Arterial Blood Gas 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