Analysis Of Abg Samples

Analysis of ABG Samples

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ABG Analysis, Introduction pH, PaCO2, PaO2 are measured directly by special electrodes contained in a device made for that purpose ■ Other indirect measurements can be made or calculated from the above measurements i.e., HCO3-, O2 Sat. ■

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pH ■

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pH electrode is constructed of two half cells, which develop an electrical potential when connected together Reference electrode maintains a constant potential and is bathed in KCl Glass electrode (Sanz electrode) develops an electrical potential that is proportional to the amount of H+ present

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PaCO2 Electrode (Severinghaus) ■

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Measures carbon dioxide tensions by allowing the CO2 gas to undergo a chemical reaction that produces hydrogen ions The hydrogen ion concentration produced is directly proportional to the PCO2 in contact with the membrane of the electrode Operates on the principle of electric potential between electrodes CO2 + H2O H2CO3 H+ + HCO3SVCC Respiratory Care Programs

O2 Electrode (Clark) ■

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Is a polarographic device that measures oxygen tensions by oxidation/reduction reactions, a chemical process that generates measurable electrical currents Has platinum cathode and silver anode immersed in an electrolyte solution Volume of O2 will be directly proportional to the number of electrons used in the cathode rxn. and by measuring current ∆ is a measure of O2 diffused across membrane

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QA in Blood Gas Analysis ■

ABG lab must be able to assure accurate and reliable results ■ The above is accomplished by applying protocols in 3 areas: - pre-analytic error - calibration - quality control SVCC Respiratory Care Programs

Pre-analytic Error ■

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All factors that cause variance in lab results prior to the sample arriving in the ABG lab. 4 factors assoc. with signif. P. E. are: - air bubbles in sample - time delay (iced sample with more than 60 min. or uniced with more than 10 min.) - blood clots in sample - small sample size where excessive anticaogulation is suspect

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Calibration ■ ■

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Purpose is assure consistency Def.: the systemic standardization of the graduation of a quantitative measuring instrument Calibrating standards for blood gas analyzers should simulate the physical properties of blood and meet manuf. specs. When 2 standards are used ---> 2-point calibration, performed after 50 blood gases or at least every 8 hours

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Calibration (cont’d) ■

A “one-point calibration” is an adjustment of the electronic response of an electrode to a single standard and is performed more freq. than a 2 pt. cal., ideally prior to each sample analysis

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pH Calibration ■

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Several types of buffer solutions are acceptable for pH electrode calibration Buffers are excellent standards because of their long shelf life and stability for days after being opened 2-point cal. of pH electrode is done by setting the “slope” potentiometer to a low pH buffer (6.84) and setting the “balance” potentiometer with a nearnormal buffer (7.384) with an accuracy of +/- 0.005

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PaCO2 Calibration PaCO2 electrode may be calibrated by introducing a known gas concentration ■ Gases used are 5% and 10% CO2, +/- .03% - 1-point cal. uses 5% standard to set the “balance” point - 2-point cal. uses 5% for “balance” point and 10% for “slope” point ■ PCO2 = (BP - 47 mmHg) x %CO2 ■

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PO2 Calibration ■

Properly calibrated PO2 electrodes perform within the manuf. stated accuracy in PaO2 ranges below 150 mmHg but may vary 20% at 500 mmHg ■ 0% oxygen is used for the “slope” point and 12% or 20% for the “balance” point ■ PO2 = (BP - 47 mmHg) x % O2 SVCC Respiratory Care Programs

Quality Control ■

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Refers to a system that documents the accuracy and reliability of the blood gas measurements and is essential to assure accuracy in the blood gas lab Media available as blood gas controls include: - aqueous buffers - glycerin soltn. - human/animal serum and blood - artificial blood A QC system must ID problems and specify corrective action, document. of accept. oper. SVCC Respiratory Care Programs

QC (cont’d) ■

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Documentation of QC is usu. on Levy-Jennings Chart which shows measured results on the y axis versus time of measurement on the x axis SD is used to summarize a mass of data: the difference between a number in a data set and the mean of the data set is called a deviation. A deviation shows how much a number varies from the mean

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QC (cont’d) ■

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A properly functioning electrode that repeatedly analyzes a known value will produce results within a rel. small range, e.g., a PaCO2 electrode that analyzes a 40 mmHg standard 100 times will produce results where 2/3 of the measurements are 39 - 41 mmHg and nearly all measurements fall in 38 - 42 range 95% of the control measurements should fall within 2 SD SVCC Respiratory Care Programs

QC (cont’d) ■

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Random errors indicates a value outside of 2 SD of the mean: a single random error has minor signif., but if number increased the machine and techniques must be evaluated Systematic errors is recurrent measurable deviation from the mean Causes of systematic errors: - contaminated standard - variations in electrode temp. - inconsistent introduction of standard SVCC Respiratory Care Programs

QC (cont’d) ■

Causes of systematic error (cont’d) - inconsistent calibration technique - change in QC standard storage or prep. - electrode problems, e.g., protein contamin., membrane malfunction, contamin. electrolyte, or electrical problems

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QC Levels ■

Level 1 simulates a patient hypoventilating ■ Level 2 simulates a patient with normal ventilatory status ■ Level 3 simulates a patient hyperventilating

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