Why ABGs Matter
Arterial blood gases tell you what's happening inside your patient at a cellular level — are they oxygenating? Are they ventilating? Is their acid-base balance intact? Understanding ABGs gives you the clinical picture that vital signs alone can't provide.
The good news is that ABG interpretation follows a predictable, systematic process. Once you learn the three-step method with the ROME mnemonic, you'll be able to interpret any ABG result in under 30 seconds.
Normal ABG Values
pH: 7.35 - 7.45 PaCO2: 35 - 45 mmHg HCO3: 22 - 26 mEq/L PaO2: 80 - 100 mmHg SaO2: 95 - 100%
Memorize these ranges — they're your baseline for every interpretation. If you know normal, abnormal jumps right out at you.
The ROME Method
ROME stands for: Respiratory = Opposite, Metabolic = Equal. This tells you how pH relates to the primary value. In respiratory disorders, pH and PaCO2 move in OPPOSITE directions (pH down + CO2 up = respiratory acidosis). In metabolic disorders, pH and HCO3 move in the SAME direction (pH down + HCO3 down = metabolic acidosis). Tape this mnemonic to your badge for the first few weeks — you'll use it constantly.
The 3-Step Interpretation Process
Step 1: Look at the pH. Is it below 7.35? That's acidosis. Above 7.45? That's alkalosis. Between 7.35 and 7.45? It's normal — but it could still be a compensated disorder, so keep going.
Step 2: Identify the primary cause. Check PaCO2 (the respiratory component) — if it's abnormal and explains the pH direction using ROME, it's a respiratory disorder. Check HCO3 (the metabolic component) — if it's abnormal and explains the pH, it's a metabolic disorder.
Step 3: Check for compensation. Uncompensated: only one value (CO2 or HCO3) is abnormal. Partially compensated: both values are abnormal, but pH is still outside normal range. Fully compensated: both values are abnormal, but pH has returned to normal range.
The 6 Acid-Base Disorders
Respiratory Acidosis (pH down, PaCO2 up) — caused by hypoventilation: COPD, sedation, neuromuscular disease, pneumonia.
Respiratory Alkalosis (pH up, PaCO2 down) — caused by hyperventilation: anxiety, pain, fever, PE, mechanical over-ventilation.
Metabolic Acidosis (pH down, HCO3 down) — caused by acid gain or base loss: DKA, renal failure, diarrhea, lactic acidosis.
Metabolic Alkalosis (pH up, HCO3 up) — caused by acid loss or base gain: vomiting, NG suction, diuretics, excessive bicarb.
Combined Respiratory + Metabolic Acidosis (pH way down, CO2 up, HCO3 down) — the most dangerous combination, seen in cardiac arrest.
Combined Respiratory + Metabolic Alkalosis (pH way up, CO2 down, HCO3 up) — rare but possible with over-correction.
Worked Example: COPD Exacerbation
A patient with COPD comes into the ED in respiratory distress. ABG results: pH 7.28, PaCO2 55, HCO3 24, PaO2 68. What's going on?
Step 1: pH 7.28 — below 7.35 = Acidosis Step 2: PaCO2 55 — elevated (normal 35-45). ROME: pH down + CO2 up = opposite = Respiratory acidosis Step 3: HCO3 24 — normal (22-26), so no compensation yet = Uncompensated Bonus: PaO2 68 — below 80 = Hypoxemia
Uncompensated respiratory acidosis with hypoxemia. Think: COPD exacerbation, pneumonia, or CNS depression. This patient needs respiratory support — potentially BiPAP or intubation.
Worked Example: DKA
A type 1 diabetic presents with Kussmaul respirations and fruity breath. ABG: pH 7.22, PaCO2 28, HCO3 12, PaO2 95. Interpret this.
Step 1: pH 7.22 — acidosis (below 7.35) Step 2: HCO3 12 — low (normal 22-26). ROME: pH down + HCO3 down = equal = Metabolic acidosis Step 3: PaCO2 28 — low (normal 35-45). The lungs are blowing off CO2 to compensate. But pH is still abnormal = Partially compensated
Partially compensated metabolic acidosis. Classic DKA presentation — the deep, rapid Kussmaul respirations are the body's attempt to compensate by lowering CO2.
Don't Forget Oxygenation
After interpreting acid-base status, always check PaO2 and SaO2. PaO2 below 80 mmHg = hypoxemia. PaO2 below 60 mmHg = severe hypoxemia (this is your urgent action threshold). SaO2 below 90% correlates with PaO2 around 60 and indicates the patient is on the steep part of the oxyhemoglobin dissociation curve — small drops in PaO2 now cause large drops in saturation.
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