Venous Thromboembolism and Fat Embolism

Published on 07/03/2015 by admin

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Chapter 26 Venous Thromboembolism and Fat Embolism

Madison Macht, MD , Marc Moss, MD

1 What are the sources of pulmonary embolism (PE)?

The majority of PEs arise from thromboses in the deep veins of the legs, particularly the iliac, femoral, and popliteal veins. Thromboses can also originate on central venous catheters and from the pelvic veins in women with a history of obstetric difficulties or recent gynecologic surgery. Nonhematologic sources of PE include fat particles, amniotic fluid, air introduced through central venous catheters, and foreign bodies such as talc and cotton fibers in intravenous drug users.

3 How common is venous thromboembolism in critically ill patients?

DVT and PE are common and often underdiagnosed in critically ill patients. In prospective studies, 33% of medical patients in the intensive care unit (ICU) had DVTs on routine clinical screening, and 18% of trauma patients had proximal DVTs. In a retrospective series of respiratory ICU patients, 27% had PE on autopsy.

4 How reliable is the physical examination for diagnosing DVT?

When patients complain of unilateral calf pain and swelling, a DVT is nearly always possible, and it is difficult to rule in or rule out its presence with physical examination alone. Furthermore, asymptomatic DVT may occur 5% to 10% of the time. Specific physical examination maneuvers aimed at improving diagnostic accuracy include palpation for cords in the greater saphenous vein and its branches, detection of dilation of superficial tibial veins, checking for increased temperature in the lower extremity, squeezing specific parts of the leg to elicit tenderness, measuring the diameter of each leg, and checking for pain on passive ankle dorsiflexion (referred to as Homans sign). Unfortunately these maneuvers have limited sensitivity and specificity. Simplified clinical prediction models that combine many signs, symptoms, and historical features with serum D-dimer testing have been validated in outpatients. In these patients the combination of a low clinical probability of DVT and a low serum D-dimer level may effectively exclude DVT without ultrasonography. Importantly, these clinical prediction models have not been validated in critically ill patients.

5 What are the signs and symptoms of an acute PE?

The symptoms of an acute PE depend on the thromboembolic burden, the degree of underlying pulmonary parenchymal disease, and the ability of the right ventricle to accommodate acute pressure changes. Patients can present with syncope, shock, tachycardia, acute right ventricular failure, increased dead space, or refractory hypoxemia. However, patients may also be relatively asymptomatic. In patients with underlying cardiac or pulmonary disease, significant hemodynamic compromise may occur with less occlusion of the pulmonary vasculature. Common signs and symptoms of PE are listed in Box 26-2.

6 What are some specific findings of PE visible on chest radiography?

Chest radiography can neither be used to make nor to exclude the diagnosis of PE. However, the chest radiograph does assist in ruling out alternative diagnoses, determining the severity of the patient’s illness, and identifying abnormalities that may require further evaluation. Contrary to common belief, the chest radiograph is frequently abnormal in patients with proven PE. Focal atelectasis, seen as linear streaks that run parallel above an elevated hemidiaphragm, may be seen (Fleischner lines). Small to moderate unilateral, exudative pleural effusions occur in approximately one third of patients. Although not specific for the diagnosis, patients with a pulmonary infarct may occasionally have a wedge-shaped opacity that abuts the pleura (Hampton hump). In addition, focal oligemia may be visible (Westermark sign) and when present should prompt further evaluation for PE.

7 What are the electrocardiographic (ECG) findings associated with PE?

ECG findings are also variable and relatively nonspecific. Sinus tachycardia and nonspecific ST-segment and T-wave changes occur frequently. The classic ECG findings of S₁, Q₃, T₃, or right bundle branch block occur in fewer than 15% of patients. The development of ECG findings of acute right ventricular strain (i.e., rightward shift of the QRS axis or peaked P waves in the inferior leads) in a critically ill patient should raise concern about potential PE.

8 Are any laboratory test result abnormalities helpful in diagnosing PE?

No arterial blood gas values have adequate operating characteristics to exclude PE. Elevated serum D-dimer levels are highly sensitive but have low specificity. Although primarily validated in outpatients, when clinical suspicion for PE is low, the additional information of a serum D-dimer level below 500 ng/mL may be helpful to exclude the presence of PE without the need for further chest imaging.

9 When should a ventilation-perfusion (V/Q) scan be ordered in the diagnostic work-up of PE?

V/Q scans can be particularly useful:

In patients for whom computed tomography angiography (CTA) is contraindicated (renal failure, radiocontrast allergy, concern for radiation exposure)

When a concern exists for chronic PE

At institutions with limited access or experience with CTA

Eighty-seven percent of patients with a high-probability V/Q scan have a PE, whereas the diagnosis can essentially be excluded in patients with a normal V/Q scan. Unfortunately, more than 50% of V/Q scans are nondiagnostic (i.e., intermediate or low probability), and further testing is needed. Importantly, V/Q scans are more difficult to interpret in patients with preexisting lung disease.

10 What about CTA for the diagnosis of PE?

The sensitivity and specificity of CTA exceed 90%. Higher imaging resolution and more experienced radiologists have contributed to this increase in CTA sensitivity and specificity. The sensitivity of CTA for small, subsegmental emboli is lower. However, several studies have demonstrated that subsequent DVT will develop in fewer than 1% of patients with a negative CTA for the diagnosis of PE over the next 3 months. Three additional advantages of CTA are the following:

Evaluation for alternative diagnoses in the chest

Direct visualization of emboli

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