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Summary Table

Element	Detail
Purpose	Objective measurement of lung function to classify ventilatory defects
Core Values	FEV1, FVC, FEV1/FVC ratio, with extended values including TLC, RV, DLCO
Two Fundamental Patterns	Obstructive (airflow limitation) vs. Restrictive (volume limitation)
The Single Most Important Number	FEV1/FVC ratio determines the initial branch point of interpretation
Normal FEV1/FVC	$\geq 0.70$ (or above the lower limit of normal based on age)
Bronchodilator Response	Positive if FEV1 improves by $\geq 200\ \text{mL}$ and $\geq 12%$ from baseline
Keywords in Vignettes	"Decreased breath sounds," "prolonged expiration," "barrel chest," "decreased chest expansion," "flow-volume loop," "pulmonary function testing"

1. Foundational Concepts

Spirometry measures the volume and flow of air during a forced expiratory maneuver. The patient inhales maximally to total lung capacity (TLC), then exhales as hard and as fast as possible until no more air can be expelled, reaching residual volume (RV).

FVC (Forced Vital Capacity) is the total volume of air expelled during this forced exhalation. It reflects the overall capacity of the lungs to hold air. When FVC is reduced, it suggests that lung volumes are small (restriction) or that air is trapped behind closed airways (severe obstruction with air trapping).
FEV1 (Forced Expiratory Volume in 1 Second) is the volume of air expelled in the first second. Because healthy airways allow most air to exit rapidly, a normal person exhales roughly 70 to 80 percent of their FVC in the first second. FEV1 is reduced in both obstructive and restrictive disease, so it cannot distinguish between them on its own.
FEV1/FVC Ratio is the critical branch point. This ratio tells you whether the problem is airflow limitation (obstructive) or volume limitation (restrictive). A low ratio ($< 0.70$) means that the airways are narrowed or collapsing, slowing expiratory flow disproportionately. A normal or elevated ratio with a low FVC points toward restriction, where lung volumes are globally reduced but the airways themselves are functioning.

Why this matters on the exam: Nearly every spirometry question begins by testing whether you can correctly identify the pattern as obstructive, restrictive, or mixed. If you misclassify at this step, every subsequent answer choice becomes wrong.

2. Interpretation Algorithm

Step	Question	If Yes	If No
1	Is FEV1/FVC $< 0.70$?	Obstructive pattern	Go to Step 2
2	Is FVC $< 80%$% predicted?	Possible restrictive pattern (confirm with lung volumes)	Normal spirometry
3 (Obstructive)	Does FEV1 improve $\geq 12%$ and $\geq 200\ \text{mL}$ post-bronchodilator?	Reversible obstruction (asthma)	Fixed or poorly reversible obstruction (COPD, remodeled asthma)
4 (Restrictive)	Is TLC 80% predicted?	Confirmed restriction	Consider alternate explanations (submaximal effort, obesity, neuromuscular weakness)

Severity Grading of Obstruction

Severity	FEV1 % Predicted
Mild	$\geq 70%$
Moderate	$60\text{--}69%$
Moderately Severe	$50\text{--}59%$
Severe	$35\text{--}49%$
Very Severe	$< 35%$

Once you have established that the pattern is obstructive (low FEV1/FVC), the severity of obstruction is graded by FEV1 percent predicted alone, not by the ratio.

3. Obstructive vs. Restrictive

Parameter	Obstructive	Restrictive
FEV1	Decreased	Decreased
FVC	Normal or decreased	Decreased
FEV1/FVC	Decreased ($< 0.70$)	Normal or increased ($\geq 0.70$)
TLC	Normal or increased (hyperinflation)	Decreased
RV	Increased (air trapping)	Decreased
DLCO	Decreased in emphysema; normal in chronic bronchitis and asthma	Decreased in parenchymal disease; normal in chest wall/neuromuscular causes
Flow-Volume Loop	Scooped-out (concave) expiratory limb	Narrow, tall loop (reduced width, preserved shape)
Classic Diseases	Asthma, COPD, bronchiectasis	Pulmonary fibrosis, sarcoidosis, kyphoscoliosis, obesity, neuromuscular disease

In obstructive disease, the airways are narrowed (bronchospasm, mucus, inflammation, loss of elastic recoil). Air takes much longer to exit, so FEV1 drops disproportionately compared to FVC. The ratio falls below 0.70. In severe obstruction, air becomes trapped (RV increases), and total lung capacity may even increase because the lungs are hyperinflated.

In restrictive disease, the lungs either cannot expand fully (fibrosis stiffens the parenchyma, chest wall deformity limits expansion, or weak respiratory muscles cannot generate sufficient inspiratory force). Both FEV1 and FVC fall together in proportion, keeping the ratio normal or even pushing it above normal. TLC is the confirmatory value: if TLC is reduced, restriction is confirmed.

4. Diffusing Capacity for Carbon Monoxide (DLCO)

DLCO measures how efficiently gas transfers from the alveoli into the pulmonary capillary blood. It is not part of basic spirometry but is almost always paired with it on exam questions that require fine discrimination.

DLCO Status	Conditions
Decreased	Emphysema (alveolar destruction), interstitial lung disease/pulmonary fibrosis (thickened membrane), pulmonary vascular disease, anemia
Normal	Chronic bronchitis, asthma, chest wall restriction, neuromuscular disease, obesity
Increased	Pulmonary hemorrhage (Goodpasture syndrome), polycythemia, left-to-right cardiac shunt, early congestive heart failure, exercise, asthma (sometimes)

A vignette describing a smoker with obstructive spirometry and a low DLCO is pointing you toward emphysema. The same smoker with obstructive spirometry and a normal DLCO is pointing toward chronic bronchitis. This is one of the highest-yield discriminating tools in pulmonary questions.

For restrictive patterns, a low DLCO suggests the problem is in the lung parenchyma (fibrosis, sarcoidosis). A normal DLCO with restrictive spirometry suggests the problem is outside the lungs: chest wall deformity, obesity, or neuromuscular weakness.

5. Flow-Volume Loops

Pattern	Expiratory Limb	Inspiratory Limb	Diagnosis
Normal	Rapid rise to peak flow, then linear decline	Smooth, symmetric curve	No ventilatory defect
Obstructive	Scooped-out, concave expiratory curve with reduced peak flow	Relatively preserved	Asthma, COPD
Restrictive	Preserved shape but narrowed (reduced volume on x-axis), may show increased peak flow relative to volume	Proportionally narrowed	Pulmonary fibrosis, chest wall disease
Fixed upper airway obstruction	Flattened (plateau)	Flattened (plateau)	Tracheal stenosis, goiter compressing trachea (both intra- and extrathoracic)
Variable extrathoracic obstruction	Normal	Flattened (plateau)	Vocal cord paralysis, tracheomalacia above thoracic inlet
Variable intrathoracic obstruction	Flattened (plateau)	Normal	Intrathoracic tracheal tumor

This is a conceptually challenging but testable topic. During forced expiration, intrathoracic pressure rises and compresses intrathoracic airways. During forced inspiration, negative intrathoracic pressure opens intrathoracic airways but creates a pressure gradient that tends to collapse extrathoracic structures.

A variable extrathoracic lesion (e.g., unilateral vocal cord paralysis) worsens during inspiration because negative pressure sucks the floppy cord inward. Expiration pushes air outward and stents the extrathoracic airway open. This gives a flattened inspiratory limb with a normal expiratory limb.
A variable intrathoracic lesion (e.g., tracheal tumor below the thoracic inlet) worsens during expiration because rising intrathoracic pressure compresses the airway around the lesion. During inspiration, negative pressure opens the airway. This gives a flattened expiratory limb with a normal inspiratory limb.
A fixed lesion (e.g., circumferential tracheal stenosis) does not change with the respiratory cycle, so both limbs are flattened.

6. Bronchodilator Reversibility Testing

Parameter	Criterion for Positive Response
FEV1	Increase $\geq 200\ \text{mL}$ AND $\geq 12%$ from pre-bronchodilator baseline
FVC	Same thresholds can be applied but are less commonly tested

A positive bronchodilator response strongly favors asthma over COPD. However, exam writers will test your understanding of the nuances:

Partial reversibility can occur in COPD. Some COPD patients show mild improvement post-bronchodilator, but the FEV1/FVC ratio typically remains below 0.70. In asthma with airway remodeling (long-standing, poorly controlled disease), reversibility may be incomplete, mimicking COPD. The clinical history (age of onset, smoking status, atopy, variability of symptoms) must be used alongside spirometry to distinguish them.

Methacholine challenge testing is used when spirometry is normal at baseline but asthma is clinically suspected (episodic wheeze, cough-variant asthma). A positive methacholine challenge is defined as a provocative concentration causing a 20% fall in FEV1 ($PC_{20}$). This test has high sensitivity (a negative test effectively rules out asthma) but lower specificity.

7. Disease-Pattern Correlation

Disease	FEV1/FVC	FEV1	FVC	TLC	RV	DLCO	Distinguishing Feature
Asthma	Low	Low	Normal or low	Normal or high	High	Normal or slightly high	Reversible with bronchodilator
COPD (Emphysema)	Low	Low	Normal or low	High	High	Low	Irreversible; destroyed alveoli reduce gas exchange
COPD (Chronic Bronchitis)	Low	Low	Normal or low	Normal or high	High	Normal	Clinical diagnosis: productive cough $\geq 3$ months/year for $\geq 2$ consecutive years
Pulmonary Fibrosis (IPF)	Normal or high	Low	Low	Low	Low	Low	Restrictive with impaired gas exchange
Sarcoidosis	Variable	Low	Low	Low	Low	Low	Can show obstructive, restrictive, or mixed pattern depending on stage
Obesity	Normal or high	Normal or low	Low	Normal or low	Low (reduced ERV)	Normal	ERV is characteristically reduced; TLC may be normal
Neuromuscular Disease (e.g., ALS, Guillain-Barre)	Normal or high	Low	Low	Low	Variable	Normal	Reduced MIP/MEP (maximal inspiratory/expiratory pressures); DLCO normal because lung parenchyma is intact
Kyphoscoliosis	Normal or high	Low	Low	Low	Low	Normal	Chest wall deformity limits expansion; lung tissue itself is healthy

8. Mixed Obstructive-Restrictive Pattern

A mixed pattern is present when spirometry shows both a reduced FEV1/FVC ratio ($< 0.70$) and a reduced TLC ($< 80%$ predicted). This combination tells you that the patient has airflow limitation from airway disease and reduced lung volumes from a separate restrictive process.

Classic clinical scenarios that produce a mixed pattern:

A patient with COPD who also has pulmonary fibrosis (combined pulmonary fibrosis and emphysema syndrome)
A patient with asthma and severe obesity
A patient with COPD and prior lobectomy
Sarcoidosis (granulomas cause both airway obstruction and parenchymal restriction)

A severely obstructive patient may have a low FVC due to air trapping, mimicking restriction on basic spirometry. The FEV1/FVC ratio is low and the FVC is low. You might be tempted to call this "mixed." However, you cannot diagnose restriction without measuring TLC. If TLC is normal or increased, this is pure obstruction with air trapping, not a mixed pattern.

9. Traps and High-Yield Pearls

The most common way students lose points on spirometry questions is by jumping to a disease diagnosis before correctly identifying the ventilatory pattern. The exam almost always tests a two-step process: first classify the pattern (obstructive vs. restrictive vs. mixed vs. normal), then correlate with the clinical scenario. Students who skip step one and guess a disease from keywords in the stem frequently fall into traps.

A second frequent error involves confusing air trapping with restriction. When a vignette shows low FEV1/FVC and low FVC without reporting TLC, students often select "mixed pattern." The correct answer in this situation is to recognize obstruction with probable air trapping and to state that lung volumes (specifically TLC) are needed before restriction can be confirmed.

Third, students forget that a normal spirometry result does not rule out asthma. If the clinical picture is suggestive (episodic cough, nocturnal wheeze, exercise-induced dyspnea in a young atopic patient), the next step is a bronchoprovocation test, not reassurance.

Fourth, the DLCO question is a perennial discriminator. Examiners use it to separate students who memorized "obstructive = low FEV1/FVC" from those who truly understand the pathology. Know that emphysema lowers DLCO (destroyed alveolar-capillary surface), chronic bronchitis preserves DLCO (intact alveoli, problem is in the airways), and asthma typically preserves or slightly elevates DLCO.

Finally, flow-volume loops testing upper airway obstruction is a topic that appears straightforward in review but causes errors under time pressure. The core competency being tested is whether you understand how transmural pressure gradients affect floppy lesions during inspiration vs. expiration. If you can reason through the physics, you will never confuse variable extrathoracic with variable intrathoracic obstruction.

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