ABG Cheat Sheet for Respiratory Therapy Students
Use this quick-reference ABG cheat sheet to review normal values, acid-base patterns, compensation clues, oxygenation categories, and the step-by-step interpretation process used in PulmoLearn ABG practice cases.
The 5-Step ABG Method
Evaluate pH.
Decide whether the patient is acidotic, alkalotic, or normal.
Decide whether the patient is acidotic, alkalotic, or normal.
Identify the primary disorder.
Compare pH with PaCO₂ and HCO₃ to decide respiratory or metabolic.
Compare pH with PaCO₂ and HCO₃ to decide respiratory or metabolic.
Assess compensation.
Determine whether the other system is normal, partially compensating, or fully compensating.
Determine whether the other system is normal, partially compensating, or fully compensating.
Assess oxygenation.
Use PaO₂ to determine normal oxygenation or hypoxemia.
Use PaO₂ to determine normal oxygenation or hypoxemia.
Build the full interpretation.
Combine compensation, primary disorder, and oxygenation status.
Combine compensation, primary disorder, and oxygenation status.
Normal ABG Values
| Value | Normal Range | What It Tells You |
|---|---|---|
| pH | 7.35–7.45 | Acid-base status. |
| PaCO₂ | 35–45 mmHg | Respiratory component / ventilation. |
| HCO₃ | 22–26 mEq/L | Metabolic component. |
| PaO₂ | 80–100 mmHg | Oxygenation status. |
| SaO₂ | 95–100% | Hemoglobin saturation with oxygen. |
Core Acid-Base Patterns
| Disorder | pH | PaCO₂ | HCO₃ | Key Clue |
|---|---|---|---|---|
| Respiratory Acidosis | Low | High | Normal or high if compensated | pH and PaCO₂ move opposite directions. |
| Respiratory Alkalosis | High | Low | Normal or low if compensated | pH and PaCO₂ move opposite directions. |
| Metabolic Acidosis | Low | Normal or low if compensated | Low | pH and HCO₃ move same direction. |
| Metabolic Alkalosis | High | Normal or high if compensated | High | pH and HCO₃ move same direction. |
Compensation Clues
No compensation
The opposite system is still normal.
The opposite system is still normal.
Partial compensation
Both PaCO₂ and HCO₃ are abnormal, but pH is still outside normal range.
Both PaCO₂ and HCO₃ are abnormal, but pH is still outside normal range.
Full compensation
Both PaCO₂ and HCO₃ are abnormal, and pH has returned to 7.35–7.45.
Both PaCO₂ and HCO₃ are abnormal, and pH has returned to 7.35–7.45.
Mixed disorder warning
If both systems push pH in the same direction, consider a mixed disorder.
If both systems push pH in the same direction, consider a mixed disorder.
PaO₂ Oxygenation Categories
| PaO₂ | Interpretation |
|---|---|
| 80–100 mmHg | Normal oxygenation |
| 60–79 mmHg | Mild hypoxemia |
| 40–59 mmHg | Moderate hypoxemia |
| Less than 40 mmHg | Severe hypoxemia |
Board reminder: Acid-base status and oxygenation are related but separate. Always interpret both.
Quick ABG Examples
Normal ABG
pH 7.40 / PaCO₂ 40 / HCO₃ 24 / PaO₂ 90
pH 7.40 / PaCO₂ 40 / HCO₃ 24 / PaO₂ 90
Respiratory Acidosis
pH 7.28 / PaCO₂ 60 / HCO₃ 25 / PaO₂ 55
pH 7.28 / PaCO₂ 60 / HCO₃ 25 / PaO₂ 55
Respiratory Alkalosis
pH 7.50 / PaCO₂ 28 / HCO₃ 24 / PaO₂ 88
pH 7.50 / PaCO₂ 28 / HCO₃ 24 / PaO₂ 88
Metabolic Acidosis
pH 7.29 / PaCO₂ 38 / HCO₃ 18 / PaO₂ 92
pH 7.29 / PaCO₂ 38 / HCO₃ 18 / PaO₂ 92
Metabolic Alkalosis
pH 7.49 / PaCO₂ 42 / HCO₃ 33 / PaO₂ 85
pH 7.49 / PaCO₂ 42 / HCO₃ 33 / PaO₂ 85
Compensated Example
pH 7.37 / PaCO₂ 55 / HCO₃ 32 / PaO₂ 68
pH 7.37 / PaCO₂ 55 / HCO₃ 32 / PaO₂ 68
Common Shortcuts That Cause Mistakes
Starting with PaCO₂
Always start with pH so you know the direction of the disorder.
Always start with pH so you know the direction of the disorder.
Forgetting compensation
Compensation changes how you name the disorder.
Compensation changes how you name the disorder.
Ignoring PaO₂
A complete ABG interpretation should include oxygenation status.
A complete ABG interpretation should include oxygenation status.
Memorizing without practice
Pattern recognition improves fastest with repeated clinical cases.
Pattern recognition improves fastest with repeated clinical cases.
Apply This Cheat Sheet to Real Cases
Use the PulmoLearn free ABG practice cases to walk through pH, primary disorder, compensation, oxygenation, and final interpretation with immediate feedback.