Arterial Blood Gas (ABG)

See also Pulse Oximetry, [[Pulse Oximetry]]


Physiology

Types of Hemoglobin Which May Be Present in the Adult Circulation

  • Oxyhemoglobin (OxyHb)
  • Deoxyhemoglobin (DeoxyHb): reduced hemoglobin
  • Carboxyhemoglobin (COHb) (see Carboxyhemoglobinemia, [[Carboxyhemoglobinemia]]): usually present in small amounts
  • Methemoglobin (MetHb) (see Methemoglobinemia, [[Methemoglobinemia]]): usually present in small amounts
  • Sulfhemoglobin (see Sulfhemoglobinemia, [[Sulfhemoglobinemia]])

Blood Gas Technique

Arterial Blood Gas (ABG)

  • Radial/Brachial/Femoral Arterial Puncture: blood passively enters the heparinized syringe, which is put immediately on ice and transferred promptly to the blood gas laboratory for analysis

Venous Blood Gas (VBG)

  • Technique for Drawing VBG from a Peripheral Blood Draw Using a Tourniquet: it is recommended to release the tourniquet 1 min before drawing the blood to avoid ischemia-related changes in the measured parameters

Normal Values for Blood Gases

Arterial Blood Gas (ABG)

  • pH: 7.40
  • pCO2: 40 mmHg
  • pO2: age-dependent
    • Predicted Room Air pO2 = 104.2 – (0.27 x Age)
    • Alternatively, pO2 can be evaluated by calculating the alveolar-arterial O2 gradient (A-a gradient) (see Hypoxemia, [[Hypoxemia]])
  • HCO3: 24 mEq/L

Venous Blood Gas (VBG)

  • VBG values are slightly different than ABG values (due to the uptake and buffering of metabolically-produced CO2 in the capillaries and the addition of organic acids produced by the tissue bed drained by the vein)
    • pH: venous pH is usually 0.02-0.04 lower than the arterial pH
    • pCO2: venous pCO2 is usually 3-8 mmHg higher than arterial pCO2
    • HCO3: venous HCO3 is usually 1-2 mEq/L higher than arterial HCO3

Principles of Arterial Blood Gas Analysis

Measurement of pH, pCO2, pO2, and HCO3 from Arterial Blood Gas Sample

  • pH: measured using pH electrode
  • pCO2: measured using a chemical reaction that consumes CO2 and produces a hydrogen ion -> this is sensed as a change in pH
  • pO2: measured using oxidation-reduction reactions that generate electric currents
  • Temperature Dependence: pH increases and both pCO2 and pO2 decrease as the temperature decreases
    • For this reason, ABG analysis must account for either the patient’s temperature (or using 37 degrees C as a standardized procedure)
  • HCO3: calculated

Determination of SaO2 from Arterial Blood Gas Sample

  • SaO2: there are 3 methods for determining SaO2 in the arterial blood gas anlyzer
    • Fractional Oxygen Saturation from Co-Oximetry: determining the SaO2 by measuring the OxyHb and comparing it to all of the hemoglobin measured
      • Equation: FO2Hb = O2Hb/100
    • Functional Hemoglobin Saturation from Co-Oximetry: determining SaO2 by measuring the OxyHb and reduced Hb and DeoxyHb only
      • This allows clinicians to assess how much of the hemoglobin capable of carrying oxygen is actually saturated with oxygen molecules
      • Equation: SaO2 = OxyHb/OxyHb + DeoxyHb.
    • Calculation of SaO2 Using an Equation/Algorithm Using Measured PO2, pH, PCO2, and a Calculated/Default Hemoglobin
      • Assumptions: assumes a normal hemoglobin value and that there is no methemoglobin or carboxyhemoglobin present

Co-Oximetry on Arterial Blood Gas Sample

  • Principle: co-oximeter uses spectrophotometry with four wavelengths of light to detect oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin
    • Can also be used to determine levels of sulfhemoglobin and fetal hemoglobin

Clinical Scenarios Where Discordance May Occur Between Pulse Oximetry Saturation (SpO2) and pO2 from Arterial Blood Gas

  • Pulse Oximeter Equipment Malfunction: in this case, SpO2 may vary widely from the pO2
  • Venous Blood Sample Inadvertently Drawn (Instead of Arterial Blood Sample): in this case, pO2 would be lower than one would expect from an arterial blood sample
  • Arterial Blood Sample Drawn from Ischemic Body Site (Such as an Ischemic Limb): in this case, the arterial pO2 would be lower than the SpO2 would predict (and would be lower than the arterial pO2 obtained at another body site)
  • Pseudohypoxemia: in cases with very high WBC count, in vitro consumption of oxygen can occur in the arterial blood gas sample during transit (prior to processing) on the blood gas machine -> results in artifactually low pO2, as compared to the SpO2
  • Methemoglobinemia (see Methemoglobinemia, [[Methemoglobinemia]]): in this case, SpO2 will give an abnormally low reading, as compared to a normal pO2

References

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