Blood Gas Electrodes and Quality Assurance Ch 4 RES 121

calibration standards

Gases of buffer solutions with precise, specific blood gas values that are used to set the machine linearly over the physiologic range.

prevent issues

Systems must be kept free of contaminants, and membranes must be maintained and manufactures recommendations followed to...

currently recommended

Temp. correction for blood gas data for clinical application is not ...

Accuracy

a measure of how closely the measured results reflect the true or actual value (characterized by systemic error)

Dispersion

a pattern of frequent random error.

half-cell

a single electrode terminal in contact with an electrolyte solution ◼ But you need 2 half-cells for the electrode to function

Voltage =

amp x ohm

Precision

an index of dispersion of repeated measurements.

Potentiometric method

an unknown voltage is measured by comparison with a known voltage and is then converted to pH.

Accuracy vs. Precision

analytical accuracy and precision are illustrated by the ability to "hit" a known target A- Very good accuracy and precision B- Poor accuracy but good precision C- Occasional accuracy but poor precision

electrochemical cell systems

are referred to as simply "Blood Gas Electrodes" ◼ PO2 electrode ◼ pH electrode ◼ PCO2 electrode

random error

characterized by an isolated result outside control limits. ◼ Dispersion - a pattern of frequent random error. ◼ Increased frequency in random error → machine and techniques should be evaluated carefully

plus or minus 2 SD

What are normal limits of control samples based on?

Maybe caused by bubbles beneath the membrane, change in temperature, or contamination of calibration standards.

What causes shifting?

the pH, HCO3 & BE concentrations may fall.

What happens if extreme dilution of the sample occurs?

the sample may actually change over time →PaO2 increased and PaCO2 decreased in iced samples with plastic syringes

What happens with PaO2 and PaCO2 in iced samples within plastic syringes ?

drop in PaCO2 → tends to increase pH.

What happens with a major blood gas error?

Dry crystalline heparin

What has increased accuracy and has virtually eliminated the need for concern of sample dilution?

platinum

What is a working half-cell usually made of?

PaCO2

What is affected more than PaO2 because of the different solubility coefficients of the two gases in the liquid phase?

pH

What is repeatedly the most reliable and accurate electrode?

Pre-Analytical Errors

What is the greatest source of incorrect blood gas data?

normal ABG values at these various temp. are unknown ◼ Thus, the target ABG values for therapy are unclear

What is the problem with temp. corrected values?

an aging electrode, aging mercury battery or protein contamination of electrode.

What might cause trending?

the PaO2 of the sample falsely increases. ◼ The magnitude of the increase depends partly on the duration of exposure, whereas the volume of air bubble seems to make less difference.

What occurs when PaO2 of less than 158 mmHg is interfaced with an air bubble?

Samples that are not likely to be run in 30 minutes

What samples should be iced and placed in glass syringe?

draw sample sizes for adults exceeding 2 ml.

What should Laboratories still using 5 ml or 10 ml syringes do?

ABGs drawn with plastic syringes

What should not be iced if sample will be run within 30 minutes ?

carefully evaluate machine and techniques

What should you do if there is an increased frequency in random errors?

Henry's Law

When a gas is exposed to a liquid, the partial pressure of the gas in the liquid phase equilibrates with the partial pressure of the gas in the gaseous phase.

Joint Commission for the Accreditation of Health Organizations (JCAHO), College of American Pathologists & Clinical Laboratories Improvement Amendments (CAP & CLIA) .

Who requires internal quality control?

Because there is essentially zero PCO2 in room air

Why do blood sample PCO2 levels tend to falsely fall if blood were exposed to an air bubble?

because it alters blood gas results

Why must care be taken not to add unnecessaryheparin volume?

to ensure calibration, stability and reliability.

Why should you check an instrument's performance periodically?

If it doesn't fit the pt scenario...be concerned of error

clinical status and picture is remarkably better than the ABG results suggest. Cross check with pulse oximeter -For example: Pt patients color is pink and POX is 100% and pt is no respiratory distress with nml VS yet PaO2 on ABG is 30 torr...consider it mixed venous and redraw sample

reference half-cell

consists of a solid metal and a solution of its salt (e.g. silver and silver chloride, mercury and mercurous chloride) attached to an electronic circuit.

Electromotive force=

current x resistance

correction factor

each 1% dilution results in a 1% decline in PCO2

electrodes

electrochemical devices which measure changes in either electrical current or voltage and equate these changes with chemical measurements.

electricity

form of energy resulting from the flow of electrons through a substance that is called a conductor

Clark electrode

has membrane permeable to O2

Potentiometer

instrument that measures an unknown voltage by comparing it with an known reference voltage.

Sodium Heparin (1,000 unit / ml)

is also used frequently as an anticoagulant The pH of this solution approximates the normal pH of arterial blood.

Values of mixed venous blood

is an average of all venous blood returning to the heart in the IVC/SV, RA, RV and pulmonary artery.

PaO2

is relatively unchanged in heparin dilution. ◼ However, if PaO2 is very high, it may result in a significant fall.

phanz of sanz

pH

Working or Measuring half-cell

placed at the site where the actual chemical analysis, work or electrochemical change takes place. ​

trending

progressive controls either increase or decrease. ◼ Maybe cased by aging electrode, aging mercury battery or protein contamination of electrode.

ohm

unit of electrical resistance

Ampere (amp)

unit of measurement for an electrical current

voltage

unit of measurement for electromotive force.

watt

unit of power

polarographic

utilizing the electrochemical properties of oxygen in order to determine the concentration of oxygen in a mixture

PCO2

what ele trade is usually less than 3 mmHg?

Polarographic technique of gas analysis

what is PO2 analysis often referred to as?

silver-silver chloride

what is a reference cell usually made of?

PO2

what is the least accurate electrode with reports of as high as 20% inaccuracy?

Read Back results & Verify the receiver heard the results correctly from the transmitter

what should you do if giving results over the phone.

Errors introduced into blood gas sample during arterial puncture or handling

◼ Air bubble in sample ◼ Mixed venous sample ◼ Anticoagulant effects ◼ Sample not mixed properly ◼ Inappropriately drawn or transported ◼ ABG results attributed inadvertently to wrong patient ◼ Pts status or therapy incorrectly assessed or recorded

Clinical Guidelines to Maintain Anaerobic Sample

◼ Air bubbles should be expelled immediately. ◼ Results are reasonably stable when blood samples are not prematurely mixed and when foreign air bubbles are expelled within 2 minutes. ◼ All samples with visually apparent froth should be discarded. ◼ Be careful not to introduce air in the electrode chamber of the blood gas machine.

Point-of-Care (POC) Testing

◼ Being able to ascertain laboratory values immediately and accurately at bedside ◼ Beneficial for care being delivered at alternative and home ◼ Must be able to provide POC at a reasonable cost

Internal Quality Control Principles and Materials

◼ Controls or Control Samples - samples in which the true values are known. ◼ Normal limits are based on plus or minus 2 SD. ◼ Controls are available with high, low and nml values

Post-Analytical Error

◼ Error in transcription associated with recording the results after analysis. ◼ Important to print results ◼ If giving results over the phone - Read Back results & Verify the receiver heard the results correctly from the transmitter ◼ Know how to interpret ◼ Know Critical Values and report to MD

Regulations and Lab Accreditation

◼ Evaluation of Blood Gas labs included in site visits and inspections performed by the JCAHO ◼ Federal guidelines for lab and personnel - Clinical Laboratory Improvement Amendments (CLIA) ◼ Many labs are also accredited under the very detailed regulations provided by the CAP (College of American Pathologists)

Venous Admixture Precautions

◼ Use short beveled needles to minimize the surface area available for aspiration → decreases the potential for venous admixture. ◼ Avoid over shooting the blood vessel and then withdrawing the syringe when ever possible. ◼ The Femoral artery, where venous contamination or sampling is likely to occur should be punctured only when absolutely necessary

2 tr y 2 types of half-cells

◼ Working or Measuring - placed at the site where the actual chemical analysis, work or electrochemical change takes place. ◼ Reference - consists of a solid metal and a solution of its salt (e.g. silver and silver chloride, mercury and mercurous chloride) attached to an electronic circuit. ◼ Reference half-cell is the standard against which the electrochemical change is compared/measured

half-cell electrode terminal

A solid site where electrons enter or leave a liquid medium (sites that chemists would refer to as electrodes)

External Quality Control

A system by which laboratories can compare the accuracy of their results with results obtained from other labs. ◼ Also knows as Proficiency Testing.

standard deviation

Measure of variance around the mean. ◼ Allowable normal range that gives high and low values within which 95% of nml population fall within when subjected to test

Sanz electrode

Modern, compact, pH electrode that facilitates pH measurement at 370 C with a small blood sample and is accomplished by drawing the blood sample in a pH sensitive glass capillary tube then connects the blood to a large reservoir junction and the calomel half-cell.

PCO2 Electrode ​

Modified version of PCO2 electrode Modified version of PCO2 electrode

high, low and nml values

Controls are available with...

those that occur during air contamination.

Effects of heparin on an ABG sample is very similar to what?

electrochemical cell

An apparatus that incorporates one or more electrochemical cells to measure a specific chemical species.

electrode

An electric conductor or terminal through which electricity enters or leaves a medium such as an electrolyte solution.

An issue with ABG machine or electrodes ◼ Failure to mix sample prior to introducing into analyzer

An error that occurs during the actual analysis of blood gases via the electrodes is usually what?

Mean

Calculated by dividing the sum of all the numbers in a group by the number of numeric entries. ◼ Average number that best characterizes the group

Pre-analytical error

If drawing or handling causes error

Analytical error

If error occurs during analysis of ABG

decreases the potential for venous admixture.

In Venous Admixtures, what happens when you use short beveled needles to minimize the surface area available for aspiration?

falsely low PaO2 measurement in the blood sample being analyzed.

In clinical situations where the PaO2 is higher than 158 mmHg, O2 tends to migrate from the blood phase to the bubble and results in?

PO2 Electrode Basic Components

Incorporates a battery and an ammeter (device used to measure flow of electrical current) as the electrical components. ◼ Working half-cell is usually made of platinum. ◼ Reference half-cell is usually made of silver / silver chloride.

Internal Quality Control

Involves routine procedures and protocols designed to detect inconsistencies in performance of electrodes.

q25 blood gases or at a minimum of q4. ◼ All 3 levels should be analyzed once a day

It's recommended that Controls are run....

Peripheral venous blood

Peripheral venous blood from different organs and tissues has different PvO2 values depending on local metabolism and tissue function.

Calibration

Procedure done on blood gas electrodes before analyzing blood samples to establish accuracy to readings in the anticipated range. ◼ This gives an accuracy RANGE

Modified version of PCO2 electrode

Severinghaus Electrode

venous admixture

The contamination of the arterial sample with a small amount of venous or capillary blood during an attempt at arterial puncture. ◼ One-tenth part of venous blood to an arterial sample could produce as much as 25% drop in measured PaO2

watts=

Volts x Amps

Shifting

relatively abrupt change in measurement outcome followed by clustering or plateauing in a particular area.

Controls or Control Samples

samples in which the true values are known.

Mixed venous blood

samples may be taken from a catheter in the patients pulmonary artery to measure SvO2 in hemodynamic monitoring

reference half-cell is the

standard against which the electrochemical change is compared/measured

Quality Assurance

systematic process used to monitor, document, and regulate the accuracy and reliability of a procedure of laboratory measurement.

current

the actual flow of electrons through a conductor.

Ohm's Law

the current in a circuit equals the voltage difference divided by the resistance

Lithium heparin

the current recommended anticoagulant for ABG sampling.

Electromotive force / potential

the force responsible for pumping these electrons from the negative pole to the positive pole.

anode

the pole that has a relative shortage of electrons and has a positive charge.

cathode

the pole that has an excess of stored electrons and is therefore negatively charged.