. The color wall filter should also be set at a low level, and the spectral Doppler sample volume should be increased in size to cover the vessel, so that flow is sampled across the lumen.
Ultrasound compression is the main method of confirming vein patency. If direct transducer pres- sure is applied over a vein it will collapse, as the blood pressure in the deep veins is low, unlike the pressure in the adjacent artery, and the walls will be seen to meet (coapt). The adjacent artery should demonstrate little or no distortion. In contrast, if