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ABG Problem-Solving Method

Jul 2, 2025

Overview

This lecture reviews the ROE (Respiratory Opposite, Metabolic Equal) method for solving arterial blood gas (ABG) problems, focusing on identifying acid-base imbalances and determining compensation status.

Understanding ABG Problem-Solving

  • ABG problems test your ability to find acid-base imbalances using pH, CO₂ (respiratory), and HCO₃⁻ (metabolic) values.
  • Always check if the issue is respiratory or metabolic, determine if it is acidosis or alkalosis, and assess compensation status.
  • The three values to assess are: pH, CO₂ (carbon dioxide), and HCO₃⁻ (bicarbonate).

Normal Value Reference Table

  • Normal pH: 7.35–7.45 (↓ = acidotic, ↑ = alkalotic).
  • Normal CO₂: 35–45 mmHg (↑ = acidotic, ↓ = alkalotic).
  • Normal HCO₃⁻: 22–26 mEq/L (↓ = acidotic, ↑ = alkalotic).

ROE Method (Respiratory Opposite, Metabolic Equal)

  • "Respiratory Opposite": If CO₂ and pH move in opposite directions, it's a respiratory problem.
    • High CO₂ + Low pH = Respiratory acidosis.
    • Low CO₂ + High pH = Respiratory alkalosis.
  • "Metabolic Equal": If HCO₃⁻ and pH move in the same direction, it's a metabolic problem.
    • Low HCO₃⁻ + Low pH = Metabolic acidosis.
    • High HCO₃⁻ + High pH = Metabolic alkalosis.

Compensation in ABG Interpretation

  • Fully compensated: pH is normal, but CO₂ and/or HCO₃⁻ are abnormal.
  • Partially compensated: pH is not normal, and the other system is abnormal in a compensatory direction.
  • Uncompensated: pH is not normal, and only one value (CO₂ or HCO₃⁻) is abnormal.

Example Problems Summary

  • Example 1: pH 7.28, CO₂ 50, HCO₃⁻ 24 → Respiratory acidosis, uncompensated.
  • Example 2: pH 7.30, CO₂ 40, HCO₃⁻ 18 → Metabolic acidosis, uncompensated.
  • Example 3: pH 7.42, CO₂ 26, HCO₃⁻ 18 → Respiratory alkalosis, fully compensated.
  • Example 4: pH 7.37, CO₂ 32, HCO₃⁻ 17 → Metabolic acidosis, fully compensated.
  • Example 5: pH 7.51, CO₂ 47, HCO₃⁻ 32 → Metabolic alkalosis, partially compensated.

Key Terms & Definitions

  • ABG (Arterial Blood Gas) — a test to measure pH, CO₂, and HCO₃⁻ in arterial blood.
  • Acidosis — blood pH below 7.35.
  • Alkalosis — blood pH above 7.45.
  • Compensation — the body's attempt to return pH to normal by adjusting CO₂ or HCO₃⁻.
  • Respiratory — related to CO₂, the respiratory component.
  • Metabolic — related to HCO₃⁻, the metabolic component.

Action Items / Next Steps

  • Memorize the normal ranges for pH, CO₂, and HCO₃⁻.
  • Access and complete the free practice quiz for additional ABG problem practice.

Full transcript