Step by Step
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Step 1 — check the pH
pH below 7.35 indicates acidosis; pH above 7.45 indicates alkalosis.
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Step 2 — check the respiratory component (PaCO₂), using ROME-R
CO₂ moves OPPOSITE to pH direction (Respiratory Opposite). Increased CO₂ makes the blood more acidic — respiratory acidosis. Decreased CO₂ makes the blood more alkaline — respiratory alkalosis.
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Step 3 — check the metabolic component (HCO₃⁻), using ROME-M
Bicarbonate moves the SAME direction as pH (Metabolic Equal). Increased HCO₃⁻ makes the blood more alkaline — metabolic alkalosis. Decreased HCO₃⁻ makes the blood more acidic — metabolic acidosis.
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Step 4 — assess compensation
The lungs compensate for metabolic disorders quickly (within minutes), while the kidneys compensate for respiratory disorders much more slowly (over days). PaO₂ below 60 mmHg indicates hypoxemia, requiring supplemental oxygen. SpO₂ (peripheral oxygen saturation via pulse oximeter) should normally be above 95%.
Applied Walkthrough
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A patient's ABG shows pH 7.25 (acidosis), PaCO₂ 60 mmHg (elevated), and HCO₃⁻ 24 mEq/L (normal). Using ROME: CO₂ is elevated and pH is acidic — since CO₂ moves opposite to pH, this confirms respiratory acidosis (likely from hypoventilation or COPD).
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A different patient's ABG shows pH 7.30 (acidosis), PaCO₂ 30 mmHg (decreased), and HCO₃⁻ 15 mEq/L (decreased). Here, HCO₃⁻ has moved in the SAME direction as pH (both low) — confirming a primary metabolic acidosis, with the low CO₂ representing the lungs' fast compensatory hyperventilation.
Exam Application
Exams test whether you can apply the ROME framework correctly (Respiratory Opposite, Metabolic Equal) to identify the primary acid-base disorder from an ABG, and whether you can distinguish primary disorders from compensatory responses based on speed (lungs compensate quickly, kidneys slowly).
⚠ Common Trap
The most common trap is confusing which value moves opposite versus the same direction as pH — remember "Respiratory Opposite, Metabolic Equal": CO₂ moves opposite to pH direction, while bicarbonate moves the same direction as pH.
✓ Quick Self-Check
1. What pH range indicates acidosis versus alkalosis?
Below 7.35 is acidosis; above 7.45 is alkalosis.
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2. According to ROME, does CO₂ move the same direction or opposite to pH?
Opposite ("Respiratory Opposite").
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3. According to ROME, does bicarbonate move the same direction or opposite to pH?
The same direction ("Metabolic Equal").
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4. Which organ compensates faster — lungs or kidneys?
Lungs (minutes), compared to kidneys (days).
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5. What PaO₂ level indicates hypoxemia requiring supplemental oxygen?
Below 60 mmHg.
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