How do you interpret an arterial blood gas (ABG) result?

How to Interpret an Arterial Blood Gas (ABG)

Use a systematic three-step approach: first assess pH to determine if acidemia or alkalemia is present, then evaluate PaCO2 to identify the respiratory component, and finally examine bicarbonate/base excess to identify the metabolic component. 1

Step 1: Assess pH (Acid-Base Status)

• pH < 7.35 indicates acidemia 1
• pH > 7.45 indicates alkalemia 1
• Normal pH range is 7.35-7.45 2

Step 2: Evaluate PaCO2 (Respiratory Component)

• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 1
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 1
• Normal PaCO2 range is 35-45 mmHg 2

Step 3: Evaluate Bicarbonate/Base Excess (Metabolic Component)

• Base excess < -2 or HCO3 < 22 mmol/L indicates metabolic acidosis 1
• Base excess > +2 or HCO3 > 26 mmol/L indicates metabolic alkalosis 1
• Normal HCO3 range is 22-26 mEq/L 2

Step 4: Assess Oxygenation Status

• PaO2 < 60 mmHg (8 kPa) indicates significant hypoxemia requiring intervention 1, 3
• Normal PaO2 is >80 mmHg 2
• Consider PaO2/FiO2 ratio to assess severity of hypoxemia 2

Oxygen Saturation Targets:

• Target SpO2 88-92% for COPD and hypercapnic respiratory failure 1, 3
• Target SpO2 94-98% for most other patients 1

Critical Management Points Based on ABG Results

For Respiratory Acidosis:

• Initiate non-invasive ventilation (NIV) when pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg) despite optimal medical therapy 3, 2
• Address underlying cause and provide ventilatory support 2

For Hypoxemia:

• Continuous oxygen therapy is indicated for PaO2 < 8 kPa 4
• High concentration oxygen (≥35%) can be safely used unless complicated by severe COPD with ventilatory failure 4
• For COPD patients with CO2 retention, start with low oxygen concentrations (24-28%) and progressively increase based on repeated ABG measurements 4

For Metabolic Acidosis:

• Treat the underlying cause 2
• Consider sodium bicarbonate therapy only for severe acidosis (arterial pH <7.1 and base deficit <10) 2
• Monitor base deficit as a sensitive marker for severity of shock and mortality risk 2

Timing of Repeat ABG Measurements

• Repeat ABG within 60 minutes after starting oxygen therapy or changing FiO2 in COPD patients 1
• After each titration of oxygen flow rate in patients with baseline hypercapnia, perform ABG analysis 3, 2
• Aim to keep SaO2 > 90% without causing arterial pH to fall below 7.35 4

Technical Considerations for ABG Sampling

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand 1, 3, 2
• Use local anesthesia for all non-emergency ABG sampling 1, 3, 2
• Use arterial samples rather than venous samples in critically ill patients 2
• Knowledge of the inspired oxygen concentration (FiO2) is essential to interpretation and should be clearly recorded with the blood gas result 4

Critical Pitfalls to Avoid

• A normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 3, 2
• Pulse oximetry cannot differentiate carboxyhemoglobin, potentially giving falsely normal readings in carbon monoxide poisoning 2
• Failing to repeat ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention, is a common management pitfall 3
• High concentration oxygen given to COPD patients with CO2 retention can reduce hypoxic drive and increase ventilation-perfusion mismatching 4

Additional Markers to Consider

• Lactate levels provide information about tissue oxygenation and perfusion, with elevated levels indicating shock 2
• Serial lactate measurements help predict survival and evaluate response to therapy 2
• Base deficit is a sensitive marker for severity of injury and mortality risk, particularly in trauma patients 2