Acute Respiratory Distress Syndrom; causes, symptoms, treatment

Acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) describes the acute,

diffuse pulmonary inflammatory response to either direct (via

airway or chest trauma) or indirect blood-borne insults from

extrapulmonary pathology. It is characterised by:

Neutrophil sequestration in pulmonary capillaries.

Increased capillary Permeability

 Protein-rich pulmonary oedema.

If this early phase does not resolve with treatment of the underlying

cause, a fibroproliferative phase ensues and causes pulmonary

fibrosis. It is frequently associated with other organ dysfunction as

part of multiple organ failure.

The term ARDS is often limited to patients requiring ventilatory

support on the ICU, but less severe forms, conventionally referred

to as acute lung injury (ALI) and having similar pathology, occur on

acute medical and surgical wards. The clinical picture is non-specific

and shares features with other conditions (diffuse bilateral shadowing

on CXR in the absence of raised left atrial pressure, hypoxaemia,

impaired lung compliance).

Acute Respiratory Distress


Management

Respiratory support is indicated to maintain the patency of the

airway, correct hypoxaemia and hypercapnia, and reduce the work

of breathing.

Oxygen therapy: Ensure adequate arterial oxygenation (SpO2

> 90%). If a patient remains hypoxaemic on high-flow oxygen, other

measures are required.

Non-invasive respiratory support: If the patient has respiratory

failure associated with decreased lung volume, application of continuous

positive airway pressure (CPAP) will improve oxygenation

by recruitment of under-ventilated alveoli and reduce the work of

breathing. CPAP is most successful when alveoli are readily recruited

(e.g. pulmonary oedema, post-operative atelectasis). Patients should

be cooperative, be able to protect their airway and have the strength

to breathe spontaneously and cough effectively. In non-invasive

positive pressure ventilation (NIV), the patient’s spontaneous

breathing is supported by increased inspiratory and expiratory pressures

delivered using a bi-level ventilator (BiPAP). For NIV to

succeed, the patient’s spontaneous breathing must be sufficient to

trigger the machine and they must be able to tolerate a tight-fitting

facemask. NIV reduces the work of breathing, relieves breathlessness

and is proven to reduce the need for endotracheal intubation

in patients with type 2 respiratory failure (e.g. exacerbations of

COPD;). It is also used during weaning from conventional

ventilation.

Endotracheal intubation and mechanical ventilation: Many

patients admitted to ICU require intubation and mechanical ventilation.

Indications for intubation are summarised in. The

various types of invasive ventilatory support are summarised in.

In the conscious patient, intubation requires induction of anaesthesia

and muscle relaxation, while in more obtunded patients sedation

alone may be adequate. Hypotension commonly follows

sedation or anaesthesia because of direct cardiovascular effects

of the drugs and loss of sympathetic drive; positive pressure ventila-27

tion may compound this problem by increasing intrathoracic pressure,

which reduces venous return.

The selection of ventilator mode and settings for tidal volume,

respiratory rate, positive end-expiratory pressure (PEEP) and inspiratory

to expiratory ratio is dependent on the cause of the respiratory

failure.

Weaning from respiratory support: The majority of patients

require mechanical ventilatory support for only a few days and do

not need a process of weaning. In contrast, patients who have

required long-term ventilatory support for severe lung disease, e.g.

ARDS, may initially be unable to sustain even a modest degree of

respiratory work due to reduced lung compliance and muscle weakness;

hence they require a programme of gradual weaning from

ventilation.

Tracheostomy: This is usually performed electively when endotracheal

intubation is likely to be prolonged (> 14 days). Tracheostomies

help patient comfort, aid weaning from ventilation, and allow access

for tracheal toilet and intermittent respiratory support.

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