Arterial Blood Gas

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					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.
• 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.
                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.
   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.
            Acid-Base Balance
• Non-Respiratory Acid Base Component
  (Metabolic Indices)
  • HCO3-
  • BE
• Respiratory Indice (Respiratory Index)
  • PaCO2
        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
               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
        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
               Normal Values
•   pH: 7.35 – 7.45       • %MetHb: < 2%
•   PaCO2: 35 – 45 torr   • %COHb: < 2%
•   PaO2: 80 – 100 torr     • Smokers: 5 – 10%
•   SaO2: 97%             • BE: +/- 2 mEq/L
•   HCO3-: 22-26 mEq/L    • CaO2: 18 – 20 vol%

                            * Vol% = mL/100 mL of
                                    blood
     Hemoglobin Saturation
• %SaO2 + %COHb + %MetHb  100%
• Example of error:
 • SaO2 97%, %COHb 50%, MetHb% 0%
      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.
         Interpretation of an ABG
• Immediate Environment
  •   FIO2
  •   Barometric Pressure
  •   Toxic gases/smoke
  •   Level of consciousness
  •   Environmental information
       • Empty Pill Bottle
  • Accident
        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
         Interpretation of an ABG
• Clinical Information
  •   History and physical exam.
  •   Vital Signs.
  •   Respiratory effort & ventilatory pattern.
  •   Mental Status.
  •   State of tissue perfusion.
  Assessing Oxygenation
• FIO2
• Barometric Pressure
• Age
         Composition of the
           Environment




• These values stay constant even with
  changes in barometric pressure.
        Dalton’s Law of Partial
              Pressures
• All pressures in a gas mixture must add up
  to the total pressure (PBARO).
• Dry Gas
  • Pgas = PBARO x FIO2
• Inspired Gas (ex. PIO2)
  • Pgas = (PBARO - 47 torr) x FIO2
   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
Altitude’s Effect on Partial Pressure
          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.
      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)
      Physiologic Processes
• ABG results provide information
  on the three physiologic processes
  • Alveolar Ventilation
  • Acid-Base
  • Oxygenation
   Equations Used to Reflect the
      Physiologic Processes
• PaCO2 Equation          Alveolar Ventilation

                          Acid Base
• Henderson
  Hasselbalch

• Alveolar Air Equation   Oxygenation

• Oxygen Content (CaO2)   Oxygenation

                          Oxygenation
• Oxygen Delivery
   PaCO2 and Alveolar Ventilation
• Alveolar Ventilation is the amount of air in L/min
  that reaches the alveoli and takes part in gas
  exchange.
•
         
      V V
   VA       
          E  D
• The body eliminates the CO2 produced, during
  metabolism, via ALVEOLAR ventilation.
                 Metabolism
• Steady State
  • The amount of CO2 added to the blood through
    metabolism = the amount of CO2 excreted by
    the lungs.
  • 200 mL/min
            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
            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
      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
                   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
 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.
                     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%.
                  Vol %
• mL of oxygen/100 mL of blood

                    Or

• mL of oxygen/dL of blood
             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.
    Factors that Influence Oxygen
       Delivery to the Tissues
•   SaO2
•   Hb
•   PaO2
•   Stroke Volume
•   Heart Rate
  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.
     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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posted:5/24/2012
language:English
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