Investigations of DVT

Investigations of DVT

1. Doppler Ultrasound examination
2. Exclude pulmonary embolus (caused by DVT)
3. Thrombophilia screen
4. Venogram (gold standard)
5. MRV (magnetic resonance venogram) or MRI (may be better than other non-invasive techniques in diagnosing DVT in calf, but too expensive, so not used widely)

Ultrasonography
- best non-invasive diagnostic method
- average sensitivity & specificity of 97% for proximal DVT
- cannot be relied on to diagnose calf vein thrombosis (sensitivity too low 75%)
- three common techniques:

Compression ultrasound:
- looks at non-compressibility of the vascular lumen under gentle probe pressure
- if no residual lumen observed, vein is considered to be fully compressible, hence absence of DVT

Duplex ultrasonography:
- similar to above, but
- blood flow characteristics evaluated using pulsed Doppler signal
- blood flow in normal veins is spontaneous & phasic with respiration; can be augmented by manual compression distal to the ultrasound transducer
- if no phasic pattern, flow is defined as continuous, hence presence of venous outflow obstruction

Colour flow duplex imaging (colour coded Doppler ultrasonography aka colour Doppler):
- identical to above
- pulsed Doppler signals is used to produce images
- when a Doppler shift is recognized, it is assigned a colour (red or blue) according to its direction towards or away from the probe
- flowing blood displayed as a colour overlay to the grey scale ultrasound image, hence, easier to identify the veins
Therefore, Doppler ultrasonography
- shows direction and velocity of blood flow
- detects turbulent flow due to narrowing or blockage of blood vessels
- shows different rates of blood flow in different colours

Focus on Doppler Ultrasonography

Why the test is performed
- alternative to arteriography or venography
- help diagnose a blood clot, venous insufficiency, arterial occlusion (closing), abnormalities in the carotid artery blood flow caused by a narrowing and to evaluate trauma to the arteries
- also used to monitor arterial reconstruction and bypass grafts

How the test is performed
- done in the ultrasound or radiology department or in a peripheral vascular lab
- clear jelly (water-soluble gel) is squirted onto the inside of one of the patient’s thighs to help the ultrasound sensor (a handheld device called a transducer) slide around easily
- gel may be alternatively placed on the device
- a technician/doctor places the sensor against the patient’s skin
- once it's in place, an image appears on a video screen, and the technician/doctor moves the sensor up and down along the leg - from the groin to the calf - to view the veins from different angles
- the examiner presses the sensor into the patient’s skin firmly every few inches to see if the veins change shape under pressure
- the examiner then checks the other leg in the same way
- as the machine measures the blood flowing through a vein, it makes a swishing noise in time with the rhythm of your heartbeat
- the test usually takes 15-30 minutes
- most people don't feel any discomfort, but if the patient’s leg was swollen and sensitive to the touch before the test, the pressure of the sensor might cause some tenderness

Note, to examine the arteries: Blood pressure cuffs may be put around different parts of the body, including the thigh, calf, ankle, and different points along the arm. A paste is applied to the skin over the arteries being examined. Images are created as the transducer is moved over each area.

How to prepare for the test
The patient will need to remove clothing from the extremity being examined

How the test will feel
There is little or no discomfort associated with this test

Normal Values
- the vessels show no evidence of narrowing or closure
- the arteries have normal systolic and diastolic components
- blood pressure is normal

What abnormal results mean

• In the veins:
• Venous occlusion (closing of vein)
• Blood clots
• In the arteries:
• Arterial occlusive disease
• Spastic arterial disease (arterial contractions brought on by cold or emotion)
• Embolic arterial occlusion (obstruction in the artery by a blood clot, a fat globule, or an air bubble)

What the risks are
There are no risks specifically associated with this procedure

Special considerations
Cigarette smoking may alter the results of this test, because nicotine can cause the arteries in the extremities to constrict

Exclude Pulmonary Embolus (caused by DVT)
Investigations of pulmonary embolus:
- Arterial blood gases
- ECG
- CXR
- Ventilation / Perfusion scanning - May confirm or refute diagnosis
- Pulmonary angiography and echocardiography useful if haemodynamic instability
- Spiral CT might replace pulmonary angiography
- Lower limb investigations for DVT as above

Focus on Ventilation/Perfusion Scan (V/Q Scan)

Definition
- a pulmonary ventilation/perfusion scan is a pair of nuclear scan tests
- use inhaled and injected radioactive material (radioisotopes) to measure breathing (ventilation) and circulation (perfusion) in all areas of the lungs

Why the test is performed
- ventilation scan is used to see how well air reaches all parts of the lung
- perfusion scan measures the blood supply through the lungs
- most often performed to detect a pulmonary embolus
- evaluate lung function with advanced pulmonary disease (e.g. COPD), and to detect abnormal circulation (shunts) in the pulmonary blood vessels

How the test is performed
- the V/Q scan is actually two tests, so these tests may be performed separately or together
Perfusion scan:
- a health care provider injects radioactive albumin (mineral technetium) into the patient's vein
- the patient is immediately placed on a movable table that is under the arm of a scanner
- the machine scans the patient's lungs as blood flows through them to detect the location of the radioactive particles (identify areas of the lung that have reduced blood flow)
Ventilation scan:
- performed by scanning the lungs while the patient inhales radioactive gas

In a typical test:
- multiple pictures of the patient’s chest are taken from different angles, using a special camera that detects the radionuclide
- for half of these pictures, the patient is asked to breathe from a tube that has a mixture of air, oxygen, and a slightly radioactive version of a gas called xenon, which can be detected by the camera, and which measures airflow in different parts of the lung
- for the other half of the pictures, the camera tracks the injected radionuclide to determine blood flow in different parts of the lung
- a blood clot is suspected in areas of the lung that have good airflow but poor blood flow

How to prepare for the test
- fasting, special diet, medications not required
- a chest x-ray is usually performed prior to or following the scan

How the test will feel
- the table may feel hard or cold
- the patient may feel a sharp prick while the material is injected into the vein
- the mask used during the ventilation scan may give the patient a claustrophobic feeling
- the patient must lie still during scanning
- the radioisotope injection usually does not cause discomfort

Normal Values
The health care provider should take a ventilation and perfusion scan and then evaluate it with a chest x-ray. All parts of both lungs should take up the radioisotope uniformly.

What abnormal results mean
- results are usually available within a few hours, because the test is done only when the patient is suspected of having a potentially life-threatening condition (pulmonary embolus)
- the doctor interprets the pictures to determine whether the patient’s probability of having a blood clot in your lungs is high, low, or intermediate
- If the probability is high: blood-thinning medicine is ordered
- If it is low: immediate treatment may not be required, but the doctor will want to examine the patient again in a short time
- If it is intermediate, or if the V/Q scan cannot be clearly interpreted: pulmonary angiogram may be ordered to help determine whether the patient has a blood clot. This test is more definitive than a V/Q scan, but because it is more difficult and risky, the V-Q scan usually is done first

Perfusion scan results:
- decreased uptake of radioisotope during a perfusion scan: problem with blood flow, including occlusion of the pulmonary arteries
- a localized decrease in perfusion scan uptake (particularly when the ventilation scan is normal): pulmonary embolus
- larger areas of decreased perfusion scan uptake: condition such as pneumonitis
Ventilation scan results:
- decreased uptake of radioisotope during a ventilation scan: reduced breathing and ventilation ability or airway obstruction
- decreased ventilation uptake (plus x-ray evidence of consolidation): pneumonia
- larger areas of poor uptake: damage from chronic smoking or COPD

What the risks are
- risks are about the same as for x-rays (radiation) and needle pricks
Radiation risk:
- no radiation is emitted from the scanner (it detects radiation and converts it to a visible image)
- small exposure to radiation from the radioisotope. The radioisotopes used during scans are short-lived, with almost all radiation leaving the body in a few days. However, as with any radiation exposure, caution is advised for pregnant or breast-feeding women
Infection risk:
- slight risk for infection/bleeding at the site of the needle insertion (same for any other purpose of needle pricks)
- rare cases: a patient may develop an allergy to the radioisotope, which may include a serious anaphylactic reaction

Sources:
http://www.health.harvard.edu/diagnostic-tests/venous-ultrasound-of-the-legs.htm
http://www.nlm.nih.gov/medlineplus/ency/article/003775.htm
http://www.nlm.nih.gov/medlineplus/ency/article/003828.htm
http://www.health.harvard.edu/diagnostic-tests/ventilation-perfusion-scan.htm
http://www.bmj.com/cgi/content/full/326/7400/1180
http://www.surgical-tutor.org.uk/default-home.htm?system/vascular/venous_thromb.htm~right
http://www.merck.com/mmhe/sec03/ch021/ch021c.html#sec03-ch021-ch021c-122

Further reading on DVT (covers all PCL topics):
http://www.emedicine.com/med/topic2785.htm#section~workup