Arterial blood gas analysis

Arterial blood gas (ABG) analysis is a test regularly performed to measure oxygen saturation, carbon dioxide, and bicarbonate blood levels. It provides quick assessment of gas exchange processes and acid-base balance.

• 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
  • PaCO₂: 35–45 mm Hg
  • SaO₂: ≥ 95%
  • pH: 7.35–7.45
  • HCO₃^-: 21 to 27 mEq/L
  • Base excess: -2 to +3 mmol/L
  • Resting PaO2 > 80 mm Hg is considered normal.
• Procedure
  • A modified Allen test must be performed before the radial artery is punctured to assess collateral circulation in the hand.
    1. Manual pressure below the wrist is applied to both arteries, the hand is elevated, and the patient is asked to clench their fist for about 30 seconds to induce transient ischemia, leading to pallor of the hand.
    2. When the pressure is released from the ulnar artery, the patient's hand rapidly returns to normal color if collateral circulation is present.
    3. Interpretation: The test result is normal if the patient's hand rapidly returns to normal color. The test is negative and considered pathological if the patient's hand remains at least partially pale.
  • Arterial blood can be drawn from radial arteries or an indwelling arterial catheter.
• 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)

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