blood gas analyzers

Analytes

- the substance that is being measured

760 torr - 47 torr = 713 torr or mmHg

713 torr x .12 = ? (one point for PO2 ) = _________

713 torr x .10 = ? (two-point for PCO2 ) = _________

713 torr x 0 = ? (for two-point PO2 ) = ___________

Now you have two reference values (a high and low point), ensures your analyzer is functioning optimally! Internal Calibration

In addition to automatic calibration, internal calibration verification using known media is done

When interpreting the results it is important to match the patient results with the status of the patient at the time the blood gas sample was drawn

In other words, Are the blood gas results consistent with the patient's condition at the time?

The PaO2 in patients with methemoglobinemia generally are normal

In patients with a normal PaO2 and the presence of S/S of central cyanosis you should suspect methemoglobinemia

Contaminated Buffers Perform function check of suspected

Incorrect gas concentrations problem area

Insufficient sample

Interfering substances: fetal hemoglobin (may give erroneous values if forms of Hb that the instrument does not recognize are present).

Precise means that the electrodes show repeatability (reproduce consistency)

Blood gas electrodes that are out of calibration can result in harm to the patient! Calibration of Blood Gas Electrodes

Calibration of the three primary electrodes involves exposing the electrodes to known gases or buffers

Calibration is automated

Timely results: reduces turnaround time (30 to 90 sec.), improve care and reduce costs

Can measure other analytes to include: electrolytes, blood glucose levels, blood urea nitrogen, hematocrit, hemoximetry, lactate, bilirubin and prothrombin and partial prothrombin times

Incorrect procedures Repair or replace failed components

Component failure External Quality Control: Proficiency Testing

Measures the acidity and alkalinity in the blood sample determined by the number of H+ ions that cross a pH sensitive glass membrane

Consists of two electrodes or half cells; pH is measured in pH units

Controls that fall within ±2 SD of the mean are said to be "in control" and the analyzer meets internal calibration criteria (it is safe to use).

Controls that fall outside the ±2 SD indicate analytic error

Sample mishandling (waiting too long to run control once glass vial is opened)

Corrective Action: 2 R's

To determine the expected calibration readings for PO2 & PCO2 , the barometric pressure must be known

Dalton's Law of Partial pressures must be applied.

Analysis at the patient's bedside

Depending on the product these analyzers can measure multiple parameters on a single cartridge

You will now have two reference values that the electrodes must on target (mean) or within a standard deviation (SD) from that mean as deemed by the manufacturer of the analyzer Calibration of pH electrode

During the one point calibration, the pH electrode must achieve a result of 7.38 or within a SD of the mean (as per manufacturer)

Blood gas data is used to make critical decisions, results must be accurate and their interpretation valid

Errors can result in poor patient management and unsatisfactory outcomes

Every 30 minutes an automatic one-point calibration is performed as in the first step.

Every 8 hours the analyzer will automatically repeat the one-point process followed by a two point process.

The pH moves inverse (opposite) of the CO2 Assuring Valid Measurements and Use of Blood Gas Data

Excess heparin, just remember what air in the sample does. It has the same effect

Two point PCO2 electrode calibration gas: - a tank/cylinder containing a concentration of 10% CO2

For the pH electrode - One point: known pH buffer with concentration of 7.384 and Two point: known pH buffer with concentration of 6.840 Calibration of Blood Gas Electrodes

PCO2 10% Calibration of Blood Gas Electrodes: Example

Given the following barometric pressure, determine the one point calibration points (expected values) for the PO2 & PCO2 electrodes

The benefits include:

Less blood volume required for analysis

Fever will increase CO2 and O2 . (direct relationship with temperature)

Results will be higher than those sampled by the machine at 37° C

Low body temp will decrease values in the blood. (direct relationship with temperature)

Results will be lower than those sampled by machine at 37° C

Is the PaO2 attainable on the stated FiO2?

Rule of 5: If the PaO2 is greater than 5 times the oxygen percentage, the result is suspect.

S/S usually occur at levels of 35% to 50%, with levels >50% causing cardiac arrhythmias, CNS depression, profound metabolic acidosis and death if not treated

Treatment includes the administration of a reducing agent; methylene blue

PCO2 5%

Two Point calibration

Genetics: The Blue People of Troublesome Creek

Video:

Co-oximetry provides the ______________________- of light required to measure the dyshemogoblins (HbCO and metHb)

four wavelengths

Methylene blue converts the iron portion of the Hb molecule back to its normal state

https://www.youtube.com/watch?v=cx8sytzuw20 Analyzing Sources of Error of Co-oximetry

Actual or true measurement of the HB saturation

oxyhemoglobin, [HbO2], methemoglobin [metHb], and carboxyhemoglobin [HbCO],

requires analysis of the blood gas sample using Hemoximetry (co-oximetry).

oxyhemoglobin, [HbO2], methemoglobin [metHb], and carboxyhemoglobin [HbCO],

YouTube - The Blue People of Kentucky Analyzer: The Primary Electrodes

pH - Sanz electrode

Blood gas analyzers use electrodes to directly measure the following analytes:

pH, PaCO2 & PaO2

Blood gas analyzers use these primary parameters to calculate secondary values such as the

plasma bicarbonate, base excess or deficit, and hemoglobin saturation (SaO2)

For example, ±0.04 for pH Temperature Correction

Blood gas electrodes are maintained in a temperature controlled chamber (37°C)

Done with two gases for one and two references (expected value)

PO2 0%

Done with one gas as a reference

PO2 12% or 20%

Rerun control

Repeat analysis on a different instrument What if the QC Controls are out of range?

National Committee for Clinical Laboratory Standards recommends the use of

2% Glutaraldehyde or a 1:4 solution of sodium hypochlorite to waste container prior to disposal

A one-point calibration: Every 3O minutes or prior to sample analysis. Involves exposing the PO2 and PCO2 electrodes to one of the known gases for the electrodes and the pH electrode to one (known) pH buffer solution

A two-point calibration: Every 8 hours the analyzer will automatically calibrate the electrodes by exposing two known gases to the PO2 and PCO2 electrodes and the pH electrode to two of the known pH buffer solutions

what else may also be reported on Coox

A-a, a/A, and Cao2

Calibration is performed to ensure that the electrodes are accurate and precise

Accuracy means that the expected calibration value is achieved when the electrodes are exposed to the known gases and buffers

MetHb can be inherited or acquired

Acquired causes: Environmental exposure to nitrates, nitrites or benzene, nitric oxide (NO), administration cardiac drugs such as nitroglycerin, nitroprusside

One proficiency testing agency is the College of American Pathologists (CAP)

Agencies such as this, send unknown control media to the laboratory which must be performed with the regular workload of the person who is responsible for testing the routine control media for their lab

The federal government mandated rigorous external quality control standards through CLIA - Clinical Laboratory Improvement Amendments of 1988

All analytical laboratories including blood gas labs, must undergo external testing called "Proficiency Testing".

PO2 - Clark electrode

Also called the polarographic electrode

Air in sample - raises low PaO2 in the sample when the patient's PaO2 is less than 160 torr

Always lowers the PaCO2 in the sample because very little (0.3%) CO2 is in the atmosphere.

Dyshemoglobins (COHb & MetHb) require additional wavelengths to measure

Analyzing

Local anesthetic agents such as lidocaine, benzocaine, EMLA creams (topical lidocaine-used for pediatrics to numb area, e.g. placement of IV or blood draws) Analyzing

As a result of the dark reddish brown color, metHb levels as low as 15% can cause cyanosis.

Multiple or recurring incidences of poor-performance can result in suspension of Medicare and Medicaid reimbursement or closure of the laboratory due to loss of license and accreditation

As long as the analyte being measured falls around a target value specified for the analyte, it is considered acceptable performance

Patients who have CO poisoning may not present with cyanosis, because carboxyhemoglobin is cherry red in color

At CO levels above 40%, visual disturbances, myocardial damage, coma and eventually death may occur

Failure to mix the sample before analysis

Blood clots in sample

Modified pH electrode: has the same components in pH electrode; consists of a CO2 permeable membrane

Measures PCO2 indirectly through measurement of pH (H+) Analyzer: The Primary Electrodes

PCO2 - Severinghaus electrode

Measures the partial pressure of dissolved carbon dioxide in the blood sample

Failure to be at this temperature will result in improper calibration

Modern blood gas instruments have a built-in tonometer that brings precise gases and buffer solutions into contact with the electrodes for calibration. Calibration of Blood Gas Electrodes

It has been reported that chilled samples can result in elevated potassium measured from a blood gas sample; to prevent this it is recommend to use plastic syringes with a low diffusibility as this minimizes the transport of potassium from the blood cells to the plasma

Most newer plastic syringes have low diffusibility with advances in plastics technology (If unsure, do not place sample in ice-slush, and analyze asap) Calibration of Blood Gas Electrodes

713 torr x 0.05 = ? (one point for PCO2 ) = ________ Example cont.

Now, calculate the two-point values for the gas electrodes.

This electrode measures the partial pressure of dissolved oxygen in the blood sample; measures the O2 molecules that diffuse across a membrane

O2 molecules that cross the membrane, causes electron flow (voltage/potential)

The partial pressure of a gas is the pressure it would exert if all the other gases in the mixture were absent.

One Point calibration

Two point PO2 electrode calibration gas- a tank/cylinder containing a concentration of 0% O2.

One point PCO2 electrode calibration gas: a tank/cylinder containing a concentration of 5% CO2

The known gases for the PCO2 and PO2 electrode are below:

One point PO2 electrode calibration gas: a tank/cylinder containing a concentration of 12% or 20% O2

Temperature correction refers to mathematical adjustments of patient's PaO2 , PaCO2 and pH values if their body temperature is not 37˚ C

Patient temperature must be entered when not (37°C); < 35°C or >39°C

The results are sent back to the testing agency where they are compared to other participants having the same blood gas analyzer Performance Validation and External Quality Control: Proficiency Testing

Performance validation - process of testing new blood gas analyzer to confirm accurate measurement

Reliable values were obtained at Temp. of 37° C, so there are some who believe that temperature correction should not be done Analyzers

Point-of-Care Testing

Causes: aging electrode, incorrect gas concentrations/contaminated buffers, protein buildup of the electrode, or faulty components

Possible causes: C, I, I, C Corrective Action:

Includes testing for accuracy and repeatability

Proficiency Testing

If the patient was on room air, the sum of the PaO2 and the PaCO2 should not exceed 140 mmHg (If so suspect air in sample or a reporting error, such as the patient was actually on supplemental oxygen)

Rule of thumb - When calculating the PAO2 via the alveolar-air equation, the PaO2 should never be equal to or higher than the PAO2

For the two-point the expected result is 6.84 or within a SD of the mean

Same as the gas electrodes, a one point calibration is done first followed by the two-point calibration

Metabolic effects - prevented by placing sample in ice slush if not analyzed within 10 - 30 mins.

Sample can still be analyzed if in ice slush up to an hour. After one hour discard sample (notify appropriate person, e.g. doctor)

Statistical probability

Sample contamination (expired controls, incorrect lot number)

An increase in the COHb or MetHb levels above normal, results in even less normal hemoglobin to carry oxygen

Smokers typically have CO levels in the 5% to 10% range

Self-calibrating prior to sample analysis

Some can perform 900 tests on a single set of batteries Post-analytic Errors

The controls mimic real blood samples chemically and physically

Span the ranges of blood gas samples: normal values, acidotic values and alkalotic values Plotting Control Media: ±2 SD What if the QC Controls are out of range?

Barometric pressure is 760 mmHg

Step 1: Since the calibration gases are humidified with water (bubbled through water bath at 37˚C), you must subtract the water vapor pressure from the barometric pressure.

To avoid errors requires a comprehensive quality assurance program

Such programs consist of proper acquisition and handling of blood gas specimens, calibration of the electrodes, maintenance of the analyzer and its components and proper interpretation and reporting of the blood gas results Assuring Valid Measurements and Use of Blood Gas Data

Failure to prevent these errors can result in gross measurements which can lead to inappropriate interpretation, misdiagnosis and mistreatment!

Table 8-8 pg. 169 Wilkins 7th edition: Review and know the 4 main types of blood gas pre-analytical errors, how each affects the PO2, PCO2 and pH values, how to recognize and how to avoid.

Proficiency testing is required for the laboratory to maintain accreditation

Testing is done 3 times per year using five sample tests

More recent portable multiwavelength pulse oximeters have become available that can accurately measure the dyshemogoblins at the bedside Analyzing

The CO-oximeter gives values for:

Westgard rules (statistically derived limits of normal values and ranges for the electrodes being measured (pH, PCO2 and PO2) are used to help detect, respond to and correct instrument error (e.g. Any outliers outside of the statistical norm)

The allowable distance of the media from the mean or expected value is measured in standard deviations (SD) also reported as: ± 2 SD

Methemoglobin is another dyshemoglobin in which the iron in the heme group has been oxidized from its normal ferrous state Fe2+ to the ferric state Fe3+.

The hemoglobin at this point is not capable of binding with oxygen

The more molecules present, the higher is the potential (greater is the flow of electrons)

The result in flow is proportional to the PO2 Assuring Valid Measurements and Use of Blood Gas Data

As a general rule at least two levels of control media should be analyzed during every 8 hour shift Quality Control Media Control Media Plotted on a Graph

The results of the control media are plotted on a graph called the LeveyJennings Graph or Chart

The one-point occurs Q30 mins or prior to sample analysis

The two-point every 8 hours

To be out of range is to be "out of control"

There are two categories of analytical error: Random and Systematic Random error - errors of precision or imprecision; sporadic, out-of-range data (outside ±2 SD) Possible causes, the 3 S's

Systematic error (bias) - trend or an abrupt shift in data with each run going more and more out of range

This is more serious and the analyzer should be taken out of service and cannot be used until fixed

Pre-analytical errors

Those errors that are to be prevented prior to sample analysis in the blood gas analyzer

The known media are also called "controls" or "quality controls [QC]"

Three QC levels ensure that the machine is tested over a range of values

These are standard recommended times. The times for the one-point and the two-point might differ, depending on the number of blood gas samples analyzed at the facility Calibration of Blood Gas Electrodes

To calibrate the PO2 and PCO2 electrode, the blood gas analyzer must also be at a constant temperature of 37° C

. Wipe spills with

alcohol 70% or bleach