Proxima
pCO2 and Calculated values for HCO3- and Base
Excess on Proxima
Operating principles
The Proxima Sensor is fabricated as a single
multi-analyte chip that carries an array of
individual sensors, each of which measures a
different analyte.
pCO2 is measured by direct potentiometry. The
pCO2 sensor used in Proxima is an ISFET (ion
selective field effect transistors) based version
of the sensor described by Severinghaus (1). The
Severinghaus electrode is a modified pH electrode in contact with
sodium bicarbonate solution.
The gate region of the pH sensitive ISFET is covered by a thin layer of
bicarbonate containing electrolyte, which is isolated from the sample
by a gas diffusion membrane. As the sensor is brought into contact
with the sample, CO2 readily diffuses across the gas diffusion
membrane to equilibrate the concentration in the sensor electrolyte
with that in the sample. As the CO2 in the sensor electrolyte is itself
in equilibrium with the bicarbonate concentration, the pH of the
sensor electrolyte is proportional to pCO2. The ISFET is used to
measure the pH of the sensor electrolyte with a silver-silver chloride
electrode used as the reference electrode.
Calculated parameters
Calculated parameters bicarbonate (HCO3-) and base excess (BE) are
calculated after the values obtained from pH and pCO2.
The bicarbonate ion is the main buffer substance in the body and
plays a key role in maintaining the pH value in the blood.
The Proxima System calculates the value of standard bicarbonate
from the current analyte readings. This refers to the bicarbonate
content of plasma which would be present in blood equilibrated to a
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pCO2 Test Information Sheet PL:028 v1
Proxima
pCO2 and Calculated values for HCO3- and Base
Excess on Proxima
pCO2 of 40 mmHg and pO2 of 100 mmHg at 37 ºC. The following
equation is used to calculate the standard bicarbonate:
[HCO3-] std = 24.5 + 0.9 x A + (((A – 2.9)2 x (2.65 + 0.31 x tHb))/1.000)
where A = BE(B) – (0.2 x tHb x (100 - O2SATest))/100
BE (B) is calculated assuming 100% oxygen saturation. cHb used in
this calculation is 15 g/dl.
O2 sat is calculated from:
O2 SAT (est)=((N4-15*N3+2045*N2+2000*N)/(N4-15*N3+24000N231100*N+2.4*106))*100
where N= pO2*10(0.48*(pH(37)-7.4)-0.0013*BE(B))
Base excess is a calculated parameter and allows the calculation of
the buffer quantity that needs to be infused in a patient with
impaired acid-base balance. Actual Base Excess is the concentration
of titratable base when the blood is titrated with a strong base or
acid to a plasma pH of 7.40 at a pCO2 of 40 mmHg and 37 ºC at the
actual oxygen saturation. The Proxima System calculates the value of
base excess from the current analyte readings.
The Base excess calculated in the Proxima System is the BE of the
blood. In addition to the parameters HCO3- and pH value, the base
excess of the blood takes into account the buffer effect of the blood.
BE b = (1-0.014*tHb)*((HCO3-act-24.8)+((7.7+1.43*tHb)*(pH(37ºC)-7.40)))
Where tHb is an entered value of 15 g/dl.
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pCO2 Test Information Sheet PL:028 v1
Proxima
pCO2 and Calculated values for HCO3- and Base
Excess on Proxima
Measuring and Reference ranges
Test /Abbreviation
Units*
Reportable
range
Reference range
(arterial)
Partial Pressure Carbon
Dioxide / PCO2
mmHg
15.0-100.0
35-45(2)
kPa
2.00-13.33
4.7-5.9
Bicarbonate/ HCO3-
mmol/L 0.0-80.0
22-31(2)
Base Excess/ BE
mmol/L (-29.9)(+29.9)
(-2)-(+3) (2)
*The Proxima System can be configured with any of these units
To convert pCO2 results from mmHg to kPa, multiply the mmHg value
by 0.133.
Each institution should establish its own reference range for
diagnostic evaluation of patient results. It is recommended to use
reference ranges for the population being tested.
Clinical significance
Carbon dioxide is a significant end product of metabolic processes in
the body and is excreted as gaseous carbon dioxide during
respiration and as bicarbonate ions by the kidneys. Carbon dioxide
reacts with water to form carbonic acid, which in turn dissociates to
form bicarbonate and hydrogen ions.
CO2 + H2O
H2CO3
H + HCO3-
The bicarbonate/carbonic acid buffer system is the most important
buffer keeping blood plasma in a narrow range around pH 7.4.
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pCO2 Test Information Sheet PL:028 v1
Proxima
pCO2 and Calculated values for HCO3- and Base
Excess on Proxima
Measurement of pCO2 together with pH is an important component
in the diagnosis of acid-base disorders. PCO2 is the respiratory
component of acid-base balance and is a measure of the pressure of
carbon dioxide dissolved in blood.
Elevated pCO2 readings are a primary indication of respiratory
acidosis or a compensatory response to a metabolic alkalosis (the
respiratory system is the cause).
Common causes of respiratory acidosis could be any condition that
impairs gas exchange or lung ventilation (chronic bronchitis, cystic
fibrosis, emphysema and pulmonary oedema), obstructive lung
disease, head trauma, oversedation or inappropriate settings for
mechanical ventilation.
Low pCO2 levels are a primary indication of a respiratory alkalosis or
a compensatory response to a metabolic acidosis (the respiratory
system is not the cause but it compensating)
Common causes of respiratory alkalosis could be hypoxemia, acute
anxiety, asthma, early stages of congestive obstruction airway
disease, low haemoglobin level, brain tumour or injury, respiratory
stimulant drugs or excessive mechanical ventilation.
HCO3- (bicarbonate) is the most abundant buffer in the blood plasma
and is an indicator of the buffering capacity of blood.
Regulated primarily by the kidneys, HCO3- is the metabolic
component of acid-base balance. Changes of the HCO3concentration in connection with pH values are used for the
determination of whether an acidosis or alkalosis of metabolic origin
is present. An increased level of HCO3- may be due to a metabolic
alkalosis or a compensatory response in respiratory acidosis.
Decreased levels of HCO3- are seen in metabolic acidosis and as a
compensatory mechanism in respiratory alkalosis. Causes of primary
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pCO2 Test Information Sheet PL:028 v1
Proxima
pCO2 and Calculated values for HCO3- and Base
Excess on Proxima
metabolic acidosis are ketoacidosis, lactate acidosis (hypoxia) and
diarrhoea.
A negative base excess indicates the presence of metabolic acidosis
as acid would need to be subtracted to return blood pH to normal
and a positive base excess indicates the presence of metabolic
alkalosis as the acid would have to be added to return the blood pH
to normal. BE allows the calculation of the buffer quantity that needs
to be infused.
Measurement Temperature
The Proxima Sensor is heated to 37ºC during the Sample
measurement. PCO2 is a temperature-dependent quantity and is
measured at 37ºC.
System Performance
These are example performance data given in the tables below.
Precision data in controls:
Two different levels of quality control material were estimated using
a minimum of 6 Proxima Systems over a minimum of 27 days.
Precision data
(mmHg)
N
Level 1
242 66.00 2.40 3.63%
Level 2
229 39.97 1.05 2.62%
Precision data (kPa)
N
Mean SD*
Level 1
242
8.80
0.32 3.63%
Level 2
229
5.33
0.14 2.62%
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Mean SD*
5
%CV
%CV
pCO2 Test Information Sheet PL:028 v1
Proxima
pCO2 and Calculated values for HCO3- and Base
Excess on Proxima
*Based on total SD
Method comparison data
The study was conducted in a laboratory setting with transfusion
blood samples. Arterial blood samples were collected and compared
with the Siemens 1200 reference device.
Slope Intercept RMSE* R2
Method
comparison
N
Min Max
pCO2 (mmHg)
827 0.985 0.383
3.2
0.98 15.2 99.7
pCO2 (kPa)
827 0.985 0.051
0.43
0.98 2.03 13.29
*RMSE: Root mean square error
Factors affecting the results
Adequate sample should be drawn to ensure a representative
arterial blood sample is measured. If the sample is drawn too quickly,
this may de-gas the sample leading to lower concentration to be
reported.
As Proxima operates in a closed system the arterial blood sample
couldn’t be exposed to air if the product is used following the
Proxima User Manual.
pCO2 on the sample is measured on the Proxima Sensor within less
than 1 min ensuring the prompt analysis of the sample.
Potential interference by exogenous and endogenous substances
studies (based on CLSI guideline EP7 –A2 “Interference Testing in
Clinical Chemistry; Approved Guideline-Second Edition”) have
assessed the specificity of the pCO2 measurement on the Proxima
Sensor.
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pCO2 Test Information Sheet PL:028 v1
Proxima
pCO2 and Calculated values for HCO3- and Base
Excess on Proxima
Sodium pentothal has been shown to have a potential interference
on the measurement of pCO2 on the Proxima Sensor at the following
concentration > 20.6 μmol/l.
References
1. Severinghaus J., Bradley F. Electrodes for pO2 and pCO2
determination. Journal of Applied Physiology, 13 (3):515 (1958)
2. Scott M, Heusel J, LeGrys V, Siggaard-Andersen O; Electrolytes
and Blood Gases. Tietz Textbook of Clinical Chemistry, Third
Edition, ed. C.A. Burtis and E.R. Ashwood. Saunders 1999.
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pCO2 Test Information Sheet PL:028 v1