recognition matrix comparison

ABG Interpretation: Metabolic vs Respiratory Acidosis vs Alkalosis — The 4-Pattern Matrix

An NCLEX question gives you pH 7.28, CO2 55, HCO3 26 — you have about 30 seconds to name the pattern and pick the right intervention. Mixing up the CO2 and HCO3 directions flips your answer to the wrong disorder and the wrong nursing action.

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Comparison

Side-by-side4 compared
Comparevs
Dimension
Respiratory Acidosis
Respiratory Alkalosis
Metabolic Acidosis
Metabolic Alkalosis
pH
  • ↓ < 7.35
  • ↑ > 7.45
  • ↓ < 7.35
  • ↑ > 7.45
PaCO₂
  • ★↑ > 45 (primary problem)
  • ★↓ < 35 (primary problem)
  • Normal or ↓ (compensating)
  • Normal or ↑ (compensating)
HCO₃⁻
  • Normal or ↑ (compensating)
  • Normal or ↓ (compensating)
  • ★↓ < 22 (primary problem)
  • ★↑ > 26 (primary problem)
Common causes
  • COPD, opioid OD, airway obstruction
  • Hyperventilation, anxiety/pain, high altitude
  • DKA, lactic acidosis, diarrhea, renal failure
  • Vomiting / NG suction, loop diuretics, antacids
Presentation
  • Hypoventilation
  • lethargy/confusion
  • Lightheaded
  • perioral tingling
  • Kussmaul respirations
  • fruity breath (DKA)
  • Slow shallow breathing
  • tetany/cramps
Nursing priority
  • Support ventilation
  • hold sedatives
  • Slow the breathing
  • treat anxiety/pain
  • Treat cause (insulin, fluids)
  • watch K⁺
  • Replace K⁺/Cl⁻
  • stop NG suction
Potassium shift
  • Hyperkalemia
  • Hypokalemia
  • Hyperkalemia
  • Hypokalemia
Quick ID rule
  • pH ↓ + CO₂ ↑ then opposite = respiratory
  • pH ↑ + CO₂ ↓ then opposite = respiratory
  • pH ↓ + HCO₃⁻ ↓ then same = metabolic
  • pH ↑ + HCO₃⁻ ↑ then same = metabolic
Example ABG
  • pH 7.28 / CO₂ 58 / HCO₃ 24
  • pH 7.50 / CO₂ 30 / HCO₃ 24
  • pH 7.30 / CO₂ 40 / HCO₃ 16
  • pH 7.50 / CO₂ 42 / HCO₃ 32
pH

Respiratory Acidosis

  • ↓ < 7.35

Respiratory Alkalosis

  • ↑ > 7.45
PaCO₂

Respiratory Acidosis

  • ★↑ > 45 (primary problem)

Respiratory Alkalosis

  • ★↓ < 35 (primary problem)
HCO₃⁻

Respiratory Acidosis

  • Normal or ↑ (compensating)

Respiratory Alkalosis

  • Normal or ↓ (compensating)
Common causes

Respiratory Acidosis

  • COPD, opioid OD, airway obstruction

Respiratory Alkalosis

  • Hyperventilation, anxiety/pain, high altitude
Presentation

Respiratory Acidosis

  • Hypoventilation
  • lethargy/confusion

Respiratory Alkalosis

  • Lightheaded
  • perioral tingling
Nursing priority

Respiratory Acidosis

  • Support ventilation
  • hold sedatives

Respiratory Alkalosis

  • Slow the breathing
  • treat anxiety/pain
Potassium shift

Respiratory Acidosis

  • Hyperkalemia

Respiratory Alkalosis

  • Hypokalemia
Quick ID rule

Respiratory Acidosis

  • pH ↓ + CO₂ ↑ then opposite = respiratory

Respiratory Alkalosis

  • pH ↑ + CO₂ ↓ then opposite = respiratory
Example ABG

Respiratory Acidosis

  • pH 7.28 / CO₂ 58 / HCO₃ 24

Respiratory Alkalosis

  • pH 7.50 / CO₂ 30 / HCO₃ 24

★ marks the fact that sets a column apart.

Clinical Pearl

ROME: Respiratory = Opposite (pH and CO2), Metabolic = Equal (pH and HCO3 move together).

Component Topics

Metabolic AcidosisMetabolic AlkalosisRespiratory AcidosisRespiratory Alkalosis

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