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Acute Chest Syndrome
This term is used to cover conditions characterized by chest pain, cough, fever, hypoxia and lung infiltrates. Acute chest syndrome may be the result of sickling in the microvasculature causing pulmonary infarction/emboli or viral or bacterial pneumonia. It may develop as an isolated event, or during the course of a painful vaso-occlusive episode. The clinical course is usually self-limited when small areas of the pulmonary parenchyma are involved, but without proper supportive care the syndrome can rapidly progress and result in death.
Pleuritic chest pain is the most common presenting complaint in adults. Fever, cough, tachypnea, hypoxemia or abdominal pain are common presentations for infants and children. It is always best to assume an infectious etiology in these cases and obtain appropriate cultures and serologic studies. There may or may not be radiographic evidence of pulmonary infiltrates at the initial time of presentation. Lung pathology must be differentiated from rib infarction, gastric ulcer, or cholecystitis. Abdominal pain may be a result of involvement of the diaphramatic pleura. There may be physical findings of pleural effusion or pulmonary consolidation.
Chest radiographs may be nondiagnostic for the first 24-48 hours, especially if the patient was dehydrated at presentation. Infiltrates in one or more lobes with or without effusion are common findings. It is not uncommon for blood, sputum, or pleural fluid cultures to reveal a bacterial pathogen (usually Streptococcus pneumoniae or Hemophilus influenzae). Viral and mycoplasma antibody titers are usually helpful only in retrospect since the results are conclusive late in the course of the illness. Hypoxemia can be monitored by pulse oximetry if the patient has had previous correlation (while well and on room air) with arterial blood gas. If the oxygen saturation is > than 3% below the patient’s baseline, supplemental oxygen is indicated.
Serial CBC’s and reticulocyte counts are helpful. An elevated neutrophil count, especially if shifted to the left suggests bacterial infection. A decreasing hemoglobin/hematocrit are common as the syndrome evolves and may contribute to tissue hypoxia. Patients who have had repeated episodes of acute chest syndrome often develop restrictive lung disease. They may become at increased risk for recurrence of acute chest syndrome as well as pulmonary hypertension and cor pulmonale. These patients require regular pulmonary function monitoring. Some benefit from chronic transfusion therapy. Treatment with hydroxyurea should also be considered.
Fat embolization syndrome can mimic acute chest syndrome. This involves widespread embolization of liquified bone marrow fat into the pulmonary vessels and then in to the systemic circulation. Adult patients with sickle cell disease (especially Hb SC disease) are vulnerable to this syndrome during severe vaso-occlusive episodes. In advanced cases, disseminated intravascular coagulation with severe microangiopathic hemolytic anemia is possible. Presence of fat globules in the urine, head and neck petechiae, presence of refractile bodies on fundoscopic exam or necrosis seen in bone marrow samples support the diagnosis. A high index of suspicion is essential for early diagnosis and treatment with exchange transfusion and supportive care.
Patients with acute chest syndrome must be admitted to the hospital and monitored for a potentially rapid deterioration in respiratory status. Depending on the degree of pathology and hypoxia, mechanical ventilation in an intensive care setting may be appropriate. IV fluid hydration should be given at 1.5 times maintenance. It is appropriate to treat accompanying vaso-occlusive pain with opioids as needed. Relief of pain avoids splinting or shallow breathing, which can prolong or complicate the course. This requires close monitoring to find a dose of analgesia that provides comfort without hypoventilation or over-sedation.
Since it may not be possible to initially differentiate between pulmonary infarction and bacterial or viral preumonia, antibiotic therapy should be initiated with a broad spectrum penicillin or cephalosporin pending culture results. Only about 5% oatients will have positive blood cultures. The typical organisms are pneumococcus and H. influenzae. A macrolide antibiotic should be added if mycoplasma is suspected.
Treatment with exchange transfusion should be used for patients who have multi-lobar involvement, hypoxemia, or progressive respiratory distress. Simple transfusion may be used if the baseline hemoglobin is low.