Normal Values in Plasma
pH: 7.40 ± 0.02
[H+]: 40 ± 2 nmol/L
[HCO 3 − ] in plasma (PHCO3): 25 ± 2 mmol/L
Arterial PCO 2 : 40 ± 2 mm Hg
Brachial venous PCO 2 : <10 mm Hg higher than the arterial PCO 2 .
Albumin in plasma (P Albumin ): 4.0 g/dL or 40 g/L. Accounts for ∼12 mEq/L of the anionic charge in plasma.
Anion gap in plasma (P Anion gap ): 12 ± 2 mEq/L (must adjust for the valence from the PAlbumin)
Henderson equation: [H + ] (nmol/L) = PCO 2 (mm Hg) × (25 or 24)/ PHCO3
To make the mathematics easier at the bedside, one can use 25 or 24 as the constant.
