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ARTERIAL BLOOD GAS ANALYSIS

ARTERIAL BLOOD GAS ANALYSIS. Module A. Objectives. List the normal values for parameters found in a blood-gas analysis. List the normal values for parameters found in a CO-Oximetry analysis. Differentiate between measured and calculated (derived) blood gas data.

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ARTERIAL BLOOD GAS ANALYSIS

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  1. ARTERIAL BLOOD GAS ANALYSIS Module A

  2. Objectives • List the normal values for parameters found in a blood-gas analysis. • List the normal values for parameters found in a CO-Oximetry analysis. • Differentiate between measured and calculated (derived) blood gas data. • List the three physiologic processes assessed with blood gas data. • State the PaCO2 equation. • Describe how alveolar minute ventilation is derived. • Describe the relationship between PaCO2, CO2 production and Alveolar Minute Ventilation.

  3. Objectives • Describe the effects of altitude on partial pressure, barometric pressure and fractional concentrations. • Given appropriate data, use Dalton’s Law to determine the resultant partial pressures of a gas in a mixture. • Given appropriate data, calculate the Alveolar Air Equation. • Explain how changes in the PIO2 or PaCO2 levels affect the PAO2. • State the formula for Oxygen Content and Oxygen Delivery.

  4. Arterial Blood-Gas Analysis • Two Components • Acid Base Balance/Ventilation • pH, PaCO2, HCO3-, BE • Electrolytes (primarily K+) • Oxygenation • PaO2, Hb, CaO2, SaO2, MetHb%, COHb% & any other abnormal Hemoglobin species. • Oxygenation Indices: PaO2/FIO2, A-aDO2, s/t.

  5. Acid-Base Balance • Non-Respiratory Acid Base Component (Metabolic Indices) • HCO3- • BE • Respiratory Indice (Respiratory Index) • PaCO2

  6. Definition of Blood-Gas • Any element or compound that is a gas under ordinary conditions and dissolves in the blood. • A blood-gas would exert a partial pressure • O2 • CO2 • N2 • CO

  7. Technology • Blood can be analyzed on either or both of two different machines (or one machine with two distinct components) • Blood-Gas Analyzer • CO-oximeter

  8. Measured vs. Derived • Most values are directly measured with various electrodes: • Clark: PO2 • Severinghaus: PCO2 • Sanz: pH • Some are calculated or derived Values are: • HCO3- • Base Excess (BE) • CaO2

  9. pH: 7.35 – 7.45 PaCO2: 35 – 45 torr PaO2: 80 – 100 torr SaO2: 97% HCO3-: 22-26 mEq/L %MetHb: < 2% %COHb: < 2% Smokers: 5 – 10% BE: +/- 2 mEq/L CaO2: 18 – 20 vol% * Vol% = mL/100 mL of blood Normal Values

  10. Hemoglobin Saturation • %SaO2 + %COHb + %MetHb » 100% • Example of error: • SaO2 97%, %COHb 50%, MetHb% 0%

  11. Interpretation of an ABG • Three Areas of information are necessary • Information about the patient’s immediate environment. • Additional Lab Data. • Clinical Information obtained through patient assessment.

  12. Interpretation of an ABG • Immediate Environment • FIO2 • Barometric Pressure • Toxic gases/smoke • Level of consciousness • Environmental information • Empty Pill Bottle • Accident

  13. Interpretation of an ABG • Lab Data • Previous analyses • Hemoglobin or hematocrit (from lab) • Electrolytes (K+, Na+, Cl-) • Blood Glucose • Blood Urea Nitrogen (BUN) • Chest x-ray • PFT test

  14. Interpretation of an ABG • Clinical Information • History and physical exam. • Vital Signs. • Respiratory effort & ventilatory pattern. • Mental Status. • State of tissue perfusion.

  15. AssessingOxygenation • FIO2 • Barometric Pressure • Age

  16. Composition of the Environment • These values stay constant even with changes in barometric pressure.

  17. Dalton’s Law of Partial Pressures • All pressures in a gas mixture must add up to the total pressure (PBARO). • Dry Gas • Pgas = PBAROx FIO2 • Inspired Gas (ex. PIO2) • Pgas = (PBARO - 47 torr) x FIO2

  18. Calculating Partial Pressures for dry gases • PO2 = 760 x .21 160 mm Hg or torr • PN2 = 760 x .78 593 mm Hg or torr • PCO2 = 760 x .0003 0.23 mm Hg or torr • PAr = 760 x .0093 7 mm Hg or torr NOTE: 160 + 593 + .23 + 7 = 760

  19. Altitude’s Effect on Partial Pressure

  20. High Altitude Response • Increase Altitude • ¯ PBARO¯ PIO2¯ PAO2¯ PaO2 • To adapt to high altitudes • Change the environment • Airplanes are pressurized to 7000-8000 feet. • Increase FIO2(above 20,000 feet). • Adapt Physiologically • Hyperventilation. • Collateral Circulation. • Shift the oxygen dissociation curve. • Increase Hemoglobin levels.

  21. Calculating PBaro at High Altitudes • PBARO falls 120 mm Hg per mile of altitude • Example: Leadville is 2 miles above sea level. Calculate the PBARO& PO2 • 120 x 2 miles = 240 mm Hg decline • 760 - 240 = 520 mm Hg (PBARO) • PO2 = 520 x .21 109 mm Hg or torr (PO2)

  22. Physiologic Processes • ABG results provide information on the three physiologic processes • Alveolar Ventilation • Acid-Base • Oxygenation

  23. Equations Used to Reflect the Physiologic Processes • PaCO2 Equation • Henderson Hasselbalch • Alveolar Air Equation • Oxygen Content (CaO2) • Oxygen Delivery Alveolar Ventilation Acid Base Oxygenation Oxygenation Oxygenation

  24. PaCO2 and Alveolar Ventilation • Alveolar Ventilation is the amount of air in L/min that reaches the alveoli and takes part in gas exchange. • The body eliminates the CO2 produced, during metabolism, via ALVEOLAR ventilation.

  25. Metabolism • Steady State • The amount of CO2 added to the blood through metabolism = the amount of CO2 excreted by the lungs. • 200 mL/min

  26. PaCO2 Equation • PaCO2 = CO2 production x 0.863 Alveolar Minute Ventilation • 0.863 is a constant which equates dissimilar units. • 40 mm Hg = 200 mL/min x 0.863 4.3 L/min

  27. PaCO2 Equation • If CO2 production doubles (e.g. fever), alveolar minute ventilation must double to keep a normal PaCO2 level. • 40 mm Hg = 400 mL/min x 0.863 8.6 L/min

  28. Henderson-Hasselbalch Equation • pH is defined as the negative log of the H+ concentration • pH = pK + Log HCO3-(Base) (PaCO2 x 0.03) (Acid) • pH = pK + Log 24.0 mEq/L 1.20 mEq/L • “Normal” pH implies 20 times more base than acid

  29. PAO2 • PAO2 = PBARO – 47 torr x FIO2 – PaCO2 0.8 • PAO2 = PIO2 - PaCO2 0.8 • PAO2 on room air = 100 – 104 mm Hg • PAO2 on 100% = 600’s

  30. Effects of PaCO2 on PAO2 and PaO2 • A rise in the PaCO2 will lower the PAO2 and therefore the PaO2. • Hypoventilation is a cause of hypoxemia.

  31. CaO2 • CaO2 = (SaO2 x Hb x 1.34) + (PaO2 x 0.003) • With normal values: • Oxyhemoglobin (attached) represents 19.7 vol%. • Dissolved oxygen (PaO2) represents 0.3 vol%. • Total Oxygen present in the blood 20 vol%.

  32. Vol % • mL of oxygen/100 mL of blood Or • mL of oxygen/dL of blood

  33. Oxygen Delivery • Oxygen Delivery = CaO2 x CO x 10 • Oxygen Delivery = CaO2 x SV x HR x 10 • Normal Value = 1,000 mL/min • Represents amount of oxygen delivered to the tissues each minute.

  34. Factors that Influence Oxygen Delivery to the Tissues • SaO2 • Hb • PaO2 • Stroke Volume • Heart Rate

  35. Summary of Important Points • ABG interpretation means evaluating the acid base and oxygenation status of the patient. • Acid Base represent the metabolic and respiratory indices. • FIO2 stays the same regardless of changes in PBaro. • PBARO decreases as altitude increases. • Dalton’s Law. • PO2 is affected by FIO2, PBARO and age.

  36. Summary of Important Points • PAirway = PBARO. • To interpret an ABG you need 3 areas of information. • Oxygen delivery is influenced by five factors. • ABG values are either measured or derived. • Understand the 5 equations and the relationship among the parameters used.

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