Various current brands available and their theoretical advantages & disadvantages

 

1.                   Bentley Gas-STAT

a)                   Optical fluorescence microsensing

i)                    Light of a specific colour is transmitted down the fibreoptic cables to the disposable sensors

ii)                   Sensors measure the blood gases through membranes that are permeable to gases & hydrogen ions

iii)                 Special fluorescent materials on the sensors are sensitive to levels of O2, CO2, & pH

iv)                 When light generated from the instrument reaches the fluorescent materials on the sensors, a molecular excitation occurs and light of a different colour is emitted and carried back to the instrument through another optical fibre

v)                  The difference in the transmitted light and the received light is converted into a numerical readout in mmHg for CO2 & O2 and units for pH

b)                  Features

i)                    Has its own calibration device for the initial calibration which is fast & simple

ii)                   Measurements can be displayed at actual blood temperature or temperature corrected to 37°C

iii)                 Gives immediate, continuous & accurate readings (3 min response time)

 

 

2.                   Mallinckrodt Sensor Systems

a)                   Uses conventional blood-gas machine technology (?polarographic cell etc)

b)                  Automatic calibration

c)                   Direct sampling from extracorporeal circuit

d)                  Accuracy similar to benchtop

e)                   Not fully continuous measurement - results available 90 seconds after sampling

 

3.                   Biomedical sensor

a)                   Uses optical measurement techniques based on light from a fiberoptic bundle passed through a pH sensitive dye and returned via a fiberoptic bundle to the instrument

b)                  ? Colourmetric

c)                   Limited accuracy & response time similar to 3M Healthcare below

 

4.                   CDI 3M Healthcare

a)                   Uses dyes that fluoresce at intensities that vary with the concentration of O2, CO2 & pH

b)                  Accuracy is less reliable than the Mallinckrodt system - requires periodic conventional blood gases for confirmation

c)                   90% response times: PO2-210 sec; PCO2-238 sec; pH-217 sec

d)                  Similar to Bentley system

 

 

Methods of measuring blood gases by: conventional blood gas electrodes; pulsed optical fluorescence; colorimetric

 

1.                   Conventional blood gas electrodes

a)                   Eg: miniaturised polarographic electrode used within a catheter in an artery or vein

i)                    A platinum cathode and a silver/silver chloride anode are immersed in an electrolyte solution of potassium chloride

ii)                   A voltage of 0.7 v is applied between the electrodes and the current flow is measured

iii)                 At the anode, electrons are provided by the oxidation reaction of the silver with the chloride ions of the potassium chloride electrolyte to give silver chloride and electrons

a)                    4Ag                         ® 4Ag+ + 4e-

b)                  4Ag+  + 4Cl-            ® 4AgCl

iv)                 At the cathode, oxygen combines (reduction) with the electrons and water giving rise to hydroxyl ions

a)                   O2 + 4e-   ® 2O2

b)                    2O2 + 2H2O             ® 4(OH)-

v)                  The more oxygen available, the more electrons which can be taken up at the cathode and consequently the greater the current flow.

vi)                 The current flow through the cell is dependent on the oxygen concentration at the platinum electrode

vii)               For every molecule of oxygen reduced, four electrons are supplied by the electrical circuit

viii)              Thus the current, having been determined by the availability of oxygen molecules, is directly proportional to the partial pressure of oxygen

ix)                  Measurement of the current and conversion to mm Hg is accomplished electronically with the results displayed numerically or on a meter

x)                   Temperature control is important so the electrode is kept at 37°C

xi)                  The platinum cathode cannot be inserted directly into the blood as protein deposits would develop. So a plastic membrane is used, separating the blood from electrolyte solution that bathes the cathode

b)                  Direct sampling from extracorporeal circuit

 

2.                   Pulsed optical fluorescence

a)                   Light of a specific colour is transmitted down the fibreoptic cables to the disposable sensors

b)                  Sensors measure the blood gases through membranes that are permeable to gases & hydrogen ions

c)                   Special fluorescent materials on the sensors are sensitive to levels of O2, CO2, & pH

d)                  When light generated from the instrument reaches the fluorescent materials on the sensors, a molecular excitation occurs and light of a different colour is emitted and carried back to the instrument through another optical fibre

e)                   The difference in the transmitted light and the received light is converted into a numerical readout in mmHg for CO2 & O2 and units for pH

3.                   Colorimetric

a)                   Colorimetric CO2 detectors

i)                    As CO2 in solution is acidic, pH sensitive dyes can be used to detect & measure its presence

ii)                   A pH sensitive dye is exposed to dissolved CO2, thereby changing the dyes colour

iii)                 The degree of colour change is dependent on the concentration of CO2

iv)                 Used in expired gas analysis to confirm intubation of trachea

b)                  Uses optical measurement techniques based on light from a fiberoptic bundle passed through a pH sensitive dye and returned via a fiberoptic bundle to the instrument

 

 

4.                   Mass spectrometer

a)                   Can give a continuous measurement oxygen tension when using an intravascular catheter

b)                  Identifies compounds by their mass numbers by bombarding gas molecules with a beam of electrons - charging the molecules which become ions that are accelerated and then deflected by a magnetic field - the amount of deflection dependent on their mass

 

 

5.                   Transcutaneous oxygen measurement

a)                   Uses an oxygen electrode placed on the skin surface to measure oxygen diffusing through the skin to give an estimate of arterial oxygen tension

 

 

Limitations of in-line blood gases for: Accuracy & range; Time response; Blood flow

 

1.                   Calibration remains a problem for most of the continuous recording systems

2.                   Current devices are imperfect and may provide misleading information

3.                   Relative new addition to CPB circuit

4.                   Adds 10% to total CPB circuit cost

5.                   Not yet cost effective

6.                   May become cost effective if improvements are made in accuracy & response times obviating the need for additional confirmatory samples analysed at the bench top

7.                   Oxygen cell [polarographic] when mounted on a catheter tip gives continuous oxygen measurements; readily shows changes in PO2 but calibration is more difficult than bench top devices

 

8.                   General

a)                   Do not give calculated parameters

b)                  Bulky

c)                   Rapid response time

d)                  Role especially in pediatrics

e)                   Expensive

f)                    Gas calibrated - require calibration for every case