Acute (Adult) Respiratory Distress Syndrome

Synonyms: adult respiratory distress syndrome (ARDS); acute lung injury (ALI).

Acute respiratory distress syndrome (ARDS) is a common and devastating condition which can affect all adult patients, e.g. medical, surgical and obstetric patients. It occurs when noncardiogenic pulmonary oedema (secondary to acute damage to the alveoli) leads to acute respiratory failure. Although the terms ARDS and ALI are used interchangeably, the American-European Consensus Conference Committee describes them as different entities: ALI having less severe hypoxaemia than ARDS.¹

Incidence is uncertain, but was reported as 17.9 per 100,000 for acute lung injury and 13.5 per 100,000 for ARDS in a Scandinavian study in 1994.² In the US it is estimated there are 190,000 cases per year and over 74,000 deaths.¹

The more risk factors present the greater the chance of ARDS.

Commonest risk factors

• Sepsis
• Massive trauma with shock and multiple transfusions
• Hypovolaemic shock
• Pneumonia
• Gastric aspiration

Increased permeability of pulmonary microvasculature causes leakage of proteinaceous fluid across the alveolar-capillary membrane.³ This may be one manifestation of a more generalised disruption of endothelium, resulting in hypoxia and multiple organ failure. There is also evidence of inflammation in the lung tissue which can be seen on metabolic imaging methods.⁴

• Symptoms: history of relevant injury and increasing dyspnoea which may occur some time after the precipitating event.
• Signs: cyanosis (reflecting hypoxia refractory to oxygen therapy), tachypnoea, tachycardia, peripheral vasodilatation; bilateral fine inspiratory crackles.

• FBC, U&E, LFTs, amylase, clotting, CRP, blood cultures, ABG.
• CXR shows bilateral alveolar shadowing, often with air bronchograms.
• Pulmonary artery catheter to measure pulmonary capillary wedge pressure (PCWP).

One consensus requires these 4 to exist:⁵

1. Acute onset: 20-50% of acute lung injury patients will develop ARDS within 7 days.⁶
2. CXR shows bilateral infiltrates.
3. PCWP ≥18 mmHg or a lack of clinical evidence of left atrial hypertension (i.e. no evidence of cardiac failure).
4. Refractory hypoxaemia: acute lung injury is present when the ratio PaO₂:FiO₂ <300; ARDS is present when PaO₂:FiO₂ <200.

Admit to ITU, give supportive therapy and treat the underlying cause.

Respiratory support

In early ARDS continuous positive airway pressure (CPAP) with 40-60% oxygen may be adequate to maintain oxygenation. But most patients need mechanical ventilation.

Circulatory support

Invasive haemodynamic monitoring with an arterial line and Swan-Ganz catheter aids the diagnosis and may be helpful in monitoring PCWP and cardiac output.

Maintaining cardiac output and thus oxygen delivery usually needs inotropes (e.g. dobutamine), vasodilators and blood transfusion. Pulmonary hypertension can be treated with low-dose (20-120 ppm) nitric oxide, a selective pulmonary vasodilator.¹² However, a systematic review and meta-analysis found that nitric oxide only provides short-term improvement and does not affect survival. Furthermore, nitric oxide was associated with renal dysfunction.¹³ Haemofiltration may also be needed in renal failure and to achieve a negative fluid balance.

Other therapies

Steroids, such as methylprednisolone have been used despite conflicting evidence. The evidence suggests that low doses of steroids are associated with a reduced duration of ventilation in early ARDS rather than persistent ARDS.¹ However improvements in mortality with steroids becomes apparent later (>7 days), particularly if there is eosinophilia in the blood or in bronchial-alveolar-lavage (BAL).

More novel therapies currently include activated protein C, granulocyte-macrophage colony-stimulating factor and the use of beta agonists to enhance alveolar fluid clearance.¹²

Sepsis

Identify organism(s) and treat accordingly. If clinically septic, but no organisms cultured, use empirical broad spectrum antibiotics, but avoid nephrotoxic antibiotics.

Nutritional support

Enteral is better than parenteral feeding.

• Overall mortality is 50-75%.
• Prognosis varies with age of patient, cause of ARDS (pneumonia 86%, trauma 38%), and number of organs involved (3 organs involved for >1 week is invariably fatal).
• In most cases, survivors' lung function returns almost to normal within 6-12 months. There may be reduced vital capacity and some obstructive lung disease but these are usually asymptomatic.⁶
• Interestingly, patients with acute lung injury have reduced exercise capacity up to two years after the episode and there is evidence to suggest long-term neurocognitive impairment.⁶