📉 Respiratory System Lesson

"Can't get air OUT vs Can't get air IN": lung disease categories

Two broad categories of lung disease produce opposite spirometry patterns — and a single ratio is the key diagnostic test that separates them.

Obstr.

FEV1/FVC low

Restr.

TLC low

📖 Full Breakdown

Two disease categories, opposite mechanisms, and one decisive ratio

The FEV1/FVC ratio is the single most important number for distinguishing these two disease categories.

Obstructive diseases

Airways narrow — air gets trapped

Examples include asthma, COPD, emphysema, and chronic bronchitis. The defining lab finding: FEV1/FVC ratio is LOW (below 0.7) — patients cannot exhale fast enough because narrowed airways slow the outward airflow.

Restrictive diseases

Lungs or chest wall can't expand fully

Examples include pulmonary fibrosis, sarcoidosis, pneumonia, and obesity. Total Lung Capacity is LOW, since the lungs simply cannot hold as much air as normal.

FEV1/FVC ratio — the key distinguishing test

Normal or HIGH in restrictive disease

Even though restrictive patients have a lower overall lung volume, they can still exhale a NORMAL PROPORTION of what they inhaled relatively quickly — the ratio itself stays normal or even rises, unlike in obstructive disease where the ratio specifically drops.

🩺 Clinical / Exam Application

A patient with pulmonary fibrosis (a restrictive disease) has significantly reduced Total Lung Capacity, but their FEV1/FVC ratio comes back normal, initially puzzling a student expecting an abnormal ratio to match the abnormal TLC. This makes sense once you understand that restrictive diseases shrink the lungs' overall CAPACITY without narrowing the airways themselves — so whatever air the patient does inhale, they can still exhale a normal proportion of it quickly. It's specifically airway narrowing (as in obstructive disease) that drops the FEV1/FVC ratio, not reduced lung capacity alone.

⚠️ Exam Alert

The FEV1/FVC ratio threshold of 0.7 is one of the single most frequently tested numeric cutoffs in respiratory medicine — below 0.7 defines obstructive disease, while a normal or elevated ratio despite reduced lung volumes points toward restrictive disease instead.

🚧 Common Trap

Don't assume "reduced lung function" always shows the same spirometry pattern. Obstructive and restrictive diseases can BOTH reduce a patient's overall breathing capacity, but they produce opposite effects on the FEV1/FVC ratio specifically — the ratio, not just overall severity, is what distinguishes the two categories.

✅ Quick Check

A patient has reduced Total Lung Capacity but a normal FEV1/FVC ratio. Does this pattern point toward obstructive or restrictive disease, and why?

📝 Exam Prep

Common Exam Questions

❓ What is the key difference between obstructive and restrictive lung disease?

✅ Obstructive disease narrows the airways, trapping air and producing a LOW FEV1/FVC ratio (below 0.7) — examples include asthma and COPD. Restrictive disease reduces Total Lung Capacity due to stiff lungs or chest wall, but the FEV1/FVC ratio remains normal or high — examples include pulmonary fibrosis.

❓ What does an FEV1/FVC ratio below 0.7 indicate?

✅ It indicates obstructive lung disease — the patient cannot exhale air fast enough because their airways are narrowed, as seen in asthma, COPD, and emphysema.