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ABG Interpretation and ROME Method

Sep 2, 2025

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.