“7.4 / 40 / 100 / 24”
= Normal pH / CO₂ / O₂ / HCO₃

O₂ Saturation

• 94 – 100%

A–a Gradient

• Normal ~ 5–15 mmHg

• Increases with age: (Age / 4) + 4

PF Ratio (PaO₂ / FiO₂)

• Normal: > 300

• Used for ARDS severity

ABG: 7.22 / 60 / 65 / 24

Step 1 → Acidemia
Step 2 → CO₂ high → respiratory acidosis
Step 3 → HCO₃ normal → acute
Step 4 → Cause: hypoventilation (COPD, sedation, obstruction)
Step 5 → O₂ low → treat hypoxia
Step 6 → Fix by ↑ RR, clear airway, consider NIV