How do you interpret an Arterial Blood Gas (ABG) result?

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 for the respiratory component, and finally assess base excess/bicarbonate for the metabolic component. 1, 2, 3

Step 1: Evaluate the pH

• pH < 7.35 = acidemia 1, 2
• pH > 7.45 = alkalemia 1, 2
• This first step determines the primary direction of the acid-base disturbance and guides all subsequent interpretation 2

Step 2: Identify the Respiratory Component

• PaCO2 > 45 mmHg with low pH = respiratory acidosis 1, 2
• PaCO2 < 35 mmHg with high pH = respiratory alkalosis 1, 2
• The respiratory system responds rapidly (minutes to hours) to acid-base disturbances 2

Step 3: Identify the Metabolic Component

• Base excess < -2 OR HCO3 < 22 mmol/L = metabolic acidosis 1, 2
• Base excess > +2 OR HCO3 > 26 mmol/L = metabolic alkalosis 1, 2
• The metabolic system compensates more slowly (hours to days) 2

Step 4: Determine Compensation Status

• Uncompensated: pH abnormal, only one system (respiratory OR metabolic) is abnormal 2
• Partially compensated: pH abnormal, BOTH PaCO2 and HCO3 are abnormal, moving in opposite directions to correct pH 2
• Fully compensated: pH normalized (7.35-7.45), but both PaCO2 and HCO3 remain abnormal 2

Step 5: Calculate Delta Ratio for Mixed Disorders (When Applicable)

When metabolic acidosis with elevated anion gap is present, calculate: (Anion Gap - 12) / (24 - HCO3⁻) 3

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

Critical Clinical Pitfalls to Avoid

• Never assume normal SpO2 rules out acid-base disturbances or hypercapnia - oxygen saturation can be normal despite severe metabolic acidosis or CO2 retention 2, 3
• Always repeat ABG after oxygen therapy changes in patients at risk for CO2 retention - failure to do so is a critical management error 1, 2, 3
• Monitor for PaCO2 rise > 1 kPa (7.5 mmHg) during oxygen titration - this indicates clinically unstable disease requiring intervention 2, 3

Special Population Considerations

COPD Patients with Chronic CO2 Retention

• Baseline hypercapnia leads to metabolic compensation with elevated HCO3 2
• In chronic respiratory disorders, base excess changes to compensate, whereas in acute disorders it remains initially normal 2
• Check ABG when starting oxygen therapy, especially with known CO2 retention 1

Critically Ill Patients

• All critically ill patients require ABG testing to assess oxygenation, ventilation, and acid-base status 1, 3
• Patients with shock or hypotension should have initial blood gas from arterial source 3
• Calculate delta ratio in suspected mixed disorders where multiple pathophysiologic processes may coexist 3

Primary Indications for ABG Testing

• All critically ill patients 1, 3
• SpO2 fall below 94% on room air or supplemental oxygen 1, 3
• Shock or hypotension 1, 3
• Suspected diabetic ketoacidosis 1
• Metabolic acidosis from renal failure, trauma, or sepsis 1
• Deteriorating oxygen saturation or increasing breathlessness in patients with previously stable chronic hypoxemia 1

Management Based on ABG Results

Acute Hypercapnic Respiratory Failure

• Initiate non-invasive ventilation (NIV) when pH < 7.35 AND PaCO2 > 6.5 kPa (49 mmHg) persist despite optimal medical therapy 1, 3
• Target SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1, 3
• Obtain ABG before and after starting NIV 1
• Maximize NIV time in first 24 hours with goal of normalizing pH and pCO2 1

Oxygen Titration Protocol

• Start oxygen at 1 L/min and titrate up in 1 L/min increments until SpO2 >90% 3
• Repeat ABG after each flow rate titration in patients with baseline hypercapnia 3
• Consider NIV if PaCO2 rises despite controlled oxygen therapy 1