How do I perform a complete arterial blood gas (ABG) interpretation?

Complete ABG Interpretation: A Systematic Approach

Use the American Thoracic Society's three-step systematic method: evaluate pH first to determine acidemia versus alkalemia, then examine PaCO2 to identify the respiratory component, and finally evaluate base excess/bicarbonate to identify the metabolic component. 1, 2, 3

Step 1: Evaluate the pH

• pH < 7.35 indicates acidemia 1, 2, 3
• pH > 7.45 indicates alkalemia 1, 2, 3
• pH 7.35-7.45 is normal but proceed to evaluate for compensated disorders 1

Step 2: Identify the Respiratory Component

• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 1, 2
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 1, 2
• Normal PaCO2 is 35-45 mmHg 1

Step 3: Identify the Metabolic Component

• Base excess < -2 or HCO3 < 22 mmol/L indicates metabolic acidosis 1, 2
• Base excess > +2 or HCO3 > 26 mmol/L indicates metabolic alkalosis 1, 2
• Normal HCO3 is 22-26 mmol/L 1

Step 4: Determine Degree of Compensation

Fully compensated disorders have normalized pH with abnormal PaCO2 and HCO3 levels moving in opposite directions. 1, 2

• Partially compensated disorders have abnormal pH with both PaCO2 and HCO3 abnormal, with both systems moving in opposite directions to correct the pH 1, 2
• Uncompensated disorders show abnormal pH with only one system (respiratory or metabolic) abnormal 1

Step 5: Assess Oxygenation Status

• Check PaO2, with normal values >90 mmHg on room air at sea level 3
• Evaluate P(A-a)O2 gradient, with normal values <15 mmHg (or <20 mmHg if age ≥65 years) 3
• Severe hypoxemia is PaO2 <60 mmHg requiring immediate intervention 3
• Normal arterial oxygen saturation is >94% 3

Step 6: Calculate Delta Ratio for Mixed Disorders (When Applicable)

When metabolic acidosis with elevated anion gap is identified, calculate delta ratio as (Anion Gap - 12) / (24 - HCO₃⁻) to detect concurrent metabolic alkalosis or additional metabolic acidosis. 3

• Delta ratio 1-2 suggests pure high anion gap metabolic acidosis 3
• Delta ratio <1 suggests concurrent normal anion gap metabolic acidosis 3
• Delta ratio >2 suggests concurrent metabolic alkalosis 3

Distinguishing Acute vs Chronic Disorders

In chronic respiratory disorders, base excess changes to compensate, whereas in acute disorders it remains initially normal. 1

• COPD patients with chronic CO2 retention show metabolic compensation with elevated HCO3 1
• Monitor patients with baseline hypercapnia during oxygen therapy; a PaCO2 rise >1 kPa (7.5 mmHg) indicates clinically unstable disease requiring reassessment 1, 3

Critical Pitfalls to Avoid

Normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia, especially in patients on supplemental oxygen. 4, 1, 3

• Pulse oximetry will be normal in patients with normal PO2 but abnormal pH or PCO2 4
• Failing to repeat ABG measurements after oxygen therapy changes in patients at risk for CO2 retention is a critical management error 1, 2, 3
• Blood gases and full blood count are required in all situations where these measurements may affect patient outcomes 4

When to Obtain ABG Testing

All critically ill patients require ABG testing to assess oxygenation, ventilation, and acid-base status. 4, 2, 3

• Patients with shock or hypotension (systolic BP <90 mmHg) require arterial sampling 4, 2
• SpO2 fall below 94% on room air or supplemental oxygen 4, 2
• Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in previously stable chronic hypoxemia 4
• Any patient with risk factors for hypercapnic respiratory failure who develops acute breathlessness, drowsiness, or features of CO2 retention 4
• Suspected diabetic ketoacidosis or metabolic acidosis from renal failure 4

Technical Considerations

Use local anesthesia for all ABG specimens except in emergencies. 4

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 4, 3
• Obtain informed consent with discussion of possible risks 4, 3
• For most non-critical patients, arterial or arterialized earlobe blood gases may be used for pH and PCO2 (though PO2 underestimates by 0.5-1 kPa in earlobe samples) 4