Guide — Acid-Base
ABG Foundation Guide
Arterial blood gases measure the acid-base status and oxygenation of the blood. This guide walks through each value, what it means, and a reliable step-by-step interpretation method you can use in clinical settings.
8 min read · Clinical Practice
Educational use only. This content is intended to support learning and clinical practice. Always apply institutional protocols and licensed clinical supervision in practice. 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.
Normal ABG Values
| Parameter | Normal Range | What It Measures |
|---|---|---|
| pH | 7.35 – 7.45 | Acidity or alkalinity of blood |
| PaCO₂ | 35 – 45 mmHg | Respiratory component (ventilation) |
| HCO₃ | 22 – 26 mEq/L | Metabolic component (bicarbonate buffer) |
| PaO₂ | 80 – 100 mmHg | Partial pressure of oxygen in arterial blood |
| SaO₂ | 95 – 100% | Oxygen saturation of hemoglobin |
pH — The Master Indicator
pH reflects the overall acid-base status of the blood. It moves in response to both respiratory and metabolic changes.
- pH < 7.35 — Acidosis. The blood is too acidic.
- pH 7.35 – 7.45 — Normal (compensated disorders may still be present).
- pH > 7.45 — Alkalosis. The blood is too alkaline.
Clinical tip: pH < 7.2 or > 7.6 is life-threatening and requires immediate intervention.
PaCO₂ — The Respiratory Marker
PaCO₂ reflects how well the lungs are ventilating. Carbon dioxide is an acid, so changes in PaCO₂ directly affect pH.
- PaCO₂ > 45 — Hypoventilation; CO₂ retained; respiratory acidosis component.
- PaCO₂ < 35 — Hyperventilation; CO₂ blown off; respiratory alkalosis component.
Remember: CO₂ and pH move in opposite directions for respiratory causes.
HCO₃ — The Metabolic Marker
Bicarbonate (HCO₃) is regulated by the kidneys and represents the metabolic component. It takes hours to days to change, unlike CO₂ which changes in minutes.
- HCO₃ < 22 — Metabolic acidosis component (bicarbonate depleted).
- HCO₃ > 26 — Metabolic alkalosis component (bicarbonate excess).
HCO₃ and pH move in the same direction for metabolic causes.
Respiratory vs Metabolic Causes
| Disorder | Primary Cause | Common Examples |
|---|---|---|
| Respiratory Acidosis | CO₂ retention (hypoventilation) | COPD, opioid overdose, neuromuscular disease, sleep apnea |
| Respiratory Alkalosis | CO₂ loss (hyperventilation) | Anxiety, pain, fever, early sepsis, mechanical overventilation |
| Metabolic Acidosis | HCO₃ loss or acid gain | DKA, lactic acidosis, renal failure, diarrhea, aspirin overdose |
| Metabolic Alkalosis | HCO₃ gain or acid loss | Vomiting, NG suction, diuretics, antacid overuse, hypokalemia |
Compensation Basics
The body compensates to bring pH back toward normal. The lungs compensate for metabolic disorders (fast); the kidneys compensate for respiratory disorders (slow).
- Respiratory compensation — Lungs adjust CO₂ within minutes to hours.
- Metabolic compensation — Kidneys adjust HCO₃ over hours to days.
Uncompensated: pH abnormal, one value abnormal, one value normal.
Partially compensated: pH abnormal, both PaCO₂ and HCO₃ abnormal.
Fully compensated: pH normal (but still close to acidic or alkalotic side), both PaCO₂ and HCO₃ abnormal.
Step-by-Step Interpretation (ROME Method)
ROME: Respiratory Opposite, Metabolic Equal
- Step 1 — Evaluate pH.Is it acidic (< 7.35), normal (7.35–7.45), or alkalotic (> 7.45)?
- Step 2 — Identify the primary cause. Check PaCO₂ and HCO₃. Which one is abnormal and consistent with the pH direction?
- Respiratory: PaCO₂ moves opposite to pH
- Metabolic: HCO₃ moves in the same direction as pH
- Step 3 — Assess for compensation. Is the other value abnormal (attempting to correct the pH)?
- Step 4 — State the disorder.E.g., “Partially compensated respiratory acidosis” or “Uncompensated metabolic alkalosis.”
- Step 5 — Evaluate oxygenation. Check PaO₂ and SaO₂ separately from the acid-base interpretation.
Practice This Skill
Standards & sources
Fact-checked Jun 21, 2026This page is written to align with American Association for Respiratory Care (AARC) · Standard clinical chemistry / ABG references. 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 →