Chart — ABG & Acid-Base
ABG Disorder Comparison Chart
Every primary acid-base disorder in one grid. Find the pH direction first, confirm it against the respiratory (PaCO₂) and metabolic (HCO₃⁻) columns, and use the causes and hallmark signs to connect the numbers to the patient in front of you.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
The Eight Core Disorders
| Disorder | pH | PaCO₂ | HCO₃⁻ | Common Causes | Hallmark Signs |
|---|---|---|---|---|---|
| Respiratory acidosis (acute) | ↓ | ↑ | Normal | Hypoventilation — COPD, sedation, neuromuscular weakness | Somnolence, headache, flushed skin |
| Respiratory acidosis (chronic / compensated) | Low-normal | ↑ | ↑ | Chronic CO₂ retention | Baseline-shifted COPD; few acute symptoms |
| Respiratory alkalosis (acute) | ↑ | ↓ | Normal | Hypoxemia, anxiety, pain, sepsis, PE | Tingling, lightheadedness, tachypnea |
| Respiratory alkalosis (chronic) | High-normal | ↓ | ↓ | Prolonged hyperventilation, high altitude | Often asymptomatic; renal compensation |
| Metabolic acidosis | ↓ | ↓ (compensating) | ↓ | DKA, lactic acidosis, renal failure, diarrhea | Kussmaul respirations |
| Metabolic alkalosis | ↑ | ↑ (compensating) | ↑ | Vomiting, NG suction, diuretics | Slow, shallow breathing; weakness |
| Mixed acidosis | ↓↓ | ↑ | ↓ | Cardiac arrest, severe combined failure | Critical illness |
| Mixed alkalosis | ↑↑ | ↓ | ↑ | Hyperventilation plus vomiting / diuresis | Marked alkalemia; arrhythmia and tetany risk |
Normal Reference Ranges
| Value | Normal Range |
|---|---|
| pH | 7.35 – 7.45 |
| PaCO₂ | 35 – 45 mmHg |
| HCO₃⁻ | 22 – 26 mEq/L |
How to Use This Chart
Read the chart the way you read a gas: start in the pH column to find the direction of the disturbance, then scan PaCO₂ and HCO₃⁻ to see which system explains it. When the respiratory and metabolic columns push the pH the same way, you are looking at a mixed disorder — not compensation.
- The acute and chronic respiratory rows differ only in the HCO₃⁻ column — renal compensation takes 48–72 hours to shift bicarbonate, so a raised HCO₃⁻ implies a chronic process.
- Compensation never fully normalizes the pH and never pushes it past 7.40. A pH on the “wrong” side of 7.40 still points to the primary disorder.
- Match every value to the patient and the FiO₂ before acting. A chronic CO₂ retainer lives at numbers that would alarm you in an acute patient.
Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Acid-base balance chapters.
- Malley WJ. Clinical Blood Gases: Assessment and Intervention. 2nd ed. Elsevier Saunders; 2005.