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

How to Interpret Arterial Blood Gas (ABG) Results

Use a systematic, step-by-step approach: first assess oxygenation (PaO2 and SaO2), then determine acid-base status by analyzing pH, PaCO2, and HCO3- in sequence to identify the primary disturbance and any compensation. 1, 2

Step 1: Assess Oxygenation Status

• Check PaO2 (normal >80 mmHg) and oxygen saturation (normal >94% in most patients) to determine if hypoxemia is present 2
• Calculate the PaO2/FiO2 ratio to assess severity of hypoxemia, particularly in critically ill patients 1, 2
• Important caveat: Pulse oximetry cannot differentiate carboxyhemoglobin and may give falsely normal readings in carbon monoxide poisoning 1, 2
• Normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 2, 3

Step 2: Determine Acid-Base Status Using pH

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

Step 3: Identify the Primary Disturbance

Assess PaCO2 (Normal Range: 35-45 mmHg)

• PaCO2 >45 mmHg indicates respiratory acidosis 1, 2
• PaCO2 <35 mmHg indicates respiratory alkalosis 2

Assess HCO3- (Normal Range: 22-26 mEq/L)

• HCO3- <22 mEq/L indicates metabolic acidosis 2
• HCO3- >26 mEq/L indicates metabolic alkalosis 2

Step 4: Apply the RoMe Technique to Determine Primary vs. Compensatory Changes

• "Respiratory opposite, Metabolic equal": In respiratory disorders, pH moves opposite to PaCO2; in metabolic disorders, pH moves in the same direction as HCO3- 4
• If pH is acidotic and PaCO2 is elevated, the primary disturbance is respiratory acidosis 4
• If pH is acidotic and HCO3- is decreased, the primary disturbance is metabolic acidosis 4
• If pH is alkalotic and PaCO2 is decreased, the primary disturbance is respiratory alkalosis 4
• If pH is alkalotic and HCO3- is elevated, the primary disturbance is metabolic alkalosis 4

Step 5: Assess for Compensation

• Uncompensated: Only the primary disturbance is abnormal, pH is abnormal 4
• Partially compensated: Both respiratory and metabolic components are abnormal, pH remains abnormal 4
• Fully compensated: Both components are abnormal, but pH has returned to normal range (7.35-7.45) 4

Critical Management Decisions Based on ABG Results

Respiratory Acidosis Management

• Start non-invasive ventilation (NIV) in COPD exacerbation when pH <7.35 and PaCO2 >6.5 kPa (approximately 49 mmHg) persist despite optimal medical therapy 1, 2
• Address the underlying cause and provide ventilatory support for acute respiratory acidosis 2

Metabolic Acidosis Management

• Sodium bicarbonate therapy should be limited to patients with severe acidosis (arterial pH <7.1 and base deficit <10) 1, 2
• Treat the underlying cause as primary management 2
• Monitor base deficit as a sensitive marker for severity of shock and mortality risk 2

Oxygen Therapy Adjustments

• Target oxygen saturation of 88-92% for patients with COPD and risk of hypercapnic respiratory failure 1
• Monitor for worsening hypercapnia after each titration of oxygen flow rate in patients with baseline hypercapnia 2
• Check ABG within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration in at-risk patients 3

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

Technical Considerations for Obtaining ABG

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand 1, 2, 3
• Use local anesthesia for all ABG specimens except in emergencies 1, 2, 3
• Arterial samples are preferred over capillary or venous samples in critically ill patients 1, 2, 3
• In ECMO patients, obtain samples from a right radial arterial line as this best represents cerebral perfusion 3

Common Pitfalls to Avoid

• Do not rely solely on pulse oximetry—it will appear normal in patients with normal PaO2 but abnormal pH or PaCO2 3
• Do not assume adequate ventilation based on normal oxygenation alone 2, 3
• Remember that venous blood gas cannot accurately replace arterial sampling in critically ill patients, particularly for PaO2 assessment 2, 3