Base Excess and Standard Bicarbonate

Expansion

• Actual bicarbonate: the measured value, which is influenced by carbon dioxide as well as by metabolic factors
• Standard bicarbonate: bicarbonate calculated at a PaCO2 of 40 mmHg, so it removes the respiratory influence
• Base excess: the amount of acid or base needed to return one litre of blood to a pH of 7.4 at a PaCO2 of 40 mmHg and 37 degrees. Normal is minus 2 to plus 2

Interpretation:

• Base excess below minus 2 (base deficit): metabolic acidosis
• Base excess above plus 2: metabolic alkalosis

The value of base excess is that it is independent of respiratory changes, so it quantifies the metabolic disturbance directly, whereas actual bicarbonate rises in respiratory acidosis and falls in respiratory alkalosis purely from mass action.

Standard base excess is calculated for the whole extracellular fluid rather than blood alone, and is preferred in vivo.

In trauma and shock, the base deficit is used as a marker of the severity of hypoperfusion and of the adequacy of resuscitation, correlating with lactate and with transfusion requirement.