Acid-Base Disorder Overview

Jun 28, 2025

Overview

This lecture explains how to identify simple acid-base disorders by understanding key terminology, the four major disorder categories, and interpreting arterial blood gas (ABG) results.

Key Terminology: Acidemia vs. Acidosis; Alkalemia vs. Alkalosis

  • Acidemia is defined as arterial pH < 7.35; alkalemia is pH > 7.45.
  • Acidosis is a process that drives pH lower; alkalosis is a process that drives pH higher.
  • A patient cannot have both acidemia and alkalemia simultaneously, but can have multiple acidoses and/or alkaloses that may offset each other.

Four Categories of Acid-Base Disorders

  • Disorders are either respiratory or metabolic in origin.
  • Respiratory disorders result from changes in lung function and ventilation.
  • Respiratory acidosis: pCO₂ is too high (hypoventilation).
  • Respiratory alkalosis: pCO₂ is too low (hyperventilation).
  • Metabolic disorders result from changes outside the lungs, usually involving kidneys, GI tract, or cellular metabolism.
  • Metabolic acidosis: serum bicarbonate ([HCO₃⁻]) is too low.
  • Metabolic alkalosis: bicarbonate is too high.

ABG Interpretation Steps

  • ABG provides pH, pCO₂, HCO₃⁻, plus O₂ values; focus on pH, pCO₂, and HCO₃⁻ for acid-base status.
  • Step 1: Check pH to determine if patient has acidemia (< 7.35) or alkalemia (> 7.45).
  • Step 2: Check pCO₂:
    • If pCO₂ changes in the same direction as pH (both high or both low), disorder is metabolic.
    • If pCO₂ changes in the opposite direction as pH, disorder is respiratory.

Example Case Summaries

  • pH 7.25, pCO₂ 60: Acidemia with high pCO₂ = respiratory acidosis.
  • pH 7.50, pCO₂ 45, HCO₃⁻ 34: Alkalemia with high HCO₃⁻ = metabolic alkalosis.
  • pH 7.16, pCO₂ 35, HCO₃⁻ 12: Acidemia with low HCO₃⁻ = metabolic acidosis.
  • pH low, pCO₂ high: Acidemia with high pCO₂ = respiratory acidosis.

Key Terms & Definitions

  • Acidemia — arterial pH < 7.35
  • Alkalemia — arterial pH > 7.45
  • Acidosis — physiological process lowering pH
  • Alkalosis — physiological process raising pH
  • Metabolic Disorder — acid-base disturbance due to non-lung systems
  • Respiratory Disorder — acid-base disturbance due to lung function
  • pCO₂ — partial pressure of carbon dioxide in arterial blood
  • [HCO₃⁻] — concentration of serum bicarbonate

Action Items / Next Steps

  • Study and memorize the four categories of acid-base disorders.
  • Practice identifying disorder type using ABG values.
  • Prepare for next lecture on compensation in acid-base disorders.

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