Investigations for PE and DVT, including sensitivity and specificity.

+ venous anatomy and V/Q mismatch

Investigations of DVT

Wells Criteria, for DVT These clinical prediction rules are designed to increase the probability of an accurate diagnosis of deep venous thrombosis

• Active cancer (1 point)• Paralysis, paresis, or recent plaster immobilisation of the lower extremities (1 point)• Recently bedridden for 3 days or more, or major surgery within the previous 12 weeks requiring general or regional anaesthesia (1 point)• Localised tenderness along in the distribution of the deep venous system (1 point)• Entire leg swollen (1 point)• Calf swelling at lease 3cm larger than that on the asymptomatic side (measured 10cm below tibial tuberosity) (1 point)• Pitting oedema confined to the symptomatic leg (1 point)• Collateral superficial veins (nonvaricose) (1 point)• Previously documented DVT (1 point)• Alternative diagnosis at least as likely as DVT (-2 points)

Score of 2 or higher = DVT likelyScore of less than 2 = DVT unlikely

Key Points on DVT• Clinical prediction rules (eg Wells Criteria) may be used to

categorise patients into low, medium or high risk • Low risk and negative serum D-Dimer effectively excludes

DVT • Medium and high risk patients should undergo doppler

ultrasound without D-Dimer estimation • Ultrasound is highly sensitive for proximal lower limb deep

vein thrombosis (97%). Specificity also >95%.• US is less sensitive for deep calf vein thrombosis (73%) and

for iliac vein thrombosis • After a negative doppler ultrasound, follow-up US in

patients with high clinical suspicion may be indicated to exclude a calf thrombosis that is propagating proximally

US procedure

Each venous segment is assessed for the presence of thrombosis, indicated by venous dilation and incompressibility during probe pressure (B-mode)

• Doppler findings suggestive of acute DVT are absence of spontaneous flow, loss of flow variation with respiration, and failure to increase flow velocity after distal augmentation.

DVT Differential Diagnosis

•Localized muscle strain, contusion, or Achilles tendon rupture can often mimic the symptoms of DVT. Cellulitis may cause edema, localized pain, and erythema. Unilateral leg swelling can also result from lymphedema, obstruction of the popliteal vein by Baker cyst, or obstruction of the iliac vein by retroperitoneal mass or idiopathic fibrosis. Bilateral leg edema suggests heart, liver, or kidney failure or IVC obstruction by tumor or pregnancy.

•The Virchow triad (stasis, vascular injury, and hypercoagulability) should be the cornerstone for assessment of risk factors for DVT. In most cases, the cause is multifactorial.

Investigations for PE

Investigation of PE in shock

Well’s Criteria for PEPrior to imaging, one must clinically calculate the probability of PE.

- Clinical signs and symptoms of deep venous thrombosis (leg swelling and deep venous pain): 3 points. – PE as or more likely than an alternative diagnosis: 3 points. – Previously objectively diagnosed DVT or PE: 1.5 points. – Active cancer (less than 6 months since therapy or palliative stage): 1 point. – Recent Immobilisation: 1.5 points – Bedrest for at least 3 consecutive days. – Surgery in the previous 4 weeks. – Heart rate greater than 100 per minute: 1.5 points. – Haemoptysis: 1 point.

Total Score: 0-1 = Low, 2-6 = Intermediate, 7+ = High probability

Key Points on PE investigationsThe role of a chest Xray in suspected Pulmonary Embolism (PE)• is to exclude other causes that may mimic PE and to guide further investigations

– Although the CXR is abnormal in most patients with pulmonary embolization with infarction, the abnormalities are often nonspecific (e.g., atelectasis, pleural effusions, small infiltrates). The Westermark sign (dilated pulmonary vasculature proximal to embolus with oligemia distal) and Hampton's Hump (a pleural-based density with a rounded border facing the hilum) are specific though uncommon findings in pulmonary emboli.

D-Dimers• Patients who are at low probability for PE should have a D-Dimer. A negative D-Dimer in a low probability

case of suspected PE rules out the diagnosis and no further investigation is indicated • Patients with moderate to high pre-test probability of PE should have further imaging

Key Points on PE investigationsCT Pulmonary Angiography• demonstrates PE by showing filling defects, within the contrast filled pulmonary arteries. Preferred

in patient with chronic lung disease and abnormal CXR, as they already have V/Q mismatch….– The Prospective Investigation of Pulmonary Embolism Diagnosis II (PIOPED II) trial reported a sensitivity of

83% and specificity of 96% – Another study both >85

Ventilation/Perfusion Scans (V/Q) • Lung perfusion images are taken after the intravenous injection of technetium-99m

macroaggregated albumin. A PE characteristically appears as a pleural based segmental perfusion defect. 1

• Any perfusion defects are compared to ventilation images (involves pt breathing in technetium labelled argon) and any regions of mismatch are considered suspect for PE

– Specific but not sensitive ie. negative test does not rule it out and need to preoceed to CTPA if clinical suspicion persist

• It is preferred in younger patients due to lower radiation dose and in patients with contraindicates for CT contrast

• CXR: There is a peripheral wedge shaped opacity representing pulmonary infarction and atelectasis secondary to a pulmonary embolus (arrow). This radiographic sign is referred to as Hampton's Hump.

• Axial and reconstructed images of bilateral pulmonary arterial emboli (arrows)

• Ventilation Perfusion Scan of Bilateral Embolism: The ventilation series demonstrates uniform distribution of tracer throughout both lung fields. The perfusion series demonstrates generalised reduced tracer uptake in the right lung with multiple segmental and subsegmental perfusion defects throughout both lung fields. These findings have a high probability for recent pulmonary embolism.

Other tests

Arterial Blood Gases•A clinically significant pulmonary embolism is almost always associated with hypoxemia (oxygen saturation <90%; PO2 <80 mm Hg). Hyperventilation and hypocapnia are even more common findings.

ECGIf the patient is hemodynamically unstable: Urgent CTPA is indicated. If not immediately available. ECG is the most useful initial test. Ecg can show indirect signs of acute pulmonary hypertesnion due to PE and access for cardiac differentials. Classic finding of acute right heart strain (S1/Q3/T3; T-wave inversion in leads V1–V3) is more specific but somewhat uncommon

Pulmonary AngiographyThe diagnostic accuracy of pulmonary angiography is considered to be the best of any procedure available. Frequently, CT images are convincing enough to be considered pathognomonic and in that case angiography does not offer a diagnostic advantage. Angiography requires right heart catheterization, which is usually done only as part of thrombolytic treatment for patients hemodynamically compromised by the size of the embolus. Angiography may be indicated in some patients who have nondiagnostic CT scans when the diagnosis of pulmonary emboli must be established with certainty (e.g., in patients in whom anticoagulation carries a high risk of adverse effects, or who will receive long-term anticoagulation).

V/Q mismatch

• Diffusion of O2 into blood requires the perfusion of blood at the ventilated lung units. Physiologic shunting or ventilation–perfusion (V/Q) mismatching is a major cause of abnormal blood gas values. There are four ways to consider alveolocapillary ventilation and perfusion functions: normal, ventilation without perfusion, perfusion without ventilation, and no perfusion/no ventilation. If ventilation and perfusion are normal, the alveolocapillary unit is normal. If there is ventilation without perfusion, the unit is considered alveolar dead space. An example of this is a pulmonary embolism that completely impedes circulation to an area of ventilated lungs. A (V/Q) scan evaluates this type of deficit. If there is perfusion without ventilation, the unit is considered a right-to-left shunt. A simple example of this is pneumonia, in which some alveoli are completely filled with purulent fluid and no gas.

• There are many causes of V/Q mismatch, including pulmonary emboli, pneumonia, asthma, COPD, and even extrinsic vascular compression. Regardless of cause, hypoxemia from ventilation-perfusion mismatch is associated with an increased A-a O2 gradient, and hypoxemia improves with supplemental oxygen.

Basic V/Q mismatch stuff (revision)

References

http://www.imagingpathways.health.wa.gov.au•Very good website: highly recommended for all imaging questions

CURRENT Diagnosis & Treatment: Emergency Medicine. Accessed: access Medicine online.

Pulmonary Physiology. Access Medicine.

Summary

Clinically suspected DVT-Patient haemodynamically unstable (ie. In shock), urgent CTPA required, if not available Urgent ECG-If patient haemodynamically stable

- Low probability- CXR and D- Dimers- Higher probability- CTPA gold-standard, if

contraindicated or unavailable V/Q scan.- If negative but still high clinical suspicions, do the

other scan CTPA/VQ +/- US of lower limbs