Henderson-Hasselbalch Equation
Calculates blood pH from bicarbonate and partial pressure of CO2 (pCO2). Fundamental tool for acid-base interpretation. Complements the stepwise ABG approach.
Score interpretation
pH > 7.45. Alkalosis. Determine if respiratory (high pH, high pCO2 compensation) or metabolic (high pH, high HCO3).
→ Identify and treat underlying cause. Respiratory: consider hypoventilation causes (sedation, neuromuscular). Metabolic: check vomiting, diuretics, corticosteroids, Conn syndrome.
pH 7.35-7.45. Normal. May still have mixed acid-base disorder with compensation.
→ Check pCO2 and HCO3 for compensatory changes. Calculate anion gap if metabolic acidosis suspected.
pH < 7.35. Acidosis. Determine if respiratory (low pH, high pCO2) or metabolic (low pH, low HCO3).
→ Respiratory acidosis: airway management, NIV/intubation if severe. Metabolic acidosis: calculate anion gap, treat cause (DKA, lactic acidosis, renal failure, toxins). Bicarbonate only if pH < 7.1.
Interpretation bands for the H-H Equation. Apply clinical judgement and local guidance.
References
- Hasselbalch KA. Die Berechnung der Wasserstoffzahl des Blutes. Biochem Z. 1917;78:112-144.
- Constable PD. A simplified strong ion model for acid-base equilibria: application to horse plasma. J Appl Physiol. 1997;83(1):297-311.
Related
Curated clinical cross-links plus same-class fallbacks.
- Sodium acid phosphate with sodium bicarbonate · Bowel preparation
- Noradrenaline (Norepinephrine) · Vasopressor (Alpha-1 and Beta-1 Agonist)
- Sodium Bicarbonate · Alkalising Agent / Electrolyte
- Tranexamic Acid (ICU/Trauma/Surgical) · Antifibrinolytic
- Acipimox · Lipid-modifying agent (Nicotinic acid derivative)
- Colesevelam hydrochloride · Bile acid sequestrant
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.