Overview
This lecture explains Arterial Blood Gas (ABG) testing, interpretation methods (especially ROME), common imbalances, their causes, and compensation.
What is an ABG?
- ABG is a blood test to assess oxygen, carbon dioxide (CO₂), and acid-base (pH) balance.
- Blood is drawn from an artery, usually the radial artery.
- ABGs help diagnose respiratory and metabolic problems, as well as acid-base imbalances.
Normal ABG Values
- pH: 7.35 – 7.45 (acid-base balance)
- PaCO₂: 35 – 45 mmHg (CO₂ level, lung function)
- HCO₃⁻: 22 – 26 mEq/L (bicarbonate, kidney function)
- PaO₂: 80 – 100 mmHg (oxygen in blood)
- SaO₂: 95 – 100% (hemoglobin oxygen saturation)
- pH < 7.35 = acidosis; pH > 7.45 = alkalosis
- High CO₂ = acidic; low CO₂ = alkaline
- High HCO₃⁻ = alkaline; low HCO₃⁻ = acidic
ROME Method for Interpretation
- ROME: Respiratory Opposite, Metabolic Equal (pH vs. CO₂/HCO₃)
- Respiratory acidosis: pH ↓, CO₂ ↑ (opposite)
- Respiratory alkalosis: pH ↑, CO₂ ↓ (opposite)
- Metabolic acidosis: pH ↓, HCO₃⁻ ↓ (equal)
- Metabolic alkalosis: pH ↑, HCO₃⁻ ↑ (equal)
Causes of ABG Imbalances
- Respiratory Acidosis: Hypoventilation (COPD, overdose, pneumonia); pH ↓, CO₂ ↑
- Respiratory Alkalosis: Hyperventilation (anxiety, pain); pH ↑, CO₂ ↓
- Metabolic Acidosis: Acid gain/base loss (DKA, diarrhea, renal failure); pH ↓, HCO₃⁻ ↓
- Metabolic Alkalosis: Base gain/acid loss (vomiting, NG suction, antacids); pH ↑, HCO₃⁻ ↑
ABG Interpretation Steps
- Step 1: Check pH (<7.35 = acidosis; >7.45 = alkalosis; 7.35–7.45 = normal/compensated)
- Step 2: Check PaCO₂ (>45 = resp. acidosis; <35 = resp. alkalosis)
- Step 3: Check HCO₃⁻ (<22 = metabolic acidosis; >26 = metabolic alkalosis)
- Step 4: Match pH to CO₂ (respiratory) or HCO₃⁻ (metabolic) using ROME
Compensation
- Uncompensated: pH abnormal, only one other abnormal
- Partial: All three (pH, CO₂, HCO₃⁻) abnormal
- Fully compensated: pH normal, CO₂ and HCO₃⁻ both abnormal
Practice & Mini Quiz
- Rapid, deep breathing: pH ↑, CO₂ ↓ (respiratory alkalosis)
- Example ABG (pH = 7.30, CO₂ = 55, HCO₃⁻ = 25): Respiratory acidosis (uncompensated)
- Causes of metabolic acidosis: Diarrhea (not vomiting/anxiety)
- ABG (pH = 7.36, CO₂ = 50, HCO₃⁻ = 30): Fully compensated respiratory acidosis
Key Terms & Definitions
- ABG — Arterial Blood Gas; measures oxygen, CO₂, pH.
- pH — Indicates acidity or alkalinity of blood.
- PaCO₂ — Partial pressure of arterial CO₂ (acid, regulated by lungs).
- HCO₃⁻ — Bicarbonate, base, regulated by kidneys.
- ROME — Mnemonic: Respiratory Opposite, Metabolic Equal.
- Compensation — Body's attempt to correct pH imbalance.
Action Items / Next Steps
- Review ROME interpretation method.
- Practice interpreting sample ABGs using the 4-step method.
- Study summary cheat sheet for quick reference.