• pH:
• mean: 7.4
• 1 SD: 7.38-7.42
• 2 SD: 7.35-7.45
• acceptable: 7.3-7.5
  
• PaCO2:
• mean: 5.3 kPa (40 mmHg)
• 1 SD: 5.0-5.6 kPa (38-42 mmHg)
• 2 SD: 4.7-6.0 kPa (35-45 mmHg)
• acceptable: 4-6.7 kPa (30-50 mmHg)

Complications (1): acute changes in PaCO2 result in predictable changes in pH (the negative log of hydrogen ion concentration) and plasma carbonic acid. This represents the respiratory acid-base change. Although the relation is not completely linear, within clinically relevant ranges it is sufficiently linear to allow the following guideline to estimate the degrees of abnormality resulting from acute changes in PaCO2:

• For every increase in PaCO2 of 20 mmHg (2.6 kPa) above normal the pH falls by 0.1.
• For every decrease in PaCO2 of 10 mmHg (1.3 kPa) below normal the pH rises by 0.1.

Any change in pH outside these parameters is therefore metabolic in origin.

Approximate relation:

• PaCO2 80 mmHg (10.6 kPa) <-> pH 7.2 <-> Plasma bicarbonate 28 mmol/l
• PaCO2 60 mmHg (8 kPa) <-> pH 7.3 <-> Plasma bicarbonate 26 mmol/l
• PaCO2 40 mmHg (5.3 kPa) <-> pH 7.4 <-> Plasma bicarbonate 24 mmol/l
• PaCO2 30 mmHg (4 kPa) <-> pH 7.5 <-> Plasma bicarbonate 22 mmol/l
• PaCO2 20 mmHg (2.7 kPa) <-> pH 7.6 <-> Plasma bicarbonate 20 mmol/l