Board Prep Frame
On exams, do not stop at “normal” or “abnormal.” Decide what the abnormal value means: acid-base problem, ventilation problem, metabolic problem, or oxygenation problem.
Normal ABG Values
Normal arterial blood gas values give respiratory therapy students a starting point for acid-base interpretation, ventilation assessment, and oxygenation assessment. Use these ranges to decide whether each value is low, high, or within expected limits before building the full interpretation.
Normal ABG Value Table
These are the core values most respiratory therapy students use when learning basic ABG interpretation.
| ABG Value | Normal Range | What It Helps You Assess |
|---|---|---|
| pH | 7.35–7.45 | Overall acid-base status |
| PaCO₂ | 35–45 mmHg | Ventilation / respiratory contribution |
| HCO₃ | 22–26 mEq/L | Metabolic / renal contribution |
| PaO₂ | 80–100 mmHg | Oxygenation status |
| SaO₂ | 95–100% | Hemoglobin oxygen saturation |
pH: Acidotic, Normal, or Alkalotic
Start every ABG interpretation with pH. The pH tells you whether the blood is acidotic, alkalotic, or within the normal range.
Acidotic
pH less than 7.35
pH less than 7.35
Normal
pH 7.35–7.45
pH 7.35–7.45
Alkalotic
pH greater than 7.45
pH greater than 7.45
PaCO₂: Ventilation and Respiratory Disorders
PaCO₂ reflects carbon dioxide levels in arterial blood. Because CO₂ acts as an acid, a high PaCO₂ pushes the pH down and a low PaCO₂ pushes the pH up.
| PaCO₂ | Meaning | Possible Pattern |
|---|---|---|
| Less than 35 mmHg | Low CO₂ | Can cause respiratory alkalosis |
| 35–45 mmHg | Normal CO₂ | No primary respiratory abnormality |
| Greater than 45 mmHg | High CO₂ | Can cause respiratory acidosis |
Memory tip:
pH and PaCO₂ move in opposite directions for respiratory disorders.
pH and PaCO₂ move in opposite directions for respiratory disorders.
HCO₃: Metabolic and Renal Contribution
Bicarbonate helps buffer acid. Low bicarbonate supports metabolic acidosis. High bicarbonate supports metabolic alkalosis or renal compensation for a respiratory disorder.
| HCO₃ | Meaning | Possible Pattern |
|---|---|---|
| Less than 22 mEq/L | Low bicarbonate | Can cause metabolic acidosis |
| 22–26 mEq/L | Normal bicarbonate | No primary metabolic abnormality |
| Greater than 26 mEq/L | High bicarbonate | Can cause metabolic alkalosis |
Memory tip:
pH and HCO₃ move in the same direction for metabolic disorders.
pH and HCO₃ move in the same direction for metabolic disorders.
PaO₂: Oxygenation
PaO₂ measures dissolved oxygen in arterial blood. It helps you classify oxygenation, but it does not tell you the acid-base disorder.
Normal PaO₂
80–100 mmHg
80–100 mmHg
Low PaO₂
Indicates hypoxemia severity based on range.
Indicates hypoxemia severity based on range.
Important Distinction
Oxygenation and ventilation are related but not the same. PaO₂ evaluates oxygenation. PaCO₂ evaluates ventilation.
Oxygenation Categories by PaO₂
PulmoLearn uses these categories for beginner ABG interpretation practice.
| PaO₂ Range | Oxygenation Category |
|---|---|
| 80–100 mmHg | Normal oxygenation |
| 60–79 mmHg | Mild hypoxemia |
| 40–59 mmHg | Moderate hypoxemia |
| Less than 40 mmHg | Severe hypoxemia |
Example of a Normal ABG
Use the normal values to classify each result.
pH 7.40
PaCO₂ 40 mmHg
HCO₃ 24 mEq/L
PaO₂ 92 mmHg
PaCO₂ 40 mmHg
HCO₃ 24 mEq/L
PaO₂ 92 mmHg
pH: Normal
PaCO₂: Normal
HCO₃: Normal
PaO₂: Normal oxygenation
Final Interpretation
This ABG is within normal limits with normal oxygenation.
Common Mistakes When Using Normal ABG Values
Mistake 1
Looking at PaO₂ first and forgetting acid-base status.
Looking at PaO₂ first and forgetting acid-base status.
Mistake 2
Calling a normal pH “normal” without checking compensation.
Calling a normal pH “normal” without checking compensation.
Mistake 3
Confusing PaCO₂ ventilation problems with PaO₂ oxygenation problems.
Confusing PaCO₂ ventilation problems with PaO₂ oxygenation problems.
Apply These Values in Guided ABG Cases
Normal ranges only become useful when you apply them to patient scenarios. Start the free PulmoLearn ABG case series to practice pH, primary disorder, compensation, oxygenation, and final interpretation.