Pulmonary function testing

Last updated: April 29, 2022

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Pulmonary function tests (PFTs) measure different lung volumes and other functional metrics of pulmonary function. They can be used to diagnose ventilatory disorders and differentiate between obstructive and restrictive lung diseases. The most common PFT is spirometry, which involves a cooperative patient breathing actively through his or her mouth into an external device. This simple and cost-effective test measures both dynamic and static lung volumes (with the exception of residual volume and total lung capacity), as well as airflow rates. Full-body plethysmography is an additional PFT that is able to estimate both residual volume and total lung capacity and is performed with the patient in a closed space. Lastly, single-breath diffusing capacity helps determine if the alveolar membrane is thickened (e.g., pulmonary fibrosis) or destroyed (e.g., emphysema), or if the pulmonary vasculature is affected (e.g., pulmonary hypertension).

Spirometry measures breath volume and airflow rates and is routinely performed with the help of a spirometer.

Diagnostically relevant spirometric values
Parameter Definition Normal finding [1][2][3]
Peak expiratory flow (PEF)
  • The maximum airflow rate attained during forced expiration (in L/s)
  • ≥ 80% of the predicted average value based on race, height, gender, and age

Vital capacity

  • The maximum volume of air that can be expired after maximal inspiration
  • VC can be measured through:
    • Slow respiratory maneuvers
      • Inspiratory vital capacity (IVC): The maximum volume of air that can be inspired after maximal expiration.
      • Expiratory vital capacity (EVC): The maximum volume of air that can be expired after maximal inspiration.
    • Forced respiratory maneuvers
      • Forced vital capacity (FVC): The maximum volume of air that can be forcefully expired after maximal inspiration.
  • Among young individuals, IVC, EVC, and FVC have nearly the same value. However among patients with obstructive lung disease: IVC > EVC > FVC. [4][5]
  • Depends on race, height, age, and gender; approximately 4.5–5 L in healthy young adults
Forced expiratory volume in 1 second (FEV1)
  • The maximum volume of air that can be forcefully expired (forced expiratory volume) within 1 second after maximal inspiration
  • ≥ 80% of the predicted average value based on height, gender, and age (or > 75% of vital capacity)

FEV1/FVC (Tiffeneau-Pinelli index, relative FEV1)

  • ≥ 70%
Forced expiratory flow rate at 75%, 50%, and 25% of vital capacity (FEF75%, FEF50%, FEF25%)
  • Average airflow rates observed during forced expiration when 75% (FEF25%), 50% (FEF50%), and 25% (FEF75%) of the vital capacity remains in the lungs.
  • ≥ 65% of the predicted average value based on race, height, gender, and age [6]

Ergospirometry is used to objectively measure cardiopulmonary performance when a patient is subjected to stress. Oxygen utilization (VO2 max), the rate of carbon dioxide production, and minute ventilation are measured in addition to lung volumes.

Body plethysmography is performed to detect functional limitations during respiration.

  • Measured parameters
  • Indications [7]
    • To obtain objective spirometric readings in uncooperative/unconscious patients
    • Clinical suspicion of emphysema

Obstructive vs restrictive lung diseases [1][8]
Type Obstructive lung disease Restrictive lung disease
  • Increased resistance to airflow caused by narrowing of airways
Spirometric findings FEV1
  • Normal or ↓
  • Normal or ↑ (↓ in FEV1 is proportional to ↓ in FVC)
Spirometer tracing
  • Air trapping: a "scalloping" of the expiratory limb in conditions such as emphysema or in patients who have undergone a pneumectomy.
Plethysmograph findings RV
  • Normal or
  • Normal or
Airway resistance
  • Normal
Lung compliance
  • Normal
  • Normal (extrinsic causes) or ↓ (intrinsic causes)
  • [10]
  • Normal (extrinsic causes) or ↓ (intrinsic causes)

A-a gradient

  • Normal (extrinsic causes) or ↑ (intrinsic causes)
Imaging findings

Bronchial challenge tests help distinguish bronchial asthma from other causes of obstructive lung disease.

Bronchial challenge tests
Methacholine challenge test (bronchoprovocation test) [11] Bronchodilator reversibility test (post-bronchodilator test) [12]
  • Lung function tests are performed before and after the administration of methacholine.
  • To determine if airway hyperresponsiveness is present
  • Allows reversible airway obstruction to be differentiated from irreversible obstruction

Because the methacholine challenge test can trigger a life-threatening asthma attack, medications that reverse bronchospasm (e.g., epinephrine, atropine) should be kept at hand during the test!

Single-breath diffusing capacity measures the ability of the alveoli to exchange gases with pulmonary capillaries.

  • Measured parameters
    • Carbon monoxide transfer coefficient (KCO): the amount of CO per unit time per unit partial pressure that is transferred from the alveolus to the pulmonary capillary
    • Diffusing capacity of the lung for carbon monoxide (DLCO) : the product of KCO and total alveolar volume (VA)
  • Interpretation: See table below.
    • Position and/or certain maneuvers also affect results.
    • Exercise will increase DLCO through recruitment of additional lung zones and increased perfusion with reduced capillary transit time. [14]
Spirometric findings
Restrictive lung disease (normal or FEV1/FVC) Obstructive lung disease (FEV1/FVC < 70%) Normal


Normal DLCO
  • Healthy findings

Patients with respiratory muscle weakness show spirometric findings of restrictive lung disease.

Clinical features of respiratory muscle weakness do not manifest until diaphragmatic strength is reduced to a quarter of its normal strength (unilateral diaphragmatic paralysis decreases ventilatory capacity by only 20%).

Lung volumes depend on age, height, and gender. The values that are listed below are for a healthy young adult.

Normal lung volumes
Definition Normal range
Total lung capacity (TLC)
  • Volume of air in the lungs after maximal inhalation
  • 6–6.5 L
Vital capacity (VC)
  • Difference in lung volume between maximal exhalation and maximal inhalation
  • 4.5–5 L
Residual volume (RV)
  • Volume of air that remains in the lungs after maximal exhalation
  • 1–1.5 L
Tidal volume (TV)
  • Volume of air that is inhaled and exhaled in a normal breath at rest
  • ∼ 500 mL or 7 mL/kg
Inspiratory reserve volume (IRV)
  • Maximum volume of air that can still be forcibly inhaled following the inhalation of a normal TV
  • 3–3.5 L
Inspiratory capacity (IC)
  • Maximum volume of air that can be inhaled after the exhalation of a normal TV
  • 3.5–4 L
Expiratory reserve volume (ERV)
  • Maximum volume of air that can still be forcibly exhaled after the exhalation of a normal TV
  • 1.5 L
Expiratory capacity (EC)
  • Maximum volume of air that can be exhaled after the inspiration of a normal TV
  • 2 L
Functional residual capacity (FRC)
  • Volume of air that remains in the lungs after the exhalation of a normal TV
  • 2.5–3 L

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