Analysis of ABG Samples

1. Analysis of ABG Samples SVCC Respiratory Care Programs

2. 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. SVCC Respiratory Care Programs

3. pH • 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 SVCC Respiratory Care Programs

4. PaCO2 Electrode (Severinghaus) • 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+ + HCO3- SVCC Respiratory Care Programs

5. O2 Electrode (Clark) • 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 SVCC Respiratory Care Programs

6. 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

7. Pre-analytic Error • 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 SVCC Respiratory Care Programs

8. Calibration • 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 SVCC Respiratory Care Programs

9. 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 SVCC Respiratory Care Programs

10. pH Calibration • 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 near-normal buffer (7.384) with an accuracy of +/- 0.005 SVCC Respiratory Care Programs

11. 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 SVCC Respiratory Care Programs

12. 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

13. Quality Control • 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

14. QC (cont’d) • 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 SVCC Respiratory Care Programs

15. QC (cont’d) • 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

16. QC (cont’d) • 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

17. 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 SVCC Respiratory Care Programs

18. QC Levels • Level 1 simulates a patient hypoventilating • Level 2 simulates a patient with normal ventilatory status • Level 3 simulates a patient hyperventilating SVCC Respiratory Care Programs