SVCC Respiratory Care Programs Analysis of ABG Samples

SVCC Respiratory Care Programs ABG Analysis, Introduction pH, PaCO 2, PaO 2 are measured directly by special electrodes contained in a device made for that purpose pH, PaCO 2, PaO 2 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., HCO 3 -, O 2 Sat. Other indirect measurements can be made or calculated from the above measurements i.e., HCO 3 -, O 2 Sat.

SVCC Respiratory Care Programs pH pH electrode is constructed of two half cells, which develop an electrical potential when connected together 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 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 Glass electrode (Sanz electrode) develops an electrical potential that is proportional to the amount of H+ present

SVCC Respiratory Care Programs PaCO 2 Electrode (Severinghaus) Measures carbon dioxide tensions by allowing the CO 2 gas to undergo a chemical reaction that produces hydrogen ions Measures carbon dioxide tensions by allowing the CO 2 gas to undergo a chemical reaction that produces hydrogen ions The hydrogen ion concentration produced is directly proportional to the PCO 2 in contact with the membrane of the electrode The hydrogen ion concentration produced is directly proportional to the PCO 2 in contact with the membrane of the electrode Operates on the principle of electric potential between electrodes Operates on the principle of electric potential between electrodes CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - CO 2 + H 2 O H 2 CO 3 H + + HCO 3 -

SVCC Respiratory Care Programs O 2 Electrode (Clark) Is a polarographic device that measures oxygen tensions by oxidation/reduction reactions, a chemical process that generates measurable electrical currents 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 Has platinum cathode and silver anode immersed in an electrolyte solution Volume of O 2 will be directly proportional to the number of electrons used in the cathode rxn. and by measuring current is a measure of O 2 diffused across membrane Volume of O 2 will be directly proportional to the number of electrons used in the cathode rxn. and by measuring current is a measure of O 2 diffused across membrane

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

SVCC Respiratory Care Programs Pre-analytic Error All factors that cause variance in lab results prior to the sample arriving in the ABG lab. 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 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 Calibration Purpose is assure consistency Purpose is assure consistency Def.: the systemic standardization of the graduation of a quantitative measuring instrument 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. 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 When 2 standards are used ---> 2-point calibration, performed after 50 blood gases or at least every 8 hours

SVCC Respiratory Care Programs Calibration (contd) 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 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 pH Calibration Several types of buffer solutions are acceptable for pH electrode 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 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 +/ 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 +/

SVCC Respiratory Care Programs PaCO 2 Calibration PaCO 2 electrode may be calibrated by introducing a known gas concentration PaCO 2 electrode may be calibrated by introducing a known gas concentration Gases used are 5% and 10% CO 2, +/-.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 Gases used are 5% and 10% CO 2, +/-.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 PCO 2 = (BP - 47 mmHg) x %CO 2 PCO 2 = (BP - 47 mmHg) x %CO 2

SVCC Respiratory Care Programs PO 2 Calibration Properly calibrated PO2 electrodes perform within the manuf. stated accuracy in PaO 2 ranges below 150 mmHg but may vary 20% at 500 mmHg Properly calibrated PO2 electrodes perform within the manuf. stated accuracy in PaO 2 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 0% oxygen is used for the slope point and 12% or 20% for the balance point PO 2 = (BP - 47 mmHg) x % O 2 PO 2 = (BP - 47 mmHg) x % O 2

SVCC Respiratory Care Programs 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 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: 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. A QC system must ID problems and specify corrective action, document. of accept. oper.

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

SVCC Respiratory Care Programs QC (contd) 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 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 Systematic errors is recurrent measurable deviation from the mean Causes of systematic errors: Causes of systematic errors: - contaminated standard - variations in electrode temp. - inconsistent introduction of standard

SVCC Respiratory Care Programs QC (contd) Causes of systematic error (contd) - inconsistent calibration technique - change in QC standard storage or prep. - electrode problems, e.g., protein contamin., membrane malfunction, contamin. electrolyte, or electrical problems Causes of systematic error (contd) - 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 QC Levels Level 1 simulates a patient hypoventilating Level 1 simulates a patient hypoventilating Level 2 simulates a patient with normal ventilatory status Level 2 simulates a patient with normal ventilatory status Level 3 simulates a patient hyperventilating Level 3 simulates a patient hyperventilating