ABG Interpretation Made Simple for Respiratory Therapy Students

If you’re studying for the TMC exam, ABGs can feel overwhelming at first.

Too many numbers. Too many rules. Too easy to second guess.

But here’s the truth:

ABG interpretation is actually one of the most predictable and testable skills on the exam.

Once you follow a simple system, it becomes easy.

Step-by-Step ABG Interpretation

Use this exact method every time you see an ABG:

Step 1: Check the pH

• pH < 7.35 → Acidosis
• pH > 7.45 → Alkalosis

This tells you whether the patient is acidotic or alkalotic.

Step 2: Look at PaCO₂ (Respiratory Component)

• PaCO₂ > 45 → Respiratory Acidosis
• PaCO₂ < 35 → Respiratory Alkalosis

This reflects ventilation.

Step 3: Look at HCO₃⁻ (Metabolic Component)

• HCO₃⁻ < 22 → Metabolic Acidosis
• HCO₃⁻ > 26 → Metabolic Alkalosis

This reflects metabolic balance.

Step 4: Match the System to the pH

Ask yourself:

Which value matches the pH?

• If pH is acidic AND CO₂ is high → Respiratory Acidosis
• If pH is acidic AND HCO₃ is low → Metabolic Acidosis

This tells you the primary problem

Step 5: Determine Compensation

• Uncompensated → one value abnormal, other normal
• Partially compensated → both abnormal, pH still abnormal
• Fully compensated → both abnormal, pH normal

Example ABG

pH: 7.30 PaCO₂: 55 HCO₃⁻: 24

Step 1 → Acidosis Step 2 → CO₂ is high Step 3 → HCO₃ is normal

Final Answer → Respiratory Acidosis (Uncompensated)

Ventilator Adjustments Based on ABGs

This is where most students struggle — but it’s actually simple.

Respiratory Acidosis (High CO₂)

Problem: Not enough ventilation

Fix:

• Increase respiratory rate
• Increase tidal volume

Respiratory Alkalosis (Low CO₂)

Problem: Too much ventilation

Fix:

• Decrease respiratory rate
• Decrease tidal volume

Metabolic Acidosis

Problem: Metabolic issue

Fix:

• Treat underlying cause
• Consider bicarbonate if severe

Metabolic Alkalosis

Problem: Loss of acid

Fix:

• Treat underlying cause
• May reduce ventilation if needed

Hypoxemia (Low PaO₂)

Problem: Oxygenation

Fix:

• Increase FiO₂
• Increase PEEP

Hyperoxemia (High PaO₂)

Problem: Too much oxygen

Fix:

• Decrease FiO₂
• Decrease PEEP

Common ABG Mistakes RT Students Make

1. Skipping the pH

Always start with pH.

2. Mixing Up CO₂ and HCO₃

CO₂ = respiratory HCO₃ = metabolic

3. Overthinking Compensation

Focus on the primary disorder first.

4. Ignoring the Pattern

ABGs follow patterns — trust them.

Test Yourself

ABG:

pH: 7.48 PaCO₂: 30 HCO₃⁻: 24

What is the diagnosis?

Answer: Respiratory Alkalosis

Want More Practice?

If you want to actually master ABGs and pass the TMC exam:

Exhale Academy gives you:

• practice questions
• step-by-step explanations
• exam-level scenarios
• a system that builds confidence

Start studying smarter with Exhale Academy.