Quick Answer
Start with pH to decide whether the blood is acidotic or alkalotic. Then compare PaCO2 and HCO3 to decide whether the main problem is respiratory or metabolic. PaO2 and SaO2 help you evaluate oxygenation.
pH tells acid or base. PaCO2 is respiratory. HCO3 is metabolic. PaO2 and SaO2 are oxygenation clues.
Jump to an ABG Topic
ABG Normal Values Chart
Use this quick reference chart to memorize the most common ABG normal values. These are the values healthcare learners are usually expected to recognize first.
| ABG Value | Normal Range | What It Tells You | What to Notice First |
|---|---|---|---|
| pH | 7.35 to 7.45 | Shows whether the blood is acidic or alkalotic. | Low pH = acidosis. High pH = alkalosis. |
| PaCO2 | 35 to 45 mm Hg | Reflects the respiratory component and CO2 ventilation. | High CO2 = acidic. Low CO2 = alkalotic. |
| HCO3 | 22 to 26 mEq/L | Reflects the metabolic component and bicarbonate buffer system. | Low HCO3 = metabolic acidosis. High HCO3 = metabolic alkalosis. |
| PaO2 | 75 to 100 mm Hg | Shows oxygenation status in arterial blood. | Low PaO2 suggests poor oxygenation. |
| SaO2 | 95 to 100% | Shows arterial oxygen saturation. | Compare with SpO2 and patient condition. |
What Each ABG Value Means
pH
pH tells whether the blood is acidic or alkalotic. A pH below 7.35 suggests acidosis. A pH above 7.45 suggests alkalosis.
PaCO2
PaCO2 reflects the respiratory side. CO2 acts like an acid. High PaCO2 points toward respiratory acidosis. Low PaCO2 points toward respiratory alkalosis.
HCO3
HCO3, or bicarbonate, reflects the metabolic side. Low HCO3 points toward metabolic acidosis. High HCO3 points toward metabolic alkalosis.
PaO2 and SaO2
PaO2 and SaO2 help evaluate oxygenation. They are important, but they are not the main values used to name the acid-base disorder.
How to Interpret ABGs Step by Step
A simple ABG interpretation method makes the topic much easier. Start with the pH, then compare PaCO2 and HCO3.
- Check the pH. Is the patient acidotic, alkalotic, or within normal range?
- Check PaCO2. Is the respiratory value abnormal?
- Check HCO3. Is the metabolic value abnormal?
- Match the abnormal value to the pH. Does PaCO2 or HCO3 explain the pH direction?
- Check PaO2 and SaO2. Is oxygenation normal or low?
ABG Acidosis and Alkalosis Patterns
| Condition | pH | PaCO2 | HCO3 | What to Notice First |
|---|---|---|---|---|
| Respiratory Acidosis | Low | High | Normal or compensating | Low pH matches high CO2. |
| Respiratory Alkalosis | High | Low | Normal or compensating | High pH matches low CO2. |
| Metabolic Acidosis | Low | Normal or compensating | Low | Low pH matches low HCO3. |
| Metabolic Alkalosis | High | Normal or compensating | High | High pH matches high HCO3. |
ABG Example Interpretations
The pH is low, so the patient is acidotic. PaCO2 is high, which matches acidosis. HCO3 is normal. This points to respiratory acidosis.
The pH is high, so the patient is alkalotic. PaCO2 is low, which matches alkalosis. HCO3 is normal. This points to respiratory alkalosis.
The pH is low, so the patient is acidotic. PaCO2 is normal. HCO3 is low, which matches acidosis. This points to metabolic acidosis.
The pH is high, so the patient is alkalotic. PaCO2 is normal. HCO3 is high, which matches alkalosis. This points to metabolic alkalosis.
ABGs, Vital Signs, and Oxygenation
ABGs connect closely with vital signs because they help explain ventilation and oxygenation. A patient with low oxygen saturation, fast respiratory rate, or signs of respiratory distress may have abnormal ABG values.
Ventilation
PaCO2 helps show whether the patient is ventilating carbon dioxide effectively.
Oxygenation
PaO2 and SaO2 help show whether oxygen levels are adequate.
Assessment
Respiratory rate, SpO2, work of breathing, and patient appearance help give context to ABG values.
Review related assessment basics in the Normal Vital Signs Chart.
Common ABG Mistakes Students Make
- Trying to interpret PaO2 before understanding pH.
- Confusing PaCO2 with oxygen levels.
- Forgetting that PaCO2 is respiratory.
- Forgetting that HCO3 is metabolic.
- Memorizing patterns without practicing examples.
- Ignoring whether the pH is acidic or alkalotic first.
Practice Questions
Best Study Path
Use this order if ABGs feel confusing.
Related Clinical Learning
ABG interpretation connects closely with vital signs, oxygenation, respiratory anatomy, EKG basics, and lab value review.
Ready to Practice ABGs?
Reading normal values helps, but practice is what makes ABG patterns easier to recognize.
Frequently Asked Questions
What are normal ABG values?
Common normal ABG values are pH 7.35 to 7.45, PaCO2 35 to 45 mm Hg, HCO3 22 to 26 mEq/L, PaO2 75 to 100 mm Hg, and SaO2 95 to 100%.
Which ABG value is respiratory?
PaCO2 is the main respiratory value because it reflects carbon dioxide ventilation.
Which ABG value is metabolic?
HCO3, or bicarbonate, is the main metabolic value in basic ABG interpretation.
What should I check first when interpreting ABGs?
Check pH first. Decide whether the patient is acidotic or alkalotic, then compare PaCO2 and HCO3.
Does PaO2 determine acid-base balance?
No. PaO2 helps evaluate oxygenation. Acid-base interpretation mainly uses pH, PaCO2, and HCO3.
For learning purposes only. Always follow your program, instructor, facility, and clinical guidelines.