Bicarbonate & Base Excess

In medicine, base excess refers to a highly increased amount of a base found in the blood. It is estimated as a positive concentration having the unit mEq/L (mmol/L). The specific gap for base excess is given as −2 to +2 mEq/L.

Relating the base excess to the specific gap values helps to determine if there’s an acid/base hindrance due to any respiratory, metabolic, or a combination of metabolic-respiratory issues. The CO2 determines the respiratory part of the acid-base balance and the base excess determines the metabolic part. Thus, all in all, the evaluation of base excess is determined as a fixed pressure of CO2, by titrating back to a fixed pH of 7.40 in the blood.

The major base influencing the base excess is bicarbonate. Hence, a slight shift of serum bicarbonate from the specific gap is in accordance with the slight shift in base excess. Nevertheless, base excess is an efficient calculative tool, involving and taking into consideration all metabolic aspects.

Base excess is referred to as the quantity of strong acid which is put into each liter of fully oxygenated blood to get the pH back at 7.40 and the temperature at 37°C and a pCO2 of 40 mmHg (5.3 kPa).

There can be a further division seen in actual and standard base excess: actual base excess is that found in the blood, whereas standard base excess is estimated when the hemoglobin value is 5 g/dl. Thus we use the standard base excess as it provides a more clear look at the whole extracellular fluid.

Base excess is a common estimate usually found in arterial blood gas diagnosis which is taken from other calculated information. The idea and implementation of base excess were first stated in 1958 by Poul Astrup and Ole Siggaard-Andersen.

Estimation

The following equation is the formula of Base excess:

Base excess = 0.93 * ([HCO^-₃] - 24.4 + 14.8 * (pH - 7.4)]

with units of mEq/L. It also has another formula :

Base excess = 0.93 * [HCO^-₃] + 13.77 * pH - 124.58)

Abnormal Base excess values can prove to show:

• If the value is above +2 mEq/L: metabolic alkalosis is diagnosed
• If the value is below -2 mEq/L: metabolic acidosis is diagnosed

Such as, having low ventilation, which can pose a respiratory issue, leads to the accumulation of CO2, which causes respiratory acidosis; the kidneys then try to compensate for having a less pH value by increasing bicarbonate levels in the blood. Thus, in conclusion, the kidneys do partial compensation in case of respiratory acidosis by increasing bicarbonate levels in the blood. An elevated base excess, hence metabolic alkalosis, mostly includes having very high levels of bicarbonate in the blood. It is usually because of the following:

• Requiting for primary respiratory acidosis.
• Vomiting can lead to increased loss of HCl present in gastric acid.
• Kidneys over-releasing bicarbonate, in both cases alkalosis or Cushing's disease occurs.