Interpret spirometry, lung volumes, DLCO, and flow-volume loops with confidence.
80 min estimated
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.
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 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.
Clinical Tip
Body plethysmography gives higher FRC than helium dilution in COPD — it measures ALL intrathoracic gas including trapped, non-communicating air.
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)
Clinical Tip
Low FEV1/FVC + low DLCO = emphysema. Low FEV1/FVC + normal DLCO = asthma or chronic bronchitis (airways disease without alveolar destruction).
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.
Cardiopulmonary Diagnostics
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Oxygen Therapy
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Pulmonary Disease Processes
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