Pulmonary Function Test: Purpose, Procedure Notes

Pulmonary Function Tests Introduction

Pulmonary function tests or lung function tests are useful in assessing the functional status of the respiratory system both in physiological and pathological conditions.

• Lung function tests are based on the measurement of the volume of air breathed in and out in quiet breathing and forced breathing. Pulmonary function tests are carried out mostly by using a spirometer.

The air in the lungs is classified into two divisions:

1. Lung volumes
2. Lung capacities.

Read And Learn More: Medical Physiology Notes

Lung Volumes

Lung volumes are the static volumes of air breathed by an individual. Each of these volumes represents the volume of air present in the lung under a specified static condition.

The lung volumes are of four types:

1. Tidal volume
2. Inspiratory reserve volume
3. Expiratory reserve volume
4. Residual volume.

Tidal Volume (Tv):

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

Normal Value:  500 mL (0.5 liters).

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 Value: 3300 mL (3.3 liters).

Expiratory Reserve Volume (ERV):

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

Normal Value: 1000 mL (1 liter).

Residual Volume (RV):

• Residual volume is the volume of air remaining in the 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 the forced expiration.

Residual volume is significant because of two reasons:

1. Residual volume helps to aerate the blood in between breathing and during expiration
2. Residual volume maintains the contour of the lungs.

Normal Value: 1200 mL (1.2 liters)

Lung Capacities

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

Lung capacities are of four types:

1. Inspiratory capacity
2. Vital capacity
3. Functional residual capacity
4. Total lung capacity.

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 + 3300

= 3800 mL

Vital Capacity (VC):

• Vital Capacity  is the maximum volume of air that can be expelled forcefully after a deep (maximal) inspiration.
• Vital capacity includes inspiratory reserve volume, tidal volume, and expiratory reserve volume.

VC = IRV + TV + ERV
= 3300 + 500 + 1000 = 4800 mL

Vital capacity is significant physiologically and its determination is useful in clinical diagnosis as explained later in this chapter.

Functional Residual Capacity (FRC):

• It is the volume of air remaining in the lungs after normal expiration (after normal tidal expiration).
• Functional residual capacity includes expiratory reserve volume and residual volume.

FRC = ERV + RV

= 1000 + 1200

= 2200 mL

Total Lung Capacity (TLC):

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

TLC = IRV + TV + ERV + RV

= 3300 + 500 + 1000 + 1200

= 6000 mL

Measurement Of Lung Volumes And Capacities

• Spirometry is the method to measure lung volumes and capacities. The simple instrument used for this purpose is called a spirometer.
• The modified spirometer is known as a respirometer. Nowadays plethysmograph is also used to measure lung volumes and capacities.

Spirometer:

The spirometer is made of metal. It contains two chambers namely the outer chamber and the inner chamber.

1. The outer chamber is called the water chamber because it is filled with water. A floating drum is immersed in the water in an inverted position.
2. The drum is counterbalanced by a weight. The weight is attached to the top of the inverted drum by means of a string or chain.
3. A pen with ink is attached to the counterweight. The pen is made to write on calibrated paper, which is fixed to a recording device.
4. The inner chamber is inverted and has a small hole at the top. A long metal tube passes through the inner chamber from the bottom towards the top.
5. The upper end of this tube reaches the top portion of the inner chamber. Then the tube passes through the hole at the top of the inner chamber and penetrates into the outer water chamber above the level of water.
6. A rubber tube is connected to the outer end of the metal tube. At the other end of this rubber tube, a mouthpiece is attached.

• The subject respires through this mouthpiece by closing the nose with a nose clip.
• When the subject breathes with the spirometer, during expiration, the drum moves up and the counterweight comes down.
• The reverse of this occurs when the subject breathes the air from the spirometer, i.e. during inspiration.
• The upward and downward movements of the counterweight are recorded in the form of a graph.
• The upward deflection of the curve in the graph shows inspiration and the downward deflection denotes expiration.
• The spirometer is used only for a single breath. The repeated cycles of respiration cannot be recorded by using this instrument because, the carbon dioxide accumulates in the spirometer and, the oxygen or fresh air cannot be provided to the subject.

Respirometer:

• It is the modified spirometer. It has provisions for the removal of carbon dioxide and the supply of oxygen.
• The carbon dioxide is removed by placing soda lime inside the instrument. Oxygen is supplied to the instrument from the oxygen cylinder, by a suitable valve system.
• The oxygen is filled in the inverted drum above water level and the subject can breathe in and out with the instrument for about 6 minutes and, the recording can be done continuously.

Spirogram

Spirogram is the graphical record of lung volumes and capacities using a spirometer. The upward deflection of the spirogram denotes inspiration and the downward curve indicates expiration.

In order to determine the lung volumes and capacities, the following four levels are to be noted in the spirogram:

1. The normal end-expiratory level
2. The normal end-inspiratory level
3. The maximum expiratory level
4. The maximum inspiratory level.

Computerized Spirometer:

It is solid-state electronic equipment. It does not contain a drum or water chamber.

The subject has to respire into a sophisticated transducer, which is connected to the instrument by means of a cable.

Disad vantages of spirometry:

• By using a simple spirometer, respirometer, or computerized spirometer, not all lung volumes and lung capacities can be measured.
• The volume, which cannot be measured by spirometry, is the residual volume. The capacities, which include residual volume also, cannot be measured.
• The capacities, which include the residual volume, are functional residual capacity and total lung capacity.

• The volume and capacities, which cannot be measured by spirometry, are measured by nitrogen washout technique or helium dilution technique or by body plethysmograph.

Plethysmography:

This technique is used to measure all lung volumes and capacities. It is explained later.

Measurement Of Functional Residual Capacity And Residual Volume

The residual volume and the 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.

Helium Dilution Technique:

Procedure to Measure Functional Residual Capacity:

The respirometer is filled with air containing a known quantity of Helium. Initially, the subject breathes normally.

• Then, after the end of expiration, the subject breathes from the respirometer. The helium from the re-spirometer enters the lungs and starts mixing with the air in the lungs.
• After a few minutes of breathing, the concentration of helium in the respirometer becomes equal to the concentration of helium in the lungs of the subject.
• It is called the equilibration of helium. After the equilibration of helium between the respirometer and the lungs, the concentration of helium in the respirometer is determined.

The functional residual capacity is calculated by the formula:

Where,

 C₁ = Initial concentration of helium in the respirometer

 C₂ = Final concentration of helium in the respirometer

V = Initial volume of air in the respirometer.

Helium Dilution Technique Measured Values:

For example, the following data of a subject are obtained from the experiment:

1. Initial volume of air in respirometer = 5 liters (5,000 mL)
2. The initial concentration of helium in respirometer = 15%
3. The final concentration of helium in respirometer = 10%

Helium Dilution Technique Calculation:

From the above data, the functional residual capacity of the subject is calculated in the following way:

Thus, the functional residual capacity in this subject is 2,500 ml.

Procedure to Measure Residual Volume:

To determine the functional residual capacity, the subject starts breathing with the respirometer after the end of normal expiration, and to measure residual volume, the subject should start breathing from the respirometer after forced expiration.

Nitrogen Washout Method

• Normally, the concentration of nitrogen in the air is 80%.
• So, if the total quantity of nitrogen in the lungs is measured, the volume of air present in the lungs can be calculated.

Procedure to Measure Functional Residual Capacity:

The 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 6-7 minutes until the nitrogen in the lungs is displaced by oxygen. The nitrogen comes to the Douglas bag.

Afterward, the following factors are measured to calculate functional residual capacity:

1. The volume of air collected in Douglas bag
2. The concentration of nitrogen in Douglas bag.

By using the data, the functional residual capacity is calculated by using the formula:

Where

V = Volume of air collected

C₁ = Concentration of nitrogen in the collected air

C₂ = Normal concentration of nitrogen in the air.

Nitrogen Washout Method Measured Values:

For example, the following data are obtained from the experiment with a subject:

1. Volume of air collected  = 40 liters (40,000mL)
2. The concentration of nitrogen in the collected air = 5%
3. Normal concentration of nitrogen in the air = 80%

Nitrogen Washout Method Calculation:

From the above data, the functional residual capacity of the subject is calculated in the following way:

Thus, the functional residual capacity in this subject is 2,500 mL.

Nitrogen Washout Method Procedure to Measure Residual Volume:

To measure the functional residual capacity, the subject starts inhaling pure oxygen after the end of normal expiration, and to determine the residual volume, the subject starts breathing pure oxygen after forceful expiration.

Plethysmography:

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

• Piethysmograph is the instrument used for this purpose. Whole body plethysmograph is the instrument used to measure the lung volumes including residual volume.
• Plethysmography is based on Boyle’s law of gas, which states that the volume of a sample of gas is Aversely proportional to the pressure of that gas at a constant temperature.
• The subject sits in an airtight chamber of the whole body plethysmograph and breathes normally through a mouthpiece connected to a flow transducer called a pneumotachograph.
• It detects volume changes during different phases of respiration. After normal breathing for a few minutes, the subject breathes rapidly with maximum force.
• During maximum expiration, the lung volume decreases very much. But the volume of gas in the chamber increases with a decrease in pressure.

By measuring the volume and pressure changes inside the chamber, the volume of the lungs is calculated by using the formula:

P₁ x V = P₂ (V – Δ V)

Where,

P₁ and  P₂= Pressure changes

V = Functional residual capacity

Vital Capacity

Vital Capacity Definition:

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

Lung Volumes Included In Vital Capacity:

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

Normal Value:

VC = IRV + TV + ERV

= 3300 + 500 + 1000

= 4800 mL.

Variations Of Vital Capacity:

Physiological Variations Of Vital Capacity:

• Sex: In females, vital capacity is less than in males
• Body built: Vital capacity is slightly more in heavily built persons
• Posture: Vital capacity is more in standing position and less in lying position
• Athletes: Vital capacity is more in athletes
• 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 Of Vital Capacity:

Vital capacity is greatly 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.

Vital Capacity Measurement:

Vital capacity is measured by spirometry. The subject is asked to take a deep inspiration and expire forcefully.

Forced Expiratory Volume (FEV) Or Timed Vital Capacity

Forced Expiratory Volume 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).

Forced Expiratory Volume is also called timed vital capacity or forced expiratory vital capacity (FEVC):

1. FEV ₁ – Volume of air expired forcefully in 1 second
2. FEV₂ – Volume of air expired forcefully in 2 seconds
3. FEV₃ – Volume of air expired forcefully in 3 seconds.

Forced Expiratory Volume Normal Values:

FEV in persons with normal respiratory functions is as follows:

1. FEV₁ = 83% of total vital capacity
2. FEV₂ = 94% of total vital capacity
3. FEV₃ = 97% of total vital capacity

After 3rd second = 100% of total vital capacity.

Significance Of Determining FEV:

• The vital capacity may be almost normal in some of respiratory diseases.
• However, the FEV has great diagnostic value, as it is decreased significantly in some respiratory disorders.
• Particularly, it is very much decreased in obstructive diseases like asthma and emphysema.
• In some of restrictive respiratory diseases like fibrosis, the FEV is slightly reduced.

Respiratory Minute Volume (RMV)

Respiratory Minute Volume (RMV) Definition

Resistance minute volume is the volume of air breathed m and out of the lungs every minute.

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

RMV = TV x RR

= 500 x 12

= 6000 mL

Resistance minute volume Normal Value:

Normal respiratory minute volume is 6 liters.

Resistance minute volume Variations:

• The respiratory minute volume increases in physiological conditions such as voluntary hyperventilation, exercise, and emotional conditions.
• It is reduced in respiratory disorders.

Maximum Breathing Capacity (MBC) Or Maximum Ventilation Volume (MVV)

Maximum Breathing Capacity (MBC) Or Maximum Ventilation Volume (MVV) Definition:

• Maximum breathing capacity (MBC) is the maximum volume of air that can be breathed in and out of the lungs by forceful respiration (hyperventilation = increase in rate and force of respiration) per minute. It is also called maximum ventilation volume (MVV).

Maximum Breathing Capacity Normal Value:

In healthy adult male, it is 150-170 liters/minute, and, in females, it is 80-100 liters/min.

Maximum Breathing Capacity Measurement:

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

• The volume of air inspired and expired is measured from the spirogram. From this value, the MBC is calculated for one minute.
• For example, the MBC in 12 seconds = n liters.
• So, MBC per minute = n/12× 60 liters.
• MBC is reduced in respiratory diseases.

Peak Expiratory Flow Rate (Pefr)

Peak Expiratory Flow Rate (Pefr) Definition: Peak expiratory flow rate (PEFR) is the maximum rate at which the air can be expired after a deep inspiration.

Peak Expiratory Flow Rate Normal Value: In normal persons, it is 400 liters/minute.

Peak Expiratory Flow Rate Measurement:

The peak expiratory flow rate is determined by using the instrument Wright’s peak flow meter or a mini peak flow meter.

Significance Of Determining Pefr:

• Determination of peak expiratory flow rate is useful for assessing respiratory diseases especially to differentiate the obstructive and restrictive diseases.
• Generally, PEFR is reduced in all types of respiratory diseases. However, the reduction is more significant in obstructive diseases than in restrictive diseases.
• Thus, in restrictive diseases, the PEFR is 200 liters/minute, and in obstructive diseases, it is only 100 liters/minute.

Restrictive And Obstructive Respiratory Diseases

The diseases of the respiratory tract are classified into two types:

1. Restrictive respiratory disease
2. Obstructive respiratory disease.

These two types of respiratory diseases are determined by lung function tests, particularly FEV.

Restrictive Respiratory Disease:

• Restrictive respiratory disease is an abnormal respiratory condition characterized by difficulty in inspiration.
• The expiration is not affected. Restrictive respiratory disease may be because of abnormality of the lungs, thoracic cavity, or/and nervous system.

Obstructive Respiratory Disease:

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

The obstructive and respiratory diseases are listed in table.