STEP 1 — Read the ABG

You will see:
pH / pCO₂ / pO₂ / HCO₃ / O₂ Sat

Example:
7.28 / 60 / 70 / 26

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STEP 2 — Determine Acidemia or Alkalemia

• pH < 7.35 → acidemia

• pH > 7.45 → alkalemia

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STEP 3 — Determine Primary Disorder

Respiratory component

• High CO₂ → respiratory acidosis

• Low CO₂ → respiratory alkalosis

Metabolic component

• Low HCO₃ → metabolic acidosis

• High HCO₃ → metabolic alkalosis

###Whichever direction matches the pH change = primary process.

Example:
pH 7.28 (acidemia)
CO₂ 60 (high = respiratory acid)
→ Primary respiratory acidosis

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STEP 4 — Check Compensation

A. Metabolic Acidosis (Use Winter’s Formula)

Expected CO₂ = 1.5 × HCO₃ + 8 (±2)

• CO₂ matches → appropriate

• CO₂ higher → mixed resp + metabolic acidosis

• CO₂ lower → mixed metabolic acidosis + resp alkalosis

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B. Metabolic Alkalosis

Expected CO₂ = HCO₃ + 15 (±2)

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C. Respiratory Disorders

Acute Respiratory Acidosis:

• HCO₃ ↑ by 1 for every 10 ↑ in CO₂

Chronic Respiratory Acidosis:

• HCO₃ ↑ by 3–4 for every 10 ↑ in CO₂

Acute Respiratory Alkalosis:

• HCO₃ ↓ by 2 for every 10 ↓ in CO₂

Chronic Respiratory Alkalosis:

• HCO₃ ↓ by 4–5 for every 10 ↓ in CO₂

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STEP 5 — Evaluate Oxygenation

A-a Gradient

Tells you if hypoxia is:

• V/Q mismatch

• Shunt

• Diffusion defect

• Alveolar hypoventilation

A-a gradient = PAO₂ – PaO₂

PAO₂ = FiO₂ × (713) – (pCO₂ / 0.8)
(Normal A-a: 5–15 + age/4)

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PF Ratio (Very Important in ICU)

PF ratio = PaO₂ / FiO₂

Used in ARDS classification:

ARDS Severity	PF Ratio
Mild	200–300
Moderate	100–200
Severe	< 100

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STEP 6 — Think Physiologically

Align ABG with clinical picture and ventilator settings.

If hypercapnic (high CO₂):

• Inadequate minute ventilation → adjust respiratory rate or tidal volume

• Obstruction? COPD flare? Asthma?

If hypoxemic (low PaO₂):

• Increase FiO₂

• Increase PEEP

• Evaluate lung pathology (pneumonia? ARDS? fluid overload?)

If metabolic acidosis:

• Check anion gap

• Start treating cause (sepsis, renal failure, DKA, lactate)

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