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ABG Interpretation Method

Sep 23, 2025

Overview

This lecture explains the tic-tac-toe method for interpreting arterial blood gas (ABG) values, including identifying acid-base imbalances and compensation states.

Normal ABG Values

  • Normal pH: 7.35–7.45; >7.45 is alkalotic (base), <7.35 is acidotic (acid).
  • Normal PaCO₂ (respiratory): 35–45 mmHg; <35 is alkalotic, >45 is acidotic (opposite from pH/HCO₃).
  • Normal HCO₃⁻ (metabolic): 22–26 mEq/L; >26 is alkalotic, <22 is acidotic.

Tic-Tac-Toe Method Setup

  • Draw a 3-column grid labeled: Acid – Normal – Base.
  • For each value (pH, PaCO₂, HCO₃⁻), determine if it falls in acid, normal, or base and write it in the correct column.
  • Remember, PaCO₂ is opposite: high = acidotic, low = alkalotic.

Applying the Tic-Tac-Toe Method

  • Place all three values in their appropriate columns before making any interpretation.
  • Example: pH 7.23 (acid), PaCO₂ 50 (acid), HCO₃⁻ 30 (base).
  • If pH and PaCO₂ are both in acid, and HCO₃⁻ is in base, the problem is respiratory in origin.

Interpreting Results

  • The "three in a row" determines whether the problem is respiratory or metabolic based on which value lines up.
  • The acid/base status is determined by the direction of the pH.
  • If the third value (not in the three in a row) is abnormal, the body is partially compensating.
  • If the third value is normal, the problem is uncompensated.

Key Terms & Definitions

  • ABG (Arterial Blood Gas) — A test measuring pH, PaCO₂, and HCO₃⁻ in arterial blood.
  • pH — Measures acidity/alkalinity of blood.
  • PaCO₂ — Partial pressure of carbon dioxide, reflects respiratory component.
  • HCO₃⁻ — Bicarbonate, reflects metabolic component.
  • Alkalotic — Condition of excess base (higher pH).
  • Acidotic — Condition of excess acid (lower pH).
  • Compensation — The body’s attempt to restore normal pH by altering respiratory or metabolic factors.

Action Items / Next Steps

  • Memorize normal ABG values for pH, PaCO₂, and HCO₃⁻.
  • Practice applying the tic-tac-toe method with ABG problems.
  • Watch the next video for more practice and disorder discussion.
  • Take the free ABG quiz on registernursern.com.

Full transcript