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ABG Interpretation Basics

Jun 18, 2025

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Overview

This lecture reviews how to interpret arterial blood gases (ABGs), including normal values, identifying acid-base imbalances, and naming the disturbance type.

Normal ABG Parameters

  • Normal pH: 7.35–7.45; above = alkalosis, below = acidosis.
  • Normal CO₂ (carbon dioxide, respiratory): 35–45 mmHg; high = respiratory acidosis, low = respiratory alkalosis.
  • Normal HCO₃⁻ (bicarbonate, metabolic): 23–27 mEq/L; high = metabolic alkalosis, low = metabolic acidosis.

Interpreting ABGs: Step-by-Step

  • Step 1: Check pH for first and last name (status and direction).
  • Uncompensated if pH is outside 7.35–7.45; compensated if pH is within range.
  • pH < 7.35 = acidosis; pH > 7.45 = alkalosis.
  • Within 7.35–7.40 = compensated acidosis; 7.40–7.45 = compensated alkalosis.

Determining the Primary Disturbance

  • CO₂ is an acid, controlled by respiratory system; high = acidosis, low = alkalosis.
  • HCO₃⁻ is a base, controlled by metabolism; high = alkalosis, low = acidosis.
  • Match the "last name" direction (acidosis or alkalosis) between pH and CO₂ or HCO₃⁻ to find the primary disturbance.
    • If CO₂ matches, it's respiratory.
    • If HCO₃⁻ matches, it's metabolic.
    • If both match the pH, it's a mixed disturbance.

Example Problems

  • pH 7.12, CO₂ 28, HCO₃⁻ 11: Uncompensated metabolic acidosis.
  • pH 7.55, CO₂ 29, HCO₃⁻ 20: Uncompensated respiratory alkalosis.
  • pH 7.01, CO₂ 51, HCO₃⁻ 10: Uncompensated mixed (respiratory and metabolic) acidosis.
  • pH 7.23, CO₂ 50, HCO₃⁻ 29: Uncompensated respiratory acidosis.

Key Terms & Definitions

  • pH — Measures acidity/alkalinity of blood; normal is 7.35–7.45.
  • CO₂ (carbon dioxide) — Respiratory acid; normal 35–45 mmHg.
  • HCO₃⁻ (bicarbonate) — Metabolic base; normal 23–27 mEq/L.
  • Acidosis — State of increased acidity (pH < 7.35).
  • Alkalosis — State of increased alkalinity (pH > 7.45).
  • Compensated — Body has corrected pH into normal range.
  • Uncompensated — pH remains outside the normal range.
  • Mixed disturbance — Both respiratory and metabolic components contribute to acidosis or alkalosis.

Action Items / Next Steps

  • Practice interpreting ABGs using sample values.
  • Review normal values for pH, CO₂, and HCO₃⁻ before exams.

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