The bicarbonate (HCO3
-) concentration is calculated from the PaCO2 value by using a mathematical relationship programmed into the analyzer. Some machines report actual and standardized values.
Standardized values are corrected to 98.6 F (37 C), a PaCO2 of 40 mm Hg, and normal oxygenation. Bicarbonate is one of the major acid buffering compounds in the body. When bicarbonate
is altered, body pH is altered or compensatory mechanisms are activated.
Total carbon dioxide is given, but the total carbon dioxide in the blood is largely a function of the actual bicarbonate concentration.
Usually 95% of the total carbon dioxide reported is due to the actual bicarbonate concentration. It is not an independent
measure of acid-base status because it depends on actual bicarbonate, which in turn depends on the PaCO2.
Base excess is the amount of strong acid needed to titrate the pH of 100% oxygenated human blood to 7.4 at 98.6 F (37 C) and
at a PaCO2 of 40 mm Hg.8 This parameter is often referred to as a base deficit but is also frequently called the negative base excess. Normal base excess for a person is 0 +/- 2, but base excess in veterinary patients will vary more depending on the species.
Base excess is influenced by the total serum protein concentration and will decrease about 2.9 mEq/L for every 1 g/dl increase
in total protein concentration. Base excess gives an indication of the metabolic component of acid-base disturbances and is
generally unaffected by changes in the PaCO2.
Base excess values can be used to calculate a replacement bicarbonate dose to correct metabolic acidosis. The formula is usually
bicarbonate (mmol) =
body weight (kg) X base excess
(mmol/L) X 0.3 (or some other factor)
The factor 0.3 is used for acute corrections because bicarbonate distributes to the extracellular fluid acutely. When long-term
bicarbonate therapy is indicated, other factors such as 0.6 are sometimes used because bicarbonate therapy is usually targeted
to the total body water (a larger volume of distribution). During anesthesia we correct acutely, so 0.3 is most commonly used.
When bicarbonate is given to patients, it is often administered slowly. Only 25% to 33% of the calculated amount is given
in any one dose because when acids are neutralized by bicarbonate, carbon dioxide is rapidly produced. This carbon dioxide
must be removed (usually by the lungs), or severe hypercapnia and paradoxical cerebral acidosis may result. Anesthetized patients
usually have some degree of respiratory depression and are unable to excrete large amounts of carbon dioxide as efficiently
as conscious animals are.
The alveolar-arterial oxygen difference (AaDO2 or A-a gradient) is an indication of the difference in oxygen partial pressure between the gas in the alveoli and the blood
leaving the left ventricle (assumed to be the same as the blood in the pulmonary capillaries). This difference is most useful
to indicate the degree of impairment of oxygen uptake due to factors such as pulmonary disease, atelectasis, or other causes
of ventilation-perfusion mismatching during anesthesia. Normal A-a gradients are generally less than 25 mm Hg in room air
(FiO2 ~ 0.21) but may increase at higher FiO2 values.