Medicine – Respiratory Physiology and Pulmonary Function Tests

Pulmonary function tests (PFTs) assess how effectively the respiratory system moves air into and out of the lungs and how efficiently gas exchange occurs. Important measurements include lung volumes, lung capacities, expiratory flow, spirometry, compliance, and gas transfer.

Understanding these measurements is particularly useful for distinguishing obstructive lung disease, such as asthma and COPD, from restrictive lung disease, such as pulmonary fibrosis.

1. Pulmonary Blood Flow

Normal pulmonary blood flow is approximately 5 L/min at rest in a healthy adult.

Because the pulmonary and systemic circulations are connected in series, pulmonary blood flow is normally approximately equal to the cardiac output.

During exercise, pulmonary blood flow increases considerably as cardiac output rises.

2. Alveolar Ventilation

Alveolar ventilation refers to the volume of fresh inspired air reaching the gas-exchanging alveoli each minute.

It is different from total minute ventilation because some inspired air remains within the anatomical dead space and does not participate directly in gas exchange.

The exact normal value varies with tidal volume, respiratory rate, and dead-space volume. A resting value around 4–5 L/min is commonly expected, so the quoted value of approximately 5.25 L/min should be regarded as an approximate physiological value rather than a fixed normal.

Lung Volumes and Capacities

The amount of air within the lungs changes continuously throughout the respiratory cycle.

The major individual lung volumes are tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume. Combinations of these individual volumes form capacities such as vital capacity, functional residual capacity, and total lung capacity.

3. Tidal Volume

Tidal volume (TV) is the volume of air inspired or expired during a normal quiet breath.

In a typical resting adult it is approximately 500 mL, although the actual value varies considerably with body size, metabolic demand, and activity.

During exercise, tidal volume increases to meet the body’s greater requirement for ventilation.

4. Inspiratory Reserve Volume

Inspiratory reserve volume (IRV) is the additional volume of air that can be inspired after the end of a normal tidal inspiration.

In other words, after taking a normal breath in, the additional air that can still be forcibly inhaled represents the inspiratory reserve volume.

5. Expiratory Reserve Volume

Expiratory reserve volume (ERV) is the additional volume of air that can be forcibly expired after the end of a normal tidal expiration.

It therefore represents the volume between the resting expiratory level and maximal expiration.

6. Residual Volume

Residual volume (RV) is the volume of gas remaining within the lungs following maximal forced expiration.

Even after breathing out as completely as possible, the lungs do not normally become completely empty.

Residual volume helps prevent complete alveolar collapse and allows gas exchange to continue between individual breaths.

7. Vital Capacity

Vital capacity (VC) is the maximum change in lung volume between complete inspiration and complete expiration.

It can therefore be expressed as:

VC = IRV + TV + ERV

Vital capacity does not include residual volume, because residual volume cannot voluntarily be expired.

Vital Capacity and Total Lung Capacity

The supplied notes describe vital capacity as approximately 75% of total lung capacity.

This can be a useful approximate teaching relationship, but it is not a fixed percentage in every person.

Vital capacity depends on factors such as age, sex, height, body size, respiratory muscle strength, and underlying lung disease.

Vital capacity generally decreases with increasing age as residual volume tends to increase.

8. Functional Residual Capacity

Functional residual capacity (FRC) is the volume of air remaining in the lungs at the end of a normal passive expiration.

It represents the resting equilibrium volume of the respiratory system.

FRC is calculated as:

FRC = ERV + RV

Physiological Importance of FRC

FRC provides a reservoir of gas within the lungs between breaths.

This helps prevent dramatic fluctuations in arterial oxygen and carbon dioxide concentrations during normal respiration.

Changes in lung or chest-wall mechanics can substantially alter FRC.

9. Total Lung Capacity

Total lung capacity (TLC) is the total amount of gas contained within the lungs following maximal inspiration.

It represents the maximum volume to which the lungs can be inflated.

TLC can be expressed as:

TLC = VC + RV

or equivalently:

TLC = IRV + TV + ERV + RV

Normal Total Lung Capacity

The supplied notes give approximately 6–7 L for a normal adult.

This is a reasonable approximate teaching value, but TLC varies considerably according to height, sex, age, body size, and reference population.

For clinical interpretation, measured lung volumes are therefore compared with appropriate predicted values rather than with a single universal normal volume.

10. Measuring Total Lung Capacity

TLC cannot be measured completely by ordinary spirometry because spirometry cannot directly measure residual volume.

Methods used to measure lung volumes containing RV include body plethysmography and gas-dilution techniques such as helium dilution.

Body Plethysmography

Body plethysmography measures thoracic gas volume while the patient sits inside an airtight chamber.

An important advantage is that it can detect gas trapped behind poorly communicating or closed airways.

It is therefore particularly useful when significant air trapping, such as in severe COPD, is suspected.

Helium Dilution

In helium dilution, the patient breathes from a closed system containing a known concentration of helium.

The degree to which the helium becomes diluted allows calculation of the communicating lung volume.

Because helium must communicate with the ventilated airspaces, severe air trapping can cause the technique to underestimate true lung volume.

11. Peak Expiratory Flow

Peak expiratory flow (PEF) is the maximum expiratory flow achieved during a forceful expiration beginning from full inspiration.

It provides a simple measurement of airflow through the large airways.

PEF in Asthma

PEF is particularly useful for monitoring asthma within an individual patient.

Repeated measurements can demonstrate variability in airflow obstruction and can help patients recognise deterioration.

Serial peak-flow measurements may also contribute evidence of variable airflow obstruction when investigating suspected asthma.

Therefore, the statement that PEF is simply “not effective as a diagnostic test” is too absolute. A single PEF measurement is relatively nonspecific, but serial variability can support an asthma diagnosis in the appropriate clinical setting.

12. Lung Compliance

Compliance describes the distensibility of the lungs—that is, how easily lung volume changes in response to a change in pressure.

A highly compliant lung expands relatively easily, whereas a lung with low compliance is stiff and requires greater pressure to produce the same increase in volume.

Compliance in Emphysema

Lung compliance is typically increased in emphysema.

Destruction of alveolar elastic tissue causes loss of elastic recoil, so the lungs inflate easily but have difficulty returning to their original volume during expiration.

This contributes to air trapping and hyperinflation.

Compliance in Pulmonary Fibrosis

Compliance is reduced in pulmonary fibrosis.

Fibrotic tissue makes the lungs abnormally stiff, so greater inspiratory pressure is required to expand them.

Patients therefore tend to breathe with small tidal volumes and a relatively rapid respiratory rate.

Compliance in Pulmonary Oedema

Pulmonary oedema also decreases lung compliance.

Fluid within the interstitial and alveolar compartments makes the lungs stiffer and increases the work of breathing.

Spirometry

Spirometry is one of the most important pulmonary function tests. It measures how much air a patient can forcibly expire and how rapidly that air can be expelled.

The two major measurements are FEV₁ and FVC.

13. Forced Expiratory Volume in One Second

FEV₁ is the volume of air forcibly expired during the first second of a maximal forced expiration starting from full inspiration.

FEV₁ is particularly sensitive to airflow obstruction.

When the airways are narrowed, the patient cannot expel air rapidly, so FEV₁ falls.

14. Forced Vital Capacity

Forced vital capacity (FVC) is the total volume of air that can be forcibly expired after taking a maximal inspiration.

It differs from FEV₁ because FEV₁ measures only the volume expelled during the first second, whereas FVC measures the total forced expiratory volume.

15. FEV₁/FVC Ratio

The FEV₁/FVC ratio describes the proportion of the forced vital capacity that can be expelled during the first second.

It is particularly useful for distinguishing an obstructive spirometric pattern from a possible restrictive pattern.

16. Obstructive Lung Disease

In obstructive lung disease, airflow through the airways is impaired.

Examples include asthma and COPD.

FEV₁ falls substantially because the patient cannot expel air rapidly through narrowed or collapsible airways.

FVC may be normal or reduced, but FEV₁ falls proportionately more than FVC.

The result is a reduced FEV₁/FVC ratio.

FEV₁/FVC in Obstruction

The image uses:

FEV₁/FVC <0.75 → obstructive disorder

This is useful older teaching but should not be treated as a universal modern threshold.

For COPD, a post-bronchodilator FEV₁/FVC <0.70 is commonly used, while pulmonary-function laboratories may use the lower limit of normal (LLN) based on age, sex, height, and reference equations.

The key principle remains:

Obstruction → FEV₁ falls more than FVC → FEV₁/FVC decreases.

17. Bronchodilator Reversibility

When airflow obstruction is identified, spirometry may be repeated after administering a bronchodilator.

A significant improvement in airflow after bronchodilator treatment supports the presence of variable or reversible airflow obstruction, which is particularly characteristic of asthma.

However, reversibility is not completely specific for asthma, and some patients with COPD also demonstrate bronchodilator responsiveness.

18. Restrictive Lung Disease

In a restrictive ventilatory defect, the total volume of the lungs is reduced.

Examples include interstitial pulmonary fibrosis, chest-wall restriction, and some neuromuscular disorders.

Both FEV₁ and FVC may decrease because the patient has a smaller volume of air available to expire.

However, they tend to decrease relatively proportionately.

Therefore, the FEV₁/FVC ratio remains normal or may become increased.

FEV₁/FVC in Restriction

The older notes use:

FEV₁/FVC >0.75 → restrictive disorder

The principle is correct, but a preserved or high FEV₁/FVC ratio alone does not prove restriction.

True pulmonary restriction requires demonstration of a reduced total lung capacity (TLC).

Therefore:

Low FVC + normal/high FEV₁/FVC → suspect restriction.

Reduced TLC → confirms restriction.

19. Obstructive Pattern – Note Form

Main abnormality: difficulty getting air out rapidly.

FEV₁: markedly reduced.

FVC: normal or reduced.

FEV₁/FVC: reduced.

TLC: may be normal or increased.

Residual volume: often increased when air trapping is present.

Examples: asthma and COPD.

Bronchodilator response: substantial reversibility particularly supports asthma, although reversibility can also occur in COPD.

Emphysema: compliance increased because elastic recoil is lost.

20. Restrictive Pattern – Note Form

Main abnormality: inability to fully expand the lungs or respiratory system.

FEV₁: reduced.

FVC: reduced.

FEV₁/FVC: normal or increased.

TLC: reduced and required to confirm true restriction.

Residual volume: often reduced or normal depending on the cause.

Examples: pulmonary fibrosis, severe chest-wall restriction, and neuromuscular disease.

Pulmonary fibrosis: lung compliance is reduced because the lungs become stiff.

21. Lung Volumes – Note Form

Tidal volume (TV): volume inspired or expired during an ordinary quiet breath.

Inspiratory reserve volume (IRV): additional air that can be inspired after a normal inspiration.

Expiratory reserve volume (ERV): additional air that can be expired after a normal expiration.

Residual volume (RV): air remaining after maximal expiration.

Vital capacity (VC): maximum volume that can be moved between maximal inspiration and maximal expiration.

VC = IRV + TV + ERV

Functional residual capacity (FRC): air remaining after a normal passive expiration.

FRC = ERV + RV

Total lung capacity (TLC): total volume of gas in the lungs after maximal inspiration.

TLC = VC + RV

22. Important Relationships to Remember

TLC = VC + RV

VC = IRV + TV + ERV

FRC = ERV + RV

Obstruction → FEV₁ ↓↓↓, FVC normal/↓, FEV₁/FVC ↓

Restriction → FEV₁ ↓, FVC ↓, FEV₁/FVC normal/↑, TLC ↓

Emphysema → compliance ↑

Pulmonary fibrosis → compliance ↓

Pulmonary oedema → compliance ↓

Key Clinical Pattern

For examinations, first look at the FEV₁/FVC ratio.

If the ratio is reduced, think airflow obstruction, particularly asthma or COPD.

If the ratio is normal or increased but FVC is reduced, suspect a restrictive pattern and check the TLC. A reduced TLC confirms true restriction.

Then remember the opposite effects on compliance:

Emphysema → lungs are floppy and easy to inflate → ↑ compliance + ↓ elastic recoil.

Pulmonary fibrosis → lungs are stiff and difficult to inflate → ↓ compliance.

Finally, remember that ordinary spirometry cannot directly measure RV, FRC, or TLC, because all three contain residual volume. These require additional lung-volume measurement techniques such as body plethysmography or gas dilution.