Base excess

I used to understand base excess .... until I read Friar Chris's post.

Base excess is the amount of H⁺ that would be required, if [CO₂] in the sample was normalised, to return that sample to a pH of 7.35 (the bottom limit of your reference range for pH).

In more direct terms what this means is it's going to give you more information on the nature of an acidosis or alkalosis based upon its value. Generally you get a RR of -2.0 to +2.0 and this can be used to demonstrate what I mean. All have the same CO₂ reading to exemplify here. (I'll omit HCO₃^- to focus on the base excess, but that's not because it shouldn't be evaluated).


Example 1:

pH: 7.18 ↓
PCO₂: 7.8kPa ↑
BE: 0 mmol/l -

Clearly acidotic and with carbon dioxide at 7.8kPa a respiratory one, but with a base excess of 0 there appears to be no compensation underway. Consider what I said about BE being the amount of acid needed to be added to reach 7.35 if the sample CO₂ was normalised.

If the CO₂ was at normal levels, we'd need no extra acid to counteract the effect of the circulating base, indicating that there is no excess base around from metabolic compensation.

You'd usually see this with a more acute respiratory acidosis that has presented to you as an emergency before metabolic compensation has kicked in.



Example 2:

pH: 7.10 ↓
PCO₂: 8.0kPa ↑
BE: -6.0mmol/l ↓

Once again, clearly acidotic with a pH of 7.12, and with a PCO₂ of 8.0kPa there is clearly a major respiratory component. But looking at the BE, there's also a deficit of base in the blood, so this is a mixed respiratory acidosis with metabolic acidosis too.

Going back to the basic model, if the CO₂ was normalised in this sample, you'd still require a negative amount of acid to return the pH to a normal level. I.e. there's a clear deficiency of base here, indicating that there's a metabolic acidosis going on here as well as the obvious respiratory acidosis.

You really don't want to see this on an ABG result, as it generally indicates that the faeces are going to hit the rotary ventilation device sometime soon... When you're not only failing to compensate but have deviated both respiratory and metabolically, that's bad mmmkay. Things like a patient with ARF on top of acute COPD spring to mind, acutely unwell people.



Example 3:

pH: 7.30 ↓
PCO₂: 8.0kPa ↑
BE: (+)7.0mmol/l ↑

Acidotic, though more mildly, and with the respiratory component clearly visible. The base excess is elevated, so the acidosis is solely respiratory in origin. With excess base in the blood here we're looking at metabolic compensation.

Back to the old model: if the CO₂ was normal, we'd need (+7.0?) a lot of extra acid to get the pH back down to the set-point; there's more base floating around in this sample even though the pH is lowered - i.e. that is base from metabolic compensation.

This is probably the kind of thing you'd see in a e.g. chronic COPD patient, who has been retaining too much CO₂ but has a long-term metabolic compensation for the resulting acidosis.




Now... You can go away and worry about anion gap