Bicarbonate in Acid-Base Balance

Bicarbonate and acid-base balance

Plasma bicarbonate (HCO₃⁻) does most of the buffering in the extracellular space, and a normal reading sits somewhere between 22 and 26 mEq/L. The Henderson-Hasselbalch equation ties it to pH and PaCO2: pH = 6.1 + log(HCO₃⁻ / (0.03 · PaCO2)). Drop below < 22 and you are looking at metabolic acidosis; climb above > 26 and it is metabolic alkalosis. Take pH 7.40 with PaCO2 40 mmHg: the bicarbonate works out to roughly 24, which is exactly where it should be.

The body compensates at two very different speeds. Respiratory compensation is fast, kicking in within minutes as the lung tunes PaCO2. Renal compensation is slow, taking hours or days while the kidney reabsorbs HCO₃⁻ and dumps acid. Never read the bicarbonate in isolation. Line it up with pH, PaCO2 and the anion gap, and the mixed disturbances start to show themselves.

Clinical applications

This is everyday work in the ICU and emergency department: reading arterial blood gases, pinning down a metabolic problem (diabetic ketoacidosis, lactic acidosis, prolonged vomiting, chronic diarrhea), keeping an eye on renal replacement therapy, sizing up a poisoning (salicylates, methanol), and deciding how much bicarbonate to run in severe DKA once pH falls below 6.9.

FAQ

Difference between HCO₃⁻ and standard base excess? Bicarbonate is whatever the sample shows right now. Base excess (BE) estimates the metabolic component after the math has already backed out the PaCO2. They tend to move together, but each one answers a slightly different question.

When to give bicarbonate IV? Reserve it for severe metabolic acidosis when the patient is unstable or pH has fallen under 7.1, for hyperkalemia, or for tricyclic poisoning. Routine DKA does not need it; fluids and insulin do the job.

Venous and arterial gas — same HCO₃⁻? Close enough. Venous bicarbonate usually runs about 1–2 mEq/L above arterial, which makes a venous draw a handy screen when an arterial puncture is off the table.