Define the terms 'base excess' and 'standard base excess' (20% marks)

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College Answer

Base excess is defined as the amount of acid or alkali that must be added to fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO2 of 40 mmHg.

Standard base excess is the amount of acid or alkali to return the ECF pH to 7.40 at a temperature of 37°C and a pCO2 of 40 mmHg and is calculated for blood at a Hb of 50 g/L.

Discussion

In brief:

Base excess definition

  • Dose of acid or base required to return the pH of a blood sample
  • 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

References

Reade, M. C. "Temporary epicardial pacing after cardiac surgery: a practical review." Anaesthesia 62.3 (2007): 264-271.

Reade, M. C. "Temporary epicardial pacing after cardiac surgery: a practical review: Part 2: Selection of epicardial pacing modes and troubleshooting."ANAESTHESIA-LONDON- 62.4 (2007): 364.

Gammage, Michael D. "Temporary cardiac pacing." Heart 83.6 (2000): 715-720.

Sanders, Richard S. "The Pulse Generator." Cardiac Pacing for the Clinician. Springer US, 2008. 47-71.