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Pulmonary Function Tests & Flow-Volume Loops

Name: Pulmonary Function Tests & Flow-Volume Loops
Creator: Veli Bakalov, MD

Read the ratio, the loop, and the DLCO to localize lung disease

Respiratory·Physiology·4 min read·8 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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FEV1/FVC below 0.7 defines obstruction, with ↑RV and often ↑TLC; restriction requires TLC below 80% predicted with a normal or high ratio.
A post-bronchodilator FEV1 rise of at least 12% and 200 mL is a significant response that supports asthma.
Suspected asthma with normal spirometry → methacholine challenge; a ≥20% FEV1 fall at a low dose shows hyperresponsiveness, and a negative test makes asthma unlikely.
Obstructive loop: scooped expiratory limb at high lung volumes; restrictive loop: small, narrow and tall.
Fixed upper-airway obstruction (tracheal stenosis, goiter) flattens both limbs of the flow-volume loop.
Variable extrathoracic lesions (vocal cord paralysis or dysfunction) flatten inspiration; variable intrathoracic lesions (tracheomalacia, lower tracheal tumor) flatten expiration.
DLCO is decreased in emphysema, ILD, pulmonary vascular disease and anemia; normal in asthma, chronic bronchitis and extraparenchymal restriction (obesity, kyphoscoliosis, neuromuscular disease with low MIP/MEP).
DLCO is increased in alveolar hemorrhage, polycythemia and left-to-right shunts.

Spirometry: obstruction versus restriction

Obstruction. In asthma, COPD, bronchiectasis and CF, narrowed or collapsing airways limit expiratory flow, so FEV1 falls more than FVC and the FEV1/FVC ratio drops below 0.70 (or below the lower limit of normal for age). Gas is trapped behind closing airways, raising residual volume and the RV/TLC ratio; loss of recoil in emphysema also raises TLC (hyperinflation).

Restriction. In interstitial lung disease, chest wall deformity, obesity and neuromuscular weakness, FEV1 and FVC fall together, so the ratio is normal or even high (fibrotic lungs recoil strongly and hold airways open). A low FVC only suggests restriction; the diagnosis requires a TLC below 80% of predicted measured by body plethysmography or gas dilution.

Typical PFT patterns

Measure	Obstructive	Restrictive: parenchymal (ILD)	Restrictive: extraparenchymal
FEV1/FVC	<0.7	Normal or ↑	Normal or ↑
TLC	Normal or ↑	↓	↓
RV	↑	↓	Variable; may be preserved with expiratory muscle weakness
DLCO	↓ in emphysema; normal in asthma and chronic bronchitis	↓	Normal

Obesity and neuromuscular disease. Obesity characteristically lowers expiratory reserve volume and FRC first. Neuromuscular weakness (myasthenia gravis, ALS, Guillain-Barré syndrome) lowers the maximal inspiratory and expiratory pressures (MIP and MEP); serial vital capacity and MIP track respiratory muscle strength and the need for ventilatory support.

Bronchodilator response and bronchoprovocation

Reversibility. Spirometry is repeated after an inhaled short-acting β2 agonist. An increase in FEV1 (or FVC) of at least 12% and at least 200 mL from baseline is a significant bronchodilator response and supports asthma. Both criteria must be met: a small patient gaining 150 mL (15%) or a large one gaining 250 mL (8%) does not qualify. COPD is defined by a post-bronchodilator FEV1/FVC that remains below 0.7, although some reversibility can occur.

Methacholine challenge. When symptoms suggest asthma but spirometry is normal, the patient inhales rising doses of methacholine, a muscarinic agonist. A fall in FEV1 of at least 20% at a low concentration (the PC20) demonstrates airway hyperresponsiveness. The test is highly sensitive, so a negative result makes asthma unlikely; a positive result is less specific and also occurs in COPD and allergic rhinitis.

Flow-volume loops

Reading the loop. By convention expiration is plotted above the axis and inspiration below. A normal expiratory limb rises quickly to peak flow and then declines almost linearly; the inspiratory limb is a smooth, symmetric curve.

Flow-volume loop patterns

Pattern	Loop shape	Why	Examples
Obstructive	Reduced peak; concave ("scooped") expiratory limb; loop shifted to higher lung volumes	Dynamic compression of narrowed small airways; air trapping	Asthma, COPD
Restrictive	Small, narrow, tall loop at low lung volumes	Low volumes but high recoil keeps flow high for the volume	ILD, chest wall disease
Fixed upper-airway	Both limbs flattened (box-like)	Rigid narrowing unaffected by transmural pressure	Post-intubation tracheal stenosis, goiter or tumor encasing the trachea
Variable extrathoracic	Inspiratory limb flattened	Intraluminal pressure below atmospheric during forced inspiration pulls the lesion inward	Vocal cord paralysis or dysfunction, laryngeal tumor, extrathoracic tracheomalacia
Variable intrathoracic	Expiratory limb flattened	Positive pleural pressure during forced expiration compresses the segment	Intrathoracic tracheomalacia, tumor of the lower trachea or a mainstem bronchus

A variable lesion collapses in whichever phase puts higher pressure outside the airway than inside: inspiration for lesions outside the chest, expiration for lesions inside it. A fixed lesion ignores the phase and caps flow both ways.

Diffusing capacity (DLCO)

What it measures. DLCO is the uptake of a small inhaled dose of carbon monoxide per unit of driving pressure. It depends on alveolar–capillary surface area, membrane thickness and the volume of hemoglobin in the pulmonary capillaries, so it falls with anemia and should be corrected for hemoglobin.

Interpreting DLCO

DLCO	Conditions	Reason
Decreased	Emphysema; interstitial lung disease; pulmonary embolism and pulmonary hypertension; anemia	Lost alveolar walls, thickened interstitium, lost capillary bed, or less hemoglobin to bind CO
Normal	Asthma, chronic bronchitis; obesity, kyphoscoliosis, pleural disease, neuromuscular weakness	Airway disease or restriction from outside the lung leaves the membrane intact
Increased	Diffuse alveolar hemorrhage (anti-GBM disease, GPA, SLE); polycythemia; left-to-right shunt; milder rises with exercise, supine posture, obesity, asthma	More hemoglobin available to bind CO in capillaries or air spaces

Using DLCO to localize disease. In an obstructed smoker, a low DLCO points to emphysema, whereas a normal DLCO fits chronic bronchitis or asthma. In restriction, a low DLCO places the problem in the parenchyma (ILD), while a normal DLCO points outside the lung to the chest wall, muscles or obesity. A rising DLCO in a patient with hemoptysis suggests fresh alveolar hemorrhage.