ABG Compensation Overview

Overview

This lecture explains how to interpret compensation in arterial blood gas (ABG) analysis, focusing on uncompensated, partially compensated, and fully compensated states, with detailed examples.

Compensation in Acid-Base Disturbances

• The body compensates for pH disturbances by adjusting either bicarbonate (bicarb, HCO₃⁻) or carbon dioxide (CO₂).
• Respiratory disturbances are compensated by changing bicarb; metabolic disturbances are compensated by changing CO₂.
• CO₂ compensation is rapid; bicarb compensation is slow.
• CO₂ is acidic and moves pH in the opposite direction; bicarb is alkaline and moves pH in the same direction.

Patterns of Compensation

• The body compensates in the same direction as the primary disturbance to restore pH toward normal.
• If CO₂ is high (acidosis), the body raises bicarb to compensate; if bicarb is low (acidosis), the body lowers CO₂.
• Compensation never fully normalizes pH to 7.40 nor overcompensates past it.

Degrees of Compensation

• Uncompensated: Abnormal pH, one abnormal value (CO₂ or bicarb), and one normal value; compensation hasn't occurred.
• Fully Compensated: pH is normal (but near the edge), both CO₂ and bicarb are abnormal and match the direction of compensation.
• Partially Compensated: Abnormal pH, both CO₂ and bicarb abnormal and match compensation, but pH hasn't returned to normal yet.

ABG Interpretation Examples

• Example 1: pH 7.27 (low), CO₂ 53 (high), bicarb 23 (normal) = uncompensated respiratory acidosis.
• Example 2: pH 7.36 (normal), CO₂ 49 (high), bicarb 28 (high) = fully compensated respiratory acidosis.
• Example 3: pH 7.44 (normal), CO₂ 49 (high), bicarb 29 (high) = fully compensated metabolic alkalosis.
• Example 4: pH 7.32 (low), CO₂ 34 (low), bicarb 20 (low) = partially compensated metabolic acidosis.
• Example 5: pH 7.49 (high), CO₂ 29 (low), bicarb 20 (low) = partially compensated respiratory alkalosis.

Mixed Disorders Note

• Mixed acid-base disorders can occur and complicate compensation assessment; not covered in this lesson.

Key Terms & Definitions

• Acidosis — excess acid or loss of base in the blood, pH below 7.35.
• Alkalosis — excess base or loss of acid in the blood, pH above 7.45.
• Compensation — body's response to restore pH toward normal by adjusting CO₂ or bicarb.
• Uncompensated — no adjustment by the body; only one abnormal value and abnormal pH.
• Fully compensated — pH within normal range due to compensatory adjustment.
• Partially compensated — body is adjusting, but pH not yet normalized.
• CO₂ (carbon dioxide) — acid component measured in ABG.
• Bicarb (HCO₃⁻, bicarbonate) — base component measured in ABG.

Action Items / Next Steps

• Practice ABG interpretation with additional examples online.
• Complete practice ABGs given in the comments (if applicable).
• Watch the next lesson on the tic-tac-toe method for ABG interpretation.