Guide — ABG & Acid-Base
ABG Interpretation Basics
Arterial blood gases stop being intimidating the moment you commit to one fixed reading order. This guide builds that order — pH first, then the respiratory and metabolic components, then compensation, then oxygenation — and drills it with worked examples.
9 min read · ABG & Acid-Base
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.
Overview
An ABG answers three questions: is the blood too acidic or too alkaline, which organ system caused it, and is the body fixing it? The values come as a package — pH, PaCO₂, HCO₃⁻, PaO₂, SaO₂ — but they are read in a deliberate order, because each value only means something in the context of the one before it.
| Value | Normal Range | What It Reflects |
|---|---|---|
| pH | 7.35 – 7.45 | Net acid-base status |
| PaCO₂ | 35 – 45 mmHg | Respiratory component (ventilation) |
| HCO₃⁻ | 22 – 26 mEq/L | Metabolic component (renal) |
| PaO₂ | 80 – 100 mmHg (room air) | Oxygenation — read separately |
| SaO₂ | 95 – 100% | Hemoglobin saturation |
Key Concepts — The Five Steps
- Classify the pH.Below 7.35 is acidemia, above 7.45 is alkalemia. If it’s in range, note which side of 7.40 it sits on — that hint matters in step four.
- Check PaCO₂. CO₂ is an acid the lungs exhale. High PaCO₂ pushes pH down (respiratory acidosis); low PaCO₂ pushes it up (respiratory alkalosis).
- Check HCO₃⁻.Bicarbonate is the kidneys’ base. Low HCO₃⁻ pushes pH down (metabolic acidosis); high HCO₃⁻ pushes it up (metabolic alkalosis).
- Match and assess compensation. The abnormal system that agrees with the pH is the primary disorder. Then ask whether the other system has moved the expected amount to correct it — Winter’s formula (expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2) for metabolic acidosis; HCO₃⁻ rising ~1 mEq/L (acute) or ~3.5 mEq/L (chronic) per 10 mmHg of PaCO₂ for respiratory acidosis. Both systems abnormal in the same direction means a mixed disorder.
- Evaluate oxygenation separately. PaO₂ and SaO₂ describe a different problem than acid-base. Always note the FiO₂ — a “normal” PaO₂ of 95 on 60% oxygen is a markedly abnormal gas exchange.
The Four Primary Patterns
| Disorder | pH | PaCO₂ | HCO₃⁻ | Common Causes |
|---|---|---|---|---|
| Respiratory acidosis | ↓ | ↑ | Normal (acute) / ↑ (chronic) | Hypoventilation: COPD, sedation, neuromuscular weakness, fatigue |
| Respiratory alkalosis | ↑ | ↓ | Normal (acute) / ↓ (chronic) | Hyperventilation: hypoxemia, pain, anxiety, sepsis, PE |
| Metabolic acidosis | ↓ | ↓ (compensating) | ↓ | DKA, lactic acidosis, renal failure, diarrhea |
| Metabolic alkalosis | ↑ | ↑ (compensating) | ↑ | Vomiting, NG suction, diuretics, steroids |
Worked example: pH 7.28 / PaCO₂ 60 / HCO₃⁻ 26. Step 1: acidemia. Step 2: PaCO₂ high — respiratory acid. Step 3: HCO₃⁻ normal. Step 4: the respiratory system matches the pH and the kidneys haven’t responded yet → acute (uncompensated) respiratory acidosis. The patient in front of you is hypoventilating right now.
RT Priorities & Interventions
- Treat the patient, then the gas. Confirm the sample matches the clinical picture — an alert, comfortable patient with a “critical” gas usually means a venous sample, air bubble, or delay on ice.
- Respiratory acidosis → ventilation problem. Stimulate, reposition, clear secretions, reduce sedation when possible, support with NIV or mechanical ventilation per protocol. The fix is moving more air, not giving more oxygen.
- Respiratory alkalosis → find the driver. Hypoxemia, pain, anxiety, sepsis, and pulmonary embolism all hyperventilate patients. Correct the cause; don’t just coach the breathing.
- Metabolic disorders → communicate. The RT’s job is recognizing the pattern (the hyperventilating DKA patient is compensating, not anxious) and protecting that compensation — never “normalize” the respiratory rate of a compensating patient on the ventilator.
- Chronic retainers are different. A COPD patient living at PaCO₂ 60 with pH 7.36 doesn’t need rescue ventilation for that number — compare every gas to the patient’s baseline.
Common Pitfalls
- Reading oxygenation as part of acid-base. PaO₂ has its own workup (see the A–a gradient and P/F ratio) — mixing the two muddles both.
- Calling a mixed disorder “compensation.” Compensation never overshoots and never moves pH past 7.40 — if it appears to, there are two primary disorders.
- Skipping the expected-compensation math. Eyeballing “CO₂ up, bicarb up, looks compensated” misses the second disorder hiding in a value that moved the wrong amount.
- Forgetting the FiO₂ and the patient’s baseline when judging PaO₂ and PaCO₂.
Board Exam Pearls
- Exam stems love the fully compensated gas: normal pH with abnormal PaCO₂ and HCO₃⁻. Use the 7.40 midpoint — pH 7.36 with high CO₂ and high bicarb is compensated respiratory acidosis.
- Winter’s formula is worth memorizing cold: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 (± 2). A measured PaCO₂ above the range adds respiratory acidosis; below adds respiratory alkalosis.
- Acute vs chronic respiratory acidosis on exams: HCO₃⁻ up ~1 mEq/L per 10 mmHg CO₂ = acute; ~3.5 mEq/L per 10 = chronic (renal compensation takes 48–72 hours).
- Vomiting/NG suction → metabolic alkalosis; diarrhea → metabolic acidosis. Opposite ends of the gut, opposite disorders.
FAQ
What order should I read an ABG in?
Use the same sequence every time: 1) classify the pH, 2) check PaCO₂ (the respiratory side), 3) check HCO₃⁻ (the metabolic side), 4) decide which abnormal system matches the pH and assess compensation, 5) evaluate oxygenation (PaO₂ and SaO₂) separately. Consistency is what prevents missed mixed disorders.
Why is PaCO₂ called a respiratory value and HCO₃⁻ a metabolic value?
PaCO₂ is controlled minute to minute by alveolar ventilation — only the lungs move it quickly. HCO₃⁻ is regulated by the kidneys, which retain or excrete bicarbonate over hours to days. That split is what lets one gas tell you which organ system started the problem.
How do I tell compensation from a mixed disorder?
Compensation moves the second system in the direction that corrects pH and the expected amount (Winter's formula for metabolic acidosis; the 1/3.5 mEq per 10 mmHg rules for respiratory acidosis). If the second value moves the wrong way, or far outside the expected range, you are looking at a second primary disorder, not compensation.
Does a normal pH mean a normal ABG?
No. A normal pH with abnormal PaCO₂ and HCO₃⁻ in opposite directions is a fully compensated disorder — common in chronic CO₂ retainers. Which side of 7.40 the pH sits on points to the primary problem.
Put it to work
Run real values through the interpreter — it shows the same five-step logic and the expected-compensation math for every gas you enter.
Open the ABG Interpreter →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Analysis and monitoring of gas exchange chapters.
- Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312-322.
- Malley WJ. Clinical Blood Gases: Assessment and Intervention. 2nd ed. Elsevier Saunders; 2005.