What is an Arterial Blood Gas (ABG)?

Arterial Blood Gas (ABG) Analysis

An arterial blood gas (ABG) is an essential diagnostic test that measures the levels of oxygen, carbon dioxide, and pH in arterial blood to evaluate respiratory function, acid-base balance, and oxygenation status, providing critical information for diagnosing and managing respiratory, metabolic, and circulatory disorders. 1

Components of ABG Analysis

ABG analysis measures several key parameters:

• pH: Measures blood acidity/alkalinity (normal range: 7.35-7.45)
• PaCO₂: Partial pressure of carbon dioxide (normal range: 35-45 mmHg)
• PaO₂: Partial pressure of oxygen (normal range: 80-100 mmHg)
• HCO₃⁻: Bicarbonate level (normal range: 22-26 mEq/L)
• Oxygen Saturation: Percentage of hemoglobin bound to oxygen (normal range: 95-100%) 1, 2

Clinical Applications

ABG analysis is valuable in:

1. Respiratory assessment: Evaluating respiratory failure, COPD exacerbations, and ventilation status
2. Acid-base disorder diagnosis: Identifying metabolic or respiratory acidosis/alkalosis
3. Critical care monitoring: Guiding management in shock, sepsis, and multi-organ failure
4. Oxygen therapy titration: Determining appropriate oxygen requirements
5. Ventilator management: Adjusting ventilator settings based on gas exchange parameters 1, 2

Interpretation Framework

A systematic approach to ABG interpretation involves:

1. Assess pH: Determine if acidemia (pH < 7.35) or alkalemia (pH > 7.45) exists
2. Identify primary disorder:
   • Respiratory acidosis: pH ↓, PaCO₂ ↑
   • Respiratory alkalosis: pH ↑, PaCO₂ ↓
   • Metabolic acidosis: pH ↓, HCO₃⁻ ↓
   • Metabolic alkalosis: pH ↑, HCO₃⁻ ↑
3. Evaluate compensation: Using the RoMe technique ("Respiratory opposite, Metabolic equal") 3
   • Respiratory disorders: Compensatory metabolic response moves pH in opposite direction
   • Metabolic disorders: Compensatory respiratory response moves pH in same direction
4. Assess oxygenation: Evaluate PaO₂ and oxygen saturation 1, 3

Clinical Decision Points

• Significant hypoxemia: PaO₂ < 60 mmHg requires immediate intervention
• Respiratory failure consideration: pH < 7.35 with PaCO₂ > 45 mmHg
• Ventilatory support indication: pH < 7.35 with PaCO₂ > 6.0 kPa (45 mmHg)
• Non-invasive ventilation initiation: pH < 7.35 and PaCO₂ > 6.5 kPa in COPD patients 1

Common Pitfalls in ABG Interpretation

• Relying solely on pulse oximetry: SpO₂ does not provide information about ventilation or acid-base status
• Failing to consider supplemental oxygen: PaO₂ must be interpreted in context of FiO₂
• Overlooking temperature effects: Blood gas values are affected by body temperature
• Not accounting for carboxyhemoglobin: Can falsely elevate SpO₂ readings
• Missing mixed disorders: Multiple acid-base disturbances can coexist 1

Procedural Considerations

• Perform Allen's test before radial artery sampling to ensure dual blood supply to the hand
• Obtain informed consent and discuss potential risks with the patient
• Use local anesthesia except in emergency situations
• Label specimens properly with patient identifiers, date, time, and collector's ID
• Process samples promptly to avoid pre-analytical errors 1

Monitoring and Reassessment

• Repeat ABG after 1-2 hours of treatment initiation
• Monitor for changes in respiratory rate and pH
• Reassess after interventions and adjust treatment accordingly
• Consider invasive ventilation if no improvement in PaCO₂ and pH after 4-6 hours of NIV 1

ABG analysis remains a cornerstone diagnostic tool in critical care and respiratory medicine, providing crucial information that guides therapeutic interventions and improves patient outcomes.