Introduction

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 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.
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

Signs and Symptoms

  • Can be difficult to differentiate from ACUTE HEART FAILURE
    • Can distinguish between the two by taking a pulmonary wedge pressure measurement.
    • 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
  • CXRdemonstrating 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

Example of ARDS on X-ray.

Example of ARDS on X-ray. Image from Wikimedia Commons. Author: Samir

Example of ARDS on X-ray.

Example of ARDS on X-ray. Image from wikimedia commons. Author: James Heilman, MD

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

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