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Doing Spirometry

Image: Doing a spirometry, by Jmarchn. License: CC BY-SA 3.0

Definition of PFT

What is PFT?

PFT (pulmonary function tests) can measure lung mechanics (spirometry and lung volumes) or the gas exchange/diffusion function of the respiratory system.

Categories of PFT

There are three categories of tests:

Spirometry: FVC and FEV1

  • Measures volume of exhaled air as a function of time
  • Restrictive lung disease has a reduced volume, i.e., a low FVC
  • Obstructive lung disease has a reduced flow, i.e., a low FEV1

Lung volumes: TLC, RV

  • The addition of two or more volumes comprises a capacity
  • Nitrogen washout and helium dilution technique
  • Body plethysmography is the standard for measuring lung volumes
Body box with male subject enclosed

Image: Body box with male subject enclosed, by Finchbook01. License: CC BY-SA 3.0

Gas Exchange: DLCO, AaDO2

  • Ability to transport gas from the alveoli to the blood
  • Estimates the surface area available for gas exchange
  • Depends on hemoglobin (Hb) and cardiac output

Indication of PFT

Pulmonary function tests (PFT) are mainly diagnostic/prognostic.

  1. Diagnostic: screening at-risk patients, such as those over 70 or are obese; evaluating a chronic cough; following up patients with chronic lung disease
  2. Prognostic: evaluating before lung surgery, evaluating response to drugs, and assessing the degree of disability

Contraindications of PFT

Recent eye surgery, abdominal aneurysms, hemoptysis, pneumothorax, and recent myocardial infarction

Lung Volumes and Capacities


Tidal volume (TV or VT): volume of inhaled or exhaled air during regular respiration at rest; the normal value is 500 ml

Inspiratory reserve volume (IRV): the maximum volume of air that can be inhaled above and after a normal tidal inspiration; the normal value is 3,000 ml

Expiratory reserve volume (ERV): the maximum volume of air that can be exhaled after a normal tidal expiration; the normal value is 1,500 ml

Residual volume (RV): the volume of air remaining in the lungs after maximal forced exhalation; cannot be measured directly by spirometry but is indirectly calculated as RV = FRC – ERV; the normal value is 20–25mL/kg or 1,200 mL

Capacities (sum of 2 or more volumes)

Total lung capacity (TLC): the volume of air present in lungs after maximum deep inspiration; the sum of all volumes: TLC = IRV + TV + ERV + RV; the normal value is 5,000–6,000 ml

Vital capacity (VC): the maximum volume of air exhaled after a maximum deep inspiration: VC = IRV + TV + ERV or VC = TLC – RV; the normal value is 4,500–5,000 ml

Inspiratory capacity (IC): the maximum volume of air inspired after a normal tidal expiration: IC = TV + IRV; the normal value is 2,400–3,600 ml

Expiratory capacity (EC): the maximum volume of air expired after normal tidal inspiration: EC = TV+ ERV; the normal value is 1,800–2,300 ml

Functional residual capacity (FRC): the volume of air remaining in the lungs after normal tidal expiration: FRC = ERV + RV. FRC is an important component that maintains a continuous exchange of oxygen and carbon dioxide at the alveolar-capillary membrane. Collapse or atelectasis of the lungs leads to a reduction in FRC, causing hypoxemia and hypercarbia. The normal value is 2,500 ml.

Functional Residual Capacity

Image: Functional residual capacity


Doing Spirometry

Image: Doing spirometry by Jmarchn. License: CC BY-SA 3.0

Spirometry is the most frequently used measure of pulmonary function. It measures the volume of air inhaled or exhaled by the patient as a function of time. The patient takes a deep inspiration and forcefully expels air, as quickly and for as long as possible. The test results include:

  • Forced vital capacity (FVC): the maximum volume of air forcefully exhaled after a maximal inspiration. This is usually reduced in restrictive lung disease; however, an obstructive lung disease with significant hyperinflation can also reduce FVC
  • FEV1: the volume of air exhaled during the initial second of the FVC maneuver.
  • The ratio of FEV1/FVC: the normal value is > 75%. The value is decreased in obstructive lung disease. Values < 50% suggests a severe obstruction
  • FEF 25–75% (forced mid-expiratory flow): the maximum flow rate during the mid-expiratory part of the FVC maneuver. Expressed in L/min, it represents the status of small airways. The normal value is 300 L/min
  • PEFR (peak expiratory flow rate): the maximal flow rate during the FVC maneuver occurs in the initial 0.1 seconds. It gives a crude estimate of larger airway function. The normal value is 400–700 L/min

Important: Spirometry does not measure volumes such as FRC, TC, and RV.

There are two patterns of results: obstructive and restrictive diseases.

In obstructive diseases, reduced flow is predominant, leading to low FEV1; however, FVC may be normal or low. Hence, the FEV1/FVC ratio is usually < 0.7.

Reversibility: An increase in FEV1 by 12–15% on repeat spirometry after administering a bronchodilator (salbutamol) is characteristic of asthma.

In restrictive diseases, reduced lung volume is predominant, which leads to low FVC; however, FEV1 may be normal or low. Hence, FEV1/FVC ratio is > 0.7 (and could be > 1.0).

This needs confirmation since RV cannot be measured by spirometry (a low RV is a hallmark of restrictive lung disease).

Loop spirometry

Loop Spirometry

Image: Loop spirometry

Flow volume loops form when a patient performs the spirometry maneuver, and a graph is plotted with the volume on the x-axis and the flow on the y-axis. The expiratory limb is usually represented as positive. The initial one-third of the expiratory flow is effort dependent; the latter part is effort independent, hence the shape. The inspiratory limb is entirely effort dependent, and the curve is smooth (useful for identifying the phase of respiration if not labeled).

In the obstructive pattern of the flow-volume loop, there is a decrease in the height of the y-axis, which represents decreased airflow. The volume remains normal or high (hyper-inflation). In the restrictive pattern, there is a predominantly reduced volume, which is more prominent than the decreased flow.

Loop Spirometry

Image: Loop spirometry

Limitation of spirometry

Spirometry cannot measure RV or TLC. It is essential to differentiate the cause of decreased VC in a patient suffering from chronic obstructive pulmonary disease (COPD). In COPD, VC can be reduced in two scenarios:

  • The patient has a superimposed, restrictive lung disease
  • The patient has significant hyper-inflation

These can be differentiated using lung volume measurement. The former will have a reduced TLC, while hyper-inflation will show increased TLC. Hence, a confirmation by lung volume studies is necessary.

Detection of upper airway obstruction–other spirometry patterns

Detection of upper airway obstruction

Image: Detection of upper airway obstruction

  • Fixed airway obstruction: constant limitation of flow during inspiration and expiration, such as stricture, goiter, or stenosis
  • Variable extrathoracic obstruction: reduced flow during inspiration (airways tend to collapse during inspiration due to negative transmural pressure). Positive pressure in the airway during expiration decreases obstruction, such as vocal cord palsy, obstructive sleep apnea
  • Variable intrathoracic obstruction: Reduction inflow is greater during expiration (high pleural pressure decreases airway diameter). During inspiration, lower pleural pressure around the airway tends to decrease obstruction, such as tracheomalacia or bronchial tumors

Lung Volumes – RV, FRC, TLC

Gas dilution technique

N2 washout: The patient breathes 100% oxygen so that all nitrogen in the lungs is washed out. The difference in nitrogen volume at initial exhaled concentration and final concentration gives the value of FRC.

He dilution: The patient breathes from a reservoir containing a known volume of gas with a trace of helium. The inhaled helium gets diluted in the gas present in the lungs. The concentration of helium in the exhaled gas is expressed as a percentage, giving the lung volume. For example, if the patient breathes 50 mL of helium, and its concentration in the exhaled gas is 1%, the volume of the lung is 5 L.


The patient sits inside an airtight body box with a known volume and pants with an open glottis against a closed shutter. The increased chest volume reduces the relative box volume and increases the box pressure. Measurements are done during expiration, and hence FRC is measured. The principal behind plethysmography is Boyle’s law (P x V = K).

Important: Lung volumes, such as RV, FRC, and TLC, cannot be measured by spirometry.

Applications of volume testing

  • Measures RV, FRC, and TLC (which spirometry cannot measure)
  • Spirometry only measures FVC, which can be misleading when used alone
  • Reduced RV, FRC, and TLC appear in restrictive diseases like interstitial lung disease, sarcoidosis, or fibrosis
  • Increased RV, FRC, and TLC appear in obstructive lung diseases like asthma, COPD, and cystic fibrosis

Gas Exchange Function – DLCO, AaDO2

DLCO (Diffusion capacity of Carbon Monoxide)

The patient inspires a diluted mixture of CO and is told to hold their breath for 10 seconds. The amount of CO taken up is then measured by infrared analysis.

DLCO = CO (ml/min/mm Hg) ÷ PACO – PcCO

CO is the ideal gas for this study, as it has a very high affinity for Hb and very low plasma/lung concentration. It demonstrates the ability of the lung to transport inhaled gas from the alveoli to the blood. Normal value is 20–30 ml/min/mm Hg.

DLCO values depend on three factors:

  • Thickness and permeability of alveolar-capillary membrane (increased in pulmonary bleeding, interstitial lung disease)
  • Hemoglobin concentration
  • Cardiac output

Factors altering DLCO

Decrease DLCO (< 80%) Increase DLCO (> 120%)
Anemia Polycythemia
Emphysema Exercise
Fibrosis Congestive heart failure

Alveolar to arterial oxygen tension gradient (AaDO2)

  • Detects the difference between alveolar oxygen (PaO2) and arterial oxygen tension (PaO2)
  • Needs arterial blood gas analysis for PaO2
  • The normal values are below 10 mm Hg, and values above 100 mm Hg indicate significant impairment in gas exchange.

Cardiopulmonary reserve/exercise testing

  • The stair-climbing test and 6-minute walk test measure changes in SaO2/SpO2, HR, and oxygen utilization (VaO2).
  • These tests analyze the individual’s ability to cope with increased metabolic demands during exercise.
  • They give a clear picture of the functional improvement of a patient during follow-up.


Parameter Obstructive lung disease Restrictive lung disease
TLC High/normal Very low
VC or FVC Low/normal Very low
RV High Low
FRC High Low
FEV1 Very Low Normal/low
FEV1/FVC Low Normal/high
Peak flow Low Low
Examples COPD, asthma Interstitial lung disease, fibrosis
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