Base Excess
Base excess is an empirical expression that approximates the amount of
acid or base required to titrate one litre of blood back to a normal pH of 7.4.
The base excess in blood with a pH of 7.4, pCO
2
of 40 mm Hg (5.33 KPa),
total haemoglobin of 15 g/dL and a temperature of 37°C is zero. Base
excess is useful in the management of patients with acid-base disorders as
it allows the estimation of the number of equivalents of sodium bicarbonate
or ammonium chloride required to correct the patient’s pH to normal.
There are two versions of base excess, available in the System Set Up
menu.
Base Excess of Extracellular Fluid (BE(ecf))
The base excess of extracellular fluid, formerly known as in vivo base
excess, reflects only the non-respiratory component of pH disturbances
BE(ecf) = cHCO
3
–
act
– 24.8 + 16.2 (pH – 7.4)
Base Excess of Blood (BE(B))
The base excess of blood, formerly known as in vitro base excess, is
calculated from the following equation
BE(B) = (1 – 0.014ctHb) (cHCO
3
–
act
– 24.8 + (1.43ctHb + 7.7) (pH – 7.4))
If no ctHb value has been entered, a value of 15g/dL is assumed.
Oxygen Content (O
2
CT)
Oxygen content is the concentration of the total oxygen carried by the
blood, including oxygen bound to haemoglobin as well as oxygen dissolved
in plasma and in the fluid within red cells.
Oxygen content is calculated, using NCCLS recommendations
18
, as follows:
O
2
CT = (1.39ctHb x O
2
SAT/100) + (0.00314pO
2
)
where ctHb is expressed in g/dL.
If no ctHb value has been entered, O
2
CT is not calculated.
Clinically, dissolved oxygen is for most situations analytically unimportant.
However, at very low levels of haemoglobin or in patients receiving
hyperbaric oxygen therapy, dissolved oxygen may be a very significant
contributor to oxygen transport.
Page G-8 248 Operator’s Manual
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