Download BLOOD GAS ELECTRODES AND QUALITY ASSURANCE

BLOOD GAS ELECTRODES AND QUALITY ASSURANCE
CHP 4 – MALLEY
I.
Blood Gas Electrodes – Basic Electrical Principles
A. Blood Gas Electrodes are electrochemical Devices that measure either :
1. electrical current
2. voltage
B. They then equate these changes with chemical measurements.
C. Electricity is a form of energy that results from the flow of electrons through a conductor.
1. An energy source is necessary to provide the power for the flow
a. battery – energy source
b. generator – energy source
c. Energy Source thought of as an electron pump
d. electron pump has 2 poles
1. cathode – negative pole
2. anode – positive charged pole
e. electrons flow away from (-) pole, through a conductor
towards the (+) pole.
D. VOLTAGE1. electromotive force – responsible for moving the electrons
2. volt – unit of measure for electromotive force
3. Voltage refers to the electromotive potential but actual flow doesn’t occur without a
conductor bridging the (+) and (-) poles.
4. Potentiometer – measures the unknown voltage by comparing it with a known
reference voltage.
E. CURRENT –
1. current is the actual flow of electrons through a conductor
2. Amp – unit of measure for a current.
3. Ohm – unit of electrical resistance.
a. good conductors have low electrical resistance to current
b. poor conductors have high electrical resistance to current
F. OHM’S LAW –
1. Electromotive Force = current X resistance
2. In measurable terms:
Voltage = amp X ohm
3. Watts is the consumption of power (1,000 watts = 1 kw)
Watts = volt X amp
G. SPECIFIC ELECTRODES MEASURE
1. PO2
2. pH
3. PCO2
H. ELECTROCHEMICAL CELL SYSTEMS
1. electrode – electric conductor which electricity enters or leaves a
medium
2. electrochemical cell – an apparatus that consists of two
electrodes placed in an electrolyte solution. P. 83
3. All the traditional analytical devices used in blood gas analysis is
referred to as electrochemical systems ( more that one
electrochemical cell).
4. These systems are referred to as blood gas electrodes.
5. Examples are pH, PCO2 and PO2 electrodes.
I. Half – Cells
1. A single electrode terminal within an electrolyte solution is a half cell.
2. Blood gas electrodes require two half cells to function.
3. Two Types:
a. working half cells – located where the actual chemical analysis occurs
b. reference half cells – standard to compare the electrochemical change against (SEE
FIG 4-1).
II.
Structure and Function of Blood Gas Electrodes
A. PO2 Electrode
1. Basic Components
a. uses an electrical source (battery or electricity) and an ammeter (An ammeter is a
measuring instrument used to measure the flow of electric current in a circuit) .
b. working half cell – made of platinum (- charge – cathode)
c. reference cell – silver/silver chloride (+ charge - anode)
2. Electrochemical Reaction (SEE FIG 4-2) also on page 259 in RESP. SCIENCES
a. The electrical source supplies the platinum cathode with an electrical charge to
accelerate O2 reduction.
b. This charge attracts oxygen molecules from the blood sample, across the semi
permeable membrane to the cathode where they react with water and electrons.
c. The chemical reaction consumes electrons and produces ions (OH ˉ or hydroxyl ions)
from the reduction reaction.
d. The hydroxyl ions travel to the silver anode. The silver is oxidized in a reaction with
chloride ions (Clˉ) from the electrolyte solution in which they are soaking thus
providing more electrons for the reduction reaction at the cathode. This creates a
current to flow through the ammeter.
e. A flow of electricity flows through the circuit in direct proportion to the amount of
dissolved oxygen (PO2) present at the cathode. The ammeter measures this flow of
electricity and gives a readout of O2 %.
3. Polarography (pa-la-gra-fee)
a. a specific voltage must be applied initially to the cathode for the relationship between
PO2 and electrical current to be accurate
b. Proper voltage is determined using a polarogram (graph showing voltage vs. current at
a constant PO2)
c. PO2 analysis with the O2 electrode is called “polagraphic technique of gas analysis”
d. not practical for blood PO2 due to protein from the blood altering results; we use the
Clark Electrode
4. Clark Electrode – Fig 4-5
blood is separated from the electrodes by a membrane that is permeable to O2
a.
b. Oxygen diffuses through the membrane into the electrolyte solution
c. Terminals in the electrode are not directly in the blood; instead are in a phosphate
buffer solution with potassium chloride added.
d. Blood sample remains in a cuvette where it is warmed to 37° C and protected from
air contamination.
B. pH Electrode – Fig 4-6
1. Electrode Function
a. Measures change in voltage rather than current
b. Potentiometric Method – unknown voltage is measured by comparison with a
known voltage and then converted to pH.
c. Requires 4 terminals instead of two
d. A reference solution with a known pH is placed b/t two of the terminals to
have a reference voltage
e. pH sensitive glass electrode serves as a common electrode terminal for both
solutions. It allows hydrogen ions to diffuse into it in proportion to the
hydrogen ion concentration of the exposed fluid. Due to the different solutions
on either side of the glass, a net voltage develops between the two fluids and
is read at the voltmeter.
f. Nerst Equation – for each pH unit difference between the solutions a
difference of 61.5 mV develops.
g. The voltage is then converted to pH and the pH is displayed.
2. Sanz Electrode – modern pH electrode
a. pH measurement at 37°C
b. blood is drawn into a pH sensitive capillary tube
c. A membrane at the end of the cap tube connects the blood sample to the half
cell to allow reaction to occur.
C. PCO2 Electrode – Figure 4-8
1. Electrode Function
a. modified version of pH electrode
b. blood comes in contact with a semi permeable membrane made of silicone
rubber that allows CO2 to cross it.
d. The other side of the membrane contains a sodium bicarb solution in direct
contact with the pH sensitive glass. This solution is also in contact with a
silver / silver chloride electrode terminal.
e. A chemical reaction occurs in the bicarb solution as CO2 diffuses into it.
f. This chemical reaction (hydrolysis reaction) produces hydrogen ions and
results in a pH change within the solution.
g. This pH change is in direct proportion to the PCO2.
h. Corresponding voltage change is converted into PCO2 units and reflected on
the voltmeter.
2. Severinghaus Electrode –
a. Another name for the PCO2 electrode
b. See Figure 4-9
III.
Accuracy of Electrodes
A. Very precise
B. See Table 4-1.
IV.
Total Quality Management
A. Important to allow clinicians to practice good medicine by ensuring the quality of tests are
accurate.
B. Concerned with accuracy, time, simplicity, cost, sample size
C. These things are considered performance characteristics of a particular test
D. See Figure 4-10
E. See AARC Guidelines Excerpts (Handout) Egans p. 369
V.
Quality Assurance
A. process used to monitor, document, and regulate accuracy and reliability of a procedure or lab
measurement.
B. An error that occurs before lab analysis is a nonanalytical error
C. An error occurring during analysis is an analytical error.
D. Non Analytic Error
1. Preanalytical Error
a. an error introduced during sampling or arterial puncture
1. wrong patient label
2. transported without ice
3. air bubble in sample
b. most common error – can have serious consequences in reference to pt treatment
c. must have clear dept procedures, protocols, documentation and monitoring for
sample handling.
2. Post analytical Error
a. error in transcribing results
b. breakdown in verbal communication over the phone
c. prevent some of these errors by printing from blood gas machine
d. unexpected blood gas results should arouse suspicion
1. repeat if inconsistent with pt. picture, or internally inconsistent
E. Analytical Error
1. An error that occurs during actual analysis
2. mostly due to equipment issues
3. human analytical error can be caused by the failure to mix the sample prior to
introducing it.
4. Don’t record immediate results – can take 1 – 3 minutes to achieve equilibrium
5. Frequent Calibration and preventative maintenance help electrodes function properly to
reduce analytical error.
6. Preventative Maintenance –
a. Keeps system free of contaminants
b. Maintains membranes
7. Calibration
a. done on electrodes before analyzing blood to establish accuracy of readings
b. Standards are gases or buffer solutions with precise, specific values used to set
the machine to read linearly over the physiologic range.
c. Each machine is different so you must adhere to recommendations by the
manufacturer.
VI. Quality Control – periodic checking of instruments performance and reliability
A. Internal Quality Control –
1. to ensure the electrodes perform with precision
2. routine procedures and protocols that detect inaccuracies in
performance
3. required by JCAHO and CLIA p. 92
B. External Quality Control
1. aka proficiency testing
2. used to compare the accuracy of the lab results with other labs
a. uses distribution of identical samples from a central
distribution site to participation labs.
b. The central site is a noncommercial, professional association
c. Each lab runs the sample and then compares with the results
obtained by other labs.
d. Labs must also perform bias studies within their lab if they use
multiple machines – the same sample is run on different
machines
3. Statistics – used to monitor quality control
a. MEAN –statistic calculated by dividing the sum of all numbers
in a group by the number of entries (aka the average).
b. Standard Deviation –
1. a high and low value that represents 95% of where the
normal population falls when subjected to a certain test
2. See p. 5 FIG 1-1 - normal range is shown by bell
shaped curve.
3. Calculating the standard deviation (SD) is a measure of
variance around the mean.
4. Read equation p. 93 example figure 4-12 Levy Jennings
Plot
c. Coefficient of Variation –
1. more accurate than SD
2. Equation p. 93
C. Principles and Material
1. Controls
a. Samples with known values are run periodically to ensure
machine is operating correctly.
b. Control Samples – have their own range of normal limits based
on +/- 2 SD’s from the mean.
1. The mean and SD are given by the manufacturer
2. Can also determine mean and SD by running 20 controls
samples through the machine over time.
3. Controls are available with high, low and normal values.
4. Recommended to run two levels of control over every 8
hour shift
c. All 3 levels should be run every 24 hours
d. Also recommended to run a 4th level high PaO2 due to very high
PaO2 levels seen in OR and recovery.
2. Control Limits
a. See Figure 4-11
b. Limits are based on 2 or 3 SD’s from the mean
c. All values falling within the two lines are “within control limits”
3. Levey – Jennings Control Charts Figure 4-12
a. results obtained from control samples are plotted on a control
chart
b. y axis = measured results
c. x axis = time of measurement
d. produces graphic outcomes that show not only a problem with
controls but can indicate a particular problem or concern.
e. Error Patterns
1. Random Error – an isolated result outside of control
limits (4-12)
a. can be disregarded
2. Dispersion – pattern of frequent random errors (4-13 A)
3. Systematic Error – recurrent measurable deviation from
the mean. Is called trending. In trending progressive
controls either decrease or increase. (4-13 B)
a. Trending can be caused by old electrode or
protein contamination of electrode.
b. Shifting is also a systemic error that has an
abrupt change in measurement outcome
followed by clustering or plateuing in a
particular area (4-13 C) – can be caused by
bubbles under the membrane, change in temp or
contamination.
f. Accuracy vs. Precision
1. Above issues caused by accuracy or precision of
measuring device
2. Accuracy – how closely the measured results reflect the
actual value (4-14)
3. Precision – index of dispersion of repeated
measurements
g. Troubleshooting –
1. Most modern machines self troubleshoot
2. Examples of old troubleshooting guides on p. 97
4. Westgard Rules – specific criteria and actions for values found “out of
control area”
a. Careful observation for a single measurement outside 2 SD
b. Corrective Action for repeat offenders
c. A random error >3 SD requires immediate action to correct
5. Electrode Drift –
a. Change In measured values as the sample rests in the electrode.
b. Should not exceed 1%-2% within 5 minutes
c. Modern machines minimize this by performing automatic cals
every 30 minutes.
6. Types of Controls
a. Gases - usually for O2 and CO2
Must be certified for this use
b. Tonometered Liquids
1. device that allows for the bubbling of a gas with a known
pressure through a liquid until equilibrium is reached.
2. Disadvantage is that it is time consuming
3. Advantage – cost effective
c. Aqueous Buffers –
1. waterlike buffers of known pH can be used as control for
pH electrode
2. not always accurate due to electrodes responding
different to blood than to buffers
3. buffers contain no protein
4. respond differently to temp changes that blood p. 102
d. Whole Blood
1. hazardous due to possibility of infected blood (HIV, etc)
e. Emulsions
1. When two non mixable liquids are together in a solution
2. Do perform similarly to blood in controls
3. Not used often p. 103
f. Commercially Prepared Controls
1. Most widely used
2. Easy to use and prepared by the manufacturer
3. Fluorocarbon based emulsions are the most common
VII. CONTINUOUS MONITORING OF BLOOD GASES
A. Technology limits us to Measurement Techniques which only allow us to see an isolated point in
time.
1. Do not provide us with real time info about our pt.
2. blood gas info is “after the fact”
3. There is some new technology (ECMO) that allows for real time continuous gases.
B. Transcutaneous Technique1. Skin PO 2 can be monitored continuously
a. can vary greatly from blood PO2
b. can cause skin burns
C. Continuous Intra Arterial Blood Gases
1. Miniature Electrode Systems
a. Electrochemical Oxygen Probe –a miniature version of the Clark Electrode
1. small enough to be placed within the radial artery
2. temp corrections must be manually entered or via a special temp probe
3. membrane is susceptible to protein deposits
4. susceptible to a variety of tech problems (drift, leakage, corrosion)
2. Optode Technology
a. Measurement Principles
1. Works by optical detection of altered light.
2. The change in light transmission through fiber optic layers change in
proportion
to the blood gas value
3. Operate by “light absorption” or “fluorescence”
a. Absorbance Optodes – called transmission optodes.
1. Transmit light through an indicator solution and the existing
light is measured.
2. The existing light is then decreased by the absorption of some
of the light of the analyte being measured.
3. Can use this technique to measure pH or PCO2
4. See Figure 4-17
b. Fluorescence –
1. Measure changes in fluorescence
b. PO2 Measurement
1. Certain fluorescent dyes decrease their fluorescence in response to oxygen
2. This decrease is in proportion to the amount of oxygen present
3. See Figure 4-18
c. pH Measurement
1. Analyze fluorescent properties of certain weak acids at specific light
Wavelengths
2. The actual pH can be determined by the degree of dissociation of the weak
Acid
d. PCO2 Measurement
1. There is a CO2 permeable membrane between the blood and the fluid being
measured by the fluorescence.
2. CO2 diffuses across the membrane and changes the pH of the measured fluid
3. PCO2 is determined indirectly by the changes in the pH
e. Monitoring Systems
1. “Gas Stat Instrument” was the first to use fluorescence technology
2. Used during surgery
3. Consistently reads a higher PO2 than traditional systems
D. Technical Issues in Continuous Intra Arterial Blood Gases
1. CIABG used in select pts
a. invasive
b. costly
c. high technical failure
2. Ex Vivo (On Demand) Systems (Tomball Regional)
a. accurate
b. reliable
c. 90 seconds
d. Can be set to automatically draw blood from the radial artery (every 3 minutes or ?)
e. Records results on a roll
3. Summary – technology exists now to measure blood gases quickly
a. Ex VIVO – outside the body
b. CIABG – inside the body
c. Be careful with choosing due to risk of thrombosis, infection and pt cost
E. Point of Care Testing
1. Testing at the bedside can facilitate treatment when time is “of the essence”
2. POC (pt of care) systems are user friendly and almost error proof
3. Many offer electrolyte measurements
4. Does it justify the cost???
F. Regulations and Lab Accreditation
1. JACHO inspects blood gas labs
2. CLIA regulates blood gas labs
3. Careful adherence to governmental regulations is important