Before We Start
What is an arterial blood gas and what does it measure?
An arterial blood gas (ABG) is a blood test drawn from an artery (usually the radial artery in the wrist) that measures the oxygen and carbon dioxide levels in the blood, along with the blood's pH (acidity or alkalinity).
ABGs tell you two critical things: how well the lungs are working (oxygenation and ventilation) and whether the body's acid-base balance is normal.
📌 Normal ABG Values — Memorize These
pH: 7.35–7.45 (below 7.35 = acidosis · above 7.45 = alkalosis)
PaCO₂: 35–45 mmHg (the respiratory component)
HCO₃⁻: 22–26 mEq/L (the metabolic component)
PaO₂: 80–100 mmHg (oxygen level — separate from acid-base)
SpO₂: 95–100% (oxygen saturation)
The key to ROME is understanding that pH, CO₂, and HCO₃⁻ are the three values that determine acid-base status. Memorize which is respiratory and which is metabolic:
CO₂ = Respiratory component
Carbon dioxide is a gas — it's controlled by breathing. The lungs blow off CO₂ or retain it. CO₂ is an acid — more CO₂ = more acidic = lower pH. Think: CO₂ = exhaled gas = lungs = respiratory.
HCO₃⁻ = Metabolic component
Bicarbonate is a base — it's controlled by the kidneys. The kidneys retain or excrete bicarbonate. More HCO₃⁻ = more basic = higher pH. Think: HCO₃⁻ = kidney regulation = metabolic.
The Two ROME Rules
Respiratory Opposite · Metabolic Equal
R — Respiratory Opposite
pH and CO₂ move in OPPOSITE directions
In a respiratory problem, pH and CO₂ go in opposite directions:
Respiratory Acidosis: CO₂ is HIGH (retained) → pH is LOW (acidic)
pH ↓ and CO₂ ↑ — they move in OPPOSITE directions
Respiratory Alkalosis: CO₂ is LOW (blown off) → pH is HIGH (alkalotic)
pH ↑ and CO₂ ↓ — they move in OPPOSITE directions
Why? CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻. More CO₂ = more acid = lower pH. Less CO₂ = less acid = higher pH. Opposite directions every time.
💊 Respiratory Acidosis: patient hypoventilating (COPD, overdose, asthma). CO₂ builds up → pH drops.
Respiratory Alkalosis: patient hyperventilating (anxiety, pain, fever). CO₂ blown off → pH rises.
M — Metabolic Equal
pH and HCO₃⁻ move in the SAME direction
In a metabolic problem, pH and HCO₃⁻ go in the same direction:
Metabolic Acidosis: HCO₃⁻ is LOW (bicarbonate lost or acid gained) → pH is LOW (acidic)
pH ↓ and HCO₃⁻ ↓ — they move in the SAME direction (both down)
Metabolic Alkalosis: HCO₃⁻ is HIGH (bicarbonate gained or acid lost) → pH is HIGH (alkalotic)
pH ↑ and HCO₃⁻ ↑ — they move in the SAME direction (both up)
Why? HCO₃⁻ is a base. More base = higher pH. Less base = lower pH. Same direction always.
💊 Metabolic Acidosis: DKA, renal failure, lactic acidosis (HCO₃⁻ consumed fighting the acid).
Metabolic Alkalosis: vomiting (losing acid), excess antacids, loop diuretics (losing H⁺ and Cl⁻).
The Four-Step Method
How to interpret any ABG systematically
Step 1
Look at the pH — is it acidic or alkalotic?
pH below 7.35 = acidosis. pH above 7.45 = alkalosis. pH 7.35–7.45 = normal (but compensation may be occurring — keep reading).
Step 2
Apply ROME — is the CO₂ or HCO₃⁻ causing the pH change?
Compare the direction the pH moved to CO₂ and HCO₃⁻:
• If pH and CO₂ moved OPPOSITE → Respiratory cause
• If pH and HCO₃⁻ moved the SAME way → Metabolic cause
Step 3
Is the other value compensating?
The body always tries to return pH to normal. If CO₂ is the problem, the kidneys will adjust HCO₃⁻ to compensate (and vice versa). If both CO₂ and HCO₃⁻ are abnormal — one is the problem, one is the compensation.
• Uncompensated: only the causative value is abnormal
• Partially compensated: both values abnormal, pH still outside normal
• Fully compensated: both values abnormal, pH returned to normal range
Step 4
Assess oxygenation separately
PaO₂ and SpO₂ tell you about oxygenation — separate from acid-base balance. A patient can have normal acid-base and still be hypoxic. Always check both.
🏥 ABG Interpretation Practice — Four Cases
Apply the four-step method and ROME to each ABG result below.
| Case | pH | PaCO₂ | HCO₃⁻ | Interpretation |
|---|
| 1 | 7.28 | 58 | 24 | Respiratory Acidosis — pH low, CO₂ high (OPPOSITE), HCO₃⁻ normal (uncompensated) |
| 2 | 7.52 | 28 | 24 | Respiratory Alkalosis — pH high, CO₂ low (OPPOSITE), HCO₃⁻ normal (uncompensated) |
| 3 | 7.22 | 38 | 14 | Metabolic Acidosis — pH low, HCO₃⁻ low (EQUAL/SAME), CO₂ normal (uncompensated) |
| 4 | 7.48 | 48 | 34 | Metabolic Alkalosis (compensated) — pH high, HCO₃⁻ high (EQUAL), CO₂ high = respiratory compensation (retaining CO₂ to lower pH) |
Case 1
Clinical picture: COPD patient in respiratory distress. Hypoventilating → CO₂ retained → pH drops. Treatment: improve ventilation — bronchodilators, possible BiPAP or intubation.
Case 2
Clinical picture: Anxious patient hyperventilating. Blowing off CO₂ → pH rises. Treatment: address cause of hyperventilation. Have patient breathe into cupped hands (rebreathes CO₂).
Case 3
Clinical picture: Diabetic patient in DKA. Ketoacids consuming bicarbonate → HCO₃⁻ falls → pH drops. Treatment: IV fluids, insulin drip, electrolyte replacement.
Case 4
Clinical picture: Patient vomiting for 3 days. Lost gastric acid → HCO₃⁻ rises → pH rises. Lungs compensate by retaining CO₂. Treatment: IV fluids with potassium chloride, antiemetics.
📌 NCLEX Application
NCLEX loves ABG questions. They typically give you an ABG and a clinical scenario and ask you to identify the disorder or the appropriate intervention.
Common clinical presentations to recognize:
• COPD / hypoventilation / opioid overdose → Respiratory Acidosis
• Hyperventilation / anxiety / mechanical ventilation set too fast → Respiratory Alkalosis
• DKA / renal failure / lactic acidosis / diarrhea → Metabolic Acidosis
• Vomiting / NG suctioning / loop diuretics / antacid overuse → Metabolic Alkalosis
The fastest ROME approach on NCLEX: Look at pH first. Acidosis or alkalosis? Then find which value (CO₂ or HCO₃⁻) moved in the direction that explains it. That's your cause.
⚠️ The Trap — Confusing Which Value Is Which
The most common ABG error: forgetting which component is respiratory and which is metabolic.
Memory anchor: CO₂ is a GAS. Gases are controlled by breathing (respiratory). HCO₃⁻ is a chemical (bicarbonate). Chemicals are regulated by the kidneys (metabolic).
CO₂ = lungs = respiratory
HCO₃⁻ = kidneys = metabolic
Write this on your hand during nursing school if you have to. It unlocks every ABG question.
✓ Quick Self-Test
Identify each disorder using ROME:
1. pH 7.30, CO₂ 55, HCO₃⁻ 25
2. pH 7.50, CO₂ 40, HCO₃⁻ 32
3. pH 7.25, CO₂ 30, HCO₃⁻ 12
4. pH 7.48, CO₂ 30, HCO₃⁻ 24
Answers:
1. Respiratory Acidosis — pH low, CO₂ high (opposite)
2. Metabolic Alkalosis — pH high, HCO₃⁻ high (same direction)
3. Metabolic Acidosis — pH low, HCO₃⁻ low (same direction); CO₂ low = respiratory compensation
4. Respiratory Alkalosis — pH high, CO₂ low (opposite)
Next Lesson
SAMPLE — Health History
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