Contents
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
- 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
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
