What are the components of an Arterial Blood Gas (ABG)?

Components of Arterial Blood Gas (ABG)

An arterial blood gas (ABG) analysis consists of five key components: pH, PaCO2, PaO2, HCO3-, and oxygen saturation, which together provide critical information about a patient's acid-base status, ventilation, and oxygenation.

Core Components of ABG

1. pH

• Measures the hydrogen ion concentration in blood
• Normal range: 7.35-7.45
• Indicates overall acid-base status
• Values <7.35 indicate acidosis
• Values >7.45 indicate alkalosis 1

2. PaCO2 (Partial Pressure of Carbon Dioxide)

• Measures the pressure exerted by CO2 dissolved in arterial blood
• Normal range: 35-45 mmHg (4.7-6.0 kPa)
• Reflects adequacy of ventilation and respiratory component of acid-base status
• Primary indicator of respiratory acidosis (↑PaCO2) or alkalosis (↓PaCO2) 1

3. PaO2 (Partial Pressure of Oxygen)

• Measures the pressure exerted by oxygen dissolved in arterial blood
• Normal range: 80-100 mmHg (10.6-13.3 kPa)
• Indicates oxygenation status
• Values <80 mmHg suggest hypoxemia 1, 2

4. HCO3- (Bicarbonate)

• Measures bicarbonate concentration in blood
• Normal range: 22-26 mEq/L
• Reflects metabolic component of acid-base status
• Primary indicator of metabolic acidosis (↓HCO3-) or alkalosis (↑HCO3-) 1

5. SaO2 (Oxygen Saturation)

• Percentage of hemoglobin binding sites occupied by oxygen
• Normal range: 95-100%
• Provides information about oxygen-carrying capacity 1, 3

Additional Parameters Often Included

6. Base Excess/Deficit (BE)

• Indicates the contribution of the metabolic component to acid-base status
• Normal range: -2 to +2 mEq/L
• Measures the amount of acid or base needed to return pH to normal 1

7. Alveolar-arterial Oxygen Gradient (P[A-a]O2)

• Difference between alveolar and arterial oxygen tension
• Normal range: 4-8 mmHg (increases with age)
• Helps differentiate causes of hypoxemia 1

Clinical Significance

ABG analysis is essential for:

• Assessing respiratory function and adequacy of ventilation
• Evaluating acid-base disorders (respiratory or metabolic)
• Monitoring oxygen therapy effectiveness
• Diagnosing and managing critical conditions including respiratory failure, shock, and metabolic disorders 1, 2

Sampling Considerations

• Arterial samples are preferred for critical patients or those with shock/hypotension
• Local anesthesia should be used for all ABG specimens except in emergencies
• Either arterial or venous blood may be used for measuring COHb levels, as they are similar when CO body stores are in equilibrium 1
• Allen's test should be performed before radial artery sampling to ensure dual blood supply to the hand 1

Common Pitfalls to Avoid

• Relying solely on pulse oximetry without ABG when assessing ventilation or acid-base status
• Failing to consider the impact of supplemental oxygen on interpretation
• Not accounting for temperature effects on blood gas values
• Overlooking the presence of carboxyhemoglobin, which can falsely elevate SpO2 readings 1

Understanding these components allows for systematic interpretation of ABG results, which is crucial for accurate diagnosis and appropriate management of patients with respiratory, metabolic, or mixed disorders.