In one breath
Pulmonary function tests ask three things of the lung: is it ventilating, is it exchanging gas, and is it being perfused. The ventilation half is measured mostly by spirometry, which gives the static lung volumes and capacities (TV 500 mL, VC about 4 L in men, TLC 6 L) and the dynamic mechanics of breathing — compliance, airway resistance and the timed flow tests such as FEV₁ and PEFR. Anything that contains the residual volume cannot be measured by a spirometer at all, because that air never leaves the lung.
Builds on: Alveolar ventilation · Dead space · Leads to: Obstructive vs restrictive disease · Tests of gas exchange and pulmonary circulation
Why the lung is tested
The lung’s basic job is to keep the O₂ and CO₂ tensions of arterial blood inside the normal range: it takes up O₂ from inspired air and gives off CO₂ in expired air. Three mechanisms do this — ventilation, diffusion and perfusion — and the tests are grouped around them.
- Knowing the type and degree of functional loss can matter as much for diagnosis, treatment and prognosis as the history, radiology, bacteriology and pathology.
- Some tests need elaborate equipment, but most are simple, non-invasive and cheap.
Classification of PFTs
Grouped by the function each test measures:
- Tests that assess ventilation
- Lung volumes: tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume
- Lung capacities: vital capacity, inspiratory capacity, functional residual capacity, total lung capacity
- Mechanics of breathing: timed vital capacity, maximum mid-expiratory flow rate, maximum voluntary ventilation, peak expiratory flow rate, flow–volume curve, closing volume, maximal expiratory pressure, maximal inspiratory pressure
- Tests of gas exchange in the lungs
- Ventilation–perfusion matching
- Diffusing capacity
- Arterial blood gases
- Tests that assess pulmonary circulation
- Pulmonary blood flow and pressure
See Tests of gas exchange and pulmonary circulation for the second and third groups.
Uses of PFTs
- Diagnosing respiratory dysfunction.
- Following the progress of a disease.
- Judging whether treatment is working.
- Screening health before recruitment to the military or police.
- Measuring how effective physical training has been.
- Community surveys of respiratory disease and of industrial respiratory hazards.
- Deciding whether a patient is fit for general anesthesia before surgery.
- Settling medico-legal cases — fitness, or the degree of compensation.
Normal values
For an adult male:
| Parameter | Normal value |
|---|---|
| TV | 500 mL |
| IRV | 3000 mL |
| ERV | 1100 mL |
| FVC | 4600 mL |
| FEV₁ | over 80% of FVC |
| FEV₁/FVC | 0.75–0.80 |
| PEFR | 400–600 L/min |
| RMV | 6000 mL per minute |
| MVV | 125–170 L/min |
| Breathing reserve (MVV − RMV) | 115–160 L/min |
| Dyspneic index | over 90% |
- Values swing widely with race, age, sex, height, build and training.
- All volumes and capacities are 15–25% lower in females.
- Higher in athletes and tall people, lower in non-athletes and poorly built people.
Two sets of numbers in one chapter
Your book’s normal-values table (IRV 3000 mL, ERV 1100 mL, FVC 4600 mL) does not quite match its own definitions later in the chapter (IRV 3.3 L, ERV 1.0 L, VC 4 L in men). Learn one set and say which you are quoting; an examiner will accept either. The ratios and flow values (FEV₁/FVC 0.75–0.80, PEFR 400–600 L/min) are the same in both places, so those are the safest marks.
Applied: the dyspneic index
Dyspneic index compares the breathing the patient actually uses with the breathing they could do. Normally it is over 90%; a value under 60% means the patient is dyspneic even at rest, because almost all of their breathing reserve has already been spent.
Assessment of ventilation
Ventilation is the movement of air into and out of the alveoli, the gas-exchanging units. Whether it is adequate depends on two things:
- the lung volumes and capacities, which are indices of the static size of the lung at different degrees of inflation;
- the mechanics of breathing, which cover how the respiratory apparatus behaves mechanically, both statically and dynamically.
Lung volumes
| Volume | What it is | Normal (men / women) |
|---|---|---|
| Tidal volume (TV) | Gas moved in or out in one quiet breath | 500 mL in adults |
| Inspiratory reserve volume (IRV) | Extra air taken in by maximal effort, above TV | 3.3 L / 1.9 L |
| Expiratory reserve volume (ERV) | Air blown out by maximal effort after a quiet expiration | 1.0 L / 0.7 L |
| Residual volume (RV) | Air still in the lung after a maximal expiration | 1.2 L / 1.1 L |
Where the tidal volume goes: of the 500 mL, about 150 mL stays in the airways running from the nose and mouth as far as the respiratory bronchioles, where no gas is exchanged — the anatomical dead space. The other 350 mL reaches the alveoli and counts as alveolar ventilation.
Lung capacities
A capacity is two or more volumes added together.
| Capacity | What it is | Normal (men / women) |
|---|---|---|
| Vital capacity (VC) | Most air expirable after a maximal inspiration | 4 L / 3 L |
| Inspiratory capacity (IC) | Air inspirable from the resting expiratory level (IRV + TV) | — |
| Functional residual capacity (FRC) | Air left in the lung at the resting expiratory level | — |
| Total lung capacity (TLC) | Air in the lung at the end of a maximal inspiration | 6 L / 4.2 L |
Forced vital capacity (FVC): the whole volume expired with the greatest possible force and speed after a maximal inspiration.
- In a normal person FVC is barely different from VC.
- FVC falls proportionately more than VC once obstruction starts trapping air, which is why FVC is the number used in disease (see Obstructive vs restrictive disease).
Worth knowing, though not in your pages: the arithmetic of the capacities — VC = IRV + TV + ERV, IC = IRV + TV, FRC = ERV + RV, TLC = VC + RV. Every capacity that contains RV (FRC and TLC) is the kind spirometry cannot reach.
Draw it: the spirogram
Volume on the y-axis, time on the x-axis. Draw a wavy resting baseline of small 500 mL breaths (TV), then one big inspiration to the top of the trace and one big expiration to the bottom. Mark off, from the top down: IRV above the tidal peak, TV, ERV below the tidal trough, and RV as a shaded block at the very bottom that the trace never enters. Then bracket the capacities on the right-hand side: IC = IRV + TV from the resting expiratory level upwards; FRC = ERV + RV from the resting expiratory level downwards; VC = IRV + TV + ERV; TLC = everything. Put the numbers on (500, 3300, 1000, 1200 mL) and add a dashed “resting expiratory level” line, because most marks are lost on that line.
Mechanics of breathing
To ventilate the lung, the muscles must overcome three things: the elastic recoil of lung and thorax, the non-elastic resistance of tissues moving over each other, and the airway resistance.
Compliance
- Compliance is the elastic recoil of the lung measured under static conditions.
- Definition: the change in volume per unit change in transpulmonary pressure, that pressure being the difference between esophageal (or intrapleural) pressure and mouth pressure.
- It is a measure of the stiffness and distensibility of lungs and thorax.
- Volume change per unit pressure change higher than normal → tissues more distensible.
- Lower than normal → tissues stiffer.
- A patient with low compliance must put in more respiratory effort for the same alveolar ventilation, and so is dyspneic. Conditions that lower it are listed in Obstructive vs restrictive disease.
Airway resistance
- Airway resistance is the frictional opposition that the conducting air passages offer to moving air.
- Raised airway resistance is a serious mechanical burden and produces dyspnea, graded by how severe the obstruction is.
Tests that measure the mechanics
These are the numbers that tell you, in clinic, how the mechanics are doing.
- Timed vital capacity (FEV₁) — also called forced expiratory volume in one second: the percentage of air expired in a stated time. It measures VC in relation to time, so it is an index of airflow rate.
- Normally 80–85% of FVC is out within one second, 95% by two seconds and 97% by three.
- It is among the most useful tests for picking up generalized airway obstruction in the clinic.
- Maximum mid-expiratory flow rate (MMFR) — the greatest flow reached during the middle third of the expired volume, written FEF25–75%. It reflects the patency of small airways. The flow between 200 and 1200 mL (FEF200–1200, in L/s) reflects how patent the larger airways are.
- Maximum voluntary ventilation (MVV), or maximum breathing capacity — the greatest volume a person can move in and out of the lungs in one minute, with voluntary effort pushed to its maximum. 150 L/min in adult men, 125 L/min in adult women.
- Peak expiratory flow rate (PEFR) — the maximum speed, in L/min, at which air is forced out. Read straight off the dial of a peak-flow meter. Normal 6–10 L/s, which works out at 400–600 L/min.
- Maximum expiratory flow–volume curve (MEFVC) — flow plotted against volume already exhaled. Used because FEV₁ misses early disease; the family includes Vmax 75% (reached after exhaling 75% of the FVC), the response of the MEFV curve to breathing helium, and closing volume.
- Closing volume (CV) — detects small-airway obstruction by measuring the gas left in the lung once the small airways in the gravity-dependent parts have closed.
- Maximal inspiratory pressure (MIP) — the patient empties the lung down to RV, then inspires as hard as possible against a closed airway. The pressure generated measures inspiratory muscle strength.
- Maximal expiratory pressure (MEP) — the patient fills the lung to TLC, then expires as hard as possible against a closed airway. It measures expiratory muscle strength.
MIP and MEP: the book's typo
Your book’s paragraph on MEP ends by calling the pressure generated “the MIP”. That is a printing slip. Remember it by the direction of effort: MIP is inspiring from RV (inspiratory muscles), MEP is expiring from TLC (expiratory muscles).
Measurement: the spirometer
- Most lung functions are measured by spirometry. A recording spirometer is a spirometer fitted with a recording kymograph.
- Build: a hollow double-walled vessel with water between the walls to make an airtight seal. An inverted hollow cylindrical bell of 9 L capacity sits in that space.
- The bell hangs from a chain over a pulley attached to a counterbalance, and the counterbalance carries the pen that writes on the kymograph paper.
- The kymograph runs at 60 and 1200 mm per minute. Today the same measurements are made by a computerized spirometer.
What a spirometer cannot measure
RV, FRC and TLC cannot be measured by spirometry, because each of them includes gas that is still in the lung after a maximal expiration — gas that never passes through the machine. They are measured instead by the helium dilution technique or total body plethysmography. The rule to carry into viva: if it contains the residual volume, a spirometer cannot see it.
Exam-answer skeleton: "Classify pulmonary function tests and describe the tests that assess ventilation" (long essay)
- Open with the lung’s aim — normal arterial O₂ and CO₂ tensions — through ventilation, diffusion and perfusion; note that most PFTs are simple and non-invasive.
- Classification into the three groups, with the tests under each (the list above).
- Uses of PFTs, in brief: diagnosis, monitoring, treatment response, fitness and screening, medico-legal.
- Lung volumes defined, with normal values for men and women.
- Lung capacities defined, with normal values; VC versus FVC and why FVC is used in disease.
- Draw and label the spirogram, showing which volumes make up each capacity.
- Mechanics of breathing: compliance and its definition, airway resistance, then the flow tests — FEV₁ (80–85% in one second), MMFR, MVV, PEFR, MEFVC, closing volume, MIP and MEP.
- The spirometer itself, and the point that RV, FRC and TLC need helium dilution or plethysmography.
- Close with the table of normal values.
Asked in exams
- Pulmonary function tests as a whole: Mar 2024, essay
- Lung volumes and capacities: Jul 2010, 2 marks · Jan 2011, 2 marks · Jan 2011, 2 marks · Nov 2012, 4 marks · Jan 2021, 3 marks · Jan 2021, 4 marks · May 2022, MCQ · Dec 2023, MCQ · Mar 2024, MCQ · Oct 2024, MCQ
- Lung compliance: Oct/Nov 2018, 4 marks · Mar 2024, essay
- Airway resistance: Dec 2016, 10 marks · Mar 2024, essay · Oct 2024, MCQ