About the condition

Description/definition:

Acute chest syndrome (ACS) refers to a spectrum of disease from a mild pneumonic illness to acute respiratory distress syndrome and multi-organ failure. It is defined as an acute illness characterized by:

  • Fever and/or respiratory symptoms, and
  • New pulmonary infiltrate on chest X-ray

ACS is the second most frequent reason for hospitalization in children and adults with sickle cell disease (SCD) (after veno-occlusive crises) and the most common cause of death.

In about 50% of cases, ACS develops one-three days following onset of a vaso-occlusive crisis. It can also develop after surgery, especially following abdominal surgery and in cases where patients do not receive appropriate pre-operative blood transfusion.

ACS can be severe in all sickle genotypes. Its incidence in HbSC disease and HbS-β+ thalassemia is less than in sickle cell anemia and HbS-β0 thalassemia.

Pathophysiology:

The initial insult, which may include pulmonary infection, fat embolism and/or pulmonary infarction, causes a reduction in alveolar oxygenation tension, which, in turn, leads to HbS polymerization. Sickling results in decreased pulmonary blood flow that exacerbates vaso-occlusion, producing more severe hypoxia such that a vicious cycle of hypoxia, HbS polymerization, vaso-occlusion and altered pulmonary blood flow ensues. In many cases, the specific cause or inciting factor is not apparent.

Infection

  • The most common well-defined etiology
  • Pathogens include:
    • Viruses
    • Bacteria
    • Atypical bacteria – the most common bacterial organism identified in adults is Chlamydophila pneumoniae

Fat embolism

  • During a painful crisis, vaso-occlusion within the bones leads to bone marrow necrosis and the release of fat emboli into the blood.
  • Fat emboli then lodge in the pulmonary vasculature causing acute hypoxia.

Microvascular pulmonary infarction

  • May result from hypoventilation, causing pulmonary atelectasis, hypoxia and pulmonary intravascular sickling.
  • Microvascular pulmonary infarction must be distinguished from pulmonary embolism, which can present with chest pain and tachypnoea but without a new infiltrate on chest X-ray (CXR).

Hypoventilation/atelectasis

  • May result from/occur in:
    • Pain-related splinting and regional hypoventilation in the areas of pain
    • Opiate narcosis
    • Post-operative period, following a general anesthetic

Paradoxically, despite a lower incidence of acute chest syndrome (ACS) in HbSC compared to HbSS, severe cases of ACS associated with multiorgan failure and necrotic fat emboli appear more common in HbSC disease. This may be the result of increased blood viscosity due to a higher hemoglobin concentration and packed cell volume.

Clinical presentation:

  • The most common respiratory symptoms of acute chest syndrome (ACS) include:
    • Cough – may be productive
    • Chest pain – may be pleuritic
    • Shortness of breath
  • Other symptoms include:
    • Rib and sternal pain
    • Chills
    • Wheezing
    • Hemoptysis
  • Physical findings (often precede the chest X-ray findings) may show:
    • Fever
    • Tachypnea
    • Tachycardia
    • Wheezing
    • Additional chest signs
  • Low oxygen saturation
  • Chest X-ray (CXR) changes (see next section)

Diagnosis:

Diagnosis of ACS is based on the finding of recent onset hypoxia, tachypnea, chest signs, fever in a patient with sickle cell disease (SCD) and the appearance of consolidation on a plain chest X-ray.

Differential diagnosis (these conditions may also trigger/exacerbate ACS):

  • Pulmonary embolism:
    • Patients may present with chest pain, dyspnea and hypoxia.
    • D-dimers are unhelpful in SCD as levels are usually elevated, so testing should be avoided.
    • If there is a high clinical suspicion of pulmonary embolism (i.e. sudden onset unilateral pleuritic pain that is not typical of sickle pain) treat for both conditions pending a computerized tomography pulmonary angiogram (CTPA).
    • ACS may be complicated by pulmonary embolism or may occur secondary to pulmonary embolism and treatment will be required for both conditions simultaneously.
  • Fluid overload
  • Alveolar hypoventilation due to pain

Labs include:

  • Complete blood count (CBC) – may show reduced Hb and platelet count from baseline.
  • Renal function and liver function
  • Hemolysis markers – may show increase in markers of hemolysis.
  • Blood group and screen
  • Blood cultures
  • Arterial blood gases measurement on room air:
    • In adults if SpO2 ≤ 94% on room air.
    • This should not be done on room air if patient is in obvious respiratory distress or if SpO2 saturations fall to <85% if oxygen is stopped briefly.
  • Serology for atypical respiratory organisms and urine for Pneumococcal and Legionella antigen
  • Sputum for bacterial culture and sputum and nasopharyngeal aspirate for immunofluorescence or polymerase chain reaction (PCR) for viruses in patients with coryzal symptoms
  • Chest X-ray (CXR):
    • Typical findings on plain chest radiography in ACS are segmental, lobar or multilobar consolidation usually involving the lower lobes, or collapse with or without pleural effusion.
    • The radiological signs often lag behind the physical signs.
    • Treatment must not be delayed because the changes on CXR are unremarkable.
    • If initial CXR is normal, a repeat CXR should be performed if there is ongoing clinical suspicion.

Treatment:

Treat as medical emergency:

  • Oxygen:
    • Maintain SpO2 ≥ 95% or within 3% of the patient’s baseline.
    • Monitor SpO2 at least 4-hourly to detect clinical deterioration indicated by increasing oxygen requirement.
  • IV fluids:
    • Patients with ACS are typically unable to maintain adequate hydration orally.
    • Intravenous crystalloid infusion should be administered until the patient can drink adequate amounts of fluid.
    • Fluid requirements should be individualized and be guided by the patient’s fluid balance and cardiopulmonary status.
    • Clinical consideration must be given to avoiding fluid overload and the development of acute pulmonary edema.
  • Pain relief:
    • Vaso-occlusive crisis affecting the thorax (ribs, sternum and thoracic spine) often leads to chest splinting and alveolar hypoventilation, which can cause lung atelectasis. There is a high correlation between thoracic bone infarction and ACS.
    • However, care must be taken to avoid alveolar hypoventilation from opioid overdose, as this would heighten the risk of ACS.
  • Incentive spirometry
  • Antibiotics:
    • Since it is not possible to reliably rule out an infectious etiology of the ACS, all patients should be treated with empiric antibiotics for severe community acquired pneumonia.
    • The antibiotic combination must always cover atypical respiratory organisms such as Mycoplasma and Chlamydophila.
  • Blood transfusion:
    • Not all patients with ACS will require a blood transfusion.
    • A simple (‘top-up’) transfusion:
      • Considered in patients with a PaO2 of less than 90 kPa on room air.
      • May also be needed at less severe degrees of hypoxemia, depending on the individual patient’s history and clinical features, or if the patient’s oxygen requirements are increasing.
      • Aim for a final hemoglobin concentration of 10–11 g/dL.
      • If the patient has HbSC disease and a packed cell volume more than 30, exchange transfusion might be the prudent choice to avoid hyperviscosity.
    • An exchange transfusion:
      • Indicated for:
        • Patients who show features of severe disease.
        • In those who deteriorate despite an initial simple transfusion.
        • In those with a higher hemoglobin concentration (>9 g/dL).
      • Aim for final hemoglobin concentration of 10–11 g/dL and HbS% target post-exchange transfusion <30– 40% (though this is not a hard and fast rule and clinicians should be guided by the clinical response).

Prognosis

The severity of ACS is variable, ranging from a mild illness
to a severe life-threatening condition.

Predictors of a more severe course include:

  • Multilobe disease
  • Increased work of breathing
  • Inability to maintain oxygen saturation above 95 percent even with supplemental oxygen
  • Pleural effusion

All patients, in particular, those with chest signs and symptoms can progress rapidly during ACS to acute hypoxic respiratory failure and therefore regular SpO2 monitoring – at least every 4 hours – is essential.

Mechanical ventilation has been reported as necessary in up
to 20% of cases.

Even with good medical treatment, overall mortality rates of up to 3% are reported.

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