Unusual Forms ofEmbolism

Peter F. Fedullo

Because the lung receives all of the blood flow returned from the venous system, the pulmonary vascular bed serves as a “sieve” for all particulate substances entering the venous blood and is the first vascular bed to be exposed to any toxic substance injected intravenously. As a result of its strategic position, the pulmonary vascular bed is, therefore, exposed to a wide variety of potentially obstructing and injurious agents.

SCHISTOSOMIASIS

Among such agents, the most common worldwide, though not in the United States, is schistosomiasis. Schistosomiasis is caused by one of a variety of blood flukes, Schistosoma haematobium (Africa and Middle East), S. japonicum (Japan, China, Philippines), S. mansoni (Africa, Arabia, South America), S. mekongi (Laos, Thailand), and S. intercalatum (Africa) being among the most common. Limited data suggest that cardiopulmonary schistosomiasis is seen most often in S. mansoni and S. Japonicum infection.

Infection occurs after contact with water containing the infective stage of the parasite, the cercaria. The cerceria penetrate unbroken skin and subcutaneous tissue and migrate through the lungs and then to the portal vein, probably by an intravascular route. The maturing schistosomes pair in the portal vein and then migrate to the venules of the mesentery, bladder, or ureters and begin to deposit eggs, many of which are subsequently swept back to the liver.

During acute infection (Katayama fever), nonspecific influenza-like symptoms, abdominal pain, lymphadenopathy, hepatosplenomegaly, and blood eosinophilia associated with fleeting chest radiographic abnormalities can occur.

Pulmonary hypertension occurs in less than 5% of infected patients. Cor pulmonale related to schistosomal infection usually does not occur in the absence of concomitant liver schistosomal liver disease because the liver is involved, usually quite extensively, before pulmonary involvement. Pulmonary vascular obstruction appears to be induced by two mechanisms: anatomic obstruction by the organism itself and by an intense, granulomatous, inflammatory vasculitic response to shunted and embolized schistosomal eggs. In endemic areas, schistosomal disease is the most common cause of cor pulmonale.

The premortem diagnosis of cardiopulmonary schistosomiasis depends on the detection of viable schistosomal ova in stool, urine, or tissue (rectal mucosa or lung) along with evidence of hepatic fibrosis and pulmonary hypertension. Currently utilized serologic tests only indicate past or present infection, although promising serologic markers capable of differentiating acute from chronic disease are being investigated. Treatment with praziquantel can effectively eradicate schistosomal infections in the acute phase of the disease with minimal toxicity. However, chronic cardiopulmonary manifestations are not likely to be reversible given the fibrotic changes that are present.

AIR EMBOLISM

An increasingly common form of nonthrombotic embolism in the United States is venous air embolism. The increasing frequency reflects the wide variety of invasive surgical and medical procedures now available, broad use of indwelling central venous catheters, use of positive pressure ventilation with high levels of positive end expiratory pressure, and the frequency of thoracic and other forms of trauma. The simple inadvertent transection or loss of closure of a large-bore intravenous catheter, particularly in the jugular or subclavian vein, can result in ingress of substantial quantities of air. Air bubbles enter the pulmonary vascular bed and, from there, can enter the arterial system and be diffusely distributed throughout the body by way of either an intracardiac shunt (atrial septal defect, patent foramen ovale) or, more likely, through microvascular pulmonary shunts. Direct arterial gas embolism has been reported with transthoracic diagnostic procedures with inadvertent entry into a pulmonary vein.

The lethal volume of injected air in humans is estimated to be in the range of 100 to 500 mL. Physiologic consequences include an abrupt rise in pulmonary artery pressure. Non-cardiogenic pulmonary edema may develop, lung compliance falls, and hypoxemia ensues. The elevated pulmonary artery pressure may result in elevated right atrial pressure and subsequent paradoxical embolization. Gas entry into the systemic circulation results in ischemia of the affected organ.

The symptoms of venous air embolism are variable and nonspecific, and may include alterations in sensorium, chest pain, dyspnea, or a sense of impending doom. These and other consequences appear to be due to two phenomena: actual lodgment of the bubbles in capillary beds that interfere with nutrient supply to the affected organs, and the formation of platelet-fibrin aggregates, creating diffuse microthrombi. Thrombocytopenia may be seen as a consequence of this latter event. The most serious consequences result from cerebral or coronary artery air embolism, the severity of the consequences depending upon the rate and volume of air gaining access to the circulation.

The best approaches to air embolism are prevention and early detection. Aggressive treatment is essential and should consist of measures designed to restore flow and promote reabsorption of the intravascular air. Measures designed to restore flow include patient positioning (Trendelenburg position with the left side down), removal of air through central venous catheters or direct needle aspiration, and closed chest cardiac massage. Measures designed to increase absorption include the use of 100% oxygen and the institution of hyperbaric oxygen therapy as early as possible. Recovery following delayed institution of hyperbaric oxygen has been reported. Utilizing such aggressive measures, mortality from venous air embolism has been reduced dramatically.

FAT EMBOLISM

Another reasonably frequent and dramatic form of nonthrombotic embolism is fat embolism. By far, the most common inciting event is traumatic fracture of long bones, with incidence rising with the number of fractures. However, orthopedic procedures and trauma to other fat-laden tissues (e.g., fatty liver) occasionally can be followed by the same syndrome. Although considerably less common, fat embolism syndrome has been reported following both liposuction and lipoinjection procedures.

A rather characteristic syndrome follows entry of neutral fat into the vascular system, consisting of the onset of dyspnea, hypoxemia, petechiae, and mental status changes. Seizures and focal neurologic deficits have been described. There is a variable lag time of 24 to 72 hours in the onset of the syndrome following the inciting event; rarely, cases occur within 12 hours or as late as 2 weeks after the event.

The variability in incidence of the syndrome after apparently comparable injuries has not been well-defined; neither has the reason for the delay in clinical presentation been explained. The pathophysiologic consequences appear to derive from two events: (1) actual vascular obstruction by neutral particles of fat and (2) the injurious effects of free fatty acids released by the action of lipases on the neutral fat. The latter effect is probably more important, causing diffuse vasculitis with leakage from cerebral, pulmonary, and other vascular beds. The time necessary to produce toxic intermediaries may explain the delay from the inciting event to clinical presentation.

The diagnosis of fat embolism syndrome is a clinical one suggested by the onset of dyspnea, neurologic abnormalities, petechiae, and fever in the proper clinical context. Chest imaging studies may demonstrate bilateral infiltrates, ground glass opacities, and centrilobular and subpleural nodules. Petechiae, typically distributed over the head, neck, anterior chest, and axillae are present in only 20% to 50% of cases. Their absence, therefore, should not preclude consideration of the disease. No laboratory test is diagnostic of the syndrome. Fat can be demonstrated in the serum of a majority of fracture patients with evidence of fat embolism syndrome. The finding of lipid-laden cells in bronchoalveolar lavage fluid appears to occur commonly in patients with traumatic injuries, irrespective of the presence of fat embolism syndrome.

Although a variety of treatments have been suggested (e.g., intravenous ethanol, albumin, dextran, heparin), none has proven effective. The role of corticosteroid therapy to prevent the onset of fat embolism syndrome after an inciting event remains controversial. Supportive treatment, including mechanical ventilatory support when necessary, is the primary approach, and survival is now the rule with meticulous support.

AMNIOTIC FLUID EMBOLISM

Another special form of embolism is amniotic fluid embolism, a rare but unpredictable and catastrophic complication of pregnancy that represents the third leading cause of maternal mortality. This disorder occurs during or after delivery when amniotic fluid gains access to uterine venous channels and, therefore, to the pulmonary and systemic circulations. The delivery may be either spontaneous or by cesarean section and is usually uneventful. Most cases occur during labor, but delayed onset of symptoms up to 48 hours after delivery can occur. Although specific risk factors have not yet been identified, advanced maternal age, multiparity, premature placental separation, fetal death, and meconium staining of amniotic fluid have been associated with increased risk of amniotic fluid embolism.

Amniotic fluid embolism syndrome is primarily a clinical diagnosis. There is, unexpectedly, sudden onset of severe respiratory distress, cyanosis, hypotension, cardiovascular collapse, and, often, disseminated intravascular coagulation. Occasionally, seizure activity occurs. It has been postulated that there is a biphasic pattern of hemodynamic disturbance: an initial period of pulmonary hypertension, commonly seen in animal models, followed by left ventricular dysfunction and cardiogenic shock. Patients who survive the first several hours develop noncardiogenic pulmonary edema coincident with improvement in left ventricular dysfunction.

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