Lecture Notes on ABG Interpretation

Introduction to ABG

• ABG stands for Arterial Blood Gases.
• Related to the pH system: measures acidity or alkalinity.
• Normal pH range: 7.35 to 7.45.
  • Below 7.35: Acidic condition.
  • Above 7.45: Alkalotic condition.

Acid-Base Balance

• The body compensates to maintain pH within the normal range.
• Compensation: Body's mechanism to bring pH back to normal when it deviates.
• Metabolic and Respiratory Systems
  • Kidneys regulate bicarbonate (HCO₃), a base related to the metabolic system.
  • Respiratory system regulates CO₂ levels, related to acidity.

Understanding ABG Components

• Normal Values to Remember for Exams
  • pH: 7.35 to 7.45
  • CO₂: 35 to 45 mmHg
  • HCO₃ (Bicarb): 22 to 28 mEq/L
• CO₂ levels inversely affect pH (↑ CO₂ = More acidic, ↓ CO₂ = More alkaline)
• HCO₃ levels directly affect pH (↑ HCO₃ = More alkaline, ↓ HCO₃ = More acidic)

Steps for ABG Analysis

1. Determine pH status: Acidic (<7.35) or Alkalotic (>7.45).
2. Identify if the problem is Respiratory or Metabolic.
   • Respiratory Problem: Abnormal CO₂ level.
   • Metabolic Problem: Abnormal HCO₃ level.
3. Assess for Compensation
   • Uncompensated: No attempt to correct pH.
   • Partially Compensated: Attempts to correct pH but not within normal range.
   • Fully Compensated: pH brought back to normal range.

Example Interpretations

• If pH is normal but CO₂ or HCO₃ is not, check for compensation.
• Respiratory Acidosis: Low pH, High CO₂.
• Respiratory Alkalosis: High pH, Low CO₂.
• Metabolic Acidosis: Low pH, Low HCO₃.
• Metabolic Alkalosis: High pH, High HCO₃.

Practice Cases

1. Case 1: Normal ABG - Document and monitor.
2. Case 2: pH 7.20, CO₂ 58, HCO₃ 26 - Uncompensated Respiratory Acidosis.
3. Case 3: pH 7.38, CO₂ 48, HCO₃ 37 - Fully Compensated Respiratory Acidosis.
4. Case 4: pH 7.62, CO₂ 49, HCO₃ 35 - Partially Compensated Metabolic Alkalosis.
5. Case 5: pH 7.57, CO₂ 17, HCO₃ 16 - Partially Compensated Respiratory Alkalosis.

Conditions Leading to Acid-Base Imbalance

• Respiratory Acidosis: Hypoventilation, drug overdose, COPD, asthma.
• Respiratory Alkalosis: Hyperventilation, anxiety, panic attacks.
• Metabolic Acidosis: Diarrhea, renal disease, diabetic ketoacidosis.
• Metabolic Alkalosis: Vomiting, antacid use, gastric suction.

Review Questions

• Example: Clonazepam overdose typically leads to Respiratory Acidosis.
• Example Conditions: Diarrhea and renal failure cause Metabolic Acidosis.

Conclusion

• Remember key values and conditions for interpreting ABGs.
• Practice interpreting ABGs with step-by-step analysis.