Lung function tests

Lung Function Tests

Lung function tests, lung volumes, capacities and measurements of respiratory function.

Lung Function Tests Lung Volumes Lung Capacities Measurement FRC & Residual Volume Vital Capacity FEV RMV MBC PEFR Respiratory Diseases

1 Lung Function Tests

Lung function tests or pulmonary function tests are useful in assessing functional status of respiratory system.

Pulmonary function tests are carried out mostly by using spirometer.

Lung function tests are of two types:

  1. Static lung function tests.
  2. Dynamic lung function tests.

Static Lung Function Tests

Static lung function tests are based on volume of air that flows into or out of lungs and do not depend upon the rate at which air flows.

Static lung function tests include:

  1. Static lung volumes.
  2. Static lung capacities.

Dynamic Lung Function Tests

Dynamic lung function tests are based on time, i.e. the rate at which air flows into or out of lungs.

Dynamic lung function tests are useful in determining the severity of obstructive and restrictive lung diseases.

Dynamic lung function tests include:

  1. Forced vital capacity.
  2. Forced expiratory volume.
  3. Maximum ventilation volume.
  4. Peak expiratory flow.

2 Lung Volumes

Lung volumes are the static volumes of air breathed by an individual.

Lung volumes are of four types.

1. Tidal Volume (TV)

Tidal volume is the volume of air breathed in and out of lungs in a single normal quiet respiration.

Tidal volume signifies the normal depth of breathing.

Normal tidal volume: 500 mL (0.5 L)

2. Inspiratory Reserve Volume (IRV)

Inspiratory reserve volume is an additional volume of air that can be inspired forcefully after the end of normal inspiration.

Normal inspiratory reserve volume: 3,300 mL (3.3 L)

3. Expiratory Reserve Volume (ERV)

Expiratory reserve volume is an additional volume of air that can be expired out forcefully, after normal expiration.

Normal expiratory reserve volume: 1,000 mL (1 L)

4. Residual Volume (RV)

Residual volume is the volume of air remaining in lungs even after forced expiration.

Normally, lungs cannot be emptied completely even by forceful expiration.

Some quantity of air always remains in the lungs even after forced expiration.

Residual volume helps to aerate the blood in between breathing and during expiration.

Normal residual volume: 1,200 mL (1.2 L)

3 Lung Capacities

Lung capacities are the combination of two or more lung volumes.

Lung capacities are of four types.

1. Inspiratory Capacity (IC)

Inspiratory capacity is the maximum volume of air that is inspired after normal expiration (end expiratory position).

It includes tidal volume and inspiratory reserve volume.

IC = TV + IRV
= 500 + 3,300
= 3,800 mL

Normal inspiratory capacity: 3,800 mL

2. Vital Capacity (VC)

It is the maximum volume of air that can be expelled out forcefully after a maximal or deep inspiration.

Vital capacity includes inspiratory reserve volume, tidal volume and expiratory reserve volume.

VC = IRV + TV + ERV
= 3,300 + 500 + 1,000
= 4,800 mL

Normal vital capacity: 4,800 mL

3. Functional Residual Capacity (FRC)

It is the volume of air remaining in lungs after normal expiration (after normal tidal expiration).

Functional residual capacity includes expiratory reserve volume and residual volume.

FRC = ERV + RV
= 1,000 + 1,200
= 2,200 mL

Normal functional residual capacity: 2,200 mL

4. Total Lung Capacity (TLC)

Total lung capacity is the volume of air present in lungs after a deep (maximal) inspiration.

It includes all the volumes.

TLC = IRV + TV + ERV + RV
= 3,300 + 500 + 1,000 + 1,200
= 6,000 mL

Normal total lung capacity: 6,000 mL

4 Measurement of Lung Volumes and Capacities

Spirometry is the technique to measure lung volumes and capacities.

Simple instrument used for this purpose is called spirometer.

Modified spirometer is known as respirometer.

Nowadays plethysmograph is also used to measure lung volumes and capacities.

Spirometer

Spirometer contains two chambers, namely outer chamber and inner chamber.

Outer chamber is filled with water.

A floating drum is immersed in the water in an inverted position.

Drum is counter balanced by a weight.

An attachment is attached to the bottom of the inverted drum by means of string or chain.

A pen with ink is attached to the counter weight.

Pen is made to write on a calibrated paper, which is fixed to a recording device.

The chamber is inverted and has a small hole at the top.

A rubber tube is connected to bottom of inner chamber via a metal tube.

At the other end of this rubber tube, a mouthpiece is attached.

Subject respires through this mouthpiece by closing the nose with a nose clip.

When the subject breathes with spirometer, during expiration, drum moves up and the counter weight comes down.

Reverse of this occurs when the subject breathes the air from the spirometer, i.e., during inspiration.

Upward and downward movement of the counter weight are recorded in the form of a graph.

Upward deflection of the curve in the graph shows inspiration and the downward deflection denotes expiration.

Spirograph

Spirogram is the graphical record of lung volumes and capacities using spirometer.

In spirogram upward curve indicates inspiration and the downward curve indicates expiration.

To determine lung volumes and capacities, four levels are to be noted:

  1. Normal end expiratory level.
  2. Normal end inspiratory level.
  3. Maximum expiratory level.
  4. Maximum inspiratory level.

Computerized Spirometer

Computerized spirometer is an electronic equipment.

It does not contain a drum or water chamber.

Subject must respire into a sophisticated transducer, which is connected to instrument by means of a cable.

Disadvantages of Spirometry

Spirometer is used only for a single breath.

Repeated cycles of respiration cannot be recorded by using this instrument because carbon dioxide accumulates in the spirometer and oxygen or fresh air cannot be provided to the subject.

So, all the lung volumes and lung capacities cannot be measured by using spirometer.

Volume which cannot be measured by spirometry is the residual volume.

Capacities which include residual volume also cannot be measured.

Capacities that include residual volume are functional residual capacity and total lung capacity.

Volumes and capacities which cannot be measured by spirometry are measured by nitrogen washout technique or helium dilution technique or by body plethysmograph.

Respirometer

Respirometer is a modified spirometer.

It has provision for removal of carbon dioxide and supply of oxygen.

Carbon dioxide is removed by placing soda lime inside the instrument from the oxygen cylinder, by a suitable valve system.

Plethysmography

Plethysmography is another technique used to measure all the lung volumes and capacities.

5 Measurement of Functional Residual Capacity and Residual Volume

Residual volume and functional residual capacity cannot be measured by spirometer and can be determined by three methods.

  1. Helium dilution technique.
  2. Nitrogen washout method.
  3. Plethysmography.

1. Helium Dilution Technique

A respirometer is filled with air containing a known quantity of helium.

Initially, the subject breathes normally.

Then, after the end of expiration, subject breathes from respirometer.

Helium from respirometer enters the lungs and starts mixing with air in lungs.

After five minutes of breathing, concentration of helium in the respirometer becomes equal to concentration of helium in the lungs of subject.

It is called the equilibration of helium.

After equilibration of helium between respirometer and lungs, concentration of helium in respirometer is determined.

Functional residual capacity is calculated by using the data such as initial volume of air in respirometer, initial concentration of helium in respirometer and final concentration of helium in respirometer.

To measure functional residual capacity, the subject starts breathing with respirometer after normal expiration.

To measure residual volume, the subject should start breathing from respirometer after forced expiration.

2. Nitrogen Washout Method

Normally, concentration of nitrogen in air is 80%.

So, if total quantity of nitrogen in lungs is measured, the volume of air present in lungs can be calculated.

Subject is asked to breathe normally.

At the end of normal expiration, the subject inspires pure oxygen through a valve and expires into a Douglas bag.

This procedure is repeated for 6 to 7 minutes, until the nitrogen in lungs is displaced by oxygen.

Nitrogen comes to the Douglas bag.

Afterwards, functional residual capacity is calculated from the data such as volume of air collected in Douglas bag and concentration of nitrogen in Douglas bag.

To measure functional residual capacity, the subject starts inhaling pure oxygen after normal expiration.

To measure residual volume, the subject starts breathing pure oxygen after forceful expiration.

3. Plethysmography

Plethysmography is a technique to study the variations in the size or volume of a part of the body such as limb.

Plethysmograph is the instrument used for this purpose.

Whole body plethysmograph is the instrument used to measure the lung volumes including residual volume.

6 Vital Capacity

Definition and Normal Value

Definition and normal value of vital capacity are given under lung capacities.

Variations of Vital Capacity

Physiological Variations

  1. Sex: In females, vital capacity is less than in males.
  2. Body built: Vital capacity is slightly more in heavily built persons.
  3. Posture: Vital capacity is more in standing position and less in lying position.
  4. Athletes: Vital capacity is more in athletes.
  5. Occupation: Vital capacity is decreased in people with sedentary jobs. It is increased in persons who play musical wind instruments such as bugle and flute.

Pathological Variations

Vital capacity is reduced in the following respiratory diseases:

  1. Asthma.
  2. Emphysema.
  3. Weakness or paralysis of respiratory muscle.
  4. Pulmonary congestion.
  5. Pneumonia.
  6. Pneumothorax.
  7. Hemothorax.
  8. Pyothorax.
  9. Hydrothorax.
  10. Pulmonary edema.
  11. Pulmonary tuberculosis.

7 Forced Expiratory Volume (FEV) or Timed Vital Capacity

Definition

Forced expiratory volume (FEV) is the volume of air, which can be expired forcefully in a given unit of time after a deep inspiration.

It is also called timed vital capacity.

FEV1 = Volume of air expired forcefully in 1 second

FEV2 = Volume of air expired forcefully in 2 seconds

FEV3 = Volume of air expired forcefully in 3 seconds

Normal Values

Forced expiratory volume in persons with normal respiratory functions is as follows:

Measurement Normal Value
FEV1 83% of total vital capacity
FEV2 94% of total vital capacity
FEV3 97% of total vital capacity
After 3rd second 100% of total vital capacity

Significance of Determining FEV

Vital capacity may be almost normal in some of the respiratory diseases.

However, the FEV has great diagnostic value, as it is decreased significantly in some respiratory diseases.

For example, it is very much decreased in the obstructive diseases like asthma and emphysema.

It is slightly reduced in some of the restrictive diseases.

8 Respiratory Minute Volume (RMV)

Respiratory minute volume is the volume of air breathed in and out of lungs every minute.

It is the product of tidal volume (TV) and respiratory rate (RR).

RMV = TV × RR
= 500 × 12
= 6,000 mL

Normal RMV: 6 L

It increases in physiological conditions, such as hyperventilation, exercise and emotional conditions.

It is reduced in respiratory diseases.

9 Maximum Breathing Capacity (MBC)

Maximum breathing capacity (MBC) is the maximum volume of air which can be breathed in and out of lungs by rapid and forceful respiration per minute.

It is also called maximum ventilation volume (MVV).

Subject is asked to breathe forcefully and rapidly with a respirometer for 15 seconds.

Volume of air inspired and expired is measured from the spirogram.

From this value, the MBC is calculated for 1 minute.

Normal MBC (MVV)

Person Normal MBC
Adult male 150 to 170 L/min
Adult female 80 to 100 L/min

Maximum breathing capacity is reduced in respiratory diseases.

10 Peak Expiratory Flow Rate (PEFR)

Peak expiratory flow rate (PEFR) is the maximum rate at which the air can be expired after deep inspiration.

It is measured by Wright’s peak flowmeter or a mini peak flowmeter.

Normal PEFR: 400 L/min

Significance of Determining PEFR

Determination of peak expiratory flow rate is useful to diagnose respiratory diseases especially the obstructive respiratory diseases.

Generally, PEFR is reduced in all types of respiratory diseases.

However, reduction is more significant in the obstructive diseases than in restrictive diseases.

Condition PEFR
Restrictive diseases 200 L/min
Obstructive diseases 100 L/min

11 Restrictive and Obstructive Respiratory Diseases

Diseases of respiratory tract are classified into two types:

  1. Restrictive respiratory diseases.
  2. Obstructive respiratory diseases.

Both the types of respiratory diseases are determined by lung functions, particularly FEV.

Restrictive Respiratory Disease

Restrictive respiratory disease is the abnormal respiratory condition characterized by difficulty in inspiration.

Expiration is not affected.

Restrictive respiratory disease may be because of abnormality of lungs, thoracic cavity and/or nervous system.

Obstructive Respiratory Disease

Obstructive respiratory disease is the abnormal respiratory condition characterized by difficulty in expiration.

Diseases and Structures Involved

Type Disease Structures Involved
Restrictive respiratory diseases Poliomyelitis CNS
Myasthenia gravis CNS and thoracic cavity
Flail chest (broken ribs) Thoracic cavity
Paralysis of diaphragm CNS
Spinal cord diseases CNS
Pleural effusion Thoracic cavity
Obstructive respiratory diseases Asthma Lower respiratory tract
Chronic bronchitis
Emphysema
Cystic fibrosis
Laryngotracheobronchitis Upper respiratory tract
Epiglottitis
Tumors
Severe cough and cold with phlegm Upper respiratory tract

FEV in Respiratory Diseases

In normal respiration, FEV1 is about 83% of total vital capacity.

In restrictive disease, FEV1 is reduced, while the expiratory curve reaches the maximum expiratory level relatively earlier than in obstructive disease.

In obstructive disease, FEV1 is markedly reduced and expiration takes a longer time.