emDOCs Podcast – Episode 100: Acute Chest Syndrome Part 1

Background

• SCD is an autosomal recessive condition that results in the formation of hemoglobin S (HbS).
  • HbS has reduced solubility in the setting of hypoxia, leading to sickling of the RBCs.
  • Other causes of sickling:  acidosis, dehydration, inflammation, infection, fever, and blood stasis
  • Sickling leads to vascular occlusion, end-organ ischemia, and decreased RBC lifespan, which, in turn, leads to pain crisis, acute anemia, sequestration, infection, and acute chest syndrome (ACS.)
• ACS is lung injury due to vaso-occlusion in the pulmonary vasculature; many with ACS will have a concomitant vaso-occlusive pain crisis.
  • 10-20% of patients admitted for a pain crisis develop ACS
  • ACS accounts for 25% of deaths in SCD patients.
  • Each episode of ACS has a 9% mortality rate.

Diagnosis of acute chest syndrome requires: 

• New segmental radiodensity on chest imaging
• Plus, one of the following:
  • Fever > 38.5C.
  • Hypoxemia – more than a 2% decrease in SpO₂ or PaO₂ <60 mmHg
  • Tachypnea
  • Cough, chest pain, rales, wheeze

Triggers of ACS: 

• 46% will not have an identifiable cause.
• A vaso-occlusive pain crisis is the most common trigger in adults.
• Asthma is the most common trigger in kids.
• Infectious: bacterial or viral pneumonia (M. pneumoniae, C. pneumoniae, RSV)
• Others: Fat embolism, cold temperature, smoking

Clinical picture

• Respiratory symptoms
  • Cough
  • Dyspnea (47-58% adults/20-36% kids)
  • Chest pain (55-84% adults/41% kids)
  • Hemoptysis
  • Multilobar involvement (36% adults/24% kids)
• Fever (64% adults/85% kids)
• Extremity pain (59% adults/30% kids )
• Pain in ribs or sternum (30% adults)
• Neurologic symptoms (22%  adults/8% kids)
  • AMS, seizure, neurologic deficit due to ischemic stroke

ED Evaluation

• Vitals:  Pulse oximetry is less reliable in SCD.
  • Based on light absorption from blood flow at the sensor site (using HbA, not HbS)
  • Underestimates alveolar hypoxemia.
  • Overestimates arterial oxygen saturation.
  • Up to 3x less accurate in black patients.
  • When the clinical picture does not match the SpO₂, get an ABG.
  • Use a co-oximeter to measure different types of Hb.
• ECG: Evaluate for ischemia, right heart strain.
• Labs:
  • CBC with differential – anemia, elevation in WBC
  • Electrolytes, renal, and liver panels – end-organ damage
  • Reticulocyte count, type and cross, coagulation factors
  • Blood cultures, UA and culture, legionella and pneumococcal antigens, viral panel
  • Troponin – if there is concern for myocardial ischemia
  • +/-ABG
• Imaging:
  • CXR
    • Lobar, segmental or multilobar consolidation
    • Pleural effusion or atelectasis – common in adults
    • Sensitivity 85%, Specificity <60%
    • Can lag behind clinical signs/symptoms.
  • Ultrasound
    • Sensitivity 88-100%, specificity 68-94%
    • Positive likelihood ratio of 14.6 (95% CI 6.5-32.5).
    • Look for B lines, consolidation, pleural effusion.
  • CT with IV contrast
    • Higher sensitivity and specificity than x-ray.
    • Consider when X-ray negative, but still concerned for ACS or if concerned for PE.

Treatment

• Multimodal analgesic therapy including adjunct non-opioid medications
  • Opioid dose based on prior patient-specific pain requirements and review of home opioid dosing to assess tolerance

• Incentive Spirometry
  • 10 maximal inspirations q2hr
  • Decreases splinting and improves tidal volumes
• Bronchodilators
  • Indicated for patients with wheezing, history of asthma.
• Fluid management
  • Goal is euvolemia
    • Dehydration – needs IV fluid resuscitation.
    • Hypervolemia – leads to pulmonary edema: Consider diuresis
    • If euvolemic – start maintenance fluids of D5 in 0.45%NS at 1.5 times maintenance.  Can decrease rate when they are tolerating PO.

• Empiric antibiotics
  • All patients with ACS should receive antibiotics.
  • 25% have an infection as the trigger for ACS.
  • Antibiotic choice:
    • Everyone needs atypical coverage.
    • Consider risk factors for multi-drug resistant microbes:
      • Recent IV antibiotics
      • Hospitalization within 90 days.
      • Prior cultures with resistance microbes.
    •  Third-generation cephalosporin (ceftriaxone or cefotaxime) PLUS macrolide (azithromycin) or doxycycline
    • Critically ill patients should get cefepime or piperacillin/tazobactam PLUS vancomycin and azithromycin

• Airway support
  • Target saturation 95% OR within 3% of patient’s baseline sat
  • Start with nasal cannula or face mask
  • HFNC or NIPPV
    • Non-invasive
    • Recruits collapsed alveoli
    • Improves lung compliance
    • Reduces work of breathing
    • Improves oxygenation
    • Reduces the chance of intubation/mechanical ventilation.
    • Does not reduce mortality or length of stay in ACS
  • Intubation/Mechanical Ventilation
    • Indicated when the patient is: unable to protect their airway, to oxygenate, to ventilate or cannot tolerate HFNC/NIPPV
    • Use lung protective ventilation
      • Tidal volume 6 ml/kg
      • Peak airway and plateau pressures < 30 cmH₂O.
• Blood transfusion
  • Reduces the overall proportion of HbS
  • Improves oxygen carrying capacity
  • Reduces progression to severe disease
  • Two options:
    • Simple blood transfusion
      • Stable patient
      • As a temporizing measure in critically ill patients
      • Gives the patient more oxygen-carrying capacity
      • Target Hb 10-11 g/dL
      • Sickle-negative, leukocyte-depleted RBC with matching Rh D, C and E
    • Red cell exchange transfusion
      • Automated erythrocyte apheresis.
      • Decreases the concentration of HbS
      • Prevents further sickling.
      • Allows for larger blood volume to be given without increasing blood viscosity
      • Target HbS percentage <30% and Hgb 10 g/dL

Identifying severe ACS

• Severe ACS patients are unstable: SpO₂ < 85% on room air or < 90% with maximal supplemental oxygen, respiratory failure, requires intubation, multilobar infiltrates, transfusion to reach HbS less than 30%, or there is multiorgan involvement.
• Risk factors for developing severe ACS:
  • Fever
  • SpO₂ < 95%
  • Asplenia
  • History of asthma
  • Lower baseline Hb
  • Leukocytosis
  • Platelet count <200,000

Rapidly Progressive ACS and Multi-organ failure

• Defined by respiratory failure within 24 hours of initial symptoms and multi-organ involvement (AKI, hepatic dysfunction, altered mental status, multiorgan failure).

What about PE?

•   Can be inciting factor or concurrent condition.
•   Risk factors in ACS patients include:
  • Baseline Hb >8.2 mg/dL
  • Platelet count > 440,000
  • PaCO2 < 38 mm
  • No clear trigger for the ACS episode.
• Consider CT with IV contrast.

COVID-19 and ACS

• COVID increases the risk of hospitalization, pneumonia, pain, and ACS in SCD patients.
• COVID-positive patients with hypoxia should receive steroids.
• Steroids can lead to rebound vaso-occlusive crisis for SCD patients, but the benefits outweigh the risks when they have COVID and are hypoxic.
• If there is a drop in Hb, consider simple transfusion.
• If the patient is critically ill or has respiratory failure, give exchange transfusion.

Pearls:

• Acute chest syndrome (ACS) is a form of acute lung injury defined by fever and/or new respiratory symptoms accompanied by a new radiodensity on imaging.
• There are a variety of triggers, including infection (most common in pediatric patients), fat embolism, vaso-occlusive pain crisis, pulmonary embolism, and others.
• Signs and symptoms include chest pain, dyspnea, fever, cough, hemoptysis, extremity pain, and neurologic symptoms; these may occur suddenly, or they may progressively worsen over days.
• While chest radiograph is often the first-line imaging modality, lung ultrasound has higher sensitivity and can detect pulmonary abnormalities earlier; CT imaging should be considered if initial imaging is negative or if concern for PE is present.
• Markers for severe disease include hypoxemia, increasing respiratory rate or work of breathing, decreasing platelet count and/or Hb, and multilobar involvement on chest radiograph or lung ultrasound.
• Management includes rapid initiation of opioid and non-opioid agents to treat the pain associated with vaso-occlusive crisis, antimicrobial coverage, incentive spirometry, oxygen supplementation to maintain saturation > 95%, fluid resuscitation based on volume status, consideration for transfusion, and specialist consultation.
• All patients with ACS should receive antibiotics, as 25% have an infection as the trigger for ACS. Everyone needs atypical coverage.
• Simple blood transfusion should be considered early in the hypoxemic patient with advancement to red cell exchange transfusion if there are clinical features of severe pathology or evidence of progression despite initial simple transfusion.

References:

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