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

Arterial Blood Gas (ABG) Interpretation

Arterial blood gas analysis should be interpreted using a systematic approach that evaluates pH, PaCO₂, PaO₂, and HCO₃⁻ to accurately assess acid-base status, oxygenation, and ventilation, which is crucial for managing critically ill patients and preventing morbidity and mortality. 1

Normal ABG Values

• pH: 7.35-7.45
• PaCO₂: 35-45 mmHg (4.7-6.0 kPa)
• PaO₂: 80-100 mmHg (10.6-13.3 kPa)
• HCO₃⁻: 22-26 mEq/L
• Oxygen Saturation: 95-100% 1

Systematic Approach to ABG Interpretation

Step 1: Evaluate pH

• pH < 7.35 = Acidemia
• pH > 7.45 = Alkalemia
• pH within normal range = Normal or compensated disorder 1, 2

Step 2: Determine Primary Disorder

• Respiratory Acidosis: pH ↓, PaCO₂ ↑ (>45 mmHg)
• Respiratory Alkalosis: pH ↑, PaCO₂ ↓ (<35 mmHg)
• Metabolic Acidosis: pH ↓, HCO₃⁻ ↓ (<22 mEq/L)
• Metabolic Alkalosis: pH ↑, HCO₃⁻ ↑ (>26 mEq/L) 1, 3

Step 3: Assess Compensation

Use the RoMe technique (Respiratory Opposite, Metabolic Equal):

• In respiratory disorders: metabolic compensation moves pH in the opposite direction of PaCO₂
• In metabolic disorders: respiratory compensation moves pH in the same direction as HCO₃⁻ 3

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 full compensation 3

Step 4: Evaluate Oxygenation

• PaO₂ < 60 mmHg indicates significant hypoxemia
• Calculate A-a gradient if needed to distinguish between hypoventilation and V/Q mismatch 1

Clinical Applications

Respiratory Disorders

• Acute respiratory failure: PaO₂ < 60 mmHg and/or PaCO₂ > 50 mmHg with pH < 7.35
• Chronic respiratory failure: Elevated PaCO₂ with near-normal pH due to renal compensation 1, 2

Metabolic Disorders

• Diabetic ketoacidosis: Metabolic acidosis (↓ pH, ↓ HCO₃⁻) with respiratory compensation
• Renal failure: Metabolic acidosis due to impaired acid excretion 1

Mixed Disorders

• Multiple primary disorders may coexist (e.g., metabolic acidosis with respiratory alkalosis)
• Identify by examining if compensation is appropriate for the primary disorder 4

Special Considerations

Cardiac Arrest

• ABG alone may be misleading during cardiac arrest
• Sodium bicarbonate should be limited to severe acidosis (pH < 7.1, base deficit < -10) or special circumstances like hyperkalemia or tricyclic antidepressant overdose 5

Sample Collection

• Arterial blood is preferred for critically ill patients
• Venous samples are acceptable alternatives when arterial sampling is unavailable
• Avoid capillary samples in patients with shock, vasopressor therapy, or poor peripheral perfusion 1

Common Pitfalls to Avoid

1. Failing to correlate ABG results with clinical context - ABG values must be interpreted alongside the patient's clinical presentation
2. Overlooking mixed disorders - Multiple acid-base disturbances can coexist
3. Misinterpreting compensation - Distinguish between compensation and a new primary disorder
4. Ignoring the FiO₂ - Always interpret PaO₂ in relation to the fraction of inspired oxygen 1, 3

By following this systematic approach to ABG interpretation, clinicians can accurately identify acid-base disturbances and respiratory abnormalities, leading to appropriate interventions that improve patient outcomes.