Respiratory Acidosis Diagnosis and Management

Defining Hypercapnia and Respiratory Acidosis

The British Thoracic Society defines hypercapnia as a PaCO2 above 6.0 kPa (45 mm Hg), and respiratory acidosis as hypercapnia accompanied by a pH <7.35 1, 2
A PaCO2 >6.1 kPa (45 mm Hg) is considered abnormal, with values up to 6.7 kPa being clinically significant 1
The normal reference range for PaCO2 is 4.6–6.1 kPa (34–46 mm Hg) 1

Acute respiratory acidosis is diagnosed by a pH <7.35 with a PaCO2 >6.0 kPa (45 mm Hg) 1, 3
Compensated respiratory acidosis is characterized by a high PaCO2 with high bicarbonate but normal pH 1
Acute-on-chronic respiratory acidosis occurs when there is a further rise in CO2 in patients with pre-existing compensated hypercapnia 1

The British Thoracic Society recommends initiating non-invasive ventilation for acute hypercapnic respiratory failure with a PaCO2 >6.5 kPa (49 mm Hg), pH <7.35, and respiratory rate >23 after one hour of optimal medical therapy 2, 3
A PaCO2 between 6.0-6.5 kPa with pH <7.35 warrants consideration of non-invasive ventilation but is less urgent 2, 3

The British Thoracic Society advises against confusing serum CO2 on a basic metabolic panel with arterial PaCO2, as the former reflects total CO2 content (predominantly bicarbonate) 5
In patients receiving excessive oxygen therapy, a PaCO2 >10 kPa (75 mm Hg) strongly suggests oxygen-induced hypoventilation, particularly in COPD patients 5
Chronic respiratory acidosis requires bicarbonate compensation over hours to days, with acute rises in CO2 producing immediate pH drops due to delayed renal bicarbonate retention 1