REBEL Core Cast 41.0 – Acute Chest Syndrome

Take Home Points

• 100k people in US have sickle cell, the majority will at some point develop acute chest syndrome (ACS)
• The mortality rate per episode is 3-9%, similar to those of STEMI
• ACS is a syndrome – CXR infiltrate + respiratory symptoms
• Treat it aggressively and early (antibiotics, respiratory support)
• If you’re considering exchange transfusion – get hematology onboard quickly

REBEL Core Cast 41.0 – Acute Chest Syndrome

Sickle Cell Disease

• A genetic RBC disorder affecting approximately 250 million people world wide
• Affects approximately 100k people in the United States
• 1 in 13 African Americans carry the sickle cell gene 

Pathophysiology 

• Characterized as having one abnormal hemoglobin, hemoglobin S
• Hemoglobin is made of iron (heme) and protein chains (globin)
  • Four types of globin molecule chains (alpha, beta, gamma, and delta)
  • HbSS, the hemoglobin of sickle cell anemia, consists of a pair of alpha chains and a pair of sickle beta chains
• Normal RBC are donut-shaped and flexible, fit through arterioles and capillaries 
• RBC with hemoglobin S are stiff and sickle-shaped
  • Distortion of RBC leads to destruction -> increased viscosity -> microvascular obstruction -> obstruction worsens hypoxia and acidemia worsens sickling
• Initially, the sickling process is reversible when HbS is reoxygenated but with repeated episodes, the red cell membrane is permanently damaged
  • Anywhere from 5% to 50% of the circulating erythrocytes in a patient with SCD can be irreversibly sickled cells.
• The overall effect is chronic, ongoing hemolysis and episodic periods of vascular occlusion, resulting in tissue ischemia affecting many organ systems.

Acute Chest Syndrome

• Occurs into the majority of patients with sickle cell anemia at some point in their lives
• Defined as new infiltrate on chest imaging + one other sign
  • Fever (101.3), cough,  wheezing, tachypnea, or chest pain
  • The definition is fairly broad – pneumonia easily falls in this category, treat both aggressively
• Acute chest syndrome is the leading cause of death among people with sickle cell disease.
• The mortality of a single episode is estimated at ~3-9%. This is equivalent to the mortality rate of patients with STEMI.

Precipitating Etiologies 

• ACS has multiple potential inciting etiologies
  • Pulmonary infection, pulmonary infarction, fat emboli, rib infarction, dehydration, infection, inflammation, acidosis and stasis of blood (decreased cardiac output). 
  • Hypoxia is the strongest stimulus for sickling 
• Iatrogenic causes 
  • Opioids leading to hypoventilation potential cause of ACS
• 30% of ACS admitted to hospital will have an infectious cause identified
  • 2 most common pathogens are atypical pneumonia pathogens
    • Chlamydia pneumoniae (in adults) 
    • Mycoplasma pneumoniae (in pediatric patients)

Presentation

• Respiratory symptoms are usually present, including cough, shortness of breath, chest pain, and fever
• Rales are the most common physical exam finding, found anywhere between 48 – 70%
• Potential differentials to consider
  • Myocardial infarction.
  • Decompensated chronic cor pulmonale.
  • Pulmonary embolism.
  • Splenic sequestration crisis 
  • Line infection (among patients with chronic indwelling lines or ports).

Ancillary Tests

• CXR
  • Important to identify the presence of infiltrate
    • Keep in mind that imaging findings lag behind the presentation.
• Bedside US
  • One study showed lung US to be more sensitive than CXR for ACS
  • Findings include B-lines and pleural effusion – keep in mind not specific to ACS
• Complete blood count
• Hemoglobin electrophoresis to determine % Hemoglobin S
• Blood for type & screen.
• Coagulation factors (INR, PTT, fibrinogen)
• Electrolytes, including calcium level (ionized calcium level if available)
• Liver function tests

Management

• Oxygen
  • Given oxygen in order to keep saturations >92%
  • British guidelines recommend >95% or within 3% of patients baseline
• Respiratory support
  • Bipap or HFNC – need to prevent atelectasis 
  • Incentive spirometer / early ambulation 
    • Incentive spirometry has been shown to prevent chest syndrome among patients admitted with painful vaso-occlusive crises.
• Analgesia 
  • Opioids are the mainstay for pain control
    • Beware of oversedation and hypoventilation 
• Fluids
  • Administer d5 ½ ns fluid infusion to maintain euvolemia
    • Monitor closely to avoid fluid overload
• Systemic corticosteroids
  • Controversial – may prevent deterioration and need for transfusion but associated with higher rates of admission 
• Antibiotics
  • Recommended regardless of cultures
  • Empiric broad-spectrum covering community-acquired pneumonia
    • Cephalosporin  / Macrolide to cover for Mycoplasma and Chlamydia
• Transfusion
  • Simple transfusion
    • Mildly anemic (<7) and not severely ill
    • Severely anemic (<5) and severely ill
    • The consensus in the literature  is a goal of 10-11 mg/dL
    • Transfusion to >11 mg/dL could potentially increase blood viscosity, which may cause clinical hyperviscosity syndrome
  • Exchange transfusion (allows for rapid decrease in sickle cells without increasing viscosity)
    • reserved for severe crises 
      • partial pressure of arterial oxygen to <60 mm Hg (<8 kPa) 
      • presence of neurologic abnormalities or multiorgan failure
      • Rapidly progressive lung involvement
      • Patients with high risk for adverse outcomes, those that are pregnant or have multiple comorbidities
    • If unsure consult with hematology for recommendations
    • Contradictions
      • Severe anemia (<6), recommend first simple transfusion followed by exchange transfusion
      • Severe hypocalecemia 

Disposition 

• All patients with ACS should be admitted to a monitored setting
• Not all require ICU, however those with increasing age and comorbidities most likely to have poor outcomes.

Take-Home Points

• 100k people in US have sickle cell, the majority will at some point develop ACS
• The mortality rate per episode is 3-9%, similar to those of STEMI
• ACS is a syndrome – CXR infiltrate + respiratory symptoms
• Treat it aggressively and early (antibiotics, respiratory support) 
• If you’re considering exchange transfusion – get hematology onboard quickly

Resources

• Life in the Fast Lane: Sickle Cell Crisis
• emDocs: Acute Chest Syndrome
• IBCC: Acute Chest Syndrome

Post Peer Reviewed By: Salim R. Rezaie, MD (Twitter: @srrezaie)

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