Overview
This lecture introduces the tic-tac-toe method for interpreting arterial blood gas (ABG) values, focusing on differentiating metabolic and respiratory imbalances and identifying compensation.
Normal ABG Values
- Normal pH: 7.35–7.45; below 7.35 is acidotic, above 7.45 is alkalotic (basic).
- Normal PaCO₂ (respiratory): 35–45 mmHg; below 35 is alkalotic, above 45 is acidotic (opposite of pH).
- Normal HCO₃⁻ (metabolic): 22–26 mEq/L; below 22 is acidotic, above 26 is alkalotic.
Tic-Tac-Toe Method Setup
- Draw a tic-tac-toe grid labeled Acid | Normal | Base.
- Place each value (pH, PaCO₂, HCO₃⁻) under Acid, Normal, or Base based on normal ranges.
- For PaCO₂, remember it is interpreted opposite to pH and HCO₃⁻.
Solving ABG Example with Tic-Tac-Toe
- Sample values: pH 7.23 (Acid), PaCO₂ 50 (Acid), HCO₃⁻ 30 (Base).
- Place pH under Acid, PaCO₂ under Acid, and HCO₃⁻ under Base.
- If three in a row: the location of PaCO₂ or HCO₃⁻ determines if the issue is respiratory or metabolic.
- Majority direction determines acid-base disorder (e.g., respiratory acidosis).
Determining Compensation
- If the third value (non-primary) is abnormal, it indicates compensation.
- If the third value is also abnormal (not normal), it is partially compensated.
- If the third value remains normal, it is uncompensated.
Key Terms & Definitions
- Acidotic — Having a pH below the normal range (more acidic).
- Alkalotic — Having a pH above the normal range (more basic).
- Compensated — The body adjusts a second parameter to attempt to restore pH balance.
- PaCO₂ — Partial pressure of carbon dioxide, reflects respiratory function.
- HCO₃⁻ — Bicarbonate, reflects metabolic (kidney) function.
Action Items / Next Steps
- Memorize normal ABG values for pH, PaCO₂, and HCO₃⁻.
- Watch the follow-up video for more practice solving ABG problems.
- Take the online ABG quiz for additional practice.