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Reply To: VNCert EEC Exam Question: Blood Gas

Simon Patchett

Hi Gail,

So in traditional acid-base analysis the changes we see are related to the equation below.

CO2 + H2O ←→ H2CO3 ←→ H+ + HCO3−

You’re absolutely right that changes in HCO3- are therefore dependent on PCO2 (respiratory component). Usually PCO2 and HCO3- would go the same way (decrease to one results in decrease of the other) as we see in the question, the body loves to compensate and achieve equilibrium where possible and so tries to achieve this via the above equation.This creates difficulty when you’re trying to evaluate the two values separately.

Base excess is a ‘better’ way of measuring metabolic disturbance because it is independent of the respiratory system (PCO2). It is defined as the amount of acid or base that must be added to a sample of oxygenated whole blood to restore the pH to 7.4 at 37°C and at a PCO2 of 40mmHg.

Essentially if there are minimal changes to the PCO2 then either Base excess or bicarb to evaluate the metabolic component should be fine.

In your question they have only given you bicarb to evaluate the metabolic component and so that’s all you’ve got to go on here, and I would agree with Annette’s interpretation.

In terms of the lactate this would effect your base excess in an equimolar fashion. In terms of getting more complicated with acid-base analysis you can actually get a numerical contribution of things like lactate, albumin, chloride to base excess.

Hope that helps