Arterial blood gas analysis

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Arterial blood gas (ABG) analysis is a common test that measures oxygen, carbon dioxide, and bicarbonate blood levels. The test provides an assessment of gas exchange processes and acid-base balance. ABGs are usually obtained by puncturing the radial or femoral artery. An arterial line may be placed if frequent ABG sampling and continuous blood pressure monitoring are required.

See also “Arterial access.”

• Procedure: See “ABG sampling.”
• Measured parameters
  • Partial pressure of oxygen in arterial blood (PaO₂) and partial pressure of carbon dioxide (PaCO₂) in arterial blood
  • Arterial oxygen saturation (SaO₂): percentage of hemoglobin binding sites occupied by oxygen
  • pH
  • Base excess: Excess value of base in the blood. Used to identify whether an acid-base imbalance is predominantly a respiratory, metabolic, or a mixed acid-base disorder.
  • Standard bicarbonate
  • Modern blood gas analyzers also measure: some electrolytes (i.e., sodium, potassium, chloride, calcium), blood glucose, hemoglobin, methemoglobin and carboxyhemoglobin concentrations
• Reference ranges for ABG
  • PaCO₂: 35–45 mm Hg
  • SaO₂: ≥ 95%
  • pH: 7.35–7.45
  • HCO₃^-: 21–27 mEq/L
  • Base excess: -2–3 mEq/L
  • Resting PaO₂ > 80 mm Hg is considered normal.
• Interpretation
  • Hypoxemic respiratory failure (type 1 respiratory failure): ↓ PaO₂
  • Hypercapnic respiratory failure (type 2 respiratory failure): ↑ PaCO₂ and ↓ PaO₂
  • See also “Diagnostics” in “Acid-base disorders.”

• Definition
  • Mixed oxygen venous saturation (S_vO₂) is the saturation of hemoglobin in the pulmonary artery.
  • S_vO₂ is an indirect measure of the oxygen content in the venous system.
• Reference range: 65–70%
• Interpretation
  • Decreased S_vO₂
    • Increased tissue oxygen extraction due to decreased oxygen delivery to tissue
      • Decreased hemoglobin concentration
      • Lung disease
        • Decreased alveolar oxygen concentration
        • Decreased alveolar oxygen diffusion
        • Increased right-to-left shunting
      • Decreased cardiac output
    • Increased oxygen consumption by tissues
      • Exercise
      • Fever
      • Seizures
    • Inability of hemoglobin to bind to oxygen (e.g., carbon monoxide poisoning)
  • Increased S_vO₂
    • Peripheral blood shunting (e.g., AV fistula)
    • Decreased metabolic demand (e.g., hypothermia)