top of page
Search

Spirometry: Obstructive vs Restrictive — MRCP Part 1

TL;DR:

 In MRCP Part 1, spirometry questions test pattern recognition more than disease labels. A reduced FEV₁/FVC ratio defines obstruction, while a preserved or increased ratio with a low FVC suggests restriction (confirmed by TLC). Master the numbers, the exceptions, and the common traps to secure easy marks.


Why this topic matters in MRCP Part 1

Spirometry interpretation is repeatedly examined across respiratory medicine, cardiology, and systemic disease questions. Examiners expect candidates to interpret FEV₁, FVC, and FEV₁/FVC accurately, recognise pseudo-restriction, and apply bronchodilator criteria correctly—often under time pressure. Errors usually arise from skipping the ratio or over-diagnosing restriction without lung volumes.

For syllabus context, see the official MRCP Part 1 overview:https://www.mrcpuk.org/mrcpuk-examinations/part-1


Core concepts you must know

1) Key spirometric variables

  • FEV₁: Forced expiratory volume in the first second

  • FVC: Forced vital capacity

  • FEV₁/FVC ratio: Primary discriminator between obstruction and restriction

2) Obstructive pattern

  • FEV₁: Reduced

  • FVC: Normal or reduced

  • FEV₁/FVC: Reduced (classically <0.70 or below LLN)

  • Mechanism: Increased airway resistance and expiratory flow limitation

  • Common causes: Asthma, COPD, bronchiectasis

3) Restrictive pattern

  • FEV₁: Reduced

  • FVC: Reduced

  • FEV₁/FVC: Normal or increased

  • Mechanism: Reduced lung compliance or total lung volume

  • Common causes: Interstitial lung disease, chest wall disorders, neuromuscular weakness

4) Bronchodilator reversibility

A response is significant if FEV₁ increases by ≥12% and ≥200 mL after bronchodilator. This supports asthma but its absence does not exclude asthma—an important exam nuance.

5) Lung volumes and TLC

Spirometry alone can suggest restriction, but total lung capacity (TLC) is required to confirm it. Reduced TLC confirms true restriction.


High-yield comparison table

Feature

Obstructive

Restrictive

FEV₁

↓↓↓

↓

FVC

Normal or ↓

↓↓↓

FEV₁/FVC

↓

Normal or ↑

TLC

Normal or ↑

↓

Bronchodilator response

Often present (asthma)

Absent

Flow–volume loop

Scooped expiratory limb

Small, proportionate loop

Five subtopics examiners frequently test

  1. Asthma vs COPD – variability and reversibility favour asthma

  2. Pseudo-restriction – air-trapping lowers FVC in obstruction

  3. Obesity and neuromuscular disease – restrictive pattern with normal lung parenchyma

  4. Interstitial lung disease – low FVC, preserved ratio, low TLC

  5. Upper airway obstruction – flattened inspiratory and/or expiratory limbs if loops are shown

Focused MRCP Part 1 study setup with medical notes and exam preparation materials

Mini-case (exam style)

A 58-year-old man with a 35-pack-year smoking history has exertional dyspnoea. Spirometry shows:

  • FEV₁ 55% predicted

  • FVC 70% predicted

  • FEV₁/FVC 0.60Post-bronchodilator FEV₁ increases by 100 mL (6%).

Question: Which spirometric pattern is present?

Answer: Obstructive. The reduced FEV₁/FVC ratio defines obstruction. The reduced FVC is most consistent with air-trapping rather than true restriction. The bronchodilator response is below the threshold for significance but does not exclude asthma or COPD.

For practice questions of this style, use the Crack Medicine MRCP QBank:https://crackmedicine.com/qbank/


Common pitfalls (very high yield)

  • Diagnosing restriction without confirming TLC

  • Ignoring lower limit of normal (LLN) in older adults

  • Assuming absent reversibility excludes asthma

  • Forgetting that obstruction can reduce FVC (pseudo-restriction)

  • Over-interpreting poor effort or suboptimal technique


Practical study checklist

  • Always assess FEV₁/FVC first

  • Decide the pattern before naming a disease

  • Memorise bronchodilator thresholds precisely

  • Ask whether lung volumes are needed

  • Practise under time pressure with mixed respiratory blocks

Supplement revision with focused explanations from Crack Medicine MRCP lectures:https://crackmedicine.com/lectures/


FAQs

What FEV₁/FVC ratio defines obstruction in MRCP exams?

Typically <0.70 or below the lower limit of normal. Use LLN when provided, especially in older patients.

Can obstructive lung disease reduce FVC?

Yes. Air-trapping can lower FVC, producing pseudo-restriction.

Does a normal bronchodilator response rule out asthma?

No. Asthma is variable, and reversibility may be absent between attacks.

Is spirometry alone enough to diagnose restrictive lung disease?

No. Spirometry suggests restriction, but reduced TLC is required for confirmation.


Ready to start?

Ready to convert this topic into guaranteed marks?Test your understanding immediately with exam-standard MRCP Part 1 questions and identify weak areas before the real exam.

👉 Practise now with the Crack Medicine MRCP QBank:https://crackmedicine.com/qbank/

For full exam simulation and timing practice, attempt a realistic MRCP Part 1 mock test:https://crackmedicine.com/mock-tests/


Sources

 
 
 

Comments

bottom of page