Learning Center/ABG Masterclass

ABG Masterclass

Master arterial blood gas interpretation with systematic approach, compensation rules, and unlimited practice cases.

5-Step Systematic Approach

1
Assess pH
< 7.35 = acidosis · > 7.45 = alkalosis · 7.35–7.45 = normal
2
Identify primary disorder
If acidosis: ↑PaCO₂ = respiratory OR ↓HCO₃ = metabolic. If alkalosis: ↓PaCO₂ = respiratory OR ↑HCO₃ = metabolic
3
Check compensation
Is the compensatory response appropriate? Use the rules below. Under/over compensation = mixed disorder
4
Assess oxygenation
PaO₂ and SaO₂ — calculate A-a gradient = PAO₂ − PaO₂ (normal 5–15 mmHg on RA)
5
Correlate clinically
Does the ABG match the clinical picture? Apply MUDPILES for anion gap acidosis if needed

Normal Values

pH7.35–7.45↓ Acidosis / ↑ Alkalosis

PaCO₂35–45 mmHg↓ Alkalosis (↑ eliminated) / ↑ Acidosis (↑ retained)

HCO₃22–26 mEq/L↓ Metabolic acidosis / ↑ Metabolic alkalosis

PaO₂80–100 mmHg↓ Hypoxemia / ↑ Hyperoxia

SaO₂95–100%↓ Desaturation / ↑ —

Compensation Rules

Respiratory Acidosis (Acute)HCO₃ rises 1 mEq/L per ↑10 PaCO₂

Respiratory Acidosis (Chronic)HCO₃ rises 3.5 mEq/L per ↑10 PaCO₂

Respiratory Alkalosis (Acute)HCO₃ falls 2 mEq/L per ↓10 PaCO₂

Respiratory Alkalosis (Chronic)HCO₃ falls 5 mEq/L per ↓10 PaCO₂

Metabolic AcidosisExpected PaCO₂ = 1.5 × HCO₃ + 8 ± 2 (Winter's formula)

Metabolic AlkalosisExpected PaCO₂ = 0.7 × HCO₃ + 21 ± 2

Key Rule

Compensation NEVER normalizes pH past 7.40. If pH is normal with abnormal PaCO₂ AND HCO₃, suspect a mixed disorder.

Practice Cases

20 cases from beginner to advanced — each with systematic interpretation and management.

1 / 20

beginnerCase #01

A 68-year-old male with a 50 pack-year smoking history presents to the ED with worsening dyspnea and productive cough for 3 days. He has a known history of COPD. SpO2 is 84% on room air. He is using accessory muscles and appears in moderate distress.

ABG Values

7.28

7.35–7.45

PaCO₂

35–45 mmHg

HCO₃

22–26 mEq/L

PaO₂

80–100 mmHg

FiO₂

0.21

0.21 RA

SaO₂

95–100%

Anion Gap Acidosis: MUDPILES

Anion gap = Na − (Cl + HCO₃); normal 8–12 mEq/L. Elevated AG = added acid.

Methanol

Uremia

DKA

Propylene glycol

Isoniazid / Iron

Lactic acidosis

Ethylene glycol

Salicylates

5-Step Systematic Approach

Assess pH

< 7.35 = acidosis · > 7.45 = alkalosis · 7.35–7.45 = normal

Identify primary disorder

If acidosis: ↑PaCO₂ = respiratory OR ↓HCO₃ = metabolic. If alkalosis: ↓PaCO₂ = respiratory OR ↑HCO₃ = metabolic

Check compensation

Is the compensatory response appropriate? Use the rules below. Under/over compensation = mixed disorder

Assess oxygenation

PaO₂ and SaO₂ — calculate A-a gradient = PAO₂ − PaO₂ (normal 5–15 mmHg on RA)

Correlate clinically

Does the ABG match the clinical picture? Apply MUDPILES for anion gap acidosis if needed