Pulm questions love to hand you spirometry numbers and expect you to decide—fast—whether the problem is obstructive or restrictive. The quickest discriminator is the FEV1/FVC ratio. If you can interpret that ratio in 5–10 seconds, you’ll pick up a ton of “free” points on USMLE Step 1/2.
The 10-second definition (what the ratio means)
- FEV1 = volume exhaled in the first second of a forced exhalation
- FVC = total volume exhaled during a forced exhalation
- FEV1/FVC tells you: “How much of the total air can I get out quickly?”
One-liner:
- Obstruction = low ratio because you can’t get air out fast (FEV1 falls more than FVC).
- Restriction = normal/high ratio because both FEV1 and FVC fall together (small lungs, but flow is relatively preserved).
The one-page cheat sheet: interpret like a machine
Step 1: Look at the ratio
| Pattern | FEV1/FVC | Core idea | Most likely |
|---|---|---|---|
| Obstructive | Decreased (often < 0.70) | Can’t blow out fast | COPD, asthma, bronchiectasis, CF |
| Restrictive | Normal or increased | Total volume is reduced | ILD, obesity, neuromuscular, chest wall |
USMLE shortcut: If the stem says “wheezing, prolonged expiration, barrel chest,” expect a low FEV1/FVC.
Step 2: Use FVC to avoid traps
- Obstructive + low FVC can happen due to air trapping (“pseudo-restriction”).
- Confirm with TLC:
- TLC increased = obstruction with air trapping
- TLC decreased = true restriction
- Confirm with TLC:
Visual mnemonic: “The hallway test”
Imagine FVC is the size of the room, and FEV1 is how fast you can run into the hallway in 1 second.
-
Obstructive disease = narrow hallway
- You’re stuck at the doorway → FEV1 drops a lot
- Room size may be normal-ish → FVC falls less
- Ratio goes down
-
Restrictive disease = small room
- Room is tiny → FVC drops
- You can still get into the hallway quickly (relative to room size) → FEV1 drops proportionally
- Ratio stays normal or rises
Mnemonic phrase:
“Narrow hallway → low ratio; small room → normal/high ratio.”
High-yield numbers (don’t overthink—just recognize)
- Obstructive hallmark: FEV1/FVC < 0.70 (classic threshold used in COPD criteria; on exams it’s a helpful anchor)
- Restrictive hallmark: FEV1/FVC ≥ normal (often ≥ 0.80 in question stems), with low TLC
On real PFTs, “normal” depends on age/sex/height (LLN), but USMLE-style vignettes usually make it obvious.
What changes first in obstruction vs restriction?
Obstructive (asthma/COPD)
- FEV1 ↓↓↓
- FVC ↓ or normal
- FEV1/FVC ↓
- TLC ↑ (air trapping/hyperinflation), RV ↑
- Flow-volume loop: scooped-out expiratory limb
Restrictive (pulmonary fibrosis, ARDS, obesity hypoventilation, scoliosis)
- FEV1 ↓
- FVC ↓↓
- FEV1/FVC normal or ↑
- TLC ↓, RV ↓
- Flow-volume loop: small, “shrunken” loop (shape preserved)
Classic USMLE interpretation table (keep this in your head)
| Disease | Pattern | FEV1 | FVC | FEV1/FVC | TLC | DLCO |
|---|---|---|---|---|---|---|
| Asthma | Obstructive | ↓ | N/↓ | ↓ | N/↑ | Normal (or ↑) |
| Emphysema (COPD) | Obstructive | ↓ | N/↓ | ↓ | ↑ | ↓ |
| Chronic bronchitis (COPD) | Obstructive | ↓ | N/↓ | ↓ | N/↑ | Normal |
| Pulmonary fibrosis (ILD) | Restrictive | ↓ | ↓ | N/↑ | ↓ | ↓ |
| Neuromuscular/chest wall (e.g., ALS, kyphoscoliosis) | Restrictive | ↓ | ↓ | N/↑ | ↓ | Normal |
DLCO pearl (high-yield):
- Low DLCO = problem at the alveolar-capillary membrane or loss of surface area (e.g., emphysema, pulmonary fibrosis)
- Normal DLCO in restriction suggests extrapulmonary restriction (obesity, neuromuscular weakness, chest wall)
Quick-hit: bronchodilator response (Step 2 favorite)
If PFTs show obstruction, the next question is often: reversible or not?
- Positive bronchodilator response (supports asthma):
- and after bronchodilator
COPD can have some reversibility, but asthma is the classic “markedly reversible” pattern.
Common exam pitfalls (aka how they try to trick you)
- “Low FEV1 and low FVC” does NOT automatically mean restriction.
Look at the ratio and confirm with TLC when needed. - Mixed disease exists (e.g., COPD + pulmonary fibrosis):
- Ratio may be low/normal, TLC may be low/normal—questions usually give you DLCO/TLC clues.
- Severe obstruction can drop FVC due to air trapping → “pseudo-restriction.”
TLC settles the argument.
Rapid-fire practice (1-liners)
- FEV1/FVC = 0.58 → Obstructive (think asthma/COPD).
- FEV1/FVC = 0.85 with low FVC → Restrictive until proven otherwise (check TLC).
- Obstructive + low DLCO → Emphysema (loss of alveolar surface area).
- Restrictive + normal DLCO → Extrapulmonary restriction (obesity/neuromuscular/chest wall).
The “shareable” cheat line (if you remember nothing else)
FEV1/FVC low = obstruction (can’t exhale fast).
FEV1/FVC normal/high = restriction (small lungs, proportionate drop).