Interpretation of Arterial Blood Gases (ABG)

Introduction

• ABG: Arterial Blood Gases measure arterial blood's acidity/alkalinity and gases such as oxygen (O2) and carbon dioxide (CO2).
• Key Components:
  • pH: Measures blood acidity or alkalinity. Normal: 7.35 - 7.45
  • PaCO2: Measures carbon dioxide. Normal: 35 - 45 mmHg
  • HCO3: Bicarbonate, a base controlled by kidneys. Normal: 22 - 26 mEq/L
  • PaO2: Measures oxygen. Normal: 80 - 100 mmHg

Basic Concepts

• pH: Indicates if blood is acidic (<7.35) or alkalotic (>7.45).

• PaCO2:

  • Controlled by lungs; expelled through breathing.
  • High levels (>45) indicate more acidic blood.
  • Low levels (<35) indicate more basic blood.

• HCO3:

  • Controlled by kidneys.
  • High levels (>26) indicate more basic blood.
  • Low levels (<22) indicate more acidic blood.

• PaO2:

  • Important for assessing hypoxemia, not directly used for determining acidosis/alkalosis.

Three-Step ABG Interpretation

Step 1: Determine Acidosis or Alkalosis

• Use pH:
  • <7.35 indicates Acidosis.

  • 7.45 indicates Alkalosis.

Step 2: Determine if Condition is Respiratory or Metabolic

• PaCO2 and HCO3 are used to determine type:
  • Respiratory: Abnormal PaCO2, normal HCO3.
  • Metabolic: Normal PaCO2, abnormal HCO3.
• Concept method: PaCO2 regulated by lungs, HCO3 by kidneys.
• Examples:
  • pH 7.25, PaCO2 50, HCO3 24: Respiratory Acidosis
  • pH 7.56, PaCO2 28, HCO3 22: Respiratory Alkalosis
  • pH 7.25, PaCO2 37, HCO3 21: Metabolic Acidosis
  • pH 7.56, PaCO2 44, HCO3 35: Metabolic Alkalosis

Step 3: Identify Compensation

• Metabolic compensates Respiratory issues:
  • Respiratory Acidosis: Increase HCO3.
  • Respiratory Alkalosis: Decrease HCO3.
• Respiratory compensates Metabolic issues:
  • Metabolic Acidosis: Decrease CO2.
  • Metabolic Alkalosis: Increase CO2.
• Compensation Levels:
  • Partial: pH abnormal
  • Full: pH normal, both CO2, and HCO3 abnormal

Examples of Compensation

• Partial Compensation:

  • pH 7.30, CO2 50, HCO3 49: Respiratory Acidosis, partial compensation
  • pH 7.50, CO2 51, HCO3 41: Metabolic Alkalosis, partial compensation

• Full Compensation:

  • pH 7.35, CO2 49, HCO3 30: Respiratory Acidosis, full compensation
  • pH 7.44, CO2 48, HCO3 35: Metabolic Alkalosis, full compensation

Conclusion

• Understanding ABG values and compensatory mechanisms can simplify the interpretation of these tests.
• Remember the interrelation between pH, PaCO2, and HCO3 for a correct diagnosis and compensation assessment.