ABG Test

An ABG test measures the levels of oxygen, carbon dioxide, pH, and bicarbonate in arterial blood. It helps assess lung function, acid-base balance, and oxygenation status in critical conditions.

Why is an ABG Test Done?

• Evaluate respiratory failure(COPD, asthma, pneumonia)
• Assess metabolic disorders(diabetic ketoacidosis, kidney failure)
• Monitor critically ill patients(shock, sepsis, trauma)
• Check oxygen therapy effectiveness(e.g., in ventilated patients)

Normal ABG Values

Interpreting ABG Results

Step 1: Check pH

• pH < 7.35 → Acidosis
• pH > 7.45 → Alkalosis

Step 2: Determine Primary Disorder

Step 3: Check Compensation

• Respiratory compensation(fast, via CO₂ changes)
• Metabolic compensation(slow, via HCO₃⁻ changes)

Step 4: Calculate Anion Gap (if Metabolic Acidosis)

• Anion Gap = Na⁺ – (Cl⁻ + HCO₃⁻)
  • Normal: 8–12 mEq/L
  • High (>12):Lactic acidosis, ketoacidosis, toxins
  • Normal:Diarrhea, renal tubular acidosis

Common ABG Patterns

1. Respiratory Acidosis(↑PaCO₂, ↓pH)
   • Causes: COPD, asthma, opioid overdose
2. Respiratory Alkalosis(↓PaCO₂, ↑pH)
   • Causes: Hyperventilation, anxiety, PE
3. Metabolic Acidosis(↓HCO₃⁻, ↓pH)
   • Causes: DKA, sepsis, renal failure
4. Metabolic Alkalosis(↑HCO₃⁻, ↑pH)
   • Causes: Vomiting, diuretics, excess antacids

How is an ABG Test Performed?

1. Site:Radial artery (most common), femoral, or brachial artery.
2. Modified Allen’s Test(checks collateral circulation before radial puncture).
3. Procedure:
   • Clean the area, insert a thin needle.
   • Collect 1–3 mL of arterial blood.
   • Apply pressure to prevent bleeding.
4. Sample Handling:Must be analyzed within 10–15 minutes (or ice-cooled).

Complications of ABG Test

• Pain or bruisingat puncture site
• Arterial spasm or thrombosis(rare)
• Hematoma formation
• Nerve damage(very rare)

ABG vs. VBG (Venous Blood Gas)

VBG is easier but less accurate for oxygenation assessment.

Key Takeaways

• ABG helps diagnose acid-base disorders& respiratory failure.
• pH, PaCO₂, HCO₃⁻are key parameters.
• Compensationindicates chronicity.
• Anion Gapdifferentiates metabolic acidosis causes.