Base Excess
The base excess is the amount of strong acid that must be added to each litre of fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO2 of 40 mmHg.
Whilst P(CO2) defines the respiratory component, the base excess tells you what the metabolic state is doing.
Therefore, it can be used to tell you whether there is a compensatory metabolic acidosis/alkalosis occurring.
e.g. if you have a pure respiratory acidosis: pH=7.3, P(CO2)=50mmHg
If the CO2 was corrected, the pH would return to normal – so no extra base has to be added to make pH normal.
Therefore, the base excess here is 0, indicating that there is no metabolic compensation
If, however, correcting the CO2 would make pH abnormal there will be base excess:
High base excess (more than +2 mEq/L)
This suggests there is an excess of HCO3
(e.g., metabolic alkalosis, compensation for respiratory acidosis)
Low base excess (less than +2 mEq/L)
This suggests a reduction of HCO3- ions
(e.g., metabolic acidosis, compensation for respiratory alkalosis)
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