How to interpret an arterial blood gas (ABG)?

Arterial Blood Gas Interpretation

Arterial blood gas (ABG) analysis should be interpreted using a systematic approach that evaluates pH, PaCO2, PaO2, and HCO3- values to determine acid-base status, oxygenation, and ventilation adequacy. 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%

Systematic Interpretation Algorithm

Step 1: Evaluate pH

• pH < 7.35: Acidemia
• pH > 7.45: Alkalemia
• pH 7.35-7.45: Normal

Step 2: Identify 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)

Step 3: Assess Compensation

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

• In respiratory disorders: metabolic compensation moves pH toward normal by changing HCO3- in the same direction as pH change
• In metabolic disorders: respiratory compensation moves pH toward normal by changing PaCO2 in the opposite direction as pH change 2

Step 4: Determine Compensation Status

• Uncompensated: Only primary disorder present, no compensatory changes
• Partially Compensated: Compensatory mechanism active but pH still abnormal
• Fully Compensated: Compensatory mechanism has returned pH to normal range

Step 5: Evaluate Oxygenation

• Normal: PaO2 80-100 mmHg
• Mild Hypoxemia: PaO2 60-80 mmHg
• Moderate Hypoxemia: PaO2 40-60 mmHg
• Severe Hypoxemia: PaO2 <40 mmHg

Clinical Applications

Respiratory Disorders

• For patients with hypercapnic respiratory failure (pH <7.35 with PaCO2 >45 mmHg), consider ventilatory support, especially when pH <7.35 with PaCO2 >6.0 kPa (45 mmHg) 1
• In COPD patients, target SpO2 88-92% to prevent worsening hypercapnia 1

Metabolic Disorders

• In severe metabolic acidosis (arterial pH <7.1 and base deficit <10), consider sodium bicarbonate administration 3
• For metabolic acidosis associated with shock, monitor blood gases, plasma osmolarity, arterial blood lactate, hemodynamics and cardiac rhythm 4

Special Circumstances

• In cardiac arrest, rapid IV sodium bicarbonate (44.6 to 100 mEq) may be given initially and continued at 44.6 to 50 mEq every 5-10 minutes if necessary to reverse acidosis 4
• In less urgent metabolic acidosis, administer approximately 2-5 mEq/kg of sodium bicarbonate over 4-8 hours, depending on severity 4

Common Pitfalls to Avoid

1. Relying solely on pulse oximetry: SpO2 doesn't detect hypercarbia or acid-base disturbances
2. Failing to consider supplemental oxygen: PaO2 must be interpreted in context of FiO2
3. Not accounting for temperature effects: Blood gas values are affected by body temperature
4. Overlooking carboxyhemoglobin: Can falsely elevate SpO2 readings
5. Attempting full correction too quickly: Correcting low total CO2 content too rapidly can cause alkalosis due to ventilatory lag 1, 4

When to Repeat ABG Analysis

• After 1-2 hours of initiating treatment
• After significant changes in respiratory support or oxygen therapy
• When clinical deterioration occurs
• After 1-2 hours of initiating non-invasive ventilation, with consideration for invasive ventilation if no improvement in PaCO2 and pH after 4-6 hours 1

ABG interpretation is essential for evaluating respiratory function, diagnosing acid-base disorders, and managing critical conditions. A systematic approach ensures accurate assessment and appropriate clinical management.