Pulmonary Function Testing

Interpret spirometry, lung volumes, DLCO, and flow-volume loops with confidence.

intermediate80 min estimated3 lessons

Spirometry and Obstruction vs Restriction

Spirometry measures volumes and flows during a forced breath. Key values: FVC (forced vital capacity), FEV1 (forced expiratory volume in 1 second), and FEV1/FVC ratio.

Obstructive pattern: FEV1/FVC <70% (or below lower limit of normal). FEV1 is reduced more than FVC. Seen in asthma, COPD, bronchiectasis, CF. Severity classified by FEV1% predicted: mild ≥70%, moderate 50–69%, severe 30–49%, very severe <30%.

Restrictive pattern: FVC <80% predicted with normal or elevated FEV1/FVC ratio. Both FEV1 and FVC reduced proportionally. Confirmed with lung volumes showing reduced TLC (<80% predicted). Seen in ILD, obesity, neuromuscular disease, chest wall deformities.

FEV1/FVC <70% = obstruction; FVC <80% with normal ratio = restriction
Obstruction: FEV1 drops more than FVC (ratio falls)
Restriction confirmed by reduced TLC (<80% predicted)
COPD severity by FEV1%: mild ≥70, moderate 50–69, severe 30–49, very severe <30
Post-bronchodilator FEV1 improvement >12% and >200 mL = significant reversibility

Clinical tip

You cannot diagnose restriction by spirometry alone — you need lung volumes (TLC). A low FVC could be obstruction with air trapping, not true restriction.

Lung Volumes and Capacities

Lung volumes partition total lung capacity (TLC) into measurable fractions. TLC cannot be measured by spirometry — requires helium dilution, nitrogen washout, or body plethysmography.

Key volumes: - TV (tidal volume): normal breath, ~500 mL - IRV (inspiratory reserve): air above TV during maximum inspiration - ERV (expiratory reserve): air expelled beyond normal expiration - RV (residual volume): air remaining after maximum expiration (~1200 mL)

Key capacities: - VC = TV + IRV + ERV - FRC = ERV + RV (normal ~2400 mL) - TLC = all volumes - IC = TV + IRV

In obstruction: RV, FRC, and TLC are increased (air trapping). In restriction: all volumes are reduced. FRC/TLC ratio >50% suggests hyperinflation.

RV cannot be measured by spirometry — requires dilution or plethysmography
Increased RV + FRC = air trapping (COPD, asthma)
Reduced TLC (<80%) = restrictive disease
FRC is the resting lung volume after normal passive expiration
Body plethysmography more accurate than gas dilution (measures trapped gas)

Clinical tip

Body plethysmography gives higher FRC than helium dilution in COPD — it measures ALL intrathoracic gas including trapped, non-communicating air.

DLCO and Flow-Volume Loops

DLCO (diffusing capacity of the lung for carbon monoxide) measures gas transfer efficiency across the alveolar-capillary membrane.

Reduced DLCO: emphysema, ILD, pulmonary hypertension, anemia (fewer hemoglobin binding sites), pulmonary embolism. Normal/elevated DLCO: asthma, polycythemia, pulmonary hemorrhage (extra blood in alveoli to bind CO), early CHF.

Flow-volume loops graphically display flows during forced inspiration and expiration. Patterns: - Concave expiratory limb (scooped out) = obstruction - Small symmetric loop = restriction - Flat inspiratory limb = variable extrathoracic obstruction (vocal cord paralysis) - Flat expiratory limb = variable intrathoracic obstruction (tracheomalacia) - Flat both limbs = fixed obstruction (tracheal stenosis)

DLCO reduced: emphysema, ILD, PE, anemia, pulmonary hypertension
DLCO elevated: polycythemia, pulmonary hemorrhage, left-to-right shunt
Concave expiratory limb = obstructive pattern
Flat inspiratory limb = variable extrathoracic obstruction
Flat expiratory limb = variable intrathoracic obstruction

Clinical tip

Low FEV1/FVC + low DLCO = emphysema. Low FEV1/FVC + normal DLCO = asthma or chronic bronchitis (airways disease without alveolar destruction).

Clinical cases

Interpreting a Spirometry Report

FEV1 58% predicted, FVC 82% predicted, FEV1/FVC 62%, no significant BD response. DLCO 45% predicted.

What pattern is present and what is the most likely diagnosis?

Obstructive pattern (FEV1/FVC <70%) with severely reduced DLCO. Most likely: emphysema.

Obstruction confirmed by low FEV1/FVC. No BD response suggests fixed obstruction (COPD, not asthma). Severely reduced DLCO indicates alveolar destruction — emphysema destroys alveolar-capillary units. Chronic bronchitis would show obstruction with normal DLCO.

Know this for NBRC

FEV1/FVC <70% = obstruction (GOLD); FVC <80% with normal ratio = restriction
Restriction needs TLC <80% for confirmation
DLCO differentials: reduced vs normal/elevated
Flow-volume loop shapes and what they mean
Significant BD response: >12% AND >200 mL improvement in FEV1

Key facts

FEV1/FVC <70% (post-BD) = obstruction per GOLD criteria
FVC <80% predicted with normal ratio = restrictive (confirm with TLC)
TLC <80% predicted = confirmed restriction
Significant bronchodilator response: FEV1 ↑ >12% AND >200 mL
DLCO reduced in emphysema (alveolar destruction) but normal in chronic bronchitis
FRC measured by helium dilution, N2 washout, or body plethysmography
Body plethysmography: most accurate method for FRC in COPD
Flat inspiratory loop = variable extrathoracic obstruction (VCD, vocal cord paralysis)
RV/TLC ratio >35% suggests significant air trapping/hyperinflation
MVV (maximum voluntary ventilation) reflects overall ventilatory capacity

NBRC pearls

NBRC tests: reduced DLCO differentials — know emphysema vs ILD vs PE
Post-BD reversibility >12% + >200 mL = asthma (not COPD which has fixed obstruction)
Body plethysmography overestimates FRC vs helium dilution in trapped gas
TLC required to diagnose restriction — FVC alone insufficient

Memory tricks

Reduced DLCO: 'EIPAH' — Emphysema, ILD, PE, Anemia, Hypertension (pulmonary)
Elevated DLCO: 'PAH' gone wrong — Polycythemia, Asthma, Hemorrhage

Common mistakes

Diagnosing restriction from spirometry alone without lung volumes
Missing that a low FVC in COPD can be from air trapping, not restriction
Forgetting that DLCO is reduced in anemia (less hemoglobin, not a lung problem)
Confusing FVC with VC (FVC is forced, VC is slow/relaxed — in obstruction, VC > FVC)

Pulmonary Function Testing

Spirometry and Obstruction vs Restriction

Lung Volumes and Capacities

DLCO and Flow-Volume Loops

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics

Pulmonary Function Testing

Spirometry and Obstruction vs Restriction

Lung Volumes and Capacities

DLCO and Flow-Volume Loops

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics

Lessons

Spirometry and Obstruction vs Restriction

Lung Volumes and Capacities

DLCO and Flow-Volume Loops

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics

Lessons

Spirometry and Obstruction vs Restriction

Lung Volumes and Capacities

DLCO and Flow-Volume Loops

Clinical cases

NBRC pearls

Memory tricks

Common mistakes

Related topics