Pulmonary embolism is a circulatory event featuring a blockade of the pulmonary arteries by a circulating embolus (i.e. blood clot) that was dislodged from a thrombus in a distant place, usually from deep veins of the lower limbs or pelvic veins. Thus, it is associated to a medical condition known as deep vein thrombosis, which is secondary to prolonged recumbency or coagulation diathesis resulting in increased intravascular thrombosis tendency.
Pulmonary embolism is primarily a disease of suspicion because the mainstay part of the clinical diagnosis depends on the interpretation of the patient’s complaints and the elaboration of a presumed scenario by the physician. Thus, doctors investigate complaints and analyze them promptly to find any clue that may point at the presence of pulmonary embolism or an underlying condition that may predispose to pulmonary embolism. Once pulmonary embolism is suspected in the differential diagnosis, it should be excluded for the sake of patient’s health and safety.
From this reason, we must conclude that diagnosis of pulmonary embolism is quite challenging, and many cases could be underdiagnosed. Therefore, the diagnosis depends on a step-wise approach. First, a clinical suspicion. Second, applying tools and scores that strengthen the clinical suspicion or reject it. Finally, performing a variety of tests to confirm the diagnosis and stratify the risk.
Clinical evaluation of pulmonary embolism was the corner of the diagnosis but with recent advancement of imaging tools, its role has been greatly decreased. Now, clinical evaluation only points out at suspicious patients that should be promptly analyzed and then confirmed by another tool. Though it is now less important, an advancement of the used tools, protocols and scores have greatly aided its predictive value to decrease false negative and false positive cases. Clinical diagnosis depends on symptoms and signs at the time of presentation.
Many signs and symptoms may suggest pulmonary embolism, but none of them is highly specific. The most important of them are pleuritic chest pain, tachypnoea, dyspnea, palpitation, hypoxia, cyanosis, hypotension, disturbed level of consciousness and shock state in severe life-threatening cases with massive pulmonary embolism (i.e. main pulmonary artery embolization). Less important signs are fever, syncope, abdominal pain, productive cough, wheezes, arrythmia states, such as atrial fibrillation or flutter, seizures and delirium, which may be present at time of admission or as accompanying symptoms that lead patients to ask for a medical consultation in the emergency room or clinic.
These symptoms and signs alone may be not valuable, but integrating them in risk scores and diagnostic tools gives them an extra benefit. Clinical scores role is now limited to rule out low risk patients and avoid doing unnecessary further investigations and imaging or invasive studies with potential side effects and unnecessary high costs. Many risk scores are now present, including the Wells score, modified Wells score, revised Geneva score, and simplified revised Geneva score. All of them show promising results of positive predictive values and accuracy.
Physical examination of the patient is very important, not only for diagnostic purposes before proceeding to next step in to confirm the disease but also for risk stratification after diagnosis. It is also important to exclude other conditions that may mimic pulmonary embolism before proceeding to other steps in the workup, including lung-related causes (severe pneumonia or atelectasis, pleurisy, pleural effusion, pneumothorax and acute respiratory failure), Cardiac causes (congestive heart failure, heart attack and aortic dissection), and non-cardiopulmonary causes (musculoskeletal pain and chest wall causes). On examination, the presenting condition may vary widely from catastrophic circulatory collapse and shock to slowly progressive dyspnea. Physical signs may be present, including tachypnoea, coughs and hemoptysis, rales, accentuated heart sounds, tachycardia, fever, diaphoresis, S3 and S4 gallop, Lower limb edema, cardiac murmurs, cyanosis, hypotension and shock.
The second step in diagnosis is Laboratory investigations as:
1. Arterial blood gases
They will characteristically reveal respiratory alkalosis, hypoxia and hypocapnia, denoting respiratory hypoperfusion. It also may be normal or reveal respiratory failure and acidosis due to other causes thus excluding pulmonary embolism.
It is the degradation product of fibrin networks, which is an important component in intravascular thrombosis. Its presence in high values suggests occurrence of venous thrombosis, and may indicates deep vein thrombosis and pulmonary embolism. Some cases may be false positive or false negative because it is a non-specific test. Predictive value of D-dimer and its daily role is important, though. It is done in all patients with suspected pulmonary thromboembolism, negative results in patient with low pre-test probability, i.e.by examination and history, is an indication for a low probability for venous thromboembolism and vice versa.
It is a cardiac marker used to diagnose cardiac myocardial infarction. Its role in pulmonary embolism is currently of low value, though it is done in almost all cases to exclude heart attacks. It may be high in cases of pulmonary embolism due to right ventricular overflow and stretch of myocardium due to obstruction of the pulmonary circulation, but in case of heart attack it will be significantly high.
4. Complete blood picture and CRP
They are done to exclude pneumonia and severe sepsis in cases with shock to rule out septic shock from obstructive shock associated with massive pulmonary embolism.
Diagnosis of pulmonary thromboembolism
The cornerstone of confirmation or exclusion in the diagnosis of pulmonary thromboembolism is imaging modalities specific for deep vein thrombosis and pulmonary embolism, which include:
I. Deep vein thrombosis
It is the method of choice to diagnose and visualize the extension of deep vein thrombosis. Now, it is rarely performed because of its invasive nature and the presence of accurate non-invasive modalities.
2. Duplex ultrasonography
It is currently the standard method for initial screening and diagnosis of deep vein thrombosis. By being reliable, non-invasive, of wide variability and easy interpretation, it gained its current position in the workup, and it is routinely ordered for all patients admitted to the ER with query deep vein thrombosis.
3. Magnetic resonance imaging
Rarely ordered for this purpose and with a limited value.
II. Imaging specific for pulmonary embolism:
1. CT pulmonary angiography
It is now considered to be the gold standard for diagnosis and risk stratification of pulmonary embolism, as it has a very high sensitivity and specificity. CT pulmonary angiography localizes the thrombus and its extension, and can be used for follow up and to exclude other mediastinal and parenchymal causes. It is also very helpful to confirm presence of lung infarction.
2. Ventilation – perfusion scanning (V/Q)
An important modality in the diagnosis of pulmonary embolism by showing segmental hypoperfusion. It could be used when CT pulmonary angiography is not available or contraindicated due to renal causes.
3. Magnetic resonance imaging
May be used if CT is not available or contra indicated due to renal causes. If gadolinium enhancement, i.e. contrast, is used, its specificity and sensitivity will be significantly higher.
It is of a limited role due to its low sensitivity and specificity, though it is done in almost all cases in order to exclude other cardiac causes of chest pain or dyspnea. Its sensitivity and specificity are about 59% and 77%.
Sometimes it can visualize a central pulmonary artery thrombus and this is a highly positive value.
From all the above, it is clearly obvious that the diagnosis of pulmonary embolism in a step-wise manner resembles a sort of puzzle. Doctors use the approach described above in most hospitals worldwide. It is not wise to start with higher imaging techniques to exclude the presence of the disease due to cost-effectiveness causes and to limit side effects of contrast, minimize waiting lists in emergency rooms and avoid of unnecessary admissions.
Prognosis of pulmonary embolism
In the medical field, a good prognosis is always linked to the appropriate diagnosis and treatment. It is wise to expect that baseline condition of the patient is a strong predictor of the prognosis, as healthy and young patients are associated with better prognosis while others who have a history of coronary artery disease or prior thromboembolism and other debilitating diseases as cancer will be associated with poorer outcomes.
Clearly, the prognosis depends on the extent and the size of the thrombus in addition to the clinical conditions at the time of presentation. The presence of massive main pulmonary artery pulmonary embolism, hemodynamically unstable and shock state are all associated with poor outcomes. Also, the presence of complications like post cardiac arrest, severe hypoxemia, cor-pulmonale, lung infarction, pleural effusions, severe pulmonary hypertension and medication related complications as heparin induced thrombocytopenia or cerebral hemorrhage. Older age and pregnancy are associated with poor prognosis. Late hospital presentation and admission will alter appropriate medication and lead to poor prognosis as well.