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Arterial Blood Gas (ABG) Interpretation

Assess Oxygenation
  • PaO2 should be >10 kPa when oxygenating on room air in a healthy patient
  • A patient is receiving oxygen therapy should have a PaO2 approximately 10kPa less than the % inspired concentration of O2 (eg a patient on 50% oxygen would be expected to have a PaO2 of approximately 40kPa
Determine pH Status
  • Acidosis (pH low)
  • Alkalosis (pH high
Determine Respiratory Component
  • Respiratory acidosis – low pH, high PaCO2
  • Respiratory alkalosis – high pH, low PaCO2
Determine Metabolic Component
  • Metabolic acidosis – low pH, low HCO3
  • Metabolic alkalosis – high pH, high HCO3
Is There Compensation?
  • Compensation → if the respiratory/metabolic component does not fit with pH
  • Acidosis can be compensated by:
    • Respiratory compensation → increasing respiratory rate to blow off CO2
    • Metabolic compensation → increased bicarbonate reabsorption in the kidney
  • Alkalosis can be compensated by:
    • Respiratory compensation → decreasing respiratory rate to retain CO2
    • Metabolic compensation → decreased bicarbonate reabsorption in the kidney
Causes of Respiratory Failure
  • Type 1 → 1 gas abnormal (decreased O2)
    • Caused by ventilation/perfusion mismatch (V/Q)
    • Low V/Q → Causes: mucus plug in asthma/COPD, airway collapse in emphysema
    • High V/Q → Causes: Pulmonary embolism
  • Type 2 → 2 gases abnormal (decreased O2, increased CO2)
    • Caused by alveolar hypoventilation
    • Oxygen cannot get into alveoli and carbon dioxide cannot get out.
    • Causes: Obstructive lung diseases (eg COPD), restrictive lung diseases, decreased respiratory drive, neuromuscular disease, thoracic wall disease
Reference Range Values
  • pH: 7.35 – 7.45
  • PaCO2: 4.7 – 6.0 kPa
  • PaO2: 11 – 13 kPa
  • HCO3–: 22 – 26 mEq/L
  • Base excess (BE): -2 to +2 mmol/L

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