ABG Interpretation Principles

Aug 17, 2025

Overview

This lecture covers the core principles and steps for interpreting arterial blood gas (ABG) results, including the normal ranges, respiratory and metabolic disturbances, and practical examples.

Acid-Base Balance and Key Players

  • Normal body pH is 7.4, maintained by the bicarbonate buffering system involving the lungs and kidneys.
  • Lungs rapidly compensate for pH changes by adjusting breathing rate and tidal volume.
  • Kidneys compensate more slowly by reabsorbing more or less bicarbonate (HCO₃⁻).
  • CO₂ is acidic and influences respiratory disorders; HCO₃⁻ is alkaline and influences metabolic disorders.

Types of Acid-Base Disorders

  • Respiratory Acidosis: High CO₂, low pH (lungs under-ventilating, CO₂ retention).
  • Respiratory Alkalosis: Low CO₂, high pH (lungs over-ventilating, CO₂ loss).
  • Metabolic Acidosis: Low HCO₃⁻, low pH (loss of base or accumulation of acid).
  • Metabolic Alkalosis: High HCO₃⁻, high pH (gain of base or loss of acid).

Normal ABG Values

  • pH: 7.35 – 7.45
  • PaO₂: 80 – 100 mmHg
  • PaCO₂: 35 – 45 mmHg
  • HCO₃⁻: 22 – 26 mEq/L

ABG Interpretation Steps

  1. Assess pH: <7.35 = acidosis; >7.45 = alkalosis.
  2. Assess PaCO₂: >45 = high (acidic); <35 = low (alkaline).
  3. Assess HCO₃⁻: >26 = high (alkaline); <22 = low (acidic).
  4. Determine Cause:
    • Abnormal PaCO₂ = respiratory problem.
    • Abnormal HCO₃⁻ = metabolic problem.

The ROME Mnemonic

  • Respiratory Opposite: pH and CO₂ move in opposite directions.
  • Metabolic Equal: pH and HCO₃⁻ move in the same direction.

Example Summaries

  • All normal values = Normal ABG.
  • Low pH, high CO₂, normal HCO₃⁻ = Respiratory acidosis.
  • High pH, low CO₂, normal HCO₃⁻ = Respiratory alkalosis.
  • Low pH, normal CO₂, low HCO₃⁻ = Metabolic acidosis.
  • High pH, normal CO₂, high HCO₃⁻ = Metabolic alkalosis.

Key Terms & Definitions

  • pH — measure of acidity or alkalinity in the blood.
  • PaO₂ — partial pressure of arterial oxygen.
  • PaCO₂ — partial pressure of arterial carbon dioxide.
  • HCO₃⁻ — bicarbonate ion, a base in blood.
  • Respiratory Acidosis/Alkalosis — acid-base disturbance due to CO₂ changes.
  • Metabolic Acidosis/Alkalosis — acid-base disturbance due to HCO₃⁻ changes.
  • ROME — mnemonic for interpreting acid-base disorders.

Action Items / Next Steps

  • Memorize normal ABG value ranges.
  • Practice ABG interpretation using the 4-step process and ROME.
  • Review the next lesson focusing on compensation mechanisms.

Full transcript