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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