Base excess and standard base excess
Trainees were called upon to define these basic terms in Question 9 from the second paper of 2006. Since then, this topic has remained submerged for almost ten years, reappearing again in Question 4.3 from the first paper of 2016. It may now be lost again for another ten years. However, the issue remains interesting. The "Base Excess" is the amount of acid or base required to titrate a blood sample (of whole blood) to a pH of 7.40, at standard temperature and pressure, with a standard PaCO2 of 40mmHg. The "Standard Base Excess" is different because it uses extracellular fluid rather than whole blood. Given that extracellular fluid is a fairly heterogeneous slurry which is inconvenient to sample, the ABG machine calculates the SBE for anaemic blood, with a Hb of 50g/L. The argument for this is the buffering ability of haemoglobin. It would be inappropriate to extrapolate whole blood findings to the total extracellular fluid, because though circulating haemoglobin buffers all extracellular fluid, it does so from the intravascular compartment to which it is confined.
In brief summary:
Base excess definition
- Dose of acid or base required to return the pH of a blood sample to 7.40
- Measured at standard conditions - 37°C and 40mmHg PaCO2
- Thus, isolates the metabolic disturbance from the respiratory
Standard base excess
- Dose of acid or base required to return the pH of an anaemic blood sample
- Calculated for a Hb of 50g/L
- Haemoglobin buffers both the intravascular and the extravascular fluid
- Thus, SBE assesses the buffering of the whole extracellular fluid, not just the haemoglobin-rich intravascular fluid
Negative SBE with normal anion gap
- Normal anion gap metabolic acidosis
- Anion gap uncorrected for a low albumin
Normal SBE with abnormal anion gap
- High anion gap metabolic acidosis with pre-existing metabolic alkalosis
