Siemens RAPIDLAB 248 - Calculated Parameters

pO
2
Complete laboratory evaluation of oxygenation requires much more than
simple blood gas measurements. Assessment of ventilatory system and
acid-base status is essential to properly interpret clinical significance of
arterial oxygenation status. However, many patients can be evaluated and
treated successfully using blood gases alone if clinical observations and
patient history are taken into account
14
.
The measurement of pO
2
is significant in evaluating the degree of
hypoxemia (a deficiency of O
2
in arterial blood) present in a patient.
Calculated Parameters
The 248 calculates other parameters of interest to clinicians, and uses
several different equations to provide these parameters. Unless otherwise
noted, all measured values used in equations are at 37°C.
Bicarbonate Ion (HCO
3
–
)
Bicarbonate (HCO
3
–
) is the major buffer substance present in the body, and
plays a major role in maintaining the pH level in blood. It is present in
large amounts in the blood as a result of the dynamic state of CO
2
in the
blood. The majority of CO
2
is transported as HCO
3
–
.
The kidneys are the major controller of bicarbonate ion. Bicarbonate levels
are clinically significant in helping to determine the non-respiratory, renal
(metabolic) component in acid-base disorders.
Changes in HCO
3
–
levels along with pH values can help determine if
acidosis or alkalosis disorders are of metabolic origin. In metabolic acidosis,
HCO
3
–
levels decrease causing an increase in H
+
which leads to a decrease
in pH. Conversely, in metabolic alkalosis, HCO
3
–
levels increase, causing a
decrease in H
+
which leads to an increase in pH.
There are two versions of bicarbonate, the actual value and the standard
value, available in the System Set Up menu.
Actual Bicarbonate (HCO
3
–
act)
Based on the National Committee for Clinical Laboratory Standards
(NCCLS) recommendations
16
cHCO
3
–
act
= 0.0307 x pCO
2
x 10
(pH – 6.105)
Standard Bicarbonate (HCO
3
–
std)
The equation described by VanSlyke and Cullin
17
is used for calculating
standard bicarbonate
cHCO
3
–
std
= 24.5 + 0.9A + (A–2.9)
2
(2.65 + 0.31ctHb)/1000
where A = BE(B) – (0.2ctHb(100–O
2
SAT)/100)
If no ctHb value has been entered, a value of 15g/dL is assumed.
Operating Principles Page G-7