The base excess and base deficit measure how much extra acid or base is in the system as a result of a metabolic problem. The base excess is calculated by measuring the amount of strong acid that has to be added to the sample to produce a normal pH of 74 - it is therefore taking into account all the buffer systems. Minus is a base deficit (that is, the sample is already acidotic so no acid has to be added). Plus is a base excess (that is, the sample is alkalotic so acid has to be added). The normal range is - 2 to + 2 mmol/l. Base deficit can be an important predictor of severity of illness (see Chapter 5).
Standard bicarbonate is the concentration of bicarbonate in a sample kept under standard conditions - 37°C with a PaCO2 of 5 3 kPa (40 mmHg). This is calculated from the actual bicarbonate. Standard bicarbonate therefore purely reflects the metabolic component of acid-base balance as opposed to any subsequent changes in bicarbonate occurring as a result of respiratory problems.
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