Analysis of ABG Samples

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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

• 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)

• 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-

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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

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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

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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

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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 ---gt 2-point
  calibration, performed after 50 blood gases or at
  least every 8 hours

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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

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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

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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

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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.

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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
• 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 (contd)

• 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

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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
• Systematic errors is recurrent measurable
  deviation from the mean
• Causes of systematic errors
• - contaminated standard
• - variations in electrode temp.
• - inconsistent introduction of standard

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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

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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