Pulmonary Infarction

Loschner first described pediatric pulmonary embolism (PE) in the 1860s. Deep venous thromboses (DVT) and pulmonary emboli are relatively rare phenomena in pediatric practice; however, when they do occur, they are associated with significant morbidity and mortality. Because of the rarity of pulmonary emboli in children, they are probably underdiagnosed. ^{[1, 2, 3, 4]} For the same reason, much of the information pertaining to diagnosis and management of pulmonary embolism has been derived from adult practice. ^[5]

A specific diagnosis that should be mentioned because of its prevalence is sickle cell disease. Prompt recognition and management of pulmonary problems may lead to a decreased rate of pulmonary complications.

Most pulmonary emboli derive from a free-floating thrombus. In rare situations, extension of an existing pulmonary thrombus may result in pulmonary infarction. Many materials and substances may form emboli and move to the pulmonary circulation; these include fat, tumor, septic emboli, air, amniotic fluid, and injected foreign material.

The size of a pulmonary embolism determines at which points in the pulmonary vasculature it lodges. After the embolus lodges, it occludes the vessel, reducing distal blood flow to the area directly supplied by the vessel. The degree of obstruction of the pulmonary circulation directly affects the resulting pathophysiology.

In all cases of pulmonary embolism, ventilation/perfusion (V/Q) mismatch occurs to some degree, in which continued ventilation of lung units without circulation is present. Oxygenation is usually not affected by the V/Q mismatch, in contrast with V/Q mismatch that arises from obstruction of airways and lung parenchyma. Impaired oxygenation in the context of suspected pulmonary embolism implies a massive obstruction.

An increase in effective alveolar dead space is a direct result of the V/Q mismatch. Ventilation (carbon dioxide removal) is usually compensated for by tachypnea.

In cases in which the pulmonary embolus is large, a sudden increase in pulmonary artery pressure may lead to right ventricular strain and right heart failure. A sudden rise in the right ventricular pressure may cause a leftward shift of the intraventricular septum, which may impair left ventricular filling and output (classic obstructive shock).

Reflex bronchoconstriction is often associated with pulmonary embolism. This increases the work of breathing and decreases pulmonary compliance. Pulmonary infarction is also associated with diminished surfactant levels, which may contribute to the increased work of breathing and diminished oxygenation.

Children with pulmonary emboli often have a serious underlying condition that predisposes them to embolus development and may worsen their clinical outcome. Some of the more common underlying conditions include the following:

• Sickle cell disease

• Nephrotic syndrome

• Cancer

• Chemotherapy

• Inherited hypercoagulable state

• Vasculitis

In sickle cell disease, an initial trigger (often infection) exacerbated by dehydration (eg, due to fever, tachypnea, or decreased intake) leads to sickling of RBCs within small blood vessels of the lung and other organs. This precipitates a cycle of relative deoxygenation that further exacerbates the sickling tendency, leading to small vessel occlusion and, ultimately, infarction of areas of the pulmonary parenchyma. Allied to this sequence is the tendency of many patients with sickle cell disease to have a component of reactive airways disease, which may further decrease oxygenation.

In contrast with adults, most children (98%) diagnosed with pulmonary emboli have an identifiable risk factor or a serious underlying disorder. DVT is associated with a pulmonary embolism in 30-60% of cases. Thrombosis may also arise from intracardiac thrombi or intracerebral sinus thrombosis.

Acquired thrombosis has three broad etiological risk factors: (1) a relative stasis of blood flow due to either immobilization or the presence of a nidus on which a thrombus may form, (2) a prothrombogenic tendency (hypercoagulability), and (3) injury to a vascular wall. These three factors have been termed the Virchow triad.

The following conditions predispose to some or all of these factors for acquired thrombosis:

• Central venous catheters: This is one of the most common acquired risk factors for pediatric PE. In 1993, David et al reported that 21% of children with DVT, pulmonary emboli, or both had an indwelling central venous catheter. ^[6] Additional series report presence of central lines in as many as 36% of patients. ^[7] A clot may form as a fibrin sleeve that encases the catheter. When the catheter is removed, the fibrin sleeve is often dislodged, releasing a nidus for embolus formation. In another scenario, a thrombus may adhere to the vessel wall adjacent to the catheter.

• Surgery: Recent surgery and postsurgical immobilization are associated with approximately 15-29% of pulmonary embolism and DVT cases.

• Heart disease: Thrombi may be associated with dilated cardiomyopathy, a situation in which sluggish blood flow is combined with an enlarged cardiac chamber.

• Sickle cell disease: This condition often creates a diagnostic difficulty. A chest infection is often the presenting symptom. Hypoxemia, dehydration, and fever lead to intravascular sludging within pulmonary (among others) vasculature. This promotes a vicious cycle, further exacerbating local hypoxemia, ultimately leading to local tissue infarction. This process is further worsened by bone marrow infarction, which may cause release of fat emboli that lodge in the pulmonary circulation. ^[8]

• Trauma: Whether the increased risk of pulmonary embolism in trauma patients is independent of the role of immobilization and surgery is unclear. ^[9]

• Neoplasm: Pulmonary emboli have been reported to occur in association with solid tumors, leukemias, and lymphomas. This is probably independent of the indwelling catheters often used in such patients. ^[10]

• Hyperalimentation: A study reported that major thrombosis or pulmonary embolism was present in more than 33% of children treated with long-term hyperalimentation and that pulmonary embolism was the major cause of death in 30% of these children. Fat embolization may exacerbate this clinical picture. ^[11]

• Dehydration: Dehydration, especially hyperosmolar dehydration, is typically observed in younger infants with pulmonary emboli. ^[12]

• Inherited disorders of coagulation: In 1993, David et al reported that 5-10% of children with venous thromboembolic disease have inherited disorders of coagulation, such as antithrombin III, protein C, or protein S deficiency. ^[6] In 1997, Nuss et al reported that 70% of children with a diagnosis of pulmonary embolism have antiphospholipid antibodies or coagulation-regulatory protein abnormalities. ^[13] However, this was a small study in a population with clinically recognized pulmonary emboli; hence, its applicability to the broader pediatric population is uncertain.

• The systemic vasculitides are a heterogenous group of rare conditions which may infiltrate and occlude the pulmonary vessels leading to ischemia and infarction of the supplied tissue. ^[14]

Miscellaneous causes

Other causes of pulmonary embolism include the following:

• Obesity (BMI ≥ 25 kg/m²)

• Estrogen use, including oral contraceptives

• Pregnancy

• Pregnancy termination

• Nephrotic syndrome

• Ventriculoatrial shunt: The tip of the atrial shunt may act as a nidus for thrombus formation.

• Autoimmune disorders: These may be associated with antibodies that predispose to a hypercoagulable state.

In a retrospective review of pediatric patients presenting to a pediatric emergency department, the most common risk factors identified for pulmonary embolism were BMI ≥ 25 kg/m², oral contraceptive use, and history of previous pulmonary embolism. ^[15]

United States data

Pulmonary embolism is a rare disorder in pediatric practice. In 1993, David et al identified 308 children reported in the medical literature from 1975-1993 with DVT of an extremity, pulmonary embolism, or both. ^[6] In 1986, Bernstein reported 78 episodes of pulmonary embolism per 100,000 hospitalized adolescents. ^[16] Unselected autopsy studies in children estimate the incidence of pulmonary embolism from 0.05-3.7%.

International data

Canadian data derived from 15 tertiary care centers show a frequency of 0.86 events per 10,000 pediatric hospital admissions for patients aged 1 month to 1 year. ^[17] Frequency of pulmonary embolism in developed countries has been increasing when compared with historical data. This increase in frequency is linked with the increased use of central venous lines in the pediatric population. ^[18] The overall frequency is still considerably less than that seen in adults.

Race-, sex-, and age-related demographics

No data outlining variations in pulmonary embolism prevalence by race are available.

Some authors have reported a female-to-male ratio of 2:1. Others have found that this ratio is reversed.

Given the rarity of pulmonary embolism in childhood, no definitive data identify age as an independent risk factor for pulmonary embolism. The frequency of pulmonary embolism has a bimodal distribution, with peaks in the neonatal period and adolescence.

Morbidity/mortality

Morbidity may include pulmonary hypertension, right ventricular failure and cor pulmonale, paradoxical embolization to the systemic circulation in patients with intracardiac defects, and side effects of medications used to treat pulmonary embolism. ^{[16, 19, 9]}

In a case series and literature review of massive pediatric pulmonary embolism, children with massive PE had higher rates of mortality and were more likely to have the PE diagnosed postmortem. ^[20]

No data are available regarding the risk of recurrence of pulmonary embolism in children.

Separating mortality attributable to pulmonary embolism from that due to conditions that may be associated with pulmonary embolism, such as trauma and surgery, is difficult.

The data regarding death from pulmonary embolism in children are conflicting. Various authors suggest that pulmonary embolism contributes to the death of affected children in approximately 30% of cases. ^[21] Others, however, have reported pulmonary embolism as a cause of death in fewer than 5% of affected children. ^[22] As in adults, the mortality rate is highest in the period immediately following embolization. If no major cardiovascular sequelae are present, a full recovery may be anticipated without complications.

The importance of adherence to the treatment regimen should be repeatedly stressed. The patient should be instructed regarding what to do in the event of any bleeding complications. Because most patients are administered warfarin upon discharge from the hospital, they must be advised regarding potential interactions between warfarin and other medications.

Risk factors for the development of pulmonary embolism should be discussed, including the following:

• Pregnancy

• Oral contraceptive pill use

• Termination of pregnancy

• Smoking

For patient education resources, see the Lungs Center, as well as Pulmonary Embolism and Sickle Cell Disease.