A rising CO2 is a sign of ventilatory failure.

In a patient with reversible airways disease who is suffering an acute2
exacerbation, a PaCO2 > 6kPa is a sign of exhaustion requiring
ventilatory support. Indeed, a normal PaCO2 insuch patients is an
early indicator of exhaustion.

In a patient with COPD who has a chronically raised PaCO2 the same indications for ventilation may not apply.

A normal oxygen saturation on the pulse oximeter does not exclude2
a rising PaCO2. If there is a suspicion of poor ventilatory function despite a normal SpO2, take blood gasses.

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