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