Bicarbonate and Base Excess

Bicarbonate

Normal range: 22 - 26 mmol/L

As we have previously discussed, bicarbonate is a base involved in the blood buffering system. It mops up H⁺ ions which increases the pH, making the blood more alkaline.

Too much bicarbonate and the plasma pH will rise, too little and the pH will fall.

Bicarbonate is produced as a byproduct of cell respiration. Cells produce CO₂ as a waste product, which is converted by carbonic anhydrase into carbonic acid (H₂CO₃). Carbonic acid dissociates in the plasma to form hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-).

Regulation of bicarbonate takes place in the kidneys which can reabsorb or excrete bicarbonate ions (mostly in the proximal tubules) to adjust the plasma balance. The kidneys can also produce more bicarbonate if reabsorption is not sufficient to maintain plasma balance. This is not a quick process, and takes days to reach equilibrium.

Base Excess

Normal range: -2 - +2 mmol/L

The base excess is a calculated figure (i.e. not measured) that estimates the quantity of hydrogen ions (in mEq/L or mol/L) required to bring the pH to 7.40. Although bicarbonate is the predominant base in blood, there are plenty of others that we don't measure directly, and base excess includes these to give a better picture of what's contributing to acid-base derangements.

It is particularly useful because it adjusts for pCO₂ derangements. We know that an increase or decrease in pCO₂ can alter the pH, but changes in pCO₂ can also be in response to an already altered pH. By adjusting for pCO₂ we can then figure out if changes in pH are due to respiratory factors (i.e. directly caused by CO₂) or due to metabolic factors (e.g. changes in bicarbonate concentration). The difference between respiratory and metabolic causes will be discussed in more detail when we get to interpretation.

In simple acid-base disorders base excess doesn't contribute much to interpretation, but it really comes into it's own when dealing with more complex disorders, when multiple causes are occurring simultaneously and we need to disentangle them from one another. Complex acid-base disorders are a bit beyond the scope of this tutorial, but it's worth having a basic understanding of what base excess represents.