How do you interpret arterial blood gases (ABGs)?

Interpreting Arterial Blood Gas (ABG) Analysis

A systematic, step-by-step approach is essential for accurate interpretation of arterial blood gases, beginning with assessment of pH, followed by evaluation of PaCO2, HCO3-, and oxygenation parameters. 1

Step-by-Step Approach to ABG Interpretation

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

• pH < 7.35 indicates acidemia 1
• pH > 7.45 indicates alkalemia 1

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

• PaCO2 > 45 mmHg indicates respiratory acidosis (hypoventilation) 1
• PaCO2 < 35 mmHg indicates respiratory alkalosis (hyperventilation) 1
• Determine if the PaCO2 change is consistent with the pH change or opposing it 2
  • If PaCO2 change explains the pH abnormality → primary respiratory disorder
  • If PaCO2 change opposes the pH abnormality → compensatory response to a metabolic disorder

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

• HCO3- < 22 mEq/L indicates metabolic acidosis 1
• HCO3- > 26 mEq/L indicates metabolic alkalosis 1
• Determine if the HCO3- change is consistent with the pH change or opposing it 2
  • If HCO3- change explains the pH abnormality → primary metabolic disorder
  • If HCO3- change opposes the pH abnormality → compensatory response to a respiratory disorder

Step 4: Assess Oxygenation

• Evaluate PaO2 (Normal: >80 mmHg on room air) 1
• Check oxygen saturation (SaO2) (Normal: >94% in most patients) 1
• Calculate the PaO2/FiO2 ratio to assess severity of hypoxemia 1
  • Normal: >400
  • Mild hypoxemia: 300-400
  • Moderate hypoxemia: 200-300
  • Severe hypoxemia: <200

Step 5: Determine Compensation Status

• Uncompensated: pH abnormal, only one parameter (PaCO2 or HCO3-) abnormal 2
• Partially compensated: pH abnormal but moving toward normal, both PaCO2 and HCO3- abnormal 2
• Fully compensated: pH normal, both PaCO2 and HCO3- abnormal 2

Common ABG Patterns

Respiratory Acidosis

• pH < 7.35, PaCO2 > 45 mmHg 1
• Acute: minimal HCO3- compensation
• Chronic: HCO3- increases for compensation 3
• Causes: COPD exacerbation, respiratory depression, neuromuscular disorders 4

Respiratory Alkalosis

• pH > 7.45, PaCO2 < 35 mmHg 1
• Acute: minimal HCO3- compensation
• Chronic: HCO3- decreases for compensation 3
• Causes: anxiety, pain, fever, early sepsis, high altitude 3

Metabolic Acidosis

• pH < 7.35, HCO3- < 22 mEq/L 1
• Compensation: PaCO2 decreases (increased ventilation) 3
• Causes: diabetic ketoacidosis, lactic acidosis, renal failure, diarrhea 4

Metabolic Alkalosis

• pH > 7.45, HCO3- > 26 mEq/L 1
• Compensation: PaCO2 increases (decreased ventilation) 3
• Causes: vomiting, nasogastric suction, diuretic use, hypokalemia 3

Mixed Acid-Base Disorders

• When primary disturbances in both respiratory and metabolic components exist simultaneously 5
• Examples:
  • Respiratory acidosis + metabolic acidosis (e.g., cardiopulmonary arrest)
  • Respiratory alkalosis + metabolic alkalosis (e.g., liver disease with vomiting)
  • Respiratory acidosis + metabolic alkalosis (e.g., COPD with diuretic use)
  • Respiratory alkalosis + metabolic acidosis (e.g., sepsis) 3

Special Considerations

Clinical Context

• Always interpret ABGs in the context of the patient's clinical condition 6
• Normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia 6
• ABG analysis is essential after oxygen titration to determine whether adequate oxygenation has been achieved without precipitating respiratory acidosis 6

Technical Aspects

• Use local anesthesia for all ABG specimens except in emergencies 6
• Perform Allen's test before radial artery puncture to ensure dual blood supply to the hand 6
• Arterial samples are preferred over capillary samples in critically ill patients 1

COPD and Risk of CO2 Retention

• Target oxygen saturation of 88-92% for patients with COPD and risk of hypercapnic respiratory failure 1
• Check ABG within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration in patients at risk for hypercapnic respiratory failure 6

Common Pitfalls to Avoid

• Failing to recognize that a normal oxygen saturation does not rule out significant acid-base disturbances 6
• Overlooking the need for ABG in patients with metabolic conditions that may cause acid-base disturbances 6
• Not repeating ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention 6
• Relying solely on pulse oximetry in situations where acid-base status and ventilation need to be assessed 6
• Misinterpreting mixed acid-base disorders as simple disorders with compensation 5