PULMONARY EMBOLISM

A pulmonary embolism (PE) is a blood clot that develops in a blood vessel elsewhere in the body (most commonly from the leg), travels to an artery in the lung, and forms an occlusion (blockage) of the artery.A blood clot (thrombus) that forms in a blood vessel in one area of the body, breaks off, and travels to another area of the body through the bloodstream is called an embolus. An embolus can lodge itself in a blood vessel, blocking the blood supply to a particular organ. This blockage of a blood vessel by an embolus is called an embolism.An embolism to the lung may cause serious life-threatening consequences and, potentially, death. Most commonly, a PE is the result of a condition called deep vein thrombosis (blood clot in the deep veins of the leg).

Pathophysiology

Natural history of venous thrombosisIn the 19th century, Virchow identified a triad of factors that lead to venous thrombosis: venous stasis, injury to the intima, and enhanced coagulation properties of the blood. Thrombosis usually originates as a platelet nidus on valves in the veins of the lower extremities. Further growth occurs by accretion of platelets and fibrin and progression to red fibrin thrombus, which may either break off and embolize or result in total occlusion of the vein. The endogenous thrombolytic system leads to partial dissolution; then, the thrombus becomes organized and is incorporated into the venous wall.Natural history of pulmonary embolismPulmonary emboli usually arise from the thrombi originating in the deep venous system of the lower extremities; however, rarely they may originate in the pelvic, renal, or upper extremity veins or the right heart chambers. After traveling to the lung, large thrombi can lodge at the bifurcation of the main pulmonary artery or the lobar branches and cause hemodynamic compromise. Smaller thrombi typically travel more distally, occluding smaller vessels in the lung periphery. These are more likely to produce pleuritic chest pain by initiating an inflammatory response adjacent to the parietal pleura. Most pulmonary emboli are multiple, and the lower lobes are involved more commonly than the upper lobes.

Respiratory consequencesAcute respiratory consequences of pulmonary embolism include increased alveolar dead space, pneumoconstriction, hypoxemia, and hyperventilation. Later, 2 additional consequences may occur: regional loss of surfactant and pulmonary infarction (see the image below). Arterial hypoxemia is a frequent but not universal finding in patients with acute embolism. The mechanisms of hypoxemia include ventilation-perfusion mismatch, intrapulmonary shunts, reduced cardiac output, and intracardiac shunt via a patent foramen ovale. Pulmonary infarction is an uncommon consequence because of the bronchial arterial collateral circulation.

Hemodynamic consequencesPulmonary embolism reduces the cross-sectional area of the pulmonary vascular bed, resulting in an increment in pulmonary vascular resistance, which, in turn, increases the right ventricular afterload. If the afterload is increased severely, right ventricular failure may ensue. In addition, the humoral and reflex mechanisms contribute to the pulmonary arterial constriction. Prior poor cardiopulmonary status of the patient is an important factor leading to hemodynamic collapse. Following the initiation of anticoagulant therapy, the resolution of emboli occurs rapidly during the first 2 weeks of therapy. Significant long-term nonresolution of emboli causing pulmonary hypertension or cardiopulmonary symptoms is uncommon.

Clinical

The presentation of patients with pulmonary embolism can be categorized into 4 classes based on the acuity and severity of pulmonary arterial occlusion. These categories are (1) massive pulmonary embolism, (2) acute pulmonary infarction, (3) acute embolism without infarction, and (4) multiple pulmonary emboli.

Massive pulmonary embolismo Large emboli compromise sufficient pulmonary circulation to produce circulatory collapse and shock.o The patient has hypotension; appears weak, pale, sweaty, and oliguric; and develops impaired mentation. Acute pulmonary infarctiono Approximately 10% of patients have peripheral occlusion of a pulmonary artery causing parenchymal infarction.o These patients present with acute onset of pleuritic chest pain, breathlessness, and hemoptysis.o Although the chest pain may be clinically indistinguishable from ischemic myocardial pain, normal electrocardiogram findings and no response to nitroglycerin rules it out. Acute embolism without infarction: Patients have nonspecific symptoms of unexplained dyspnea and/or substernal discomfort. Multiple pulmonary embolio This group comprises 2 subsets of patients.o The first subset has repeated documented episodes of pulmonary emboli over years, eventually presenting with signs and symptoms of pulmonary hypertension and cor pulmonale.o The second subset has no previously documented pulmonary emboli but has widespread obstruction of the pulmonary circulation with clot. They present with gradually progressive dyspnea, intermittent exertional chest pain, and, eventually, features of pulmonary hypertension and cor pulmonale.

Most patients with pulmonary embolism have no obvious symptoms at presentation. In contrast, patients with symptomatic DVT commonly have pulmonary embolism confirmed on diagnostic studies in the absence of pulmonary symptoms.

The most common symptoms of pulmonary embolism in the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study were dyspnea (73%), pleuritic chest pain (66%), cough (37%), and hemoptysis (13%).However, patients with pulmonary embolism may present with atypical symptoms. In such cases, strong suspicion of pulmonary embolism based on the presence of risk factors can lead to consideration of pulmonary embolism in the differential diagnosis. These symptoms include the following:

Seizures Syncope Abdominal pain Fever Productive cough Wheezing Decreasing level of consciousness New onset of atrial fibrillation Flank pain Delirium (in elderly patients)

Pleuritic chest pain without other symptoms or risk factors may be a presentation of pulmonary embolism.

Physical

Physical examination findings are quite variable in pulmonary embolism and, for convenience, may be grouped into 4 categories as follows:

Massive pulmonary embolismThese patients are in shock. They have systemic hypotension, poor perfusion of the extremities,

tachycardia, and tachypnea.Additionally, signs of pulmonary hypertension such as palpable impulse over the second left intercostal

space, loud P2, right ventricular S3 gallop, and a systolic murmur louder on inspiration at left sternal border (tricuspid regurgitation) may be present.

Acute pulmonary infarctionThese patients have decreased excursion of the involved hemithorax, palpable or audible pleural friction

rub, and even localized tenderness.Signs of pleural effusion, such as dullness to percussion and diminished breath sounds, may be present.

Acute embolism without infarctionThese patients have nonspecific physical signs that may easily be secondary to another disease process.Tachypnea and tachycardia frequently are detected, pleuritic pain sometimes may be present, crackles

may be heard in the area of embolization, and local wheeze may be heard rarely. Multiple pulmonary emboli or thrombi

Patients belonging to both the subsets in this category have physical signs of pulmonary hypertension and cor pulmonale.

Patients may have elevated jugular venous pressure, right ventricular heave, palpable impulse in the left second intercostal space, right ventricular S3 gallop, systolic murmur over the left sternal border that is louder during inspiration, hepatomegaly, ascites, and dependent pitting edema.

These findings are not specific for pulmonary embolism and require a high index of suspicion for pursuing appropriate diagnostic studies.

Causes

The causes for pulmonary embolism are multifactorial and are not readily apparent in many cases. The following causes have been described in the literature:

Venous stasiso Venous stasis leads to accumulation of platelets and thrombin in veins.o Increased viscosity may occur due to polycythemia and dehydration, immobility, raised venous pressure in cardiac failure, or compression of a vein by a tumor. Hypercoagulable stateso The complex and delicate balance between coagulation and anticoagulation is altered by many diseases, by obesity, after surgery, or by trauma.o Concomitant hypercoagulability may be present in disease states where prolonged venous stasis or injury to veins occurs.o Hypercoagulable states may be acquired or congenital. Factor V Leiden mutation causing resistance to activated protein C is the most common risk factor. Factor V Leiden mutation is present in up to 5% of the normal population and is the most common cause of familial thromboembolism.o Primary or acquired deficiencies in protein C, protein S, and antithrombin III are other risk factors. The deficiency of these natural anticoagulants is responsible for 10% of venous thrombosis in younger people Immobilization

o Immobilization leads to local venous stasis by accumulation of clotting factors and fibrin, resulting in thrombus formation.o The risk of pulmonary embolism increases with prolonged bed rest or immobilization of a limb in a cast.o Paralysis increases the risk. Surgery and traumao Both surgical and accidental trauma predispose patients to venous thromboembolism by activating clotting factors and causing immobility.o Fractures of the femur and tibia are associated with the highest risk, followed by pelvic, spinal, and other fractures.o Severe burns carry a high risk of DVT or pulmonary embolism.o A prospective study by Geerts and colleagues in 1994 indicated that major trauma was associated with a 58% incidence of DVT in the lower extremities and an 18% incidence in proximal veins.o Pulmonary embolism may account for 15% of all postoperative deaths. Leg amputations and hip, pelvic, and spinal surgery are associated with the highest risk. Pregnancyo The incidence of thromboembolic disease in pregnancy has been reported to range from 1 case in 200 deliveries to 1 case in 1400 deliveries.o Fatal events may occur rarely, 1-2 cases per 100,000 pregnancies.o The mechanism of DVT is venous stasis, decreasing fibrinolytic activity, and increased procoagulant factors. Oral contraceptives and estrogen replacemento Estrogen-containing birth control pills have increased the occurrence of venous thromboembolism in healthy women.o The risk is proportional to the estrogen content and is increased in postmenopausal women on hormonal replacement therapy.o The relative risk is 3-fold, but the absolute risk is 20-30 cases per 100,000 persons per year. Malignancyo Malignancy has been identified in 17% of patients with venous thromboembolism.o The neoplasms most commonly associated with pulmonary embolism, in descending order of frequency, are pancreatic carcinoma; bronchogenic carcinoma; and carcinomas of the genitourinary tract, colon, stomach, and breast.

Diagnosis

Clinical Characteristic Score

Previous pulmonary embolism or deep vein thrombosis + 1.5

Heart rate greater than 100 beats per minute + 1.5

Recent surgery or immobilization (within the last 30 d) + 1.5

Clinical signs of deep vein thrombosis + 3

Alternative diagnosis less likely than pulmonary embolism + 3

Hemoptysis + 1

Cancer (treated within the last 6 mo) + 1

Table 1. Wells Prediction Rule for Diagnosing Pulmonary Embolism: Clinical Evaluation Table for Predicting Pretest Probability of Pulmonary Embolism

Risk Factors Points

Age older than 65 y 1

Previous DVT or PE 3

Surgery (under general anesthesia) or fracture (of the lower limbs) within 1 mo

2

Active malignant condition (solid or hematologic, currently active or considered cured less than 1 y)

2

Symptoms

Unilateral lower limb pain 3

Hemoptysis 2

Clinical Signs

Heart rate 75–94 beats/min 3

Heart rate ≥95 beats/min 5

Pain on lower limb deep venous palpation and unilateral edema

4

Clinical Probability

Low 0–3 total

Intermediate 4–10

total

High ≥11 total

Table 2 The Revised Geneva Score*

Imaging Studies

Chest radiography

Initially, the chest radiography findings are normal in most cases of pulmonary embolism. However, in later stages, radiographic signs may include a Westermark sign (dilatation of pulmonary vessels and a sharp cutoff), atelectasis, a small pleural effusion, and an elevated diaphragm. Although chest radiography findings may indicate an alternate diagnosis, this study alone is not sufficient to confirm the diagnosis of pulmonary embolism.

A chest radiograph with normal findings in a 64-year-old woman who presented with worsening breathlessness.

A posteroanterior chest radiograph showing a peripheral wedge-shaped infiltrate caused by pulmonary infarction secondary to pulmonary embolism.

Computed tomography

A spiral CT scan shows thrombus in bilateral main pulmonary arteries. A spiral

CT scan shows thrombus in bilateral main pulmonary arteries.

Ventilation-perfusion (V/Q) scanning of the lungs: This is an important diagnostic modality for establishing the diagnosis of pulmonary embolism. However, V/Q scanning should be used only when CT scanning is not available or if the patient has a contraindication to CT scanning or intravenous contrast material.

High-probability perfusion lung scan shows segmental perfusion defects in the right upper lobe and

subsegmental perfusion defects in right lower lobe, left upper lobe, and left lower lobe.

A normal ventilation scan will make the above-noted defects in Image 5 a mismatch and, hence, a high-probability ventilation-perfusion scan.

Anterior views of perfusion and ventilation scans are shown here. A perfusion defect is present in the left lower lobe, but perfusion to this lobe is intact, making this a high-probability scan.

Color-flow Doppler imaging and compression ultrasonography have a high sensitivity (89-100%) and specificity (89-100%) for detection of proximal DVT in symptomatic patients. However, compression ultrasonography has a low sensitivity (38%) and a low positive predictive value (26%) in patients without symptoms of DVT. Patients with positive findings for DVT can be anticoagulated irrespective of their V/Q scan results; other patients must have more invasive investigations performed to definitively rule out pulmonary embolism.

Deep vein thrombosis

Magnetic resonance imaging

Electrocardiography

The most common ECG abnormalities of pulmonary embolism are tachycardia and nonspecific ST-T wave abnormalities. These findings are not sensitive or specific enough to aid in the diagnosis of pulmonary embolism.

Surgical Care

The current grade 1A recommendation is that patients with acute pulmonary embolism should not routinely receive vena cava filters in addition to anticoagulants.

Inferior vena cava (IVC) interruption by the insertion of an IVC filter (Greenfield filter) is only indicated in the following settings:

Patients with acute venous thromboembolism who have an absolute contraindication to anticoagulant therapy (eg, recent surgery, hemorrhagic stroke, significant active or recent bleeding) Patients with massive pulmonary embolism who survived but in whom recurrent embolism invariably will be fatal Patients who have objectively documented recurrent venous thromboembolism, adequate anticoagulant therapy notwithstanding

An ideal IVC filter should have the following characteristics:

Easy and safe placement by percutaneous technique Biocompatible and mechanically stable Ability to trap emboli without causing occlusion of the vena cava

In thoracic surgery, a pulmonary thrombectomy, is an emergency procedure that

removes clotted blood (thrombus) from the pulmonary arteries.

Mechanical thrombectomies can be surgical (surgical thrombectomy) or percutaneous (percutaneous thrombectomy).

Medication

Thrombolytics

Thrombolysis is indicated for hemodynamically unstable patients with pulmonary embolism. Thrombolysis dramatically improves acute cor pulmonale. Thrombolytic therapy has replaced surgical embolectomy as the treatment for hemodynamically unstable patients with massive pulmonary embolism.

Thrombolytic regimens currently in use for pulmonary embolism include 2 forms of recombinant tissue-plasminogen activators, alteplase (t-PA) and reteplase (r-PA), along with urokinase and streptokinase. The comparative clinical trials have shown that administration of a 1-h infusion of alteplase is more rapidly effective than urokinase or streptokinase over a 12-h period. The safety and efficacy of different thrombolytic

agents is comparable. Streptokinase may cause anaphylaxis, hypotension, and other adverse reactions, leading to the cessation of therapy in many cases.

Anticoagulants

Heparin augments activity of the natural anticoagulant antithrombin III and prevents conversion of fibrinogen to fibrin. Full-dose LMWH or unfractionated IV heparin should be initiated at the first suspicion of DVT or pulmonary embolism. Heparin does not dissolve an existing clot, but it does prevent clot propagation and embolization. Recurrence or extension of DVT and pulmonary embolism may occur despite therapeutic anticoagulation with heparin.

With proper dosing, several LMWH products have been found to be safer and more effective than UFH for prophylaxis and treatment of patients with DVT and pulmonary embolism. Not necessary or useful to monitor aPTT while using LMWH. Drug is most active in tissue phase, and, as opposed to UFH, LMWH does not exert most of its effects on coagulation factor IIa.

Complications

Sudden cardiac death Obstructive shock Pulseless electrical activity Atrial or ventricular arrhythmias Secondary pulmonary arterial hypertension Cor pulmonale Severe hypoxemia Right-to-left intracardiac shunt Lung infarction Pleural effusion Paradoxical embolism

Prevention of pulmonary embolism:

Because pulmonary embolism is caused by an embolus formed elsewhere in the body (generally in the legs), and because it is often difficult to detect presence of a venous embolus prior to the onset of complications such as a pulmonary embolism, the prevention of these emboli is necessary in the prevention of PE.In order to prevent pulmonary embolism, the only effective way is to prevent deep vein thrombosis. Prophylactic treatment to prevent DVT includes:

Non-Invasive Mechanical MeasuresMechanical measures to prevent DVT include:

o compression stockings (elastic stockings that squeeze or compress the veins and prevent blood from flowing backward)

o pneumatic compression devices (sleeves on the legs that are connected to a machine that provides alternating pressure on the legs)

o getting up and moving as soon as possible after surgery or illness, as movement can help to prevent clots from forming by stimulating blood circulation

MedicationAnticoagulants, such as heparin and warfarin, are often given prophylactically to prevent DVT.