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Respiratory / Ventilation

Arterial Blood Gas (ABG) Interpretation

Stepwise ABG interpretation: pH, primary disorder, compensation, oxygenation assessment, and common patterns seen in the surgical ICU.

Source
Standard ABG interpretation framework
Status
review pending
Updated
5/29/2026
On this page
On this page

Content Status

Guideline-supported example educational content.* Not official local protocol. Verify all clinical decisions with attending physician.
Section
  • Guideline-supported example educational content.* Not official local protocol. Verify all clinical decisions with attending physician.

Normal ABG Values

Parameter • Normal Range • pH
Section
ParameterNormal Range
pH7.35 – 7.45
PaCO₂35 – 45 mmHg
HCO₃⁻22 – 26 mEq/L
PaO₂80 – 100 mmHg (room air)
Base Excess−2 to +2 mEq/L

Step-by-Step Interpretation

< 7.35 → Acidosis • > 7.45 → Alkalosis
Section

Step 1: pH

  • < 7.35 → Acidosis
  • > 7.45 → Alkalosis

Step 2: Primary Disorder

DisorderpHPaCO₂HCO₃⁻
Respiratory Acidosis↓↑—
Respiratory Alkalosis↑↓—
Metabolic Acidosis↓—↓
Metabolic Alkalosis↑—↑

Step 3: Compensation

Primary DisorderExpected Compensation
Acute resp. acidosisHCO₃⁻ ↑ 1 mEq/L per 10 mmHg ↑ PaCO₂
Chronic resp. acidosisHCO₃⁻ ↑ 3.5 mEq/L per 10 mmHg ↑ PaCO₂
Acute resp. alkalosisHCO₃⁻ ↓ 2 mEq/L per 10 mmHg ↓ PaCO₂
Metabolic acidosisExpected PaCO₂ = 1.5×HCO₃⁻ + 8 ± 2 (Winter's formula)
Metabolic alkalosisExpected PaCO₂ = 0.7×HCO₃⁻ + 21 ± 2

Step 4: Oxygenation

  • PaO₂ < 60 mmHg → hypoxemia
  • Calculate P/F ratio (see P/F Ratio calculator)

Common Patterns in the SICU

Scenario • Expected Pattern • Hypoventilation
Section
ScenarioExpected Pattern
Hypoventilation↓pH, ↑PaCO₂
Post-op pain/splinting↓pH, ↑PaCO₂
Early sepsis↑pH, ↓PaCO₂ (respiratory alkalosis)
Hemorrhagic shock/lactic acidosis↓pH, ↓HCO₃⁻, ↓PaCO₂ (compensation)
NGT suctioning↑pH, ↑HCO₃⁻ (metabolic alkalosis)
  • Guideline-supported example educational content. Verify all clinical decisions with attending physician and current institutional protocols.*