RespiratoryTier 2Topic

Arterial blood gases

Focus

ABG interpretation for respiratory presentations centres on the respiratory contribution to acid-base status (PaCO2) and gas exchange assessment (PaO2, A-a gradient)
Essential in acute respiratory failure, COPD exacerbation, and any undifferentiated breathless/unwell patient

Respiratory failure types

Type 1 respiratory failure: PaO2 low, PaCO2 normal/low (V/Q mismatch - pneumonia, PE, pulmonary oedema, early asthma/COPD)
Type 2 respiratory failure: PaO2 low, PaCO2 high (ventilatory failure - severe COPD, opioid overdose, neuromuscular weakness, obesity hypoventilation)

Interpretation

The A-a gradient distinguishes a primary gas exchange problem (raised gradient - V/Q mismatch, shunt, diffusion defect) from pure hypoventilation (normal gradient - CNS depression, neuromuscular disease)
Calculate the expected compensation to identify a mixed acid-base disorder

Management

Acute respiratory acidosis (Type 2 failure) -> controlled oxygen therapy in known/at-risk COPD (avoid over-oxygenation suppressing hypoxic drive) + consider NIV if pH <7.35 despite treatment
Chronic compensated respiratory acidosis (raised bicarbonate) suggests chronic CO2 retention, not an acute event

Associations

COPD exacerbation, asthma severity assessment, opioid toxicity, neuromuscular respiratory failure

The tiring asthmatic

A normal or falling PaCO2 in a tiring severe asthmatic is an ominous sign (exhaustion, impending respiratory arrest), not reassurance

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