Medica EasyStat - Page 113

107
p r i n c i p l e s o f o p e r a t i o n / t h e o r y
r
BE
ecf
The base excess of the extracellular fluid, or in vivo base
excess, or standard base excess, is calculated using the follow-
ing equation
1
. The patient’s acid-base interbalance metabolic
component is measured by the BE
ecf
. This parameter may be
determined by titration of an extracellular fluid model to pH =
7.400 and PCO
2
= 40 mmHg at 37.0°C.
BE
ecf
= [HCO
3
-
] - 25 + 16.2 (pH - 7.4)
where: BE
ecf
is expressed in mmol/L
HCO
3
-
is expressed in mmol/L
SBC
The bicarbonate concentration in plasma from a blood sample
that is equilibrated to 37.0°C with PCO
2
= 40 mmHg and
PO
2
greater than 100 mmHg is known as the standard bicar-
bonate of blood. The EasyStat analyzer uses the following
equation
1
:
SBC = 25 + 0.78 BE
b
+ 0.002 [Hb] (%SO
2
c - 100)
where: SBC is expressed in mmol/L
%SO
2
c is expressed as a percentage
The hemoglobin value used is either preset (14.5 g/dL), entered or
calculated. When entered, the value is input from the PATIENT
INFORMATION screen.
HCO
3
-
The following equation is used to calculate the bicarbonate concentra-
tion
1
:
where: log [HCO
3
-
] = pH - 7.608 + log PCO
2
HCO
3
-
is expressed in mmol/L
BE
b
The base excess of blood, or in vitro base excess, is calculated using
the following equation
1
. This parameter is defined as the titratable base
of blood which may be determined by titration with strong acid or base
to (plasma) pH = 7.400 and PCO
2
= 40 mmHg at 37.0°C.
BE
b
= (1 - 0.014 [Hb]) ([HCO
3
-
] - 25 + (1.43 [Hb] + 7.7) (pH - 7.4))
where: Hb is expressed in g/dL
HCO
3
-
is expressed in mmol/L
BE
b
is expressed in mmol/L
The hemoglobin value used is either preset (14.5 g/dL), entered or
calculated. When entered, the value is input from the PATIENT
INFORMATION screen.