Other topics - Acute Respiratory Distress Syndrome

From CambridgeNotes

• ‘leaky lung syndrome’ or non cardiogenic pulmonary oedema
• Incidence 2-8cases per 100,000/y
• Mortality >50%, usually due to mutliorgan failure (MOF), <20% die from hypoxaemia alone determined by precipitating condition; 35% trauma, ~60% for sepsis, ~80% for aspiration pneumonia

History and Examination

• progressive breathlessness, tachypnoea
• central cyanosis, hypoxic confusion
• lung crepitations

Internationally agreed criteria for dx of ARDS

1. severe hypoxaemia: P2O2/FiO2<200, PEEP, e.g. PaO2 (55mmHg)/FiO2 (80% inspired O2)=55/0.8=75
2. bilateral diffuse pulmonary infiltrates on CXR
3. normal or only slightly elevated left atrial pressure (pulmonary artery occlusion pressure <18mmHg)

Acute inflammatory phase: 3-10d and result in hypoxaemia and MOF Healing fibroproliferative phase: pulmonary fibrosis, pneumothoraces are common.

Association/aetiology

Direct:

• Pneumonia
• Near Drowning
• Pulmonary Trauma
• Aspiration
• Toxic-gas infulation: smoke, No2, NH3, Cl2, phosgene
• Oxygen toxicity (FiO2>0.8)

Indirect:

• sepsis
• non-thoracic trauma
• burns
• haemorrhage, multiple transfusion
• post arrest
• bowel infarction
• analphylactic
• pancreatitis
• uraemia, toxins, eclampsia
• drugs: salicylates, barbiturates

Investigations

• Temperature, RR, SaO2
• Urine output
• Arterial and central venous pressure- for assessing fluid balance and ensure adequate tissue oxygen delivery
• Watch out for 2y pulmonary infection; sputum/BAL
• CXR: progression of diffuse bilateral pulmonary infiltrates
• CT: (early) diffuse patchy infiltrates with dependent consolidation; (late)pneumothoraces, pneumatoceles and fibrosis

Treatments

Establish and treat underlying cause!!

1. Early stage

• o2 therapy
• physiotherapy

2. with progressive respiratory failure

• non-invasive ventilation: CPAP /NIPPV
• or full mechanical ventilation (see below) and high-inspired oxygen concentration may be required to maintain adequate ventilation and oxygenation

3. excess fluid loading must be avoided.

• Aim to maintain adequate perfusion of other organs whilst using lowest possible left atrial pressures.
• In acute situation, diuretics may be essential to correct hypoxaemia by  extravascular lung water
• combination of pulmonary and systemic vasodilators (b4 and after load  of left heart), inotropes and vasoconstrictor agents may be used to achieve adequate CO and perfusion pressure at low left atrial filling pressure

4. good nursing care, physiotherapy, nutrition and infection control: reduce fever, control anxiety with sedation reduce metabolic demand Drug treatment?

• NO drug therapy has been consistently beneficial in early ARDS, including steroids, anti-inflammatory agents, anticytokine or surfactant therapy
• 7-10d after onset, steroid therapy may prevent development of pulmonary fibrosis
• Inhaled nitric oxide and being in prone position improve gas exchange by  perfusion to ventilated areas of lung, but no survival benefit has been demonstrated
• Extracorporeal membrane oxygenation (ECMO) to oxygenate blood or remove CO2 are effective in children, but benefit in adults has not been establish

Complications

• Secondary lung infection
• High pressure required to achieve normal tidal volumes during mechanical ventilation often result in lung damage (barotraumas), pneumothorax, lung cysts. These injuries and oxygen toxicity (FiO2 >0.8) must be prevented as they contribute to mortality and MOF