How do you interpret arterial blood gases (ABGs)?

How to Interpret Arterial Blood Gases (ABGs)

A systematic, step-by-step approach to ABG interpretation is essential, beginning with assessment of pH, followed by evaluation of PaCO2, HCO3-, and oxygenation parameters to determine acid-base status and respiratory function. 1

Step-by-Step ABG Interpretation Algorithm

Step 1: Assess pH (Normal range: 7.35-7.45)

• pH < 7.35 indicates acidemia 2, 1
• pH > 7.45 indicates alkalemia 2, 1
• pH within normal range may indicate a compensated disorder or normal acid-base status 1

Step 2: Evaluate PaCO2 (Normal range: 35-45 mmHg)

• PaCO2 > 45 mmHg indicates respiratory acidosis (hypoventilation) 1, 3
• PaCO2 < 35 mmHg indicates respiratory alkalosis (hyperventilation) 2, 3
• Determine if PaCO2 changes are consistent with pH changes:
  • If pH is low and PaCO2 is high → primary respiratory acidosis 2, 4
  • If pH is high and PaCO2 is low → primary respiratory alkalosis 2, 4

Step 3: Evaluate HCO3- (Normal range: 22-26 mEq/L)

• HCO3- > 26 mEq/L indicates metabolic alkalosis 1, 5
• HCO3- < 22 mEq/L indicates metabolic acidosis 1, 5
• Determine if HCO3- changes are consistent with pH changes:
  • If pH is low and HCO3- is low → primary metabolic acidosis 4, 5
  • If pH is high and HCO3- is high → primary metabolic alkalosis 4, 5

Step 4: Identify Primary Disorder and Compensation

• Use the "RoMe" technique: Respiratory disorders show opposite pH and PaCO2 changes, while Metabolic disorders show equal pH and HCO3- changes 4
• Assess for compensation:
  • Respiratory disorders are compensated by metabolic changes (HCO3- changes) 4, 5
  • Metabolic disorders are compensated by respiratory changes (PaCO2 changes) 4, 5
• Determine degree of compensation:
  • Uncompensated: pH abnormal, only primary disorder present 4
  • Partially compensated: pH abnormal but moving toward normal, compensatory mechanism active 4, 5
  • Fully compensated: pH within normal range, both primary disorder and compensatory mechanism present 4, 5

Step 5: Evaluate Oxygenation

• Assess PaO2 (Normal range: >80 mmHg on room air) 1, 3
• Check oxygen saturation (SaO2) (Normal range: >94% in most patients) 2, 1
• Calculate PaO2/FiO2 ratio to assess severity of hypoxemia 1

Common ABG Patterns and Interpretations

Respiratory Acidosis

• pH < 7.35, PaCO2 > 45 mmHg, HCO3- normal or elevated (if compensating) 2, 1
• Causes: COPD exacerbation, severe asthma, respiratory depression, neuromuscular disorders 2, 6
• Management: Consider NIV when pH < 7.35 and PaCO2 > 6.5 kPa (48.8 mmHg) despite optimal medical therapy 1

Respiratory Alkalosis

• pH > 7.45, PaCO2 < 35 mmHg, HCO3- normal or decreased (if compensating) 2, 1
• Causes: Anxiety, pain, fever, sepsis, early salicylate toxicity, high altitude 3, 5

Metabolic Acidosis

• pH < 7.35, HCO3- < 22 mEq/L, PaCO2 normal or decreased (if compensating) 1, 5
• Causes: Diabetic ketoacidosis, lactic acidosis, renal failure, diarrhea 6, 3
• Management: Consider sodium bicarbonate only for severe acidosis (pH < 7.1 and base deficit < 10) 1

Metabolic Alkalosis

• pH > 7.45, HCO3- > 26 mEq/L, PaCO2 normal or increased (if compensating) 1, 5
• Causes: Vomiting, nasogastric suction, diuretics, hypokalemia 5

Special Considerations and Pitfalls

Important Technical Considerations

• Use local anesthesia for all ABG specimens except in emergencies 2, 1
• Perform Allen's test before radial artery sampling to ensure dual blood supply to the hand 2, 1
• For non-critical patients, arterialized earlobe blood gases may be used as an alternative to assess pH and PCO2 (though PO2 is less accurate) 2, 6

Common Pitfalls to Avoid

• A normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 2, 6
• Pulse oximetry will appear normal in patients with normal PO2 but abnormal pH or PCO2 2, 6
• In carbon monoxide poisoning, pulse oximetry readings may be falsely normal, necessitating ABG regardless of oximeter readings 1, 6
• Not repeating ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention 2
• Overlooking the need for ABG in patients with metabolic conditions that may cause acid-base disturbances 2, 6

COPD and Oxygen Therapy Considerations

• Target oxygen saturation of 88-92% for patients with COPD and risk of hypercapnic respiratory failure 1
• Start with low flow oxygen (1 L/min) and titrate up in 1 L/min increments until SpO2 >90%, then confirm with repeat ABG 2
• Perform ABG within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration in COPD patients 2