Respiratory System

Acute Respiratory Distress Syndrome

Acute Respiratory Distress Syndrome

ARDS – acute respiratory distress syndrome and ALI – acute lung injury, are essentially the same disease; ARDS is the more serious end of the spectrum, and for the purpose of this article, we will also refer to the disease as ARDS

ARDS is essentially acute lung inflammation as a result of sepsis, pneumonia, (these two causes account for 60% of cases), trauma or aspiration. It also sometimes results in the case of shock, either through direct ischaemic damage, or as a result of reperfusion damage. 

Etiology

The causes are often divided into direct and indirect lung injury, e.g.:
  • Direct – trauma, aspiration pneumonia, fat embolism, alveolar haemorrhage
  • Indirect sepsis, systemic trauma, shock, stroke, drug overdose (aspirin, heroin), burns, liver failure, pancreatitis, massive blood transfusion, head injury, pregnancy, eclampsia, malaria

Pathology

  • Results from local or systematic inflammatory processes. Cytokines and other inflammatory mediators recruit macrophages and neutrophils to the area
  • These WC’s then release other inflammatory agents, and there is disruption of the boundary between lung tissue and normal capillaries, leading to ‘leaking’ of blood products (blood / protein etc) into the air spaces.
  • This process generally occurs throughout the lung tissue
  • There is reduced lung compliance, and disruption of surfactant leading to collapse of airways
Acute Respiratory Distress Syndrome

Signs and Symptoms

  • Can be difficult to differentiate from ACUTE HEART FAILURE
    • Infection may also be a similar presentation
  • Dyspnoea
  • Tachycardia
  • Tachypnoea
  • Bilateral Basal crepitations / other abnormal breath sounds
  • Chest pain
  • Peripheral vasodilation

Diagnostic Criteria:

  • Acute onset
  • PCWP – pulmonary capillary wedge pressure – <19mmHg
  • CXR – demonstrating bilateral diffuse infiltrates
  • Refractory hypoxaemia – PO2:FiO2 <200
  • (Total thoracic compliance <30ml/cm H2O) – helpful but not necessary to fulfil diagnostic criteria

Investigations

Bloods

  • Amylase, FBC, U+E, CRP
    • ESR is not generally useful as this measures more chronic inflammation

ABG

  • Low O2 – often does not respond well to prescribed O2
  • pH – can be low initially (due to respiratory acidosis), or may be high in the presence of sepsis, or as a result of the underlying cause of ARDS

CXR

  • Bilateral, widespread infiltrates. May take several hours to appear on CXR after the onset of symptoms

Pulmonary catheter – to measure pulmonary capillary wedge pressure. This is to rule out heart failure. Pressure of <19mmHg is required to consider ARDS as a diagnosis

Treatment

Treat the underlying cause – which is usually obvious

Mortality is about 50-75%
If early:

  • Try 40-60% O2 on CPAP
  • If ABG O2 remains <8.2kPa, then give mechanical ventilation (intubate)
    • This can be dangerous, as the high tidal volumes/pressures involved due to poor lung compliance, can lead to pneumothorax
    • Keep the tidal volume and pressures as low as possible. This helps to achieve positive end expiratory pressures (PEEP) – as does CPAP.

Circulatory support

  • Give fluids
  • Consider pulmonary vasodilator to combat high intrapulmonary pressures ( e.g. inhaled nitric oxide)
  • Consider an ionotrope (increase cardiac output without affecting rate), such as dobutamine