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

Systematic Approach to Arterial Blood Gas Interpretation

Arterial blood gas (ABG) interpretation should follow a systematic approach that evaluates pH, PaCO2, PaO2, and HCO3- values to determine acid-base status, oxygenation, and ventilation adequacy. 1

Step-by-Step ABG Interpretation Algorithm

Step 1: Evaluate pH (Normal: 7.35-7.45)

• pH < 7.35: Acidemia
• pH > 7.45: Alkalemia
• pH within range: Normal or compensated disorder

Step 2: Identify Primary Disorder

• Respiratory disorders: Evaluate PaCO2 (Normal: 35-45 mmHg)

  • PaCO2 > 45 mmHg + pH < 7.35: Respiratory acidosis
  • PaCO2 < 35 mmHg + pH > 7.45: Respiratory alkalosis

• Metabolic disorders: Evaluate HCO3- (Normal: 22-26 mEq/L)

  • HCO3- < 22 mEq/L + pH < 7.35: Metabolic acidosis
  • HCO3- > 26 mEq/L + pH > 7.45: Metabolic alkalosis

Step 3: Assess Compensation

Apply the "RoMe" technique (Respiratory opposite, Metabolic equal) 2:

• In respiratory disorders: Metabolic compensation moves HCO3- in the same direction as pH
• In metabolic disorders: Respiratory compensation moves PaCO2 in the opposite direction as pH

Compensation status:

• Uncompensated: Abnormal pH with primary disorder only
• Partially compensated: Abnormal pH with compensatory mechanism present
• Fully compensated: Normal pH with both primary disorder and compensatory mechanism

Step 4: Evaluate Oxygenation

• PaO2 (Normal: 80-100 mmHg)
• Oxygen Saturation (Normal: 95-100%)
• Calculate A-a gradient if needed for hypoxemia evaluation
• Significant hypoxemia: PaO2 < 60 mmHg 1

Step 5: For Metabolic Acidosis, Calculate Anion Gap

• Anion Gap = Na⁺ - (Cl⁻ + HCO3⁻)
• Normal: 8-12 mEq/L
• Elevated (>15 ± 2): Consider MUDPILES causes (Methanol, Uremia, Diabetic ketoacidosis, Paraldehyde, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates) 3

Common ABG Patterns and Clinical Implications

Respiratory Acidosis

• pH < 7.35, PaCO2 > 45 mmHg
• Causes: Hypoventilation, COPD exacerbation, respiratory depression
• Management: Address underlying cause, consider ventilatory support if pH < 7.35 with PaCO2 > 6.0 kPa (45 mmHg) 1

Respiratory Alkalosis

• pH > 7.45, PaCO2 < 35 mmHg
• Causes: Hyperventilation, anxiety, sepsis, pulmonary embolism
• Management: Treat underlying cause, breathing techniques for anxiety

Metabolic Acidosis

• pH < 7.35, HCO3- < 22 mEq/L
• Causes: Diabetic ketoacidosis, lactic acidosis, renal failure
• Management: Identify and treat underlying cause; consider sodium bicarbonate if pH < 7.1 1

Metabolic Alkalosis

• pH > 7.45, HCO3- > 26 mEq/L
• Causes: Vomiting, diuretic use, hypokalemia
• Management: Correct fluid/electrolyte abnormalities, address underlying cause

Monitoring and Follow-up

• Repeat ABG after 1-2 hours of initiating treatment to assess response 1
• Repeat after significant changes in respiratory support or oxygen therapy 1
• For patients on non-invasive ventilation, perform ABG after 1-2 hours; consider alternative ventilation if no improvement in PaCO2 and pH after 4-6 hours 1

Common Pitfalls to Avoid

• Relying solely on pulse oximetry, which doesn't detect hypercarbia or acid-base disturbances 1
• Failing to consider supplemental oxygen when interpreting PaO2 1
• Not accounting for temperature effects on blood gas values 1
• Overlooking the presence of carboxyhemoglobin, which can falsely elevate SpO2 readings 1
• Misinterpreting mixed disorders as simple compensation 2

Technical Considerations

• Perform Allen's test before radial artery sampling to ensure dual blood supply to the hand 1
• Obtain informed consent and discuss potential risks with the patient 1
• Use local anesthesia except in emergency situations 1
• Properly label specimens with patient identifiers, date, time, and collector's ID 1

By following this systematic approach, clinicians can accurately interpret ABGs to guide diagnosis and treatment decisions for patients with respiratory, metabolic, and mixed acid-base disorders 4.