Laboratory Studies
Arterial or capillary blood gas measurements
Arterial blood gas findings are often normal. However, a calculated alveolar-arterial oxygen gradient may be elevated.
Abnormal findings are nonspecific and may include hypoxemia, hypocarbia or hypercarbia, and respiratory alkalosis that reflects dyspnea and anxiety or respiratory acidosis that reflects a V/Q mismatch.
Hypercarbia with hypoxemia is a poor prognostic sign and indicates a massive pulmonary embolism (PE).
Metabolic acidosis is occasionally observed and is a sign of poor cardiac output. In the case of pulmonary embolism with obstructive shock, venous oxygen saturation is decreased.
D-dimer levels
Two methods measure D-dimer levels, a qualitative assay and a quantitative enzyme-linked immunosorbent assay (ELISA). The ELISA is usually performed only if the result of the qualitative test is positive.
D-dimer levels are elevated (>500 ng/mL) in 90% of adults with pulmonary emboli. Although D-dimers measurement is a very sensitive test, its specificity is only on the order of 50%. Because of the poor specificity, positive D-dimers measurements are generally not helpful in diagnosis. In addition, the use of D-dimers in children is not well studied. A small pediatric series reported that D-dimers findings are negative in 40% of patients. [22] A more recent retrospective series reported an elevated D-dimer in 86% of patients at presentation. [7]
CBC count
The WBC count may be slightly elevated. Hemoglobin and hematocrit are reduced in children with sickle cell disease who present with acute chest syndrome.
Hypercoagulation panel
A hypercoagulation workup should be performed if no obvious cause for embolic disease is apparent. This may include screening for conditions such as antithrombin III deficiency, protein C or protein S deficiency, lupus anticoagulant, homocystinuria, occult neoplasm, and connective tissue disorders.
Imaging Studies
Assessment of risk factors for pulmonary embolism should guide the use of imaging studies. This has been well established in literature guiding the use of V/Q scans. Emerging literature suggests that similar analysis should be used to guide the use of computed tomography (CT) scanning and CT angiography.
Chest radiography
Radiographic findings are abnormal in as many as 70% of cases. Problems of low sensitivity and specificity complicate its use as a diagnostic tool.
Chest radiography is useful to rule out other differential diagnoses; it is a necessary adjunct to the interpretation of the V/Q scan.
The most common radiographic changes include infiltrates, atelectasis, and pleural effusions. Effusions are bilateral in 10% of cases. Signs that are said to be characteristic include Westermark sign, an area of focal hypoperfusion, and Hampton hump, a peripheral wedge-shaped density above the diaphragm. Evidence of a dilated pulmonary artery is occasionally observed.
CT scanning and CT pulmonary angiography
Many diagnostic algorithms have been suggested to facilitate the evaluation of the patient with suspected pulmonary embolism. The widespread availability of CT has evoked much interest in the use of this modality to diagnose pulmonary embolism. Most centers now use CT angiography as an initial step in the workup of pulmonary embolism.
The embolism appears as a low-density filling defect within the pulmonary artery.
One study found that 1% of patients with pulmonary embolism had negative findings on helical CT scanning. [23] Small emboli are more likely to be missed, particularly if they are peripherally located. Hence, the negative predictive value is of the order of 99%, which does not substantially differ from that found with V/Q scanning or pulmonary angiography.
Radiopaque intravenous contrast should be used with caution in patients with possible renal impairment.
Multidetector CT (MDCT) scanning has increased in popularity in recent years. MDCT techniques may reduce the volume of contrast and may improve diagnosis of pulmonary embolism, especially by trainees. [24]
Ventilation/perfusion (V/Q) scanning
This noninvasive scan delineates both regional lung ventilation and perfusion. Note that a normal finding on V/Q scanning does not rule out pulmonary embolism. However, multiple V/Q scans with normal findings suggest that, if a pulmonary embolism is present, it is clinically unimportant. A problem that commonly arises is which test to perform first because confusion may stem from overlap of radioactive signals. Generally, if only one scan can be performed, the perfusion scan is thought to provide more useful information.
A sample of aggregated albumin labeled with radioactive technetium is administered intravenously and lodges in the pulmonary capillary bed. The patient is placed in a supine position to ensure optimal blood flow to the lung apices and, thus, reduce the risk of a false-positive result. The gamma rays emitted by the technetium are revealed by a gamma camera. Areas of decreased perfusion, which suggest a pulmonary embolism, are observed as areas of decreased radiation emission.
The ventilation scan is usually performed by having the patient inhale radiolabeled xenon (Xe) 133. Areas of decreased ventilation are revealed as areas of decreased radioactivity. The ventilation scan is compared with the perfusion scan. An area with normal ventilation but decreased perfusion is consistent with a diagnosis of pulmonary embolism. An area of diminished ventilation is consistent with a large number of diagnoses.
A V/Q scan is usually reported in terms of probability of pulmonary embolism (ie, high, intermediate, moderate, low). A high-probability scan is defined as having two or more areas with segmental defects on a perfusion scan with a normal finding on a ventilation scan. V/Q scan reporting is based on adult risk stratification (Prospective Investigation of Pulmonary Embolus Diagnosis [PIOPED] study). [25]
Forty percent of adults with a high clinical index of suspicion for pulmonary embolism and a low-probability V/Q scan are found to have a pulmonary embolism based on angiography findings. No similar data are available for children. Children generally have a more homogenous perfusion scan; thus, deficits in perfusion are more likely to represent real or significant pulmonary embolism compared with adults.
Magnetic resonance imaging (MRI)
Few data are available regarding the use of MRI in children suspected of having a pulmonary embolism. Its use should be considered investigational at this time. Studies in adults suggest that MR technology has limited sensitivity for distal pulmonary embolism and is not a viable alternative to CT. [26]
Deep venous thromboses (DVT) imaging studies
The possibility of DVT should be considered. Most adults with a pulmonary embolism have a coexisting DVT; whether this holds true for children is unknown. The presence of a DVT may provide indirect evidence of a pulmonary embolism when the V/Q scan is a low-probability scan. Like pulmonary embolism, the reliability of clinical diagnosis of DVT is suboptimal. In a pediatric series, 18% of patients with clinically suspected DVT actually had a DVT confirmed radiologically.
The three methods used for the diagnosis of DVT are as follows:
-
Doppler ultrasonography is based on the principle that ultrasonic waves reflected from blood traveling at different velocities vary in frequency. The change of frequency (ie, change in blood velocity) reflects the degree of obstruction.
-
Impedance plethysmography is based on the principle that the electrical impedance of a limb is related to the blood volume in that limb. A pneumatic cuff adjusts the pressure on the limb to be measured. An increase in blood volume within that limb signifies an obstruction (ie, DVT). No data are available regarding its use in children.
-
Venography is the criterion standard for diagnosing DVT. With the advent of noninvasive imaging, it has become less common in pediatric practice.
Echocardiography
Echocardiography (ECHO) provides useful information. It may allow diagnosis of other conditions that may be confused with pulmonary embolism, such as pericardial effusion.
ECHO allows visualization of the right ventricle and assessment of the pulmonary artery pressure.
This imaging modality serves a prognostic function; the mortality rate is almost 10% in the presence of right ventricular dysfunction and 0% in the absence of right ventricular dysfunction.
ECHO may be used to identify the presence of right-chamber emboli and upper extremity DVT
Other Tests
Electrocardiogram (ECG) findings may be normal. ECG is helpful to rule out other conditions in the differential diagnosis.
ECG changes found in pulmonary embolism include sinus tachycardia, T-wave inversion, S1 Q3 T3 pattern, right axis deviation, right bundle branch block, and P pulmonale (ie, a tall P wave representing an enlarged right atrium).
Procedures
Angiography is the criterion standard for diagnosing pulmonary embolism. It detects emboli as small as 1 mm. A positive test result is defined as an intraluminal filling defect that is visible in more than one radiographic view.
The usual indication for pulmonary angiography is an equivocal finding on a V/Q scan with a high index of suspicion for pulmonary embolism, especially in a patient at high risk for complications from therapy (ie, anticoagulation, thrombolysis); another indication are instances in which an embolectomy is considered.
Complications, apart from mortality, include hemorrhage, infection, arrhythmias, and great vessel or myocardial perforation. Patients at highest risk are those with right ventricular dysfunction or elevated pulmonary artery pressures. The mortality risk in adults is less than 0.5% in series from large centers. No similar data on children are available.
