Chart — Acid-Base
ABG Interpretation Flowchart
A stepwise decision flowchart for interpreting arterial blood gases — follow each step in sequence to identify the primary disorder, compensation status, and oxygenation level.
Educational use only. ABG interpretation must occur in clinical context. Critical values require immediate provider notification. This flowchart supports learning and systematic approach development. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
Step-by-Step Flowchart
| pH < 7.35 | → Acidosis — the primary direction is acid |
| 7.35 – 7.45 | → Normal — may still be compensated; continue to next steps |
| pH > 7.45 | → Alkalosis — the primary direction is base |
| PaCO₂ > 45 | → Respiratory acidosis component (CO₂ retained = more acid) |
| 35 – 45 | → Normal respiratory component |
| PaCO₂ < 35 | → Respiratory alkalosis component (CO₂ blown off = less acid) |
| HCO₃¹ < 22 | → Metabolic acidosis component (bicarbonate depleted) |
| 22 – 26 | → Normal metabolic component |
| HCO₃¹ > 26 | → Metabolic alkalosis component (bicarbonate excess) |
Match the pH with the component that explains it:
Low pH + High PaCO₂
→ Respiratory Acidosis
Low pH + Low HCO₃¹
→ Metabolic Acidosis
High pH + Low PaCO₂
→ Respiratory Alkalosis
High pH + High HCO₃¹
→ Metabolic Alkalosis
Ask: Is the other component also abnormal, and in the direction that would partially correct the pH?
Compensation direction: primary disorder drives pH one way; compensating component corrects it back. Both move in the same direction from normal.
| PaO₂ 80–100 | → Normal oxygenation |
| PaO₂ 60–79 | → Mild hypoxemia |
| PaO₂ 40–59 | → Moderate hypoxemia — notify provider |
| PaO₂ < 40 | → Severe hypoxemia — critical value, immediate action |
| SaO₂ < 90% | → Hypoxemia — urgent intervention required |
Acid-base status and oxygenation are two separate assessments. A patient can have a normal pH and still be dangerously hypoxemic.
Practice Examples
Example A
pH 7.29 | PaCO₂ 52 | HCO₃¹ 24 | PaO₂ 55
Step 1: pH low (acidosis). Step 2: PaCO₂ high (respiratory acidosis). Step 3: HCO₃¹ normal. Step 4: Primary disorder = respiratory acidosis. Step 5: HCO₃¹ normal = uncompensated. Step 6: PaO₂ 55 = moderate hypoxemia. Uncompensated respiratory acidosis with moderate hypoxemia.
Example B
pH 7.32 | PaCO₂ 30 | HCO₃¹ 15 | PaO₂ 88
Step 1: pH low (acidosis). Step 2: PaCO₂ low (respiratory alkalosis component). Step 3: HCO₃¹ low (metabolic acidosis component). Step 4: HCO₃¹ matches pH direction = metabolic acidosis is primary. Step 5: PaCO₂ low as compensation = partially compensated. Step 6: Normal oxygenation. Partially compensated metabolic acidosis. Normal oxygenation.
Example C
pH 7.38 | PaCO₂ 55 | HCO₃¹ 31 | PaO₂ 72
Step 1: pH normal (7.35–7.45), but low-normal (acidosis side). Step 2: PaCO₂ high. Step 3: HCO₃¹ high. Step 4: Primary disorder = respiratory acidosis (pH low-normal; PaCO₂ matches). Step 5: Both abnormal + pH normal = fully compensated. Step 6: PaO₂ 72 = mild hypoxemia. Fully compensated respiratory acidosis with mild hypoxemia. Classic chronic COPD pattern.
Related Resources
Standards & sources
Fact-checked Jun 21, 2026This page is written to align with AARC Clinical Practice Guidelines. It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →