What CO2 Level is Considered High in Respiratory Acidosis
A PaCO2 above 6.0 kPa (45 mm Hg) is considered abnormal and defines hypercapnia, which when accompanied by pH <7.35 constitutes respiratory acidosis. 1
Defining Thresholds for Elevated CO2
The British Thoracic Society establishes clear cutoffs for abnormal CO2 levels:
- Any PaCO2 >6.1 kPa (45 mm Hg) is abnormal 1
- Values up to 6.7 kPa should be considered clinically significant 1
- The normal reference range for PaCO2 is 4.6–6.1 kPa (34–46 mm Hg) 1
Clinical Context: When Elevated CO2 Becomes Respiratory Acidosis
Elevated CO2 alone does not equal respiratory acidosis—the diagnosis requires both hypercapnia AND acidemia:
- Acute respiratory acidosis: pH <7.35 with PaCO2 >6.0 kPa (45 mm Hg) 1
- Compensated respiratory acidosis: High PaCO2 with high bicarbonate but normal pH 1
- Acute-on-chronic respiratory acidosis: Further CO2 rise in patients with pre-existing compensated hypercapnia 1
The distinction matters because patients with chronic COPD may have chronically elevated CO2 (e.g., PaCO2 of 7-8 kPa) with normal pH due to renal compensation through bicarbonate retention. 1 These patients are not in acute respiratory acidosis unless their pH drops below 7.35. 1
Severity Stratification for Clinical Decision-Making
For acute hypercapnic respiratory failure requiring intervention:
- PaCO2 >6.5 kPa (49 mm Hg) with pH <7.35 and RR >23 is the threshold for initiating non-invasive ventilation after one hour of optimal medical therapy 1
- PaCO2 between 6.0-6.5 kPa with pH <7.35 warrants consideration of NIV but is less urgent 1
The 2016 BTS/ICS guidelines specifically raised the NIV initiation threshold from 6.0 to 6.5 kPa because CO2 levels in the 6.0-6.5 range contribute minimally to acidosis, and many cases resolve with medical management alone. 1
Common Clinical Pitfalls
Do not confuse serum CO2 on a basic metabolic panel with arterial PaCO2. The "CO2" measured on routine chemistry panels reflects total CO2 content (predominantly bicarbonate), not the partial pressure of carbon dioxide that defines respiratory acidosis. 2 A patient can have elevated serum CO2 (bicarbonate >30 mmol/L) from metabolic alkalosis while having normal or even low arterial PaCO2. 2
In patients receiving excessive oxygen therapy, PaCO2 >10 kPa (75 mm Hg) strongly suggests oxygen-induced hypoventilation. 1 This is particularly dangerous in COPD patients where high-flow oxygen can worsen CO2 retention. 1 The oxygen should be stepped down to 28-35% via Venturi mask or 1-2 L/min via nasal cannula, targeting saturations of 88-92% in acidotic patients. 1
Chronic respiratory acidosis requires bicarbonate compensation over hours to days. 1 Acute rises in CO2 produce immediate pH drops because renal bicarbonate retention takes 24-72 hours to fully develop. 3, 4 This explains why a patient with acute-on-chronic respiratory failure may have severe acidosis (pH 7.15) despite elevated baseline bicarbonate—the kidneys were compensating for their baseline CO2 of 7 kPa but cannot keep pace with an acute rise to 10 kPa. 1