Diagnosis of Carboxyhemoglobinemia (see Carboxyhemoglobinemia)

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Diagnosis of Methemoglobinemia (see Methemoglobinemia)

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Diagnosis of Respiratory Failure (see Respiratory Failure)

• Determination of Arterial pO2 in the Setting of Hypoxemia/Acute or Hypoxemic Respiratory Failure (see Hypoxemia and Respiratory Failure)
  • Arterial Blood Gas is Especially Useful as an Adjunct to Pulse Oximetry (see Pulse Oximetry)
• Determination of Arterial pCO2 in the Setting of Acute or Hypoxemic, Hypercapnic Respiratory Failure (see Hypercapnia and Respiratory Failure)

Diagnosis of Sulfhemoglobinemia (see Sulfhemoglobinemia)

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Physiology

Types of Hemoglobin Which May Be Present in the Adult Circulation

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

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)
• HCO3: 24 mEq/L

Differences Between Arterial Blood Gas (ABG) and Venous Blood Gas (VBG) Values (see Venous Blood Gas)

• General Comments
  • Differences Between Arterial and Venous Values are 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
• Venous pH
  • pH from Arterial and Venous Samples Correlate Reasonably Well (Respirology, 2014) [MEDLINE] and (Emerg Med J, 2014) [MEDLINE]: with agreement being highest at normal values
    • Venous pH is Approximately 0.03 Lower than the Arterial pH (95% Confidence Interval: 0.039 to 0.027) (Emerg Med J, 2014) [MEDLINE]
• Venous pCO2
  • pCO2 from a Venous Sample is Approximately 4.4 mm Hg Higher Than the pCO2 from an Arterial Sample: for this reason, a normal venous pCO2 has a good negative predictive value for a normal arterial pCO2
    • Venous pCO2 is Approximately 4.4 mm Hg Higher than the Arterial pCO2 (95% Confidence Interval: 2.55-6.27) (Eur J Emerg Med, 2014) [MEDLINE]
• Venous pO2
  • pO2 from Arterial and Venous Samples Do Not Correlate with Each Other
    • Arterial PO2 is Approximately 36.9 mm Hg Higher than the Venous pO2, with Significant Variability (95% Confidence Interval: 27.2-46.6 mm Hg) (Respirology, 2014) [MEDLINE]
• Venous Bicarbonate
  • Bicarbonate from Arterial and Venous Samples Correlate Reasonably Well
    • Venous Bicarbonate is Approximately 1.03 mmol/L Higher than the Arterial Bicarbonate (95% Confidence Interval 0.56-1.50) (Eur J Emerg Med, 2014) [MEDLINE]: agreement is highest at normal values
• Venous Carboxyhemoglobin (by Co-Oximetry)
  • Carboxyhemoglobin from Arterial (ABG) and Venous (VBG) Samples Correlate Well (Ann Emerg Med, 1995) [MEDLINE] and (Crit Care Med, 2000) [MEDLINE]
• Venous Lactate (see Serum Lactate)
  • Lactate from Arterial and Venous Samples Correlate Poorly at Abnormal Levels: however, this agreement is closer at normal levels such that, if the venous lactate is normal, the arterial lactate is generally also normal
    • Venous Lactate is Approximately 0.25 mmol/L Higher Than the Arterial Lactate (95% Confidence Interval: 0.15-0.35) (Eur J Emerg Med, 2014) [MEDLINE]

Cautions

• While Blood Gas Analyzers May Report Potassium Values, These Analyzers Do Not Typically Report if the Sample Has Been Hemolyzed (as Clinical Laboratories Routinely Do): for this reason, use of a VBG sample to assess potassium must be interpreted with caution

Parameters

• pH: measured using a 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 Use 37 Degrees C as a Standardized Procedure)
• HCO3: calculated
• SaO2: in a standard blood gas analyzer (without co-oximetry), this is usually calculated from the pO2 (see below)

Methods to Determine the SaO2 from an Arterial Blood Gas Sample

• General Comments
  • It is Critical to Know Which Device is Being Used by Your Specific Laboratory to Report the SaO2, Since the Presence of Dyshemoglobinemias Can Lead to Misinterpretation of the Data

Blood Gas Analyzer without Co-Oximetry

• Principle: pO2 is measured by the analyzer and the SaO2 is calculated using a standard equation
  • Technical Issues
    • Blood Gas Analyzer Uses a Calculated or Default Hemoglobin Value
    • Blood Gas Analyzer Assumes a Normal Hemoglobin Value and the Absence of Dyshemoglobinemias (Such as Methemoglobin, Carboxyhemoglobin, Sulfhemoglobin)
• Clinical Scenarios
  • Methemoglobinemia (see Methemoglobinemia): pO2 and (calculated) SaO2 will both be reported as normal

Functional Hemoglobin Saturation from Simple Co-Oximetry

• Principle: determination of SaO2 utilizing measurement of oxyhemoglobin (O2Hb) and deoxyHb (DeO2Hb) only
  • Equation: Total Hemoglobin = O2Hb + DeO2Hb + MetHb + COHb + SulfHb
  • Equation: SaO2 = O2Hb/O2Hb + DeO2Hb

Fractional Oxygen Saturation Method from Co-Oximetry (Fractional Oxyhemoglobin, FO2Hb)

• Principle: determination utilizing oxyhemoglobin (O2Hb) and total hemoglobin
  • Equation: Total Hemoglobin = O2Hb + DeO2Hb + MetHb + COHb + SulfHb
  • Equation: FO2Hb = O2Hb/Total Hb
  • FO2Hb is Usually Expressed as a Percentage: typically ranges from 90-95% in healthy normals
• Clinical Scenarios
  • Methemoglobinemia (see Methemoglobinemia): the reported FO2Hb will be considerably lower than the SaO2 reported by the blood gas analyzer

Co-Oximetry on Arterial Blood Gas Sample

• Principle: co-oximeter is a simplified spectrophotometer which measures light absorbance at various different wavelengths of light
  • Early Co-Oximetry Devices Were Capable of Measuring Light Absorbance at Four Wavelengths of Light
  • Modern Co-Oximetry Devices (Continuous Wave Spectrophotometers) are Capable of Measuring Absorbance at >100 Different Wavelengths of Light: additional wavelengths improve accuracy, minimize artifacts from interfering substances, and enable reporting of additional components
  • More Complex Modern Co-Oximetry Devices Can Measure Absorbance at 128 Wavelengths, Allowing Determination of Total Hemoglobin Concentration, SaO2, Fractional Oxyhemoglobin, and Fractional Carboxyhemoglobin, Fractional Methemoglobin, and Fractional Sulfhemoglobin
• Hemoglobin Species Detected by Co-Oximetry Devices
  • Oxyhemoglobin
  • Deoxyhemoglobin
  • Carboxyhemoglobin
  • Methemoglobin: peak absorbance at 630 nm
  • Sulfhemoglobin: peak absorbance at 614 nm (overlaps to 630 nm and may be reported as methemoglobin on older machines)