define pulmonary embolism and deep vein thrombosis
discuss the pathogenesis and pathophysiology of the disorder including the risk factors
describe the characteristic and rare clinical presentations of pulmonary embolism
discuss the diagnostic tests used to diagnose pulmonary embolism
explain the general approach to management, prevention, complications and treatment
Pulmonary embolism (PE) is defined as a circulatory disorder of the pulmonary arteries, characterized by embolic occlusion of one or more pulmonary arteries most commonly caused by venous thrombi in the legs.
PE is not a disease; rather, it is a complication of deep vein thrombosis (DVT). It is defined as a circulatory disorder of the pulmonary arteries, characterized by embolic occlusion of one or more pulmonary arteries by venous thrombi.
The clinical presentation is varied and ranges from the asymptomatic patient through chest pain and respiratory distress, to sudden collapse and sudden death.
The diagnosis is made by high clinical suspicion, blood tests, a Computed Tomography (CT) scan and treatment is with anticoagulation.
Pulmonary embolism (PE) is a common and potentially lethal complication of DVT. In the United States there are approximately 700,000 cases a year accounting for up to 200,000 deaths per year. It is a true diagnostic dilemma on many levels and is often missed because patients with PE present with nonspecific signs and symptoms. If left untreated, approximately one third of patients who survive an initial PE, subsequently die from a future embolic episode. Most patients succumb to PE within the first few hours of the event. In patients who survive, recurrent embolism and death can be prevented with prompt diagnosis and therapy.
PE is a leading cause of morbidity and mortality particularly among hospitalized patients, with as many as 90% of pulmonary emboli being thrombotic. Thrombotic PE arises mostly from thrombi in the deep venous system of the lower extremity and less commonly from pelvic veins, upper extremity veins, right heart chambers or in situ. The remaining 10% of emboli arise from more rare causes including: air embolism, fat embolism, tumor, or amniotic fluid (1).
Pathogenesis and Pathophysiology
The coagulation cascade is an intrinsic protective mechanism, which is responsible for limiting blood loss by precisely regulating interactions between components of vessel wall, platelets and plasma proteins. However, unregulated activation of this cascade can have potentially deleterious consequences in the form of venous and arterial thrombosis, which may have life-threatening complications.
Rudolf Virchow, from autopsy studies on 11 cases of fatal pulmonary embolism in 1856, outlined a triad, which even till date forms the basis of our understanding of venous thrombosis.
The triad is composed of the following factors which contribute to formation of venous thrombosis, an endothelial lesion, venous stasis and a hypercoagulable state. Risk factors for venous thrombosis are operative by influencing one or all of the above three variables – by either the de novo development or an accentuation of preexisting elements of Virchow’s triad. Thus, trauma can result in immobility (venous stasis), tissue injury and its sequelae (hypercoagulability), and endothelial injury. Patients who already have a risk factor (e.g. hypercoagulability) may be more at risk for developing venous thromboembolism (VTE) after an event (e.g. surgery or trauma) than those who are free from predisposing factors. The presence of several risk factors in a patient results in a synergistic increase in the risk for VTE.
Pathogenesis and Pathophysiology Continued
Acute thrombus in the veins is minimally adherent to the wall of the vessel and hence the propensity to embolize to the right heart and then to the lungs resulting in a variable degree of occlusion of the pulmonary circulation. In general, the larger the thrombus load the greater the clinical consequence.
Risk Factors for Venous Thromboembolism
Strong risk factors (odds ratio >10
Fracture (hip or leg)
Hip or knee replacement
Major general surgery
Spinal cord injury
Moderate risk factors (odds ratio 2-9)
Arthroscopic knee surgery
Central venous lines
Congestive heart or respiratory failure
Hormone replacement therapy
Oral contraceptive therapy
Previous venous thromboembolism
Weak risk factors (odds ratio <2)
Bed rest >3 days
Immobility due to sitting (e.g. prolonged car or air travel)
Laparoscopic surgery (e.g. cholecystectomy)
Pulmonary embolism (PE) occurs when propagating clots break loose and embolize to lodge in the bifurcation of the main pulmonary artery or the lobar branches.
Embolic obstruction of medium sized arteries may result in pulmonary hemorrhage but usually does not cause pulmonary infarction because of collateral bronchial artery flow. However, with left-sided cardiac failure and diminished bronchial circulation, infarcts can result.
Sudden death, right-sided heart failure or cardiovascular collapse occurs when 60% or more of the pulmonary circulation is obstructed with emboli.
The most specific finding related to acute PE at autopsy is a saddle thromboembolus at the main pulmonary artery or large coiled thromboemboli. The marks of the venous valves that certificated their embolic sources are sometimes imprinted on these thromboemboli.
White organized thrombi identified by the appearance of bands and webs serve as markers of latent past thromboemboli.
The clinical diagnosis of pulmonary embolism is difficult, particularly when there is coexisting heart or lung disease, and it is notoriously inaccurate when based on clinical signs alone. Very rarely, pulmonary embolism presents in such a dramatic fashion that the diagnosis is intuitively obvious and treatment will be started but the usual presentation is frequently vague and variable in severity, so that further testing is necessary to establish or exclude the diagnosis.
The first and most common presentation is dyspnea with or without pleuritic pain and hemoptysis (acute minor pulmonary embolism).
The second presentation is hemodynamic instability which is associated with acute massive pulmonary embolism.
The third and least common presentation mimics heart failure or indolent pneumonia, especially in the elderly (subacute massive pulmonary embolism).
Diagnostic evaluation is best carried out by first attempting to identify a provable alternative diagnosis that can explain the patient’s symptoms. Most patients with pulmonary embolism will have one or more of the following clinical features:
dyspnea of sudden onset
tachypnea (> 20 breaths/minute)
or chest pain (pleuritic or substernal)
If the clinician remembers these three features, the possibility of pulmonary embolism will rarely be over-looked. By adopting a thorough stratification system the clinician can more appropriately select further investigations to prove or exclude pulmonary embolism. The most widely used and clinically relevant scoring system used is the modified Wells Criteria. This scoring system includes the following:
|Clinical Parameter Score||Score|
|Heart rate > 100||+1.5|
|Immobilization of the lower extremities (paralysis or recent fracture with cast)||+1.5|
|Bedridden for >3 days or major surgery <4 weeks||+1|
|Tenderness in Calf||+1|
|Calf swelling >3 cm compared with other asymptomatic leg||+1|
|Asymmetric pitting edema (greater in the symptomatic leg)||+1|
|Previous DVT or PE||+1.5|
|Non varicose collateral superficial veins||+1|
|Alternative diagnosis (as likely or greater than that of DVT)||-2|
|Total of Above Score|
|Moderate probability||1 or 2|
Diagnosis: Acute Minor Pulmonary Embolism
If an embolus obstructs less than 50% of the pulmonary circulation, it often produces no symptoms. For example, about 40% of patients with DVT have no symptoms of pulmonary embolism but do have evidence of the condition on lung scans. If symptoms do develop the most common is dyspnea, possibly upon minor exertion. Sometimes, the first abnormality the patient notes results from pulmonary infarction which occurs in obstruction of medium-sized pulmonary artery branches. Sharp pleuritic pain develops and there may be associated hemoptysis. Pulmonary infarction occurs in only about 10% of patients without pre-existing cardiopulmonary disease. If, however, there is already compromise of the oxygenation of the embolized area, either because the airways are abnormal or pulmonary venous outflow is impaired as a result of pre-existing left heart disease, then the incidence of infarction rises to 30%. If there are any physical signs, they are those of pulmonary infarction. The patient is distressed with rapid and shallow breathing because of the pleuritic pain, but is not cyanosed because the disturbance of gas transfer is only slight. Signs of pulmonary infarction may be found in the lungs, a mixture of consolidation and effusion, possibly with a pleural rub. Fever is common and sometimes differentiation from infective pleurisy is difficult. The fever and pain often produce a sinus tachycardia. Pulmonary artery mean pressure rarely exceeds 25 mm Hg. As minor pulmonary embolism does not compromise the right ventricle, cardiac output is well maintained.
Diagnosis: Acute Massive Pulmonary Embolism
When more than 50% of the pulmonary circulation is suddenly obstructed, the pathophysiology and clinical signs become dominated by the severe derangement of cardiac and pulmonary function. Obstruction of the pulmonary artery and mediator induced vasoconstriction causes a substantial increase in right ventricular afterload and, if the cardiac output is to be maintained, consequent elevation of pulmonary artery systolic pressure and an increase in right ventricular work. If this work cannot be sustained, acute right heart failure occurs. The thin walled right ventricle is designed to work against the normally low pulmonary vascular resistance; it performs poorly against a sudden obstruction. As a result, it dilates, and a moderate rise in the right ventricular and pulmonary artery systolic pressure occurs, which rarely exceeds 55 mm Hg because the ventricle, lacking time to develop compensatory hypertrophy, is unable to generate a higher pressure. The right ventricular end diastolic pressure and right atrial pressure rise to about 15-20 mm Hg as the ventricle fails. Right ventricular dilatation leads to tricuspid regurgitation and may compromise the filling of the left ventricle. Cardiac output falls and the patient becomes hypotensive. This may occur so rapidly that syncope is either the presenting feature or easily induced by a relatively minor cardiovascular stress. If the degree of obstruction is sufficient, death occurs almost immediately. The fall in aortic pressure and the rise in right ventricular pressure may cause ischemia of the right ventricle through a critical reduction of right coronary perfusion. Electro-mechanical dissociation is the most frequent cause of final cardiac arrest.
Arterial hypoxemia correlates roughly with the extent of embolism if there is no prior cardiopulmonary disease. Massive pulmonary embolism without hypoxemia is so rare that if the arterial oxygen tension (PaO2) is normal an alternative diagnosis should be considered. The main causes of hypoxemia in pulmonary embolism seem to be as follows:
(1) Ventilation-perfusion mismatch. Non-embolized areas of the lung are relatively overperfused, so that the ventilation in these areas may be insufficient to oxygenate fully the extra blood flow.
(2) Shunting occurs through areas of collapse and infarction that are not ventilated but retain some blood flow. This may become more important a few days after the initial episode. In patients with a patent foramen ovale, raised right atrial pressure may open the foramen and cause right-to-left shunting at the atrial level. This should be considered if the degree of hypoxemia is more profound than would be expected from other clinical features, if it cannot be corrected by oxygen administration and if it is accompanied by hypercapnia.
(3) Low mixed venous oxygen saturation caused by the reduced cardiac output causes hypoxemia because there is insufficient time for the extremely desaturated blood to become fully saturated as it passes through the alveolar capillaries in the over-perfused areas of the lung.
Although pulmonary embolism impairs the elimination of CO2, hypercapnia is rare because compensatory hyper ventilation eliminates CO2 in all but the most extensive embolism. In cases with a sufficient degree of vascular obstruction to produce hypercapnia, the hemodynamic sequelae of acute right ventricular failure usually prove fatal.
The clinical features of acute massive pulmonary embolism can be explained in terms of these pathophysiological changes. The patient becomes acutely distressed, severely short of breath, and may be syncopal because of the combination of hypoxemia and low cardiac output. The combination of hypotension, hypoxemia, and increased cardiac work may cause anginal chest pain. The physical signs are those of reduced cardiac output, that is, pronounced sinus tachycardia, hypotension, and a cool periphery, sometimes with confusion. The patient is obviously dyspneic (but not orthopneic), cyanosed both centrally and peripherally, and has signs of acute right heart strain: a raised venous pressure, which is often difficult to appreciate because of the respiratory distress, a gallop rhythm at lower sternum and a widely split second heart sound due to delayed right ventricular ejection, which is difficult to detect because of the accompanying tachycardia. The pulmonary component of the second heart sound is usually not loud because the pulmonary artery pressure is only moderately raised.
Diagnosis: Subacute Massive Pulmonary Embolism
Subacute massive pulmonary embolism is caused by multiple small or moderately sized emboli that accumulate over several weeks. Because the obstruction occurs slowly, there is time for the right ventricle to adapt and for some hypertrophy to develop; consequently, the right ventricular systolic pressure is higher than in acute pulmonary embolism. The rises in the right ventricular end diastolic and right atrial pressures are of a lesser extent than in acute massive pulmonary embolism since there is time for adaptation to occur and the degree of right ventricular failure is less for a given degree of pulmonary artery obstruction. The main symptoms are increasing dyspnea and falling exercise tolerance. There is often an associated dry cough. The breathlessness is usually out of proportion to all other findings and there may be central cyanosis. The blood pressure and pulse rate are usually normal because the cardiac output is well maintained. Commonly, the venous pressure is raised and a third heart sound is audible at the lower sternum which may be accentuated by inspiration. The pulmonary component of the second heart sound is sometimes loud. There may also be intermittent symptoms and signs of pulmonary infarction that occurred during the build up of the obstruction. In advanced cases, cardiac output falls and frank right heart failure develops. A further pulmonary embolism may change the picture to that resembling acute massive pulmonary embolism.
Diagnosis: CT Pulmonary Angiography
The imaging study of choice for pulmonary embolism is CT pulmonary angiography (CTPA). Conventional pulmonary angiography was known as the gold standard, but has been surpassed by CTPA and now is only used for selected cases. CT pulmonary angiography is the most rapid, sensitive and accurate way to diagnose pulmonary emboli. Criteria for a positive CT scan result include a partial filling defect (defined as intraluminal area of low attenuation surrounded by a contrast medium), a complete filling defect, and the “railway track sign” (masses seen floating in the lumen, allowing the flow of blood between the vessel wall and the embolus). The procedure has over 90% specificity and sensitivity in diagnosing pulmonary embolism in the main, lobar, and segmental pulmonary arteries. The diagnostic accuracy can reach as high as 96% with a high clinical likelihood (5). However, this percentage can vary on the quality of images, experience of the radiologist and the patient’s pulmonary anatomy. One major advantage of this imaging is its capability to identify other causes that may account for the patient’s presenting symptoms. Cautions with this modality include patients at risk for adverse effect for IV contrast, such as patients with chronic kidney disease or acute renal failure as indicated by an increased creatine level. Some patients are allergic to IV contrast. Pregnant patients in the first 14 weeks of pregnancy should not be subjected to the radiation.
The CT technology is rapid in the acquisition of the images and also allows the images to be manipulated and reconstructed in any plane providing considerable advantages over nuclear medicine and conventional angiography.
Timing of the bolus is absolutely essential for diagnostic accuracy. Timing infers that the peak concentration of the contrast should be present in the pulmonary arteries when the images are taken. Each patient has different hemodynamics and so a variety of electronic density sensors to enable accuracy have been developed to aid in the bolus timing.
Diagnosis: Chest Radiography
Chest radiographic findings are also non-specific but may be helpful. A normal examination is compatible with all types of acute pulmonary embolism; in fact, a normal film in a patient with severe acute dyspnea without wheezing is very suspicious of pulmonary embolism. The lung fields may show evidence of pulmonary infarction, peripheral opacities, sometimes wedge-shaped or semicircular, arranged along the pleural surface (so called Hampton’s hump). It may be possible to detect areas of oligemia in the parts of the lung affected by emboli (Westermark sign). Similar to the ECG the radiograph is especially valuable in excluding other conditions mimicking pulmonary embolism.
Other Diagnostic Tests
ECG changes are usually non-specific. In minor pulmonary embolism there is no hemodynamic stress and thus the only finding is sinus tachycardia. In acute or subacute massive pulmonary embolism, evidence of right heart strain may be seen (rightward shift of the QRS axis, transient right bundle branch block, T-wave inversion in leads V1-3, P pulmonale), but the classic SIQIIITIII pattern occurs in only a few cases.(2) The main value of ECG is in excluding other potential diagnoses, such as myocardial infarction or pericarditis.
Arterial Blood Gas
The main utility is looking for hypoxia and an elevated alveolar-arterial gradient. The PaO2 is almost never normal in the patient with massive pulmonary embolism, but can be normal in minor pulmonary embolism, mainly due to hyperventilation. In such cases the widening of the alveolo-arterial PO2 gradient (AaPO2 > 20 mm Hg) may be more sensitive than PaO2 alone.
Transthoracic echocardiography rarely enables direct visualization of the pulmonary embolus but may reveal thrombus floating in the right atrium or ventricle.. In massive pulmonary embolism the right ventricle is dilated and hypokinetic, with abnormal motion of the interventricular septum. The inferior vena cava does not collapse during inspiration. There is some evidence that regional right ventricular dysfunction (akinesia of the mid-free wall with apical sparing) may be more common in acute pulmonary embolism. The Doppler technique allows the pulmonary artery pressure to be estimated and together with contrast echocardiography it is useful in diagnosing patent foramen ovale which may indicate impending paradoxical embolism.
A D-Dimer is fibrin degradation product found in the blood. In pulmonary emboli or any thrombosis, this level is raised. The D-Dimer is sensitive but not specific, which makes it a test with good negative predictive value. Patients that have a normal D-Dimer level have a 95% likelihood that they do not have an acute pulmonary embolism. This percentage increases to 99% if there is a low clinical probability for embolus, such as, when the Wells score is <2 . In elderly or inpatients, D-dimer retains a high negative predictive value, but is normal in less than 10% of patients and hence, not very useful.
The V/Q scan is a nuclear medicine modality where Technetium labeled albumin is used to identify ventilation-perfusion (V/Q) mismatches in the lung. A normal perfusion scan essentially excludes the diagnosis of a clinically relevant recent pulmonary embolism because occlusive pulmonary embolism of all types produces a defect of perfusion. Normal results are almost never associated with recurrent pulmonary embolism, even if anticoagulants are withheld. However, many conditions other than pulmonary embolism, such as tumors, consolidation, left heart failure, bullous lesions, lung fibrosis, and obstructive airways disease, can also produce perfusion defects. Addition of a ventilation scan increases the specificity of scintigraphy. Pulmonary embolism usually produces a defect of perfusion but not ventilation (“mismatch”) while most of the other conditions produce a ventilation defect in the same area as the perfusion defect (matched defects). Pulmonary embolism can also produce matched defects when infarction has occurred, but in this situation the chest radiograph nearly always shows an abnormality in the area of scan defect.
These mismatches when read by an experienced radiologist can be stratified into low, intermediate, and high probability for pulmonary embolism. The landmark Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study, investigated the accuracy of V/Q scans as compared to the gold standard for diagnosis, pulmonary angiogram. The study found that patients with a high clinical suspicion for embolus and a high probability V/Q scan had a 95% chance of having an acute pulmonary embolism.
Scanning should be performed within 24 hours of the onset of symptoms of pulmonary embolism, since some scans revert to normal quickly.
Although the lung scan is often an imprecise guide it is useful in clinical decision making; a normal scan or a low probability scan with low clinical likelihood of pulmonary embolism means that treatment for suspected pulmonary embolism can be withheld and a high probability scan with a high clinical likelihood of pulmonary embolism means that treatment is mandatory.
The potential complications of a pulmonary embolism are related to the size and location of the embolus. In the case of a massive acute embolism, sudden cardiac death is possible. Acute cor pulmonale (acute right ventricular failure) is also a result of a massive pulmonary embolism, usually obstructing greater than 60-75% of the pulmonary arterial circulation or the embolus is a “saddle embolus,” at the bifurcation of the pulmonary artery. Cor pulmonale can lead to hypotension and cardiac arrest if not treated promptly. Pulmonary infarction may be a complication in patients with a sub massive embolism that completely obstructs a distal arterial branch. Hemoptysis will be a clue to this process. If the acute pulmonary embolism is non-fatal and goes undiagnosed, an entity called chronic thromboembolic pulmonary hypertension may occur. This entity may present years after diagnosis, presenting with signs of pulmonary hypertension.
The aim of treatment for PE is to relieve symptoms, prevent death, reduce the risk of developing chronic pulmonary hypertension, and prevent recurrence. Treatment is often based on the clinical probability of pulmonary embolism rather than on a definite diagnosis or a ruling out of the condition. Consequently, some patients receive anticoagulants without proof of pulmonary embolism and other patients are not treated although they may have it. Anticoagulation is the mainstay of treatment for PE. The role of pharmacological or mechanical embolectomy is restricted to patients who are hemodynamically unstable at presentation.
When anticoagulation is begun, the choices usually include low molecular weight heparin or unfractionated heparin. Early heparin anticoagulation is so essential that heparin should be started as soon as the diagnosis of significant pulmonary thromboembolism is considered. Heparin reduces the mortality rate of PE because it slows or prevents clot progression and reduces the risk of further embolism by augmenting the activity of antithrombin III and preventing the conversion of fibrinogen to fibrin. Heparin does nothing to dissolve the clot that has developed already but it is still the single most important treatment that can be provided because the greatest contribution to the mortality rate is the ongoing embolization of new thrombi. Prompt effective anticoagulation has been shown to reduce the overall mortality rate from 30% to less than 10%.
With proper dosing, several [low molecular weight heparin (LMWH)] products have been found safer and more effective than unfractionated heparin both for prophylaxis and for treatment of DVT and PE. Monitoring the aPTT is neither necessary nor useful when giving LMWH, because the drug is most active in a tissue phase and does not exert most of its effects on coagulation factor IIa. Enoxaparin and tinzaparin are currently approved by the FDA for treatment of DVT. Dalteparin is FDA approved for prophylaxis and has approval for cancer patients.
Fibrinolytic therapy should be considered for three groups of patients, unless overwhelming contraindications are evident:
those who are hemodynamically unstable
those with right heart strain and exhausted cardiopulmonary reserves
those who are expected to have multiple recurrences of pulmonary thromboembolism over a period of years.
Patients with a prior history of PE and those with known deficiencies of protein C, protein S, or antithrombin III should be included in this latter group.
Fibrinolytic regimens currently in common use for PE include two forms of recombinant tissue plasminogen activator, t-PA (alteplase) and r-PA (reteplase), along with urokinase and streptokinase. Alteplase usually is given as a front-loaded infusion over 90 or 120 minutes. Urokinase and streptokinase usually are given as infusions over 24 hours or more. Reteplase is a new-generation thrombolytic with a longer half-life that is given as a single bolus or as 2 boluses administered 30 minutes apart. Of the 4, the faster-acting agents, reteplase and alteplase are preferred for patients with PE, because the condition of patients with PE can deteriorate extremely rapidly.
Prevention of Recurrence
Long-term anticoagulation is essential for patients who survive an initial DVT or PE. The optimum total duration of anticoagulation has been controversial in recent years, but general consensus holds that at least 6 months of anticoagulation is associated with significant reduction in recurrences and a net positive benefit. Warfarin is dosed to achieve an INR between 2 and 3. When the pulmonary embolus is the patient’s first, and the patient has an identifiable cause, the warfarin therapy is continued for 3-6 months. If the patient has recurrent emboli, therapy is extended from 6-12 months to possibly life-long depending on the cause.
Now we will look at a few interesting and unusual cases of pulmonary emboli.
Embolization of Venous Thrombosis of the Upper Limbs
Upper limb venous thrombosis is common because the upper limbs and superior vena cava are used so frequently for venous access. Symptomatic embolization from the upper limbs is quite uncommon.
Pulmonary Emboli and Systemic Emboli in the Same Patient
Septic emboli are an insidious infection of the lung, originating from the venous system, or right side of the heart. Causes include vegetations of the tricuspid valve, (intravenous drug abusers), infected right-sided catheters, pelvic thrombophlebitis, peripheral venous thrombophlebitis (heroin drug abusers), cellulitis and skin carbuncles. The organisms are commonly staphylococcus aureus and streptococcus.
Presenting symptoms include sepsis, shaking chills, cough, dyspnea, hemoptysis, and chest pain.
Diagnosis is based on blood cultures and imaging studies that include echocardiography, chest X-ray, and CT of the chest. Cavitating multicentric nodules with a predilection for the lung bases are characteristic. Subsegmental wedge-shaped infarcts are noted as well, and sometimes a feeding vessel is seen subtending the nodules. Treatment is with antibiotics.
Fat embolism syndrome (FES) has been described as occurring after traumatic, surgical, and atraumatic conditions. It most commonly occurs after lower extremity trauma and intramedullary surgery and can result in a multi-system manifestation of embolization when fat droplets act as emboli, becoming impacted in different organ microvasculature and microvascular beds, causing damage to the small vessels. Although FES usually presents as a multisystem disorder, the most seriously affected organs are the lung, brain, cardiovascular system, and skin.
Up to 75% of patients with FES present with some degree of respiratory failure, ranging from nearly asymptomatic hypoxemia to pulmonary distress requiring ventilatory support. An asymptomatic latent period of about 12-48h precedes the clinical manifestations. Patients become tachypneic, dyspneic, and hypoxic as a result of ventilation-perfusion abnormalities 12-72h after trauma. The fulminant form of FES presents as acute cor pulmonale with respiratory failure, resulting in death within a few hours of injury. Fat embolism syndrome is a self-limiting disease and treatment should be mainly supportive.
Venous air embolism (VAE), the entry of gas into the peripheral or central vasculature, can occur secondary to iatrogenic complications, trauma, and even certain recreational activities.
Gas emboli are usually composed of air but they can also occur with medically used gases such as carbon dioxide, nitrous oxide, and nitrogen. Although very small volumes of air can lead to severe sequelae, it is generally accepted that more than 50 mL of air can cause hypotension and dysrhythmias and more than 300 mL of air can be lethal.
Transesophageal echocardiography and CT scan are the most sensitive monitoring modalities for VAE and can detect as little as 0.02 mL/kg of air. Precordial Doppler ultrasonography is also an effective monitoring technique and can detect as little as 0.25 mL of air.
Treatment should begin with:
1. the administration of 100% oxygen and intubation for significant respiratory distress or refractory hypoxemia. Oxygen may reduce bubble size by increasing the gradient for nitrogen to move out.
2. prompt placement of the patient in Trendelenburg (head down) position and rotating toward the left lateral decubitus position. This maneuver helps trap air in the apex of the ventricle, prevents its ejection into the pulmonary arterial system, and maintains right ventricular output.
3. maintaining systemic arterial pressure with fluid resuscitation and vasopressors/beta-adrenergic agents if necessary.
4. hyperbaric chambers may be considered.
Amniotic Fluid Embolism
Amniotic fluid embolism (AFE) is a rare obstetric emergency in which it is postulated that amniotic fluid, fetal cells, hair, or other debris enter the maternal circulation, causing cardiorespiratory collapse. The diagnosis of AFE has traditionally been made at autopsy when fetal squamous cells are found in the maternal pulmonary circulation; however, fetal squamous cells are commonly found in the circulation of laboring patients who do not develop the syndrome. In a patient who is critically ill, a sample obtained by aspiration of the distal port of a pulmonary artery catheter that contains fetal squamous cells is considered suggestive of but not diagnostic of AFE syndrome. The diagnosis is essentially one of exclusion based on clinical presentation. Other causes of hemodynamic instability should not be neglected. Treatment is supportive.
Certain tumors have a predilection to invade the veins, either through normal venules or the venules that are part of the neovascularity of the tumor. Once they are in the veins they are taken to the pulmonary circulation where they become trapped. If they do not proliferate and spread locally they are considered as emboli rather than metastases. Renal cell carcinoma, hepatocellular carcinoma, adrenal carcinoma, and sarcomas are known to embolize to the lungs.
Other Emboli to the Lungs
As noted earlier, the diagnosis of a pulmonary embolism may be elusive at times. However, there are certain red flags that may hint to a diagnosis. First, an unexplained syncopal event may, in fact, be a result of a massive pulmonary embolism. Next, any patient with unexplained sudden onset, shortness of breath and rapid breathing should be considered for pulmonary embolism. Also, patients with any inherited genetic thrombophilia, such as, factor V Leiden, prothrombin gene mutation G20210A, and proteins C and S deficiency should be considered at higher risk for embolism if presenting with a consistent clinical picture.
Thrombotic pulmonary embolic disease is a dangerous and often silent killer. The clinical presentation ranges from a patient with no symptoms, to a patient who presents with acute cardiorespiratory arrest. High clinical suspicion must be maintained on all patients with unexplained cardiorespiratory symptoms, but especially those patients with a predisposition, such as the patient on oral contraception, oncology patients, and immobilized patients, for example. Clinical algorithms such as the modified Wells criteria have been very helpful and CT pulmonary angiography has become the mainstay of imaging.