1

Respiratory

System

Module 2

Lung Volumes and

Lung Capacities

Prof. Dr. Hamdan Noor

August, 2010

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Module 2: Lung Volumes and Lung Capacities

Module content:

i. Learning Resources

ii. Learning Objectives of Module 2

iii. Learning Activities

1. Determination of Lung Volumes and Lung Capacities

1.1. Concept of spirometry

1.2. Lung Volumes and Lung Capacities

1.3. Determination of FRC, RV & TLC: Helium dilution technique

2. Application of Lung Volume Parameters: Pulmonary Function Tests

3. Lung capacities and respiratory diseases

iv. Summary

v. Conclusion

vi. Appendix 1: Arterial blood gases

Appendix 2: Tests of pulmonary function

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i. Learning Resources

Boron and Boulpaep: Ch 26

Ganong: Ch. 34

Guyton & Hall: Ch. 37

Marieb: Ch. 23 pp. 855-858 + Study partner

Tortora & Grabowski: Ch. 23 pp. 826-827

Vander, Sherman & Luciano, Ch. 15 pp. 475-477

ii. Objectives of Module 2

What is the central concern of the Module?

On completion of this lecture, you should be able to:

1. define the 4 basic lung volumes and describe how these volumes are

measured using a spirometer

2. list the volumes that comprise each of the four capacities.

3. describe methods of measuring or evaluating residual volume and explain

how the techniques differ in terms of volume actually measured.

4. differentiate between “restrictive disease” and “obstructive disease” and state

how each would affect TLC, FRC, RV, and VC.

Try to set up more specific objectives yourselves and try to achieve all of them.

Benchmarking the Objectives:

Medical Physiology Curriculum Objectives prepared by The American Physiological Society and the

Association of Chairs of Departments of Physiology.

PUL 3. Draw a normal spirogram, labeling the four lung volumes and four capacities. List the volumes

that comprise each of the four capacities. Identify which volume and capacities cannot be measured by

spirometry.

PUL 4. Define the mechanisms that determine the clinically important boundaries of lung volume (i.e.,

TLC, FRC, and RV).

PUL 5. Contrast the causes and characteristics of restrictive and obstructive lung disease, including the

abnormalities in lung volumes are associated with each.

PUL 16. Based on changes in FEV, FEV1, FVC, TLC, and flow volume curves, characterize the

pathology as a restrictive and/or obstructive lung disease. Describe how FRC and residual volumes are

altered in each case.

PUL 17. Define dynamic airway compression, and use this principle to explain the shift in the shape of

flow volume curves that occur with COPD (chronic obstructive pulmonary disease).

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iii. Learning activities

1. Determination of Lung Volumes and Lung Capacities

The respiratory system is vulnerable to harm by the factors in the outside world because

of direct contact between the respiratory tissues and the harmful agents. Most of the

respiratory malfunctions are reliably diagnosed by clinicians after taking history of the

patient’s complaint and making a clinical examination. Other useful investigative

methods including chest X-ray, microbiological and histological techniques, blood tests,

lavage, bronchoscopy are used to confirm or refute the original diagnosis.

Such diagnosis is, however, usually qualitative. Thus, lung function test is performed

to quantitatively measure the effect of the disease on the function of the respiratory

system. The test is performed using a spirometer. However, it should be noted that the

test is mainly valuable for following the progress of a patient with chronic pulmonary

disease and assessing the results of the treatment.

1.1. Concept of spirometry

Lung function test is carried out using an apparatus called a spirometer. You should

know how a spirometer works to appreciate the lung volume and capacity measurement.

Activity 1. Spirometry It is a method of studying pulmonary ventilation by recording the volume movement of air in and out of the lung. Please explain how a spirometer works and how a spirogram is obtained.

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1.2. Lung Volumes and Lung Capacities

Activity 2. Lung volumes and capacities Draw a normal spirogram, labeling the four lung volumes and four capacities. List the volumes that comprise each of the four capacities. Identify which volume and capacities cannot be measured by spirometry. Note the normal values of the volumes and capacities. Create a mnemonic or rewrite a lyric for a song to remember the lung volumes and capacities.

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Parameters which take into account RV cannot be determined directly from the

spirogram: FRC, RV & TLC. The next Section describes special techniques used to

determine these parameters.

1.3. Determination of FRC, RV & TLC: Helium dilution technique (refer to diagram below)

1. A spirometer of known volume (VS) is filled with air mixed with helium of known

concentration. Helium does not dissolve in blood and therefore not used in

respiration.

2. The person expires normally (at the end of normal expiration, the remaining volume

in the lung = FRC = VL).

3. The subject immediately begins to breathe from the spirometer, and the gases in the

spirometer mixes with the gases in the lungs.

4. Helium becomes diluted by the FRC gases, and helium concentrations in the

spirometer and the lungs become the same.

5. Since no helium is lost, [He]initial .Vs = [He]final .(VS+VL)

VL = VS.(([He] initial /[He] final)-1)

6. RV = FRC - ERV

7. TLC = FRC + IC

8. CONCLUSIONS: THE VALUES OF ALL LUNG VOLUMES AND CAPACITIES

CAN BE DETERMINED. What for?

Activity 3. Measuring FRC by Helium dilution technique Based on the following data, calculate the lung volume when the valve (stopcock) is opened after a normal expiration:

Initial [He] in the spirometer = 10% Spirometer volume = 2 litres Final [He] = 5%

What lung volume is measured? Explain. Also calculate RV and TLC based on normal spirogram. What lung volume would we be measuring if the valve (stopcock) is opened after the subject has just a maximal expiration?

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Measuring Residual Volume.

Residual volume can not be measured by spirometry. Residual Volume is determined by

one of 3 techniques. In practice the techniques are used to measure FRC, and then RV is

determined by subtracting expiratory reserve volume when measured by any of the

techniques.

1. Gas dilution techniques.

a. Nitrogen washout technique. Poorly ventilated or non-ventilated areas will not be

included in FRC with this technique.

Figure 2 - Measurement of Functional Residual Capacity Open Circuit Method

b. Helium dilution technique. Poorly ventilated or non-ventilated areas will not be

included as part of FRC when measured with this technique.

Figure 3 -- Measurement of FRC: Helium closed-circuit technique.

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22.. Body Plethysmography. Non-ventilated regions are included in FRC as measured

by this technique.

Assuming that the change in volume (V) = 71 ml, and that the change in lung gas

pressure (P) = 20 mmHg, and that Ps = 760 mmHg, the calculation proceeds as follows:

PV P P V V ( )( )

Multiplying out

PV PV P V V P P V

Adding out PV’s, rearranging, and factoring

VV P P

P

( )

Since P is quite small relative to P (20 mmHg versus 713 mmHg), then

VV

PP

( )

V 71

2 531 ml (713 mmHg)

20 mmHg ml = Volume at FRC,

Figure 4 - The “Mead-type” body plethysmograph. The subject breathes normally while the stopcock is connected to the environment. Then the stopcock is turned to occlude the airway while the subject makes inspiratory and expiratory efforts, his

alveolar pressure being recorded using a pressure gauge connected to the airway proximal to the stopcock. The Krogh spirometer

measures V, and thus one determines V/P, and can calculate TGV.

3. Radiographic Determination. A qualitative technique. Non-ventilated regions are

included in FRC as measured by this technique.

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2. Application of Lung Volume Parameters: Pulmonary Function Tests

Lung function tests (also called pulmonary function tests, or PFTs) evaluate how well

your lungs work. The tests determine

how much air your lungs can hold,

how quickly you can move air in and out of your lungs, and

how well your lungs put oxygen into and remove carbon dioxide from your

blood.

The tests can diagnose lung diseases, measure the severity of lung problems, and check

to see how well treatment for a lung disease is working.

Other tests such as residual volume, gas diffusion tests, body plethysmography,

inhalation challenge tests, and exercise stress tests may also be done to determine lung

function.

Lung function tests are done to:

Determine the cause of breathing problems.

Diagnose certain lung diseases, such as asthma or chronic obstructive pulmonary

disease (COPD).

Evaluate a person's lung function before surgery.

Monitor the lung function of a person who is regularly exposed to substances such as

asbestos that can damage the lungs.

Monitor the effectiveness of treatment for lung diseases.

The more common lung function values measured with spirometry are:

Forced vital capacity (FVC). This measures the amount of air you can exhale with

force after you inhale as deeply as possible.

Forced expiratory volume (FEV). This measures the amount of air you can exhale

with force in one breath. The amount of air you exhale may be measured at 1 second

(FEV1), 2 seconds (FEV2), or 3 seconds (FEV3). FEV1 divided by FVC can also be

determined.

Forced expiratory flow 25% to 75%. This measures the air flow halfway through

an exhale (FVC).

Peak expiratory flow (PEF). This measures how quickly you can exhale. It is

usually measured at the same time as your forced vital capacity (FVC).

Maximum voluntary ventilation (MVV). This measures the greatest amount of air

you can breathe in and out during one minute.

Slow vital capacity (SVC). This measures the amount of air you can slowly exhale

after you inhale as deeply as possible.

Total lung capacity (TLC). This measures the amount of air in your lungs after you

inhale as deeply as possible.

Functional residual capacity (FRC). This measures the amount of air in your lungs

at the end of a normal exhaled breath.

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Expiratory reserve volume (ERV). This measures the difference between the

amount of air in your lungs after a normal exhale (FRC) and the amount after you

exhale with force (RV).

Forced Expiratory Vital Capacity (FVC) and Forced Expiratory Volume (FEV)

• A person inspires maximally from the spirometer (TLC), then exhales with

maximum expiratory effort as rapidly and as completely as possible.

• FVC is the amount of air that can be forcefully exhaled after maximum inhalation.

• FEV1 is the amount of air exhaled during the first second after maximal

inhalation.

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Forced Vital Capacity (FVC): the maximum volume of air that can be forced out of

the lung of a subject from a position of full inspiration

Forced Expiratory Volume in one second (FEV1): the volume of air expelled in the

first second of the forced expiration. Value decrease when there is airway obstruction

FEV1/FVC: clinically very useful as it is independent of body size. Normal: 0.75.

Airway obstruction: less.

Malaysian values.

Males Females

VC 3.28 2.31

FEV1 3.35 2.42

FVC 3.49 2.51

Activity 4: Lung function test

What are lung function tests? What are they used for?

How are lung function tests carried out?

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3. Lung capacities and respiratory diseases

Restrictive Disease.

Respiratory disease which makes it more difficult to get air in to the lungs. They

“restrict” inspiration. Includes fibrosis, sarcoidosis, silicosis, asbestosis,

muscular diseases, and chestwall deformities.

Each of these disorders results in stiffer lungs which cannot expand to normal

volumes. All the subdivisions of volume are decreased and the ratio of RV : TLC

will be normal, or where VC decreases more quickly than RV, increased.

Obstructive Disease.

Respiratory disease which make it more difficult to get air out of the lungs. This

group of disorders is characterized by obstruction of normal airflow due to

airway narrowing and includes:

Asthma

COPD

Bronchiectasis

Cystic fibrosis

Tumors (inside or outside the airways).

Within this group, the mechanisms causing the airway narrowing differ. They

include obstruction by a mucus plug, airway compression, and smooth muscle

constriction.

In general, obstructive disorders lead to hyperinflation of the lungs as air is

trapped behind closed airways. RV is increased, as is the ratio of RV : TLC. In

patients with severe obstruction, air trapping is so extensive that vital capacity is

decreased.

Lung capacity changes during disease—a summary

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C. Restrictive Disease: Decreased VC; decreased TLC, RV, FRC.

B. Obstructive Disease: Decreased VC; increased TLC, RV, FRC.

Interpretation of Vital Capacity Measurements.

• Most lung disease reduces vital capacity. Note that ventilation is not

necessarily affected since we don’t use our entire vital capacity to breathe

even during maximal exercise. Note also that vital capacity shows

tremendous variation in normal individuals even when corrected for age,

sex, height, and weight.

• Values may not be considered abnormal unless they are 20% greater or

less than predicted values.

• Body position. Vital capacity decreases when lying down because

pulmonary blood volume increases and the diaphragm is pushed toward

thorax.

• Vital capacity is most useful when followed over the clinical course of a

disease since normal values of VC can be found in diseased patients who

have high values of VC under normal conditions.

Pulmonary factors that can reduce vital capacity

• Absolute reduction in distensible lung tissue, e.g. pneumonectomy,

atelectasis.

• Increases stiffness of lungs can’t get enough air in, e.g. alveolar edema,

respiratory distress syndrome (surfactant abnormality), or infiltrative

interstitial lung diseases.

• Increased residual volume. Can’t get enough air out, e.g. emphysema,

asthma, or lung cysts.

Extrapulmonary factors that can reduce vital capacity

• Limited thoracic expansion. Examples include thoracic deformities

(e.g. Kyphoscoliosis) and pleural fibrosis.

• Limitations on diaphragmatic descent. Examples include ascites and

pregnancy.

• Nerve or muscle dysfunction. Examples include pain from surgery or

rib fracture and primary neuromuscular disease (e.g. Guillain-Barré

Syndrome).

Interpretation of Residual Volume measurements

• RV/TLC increases with normal aging.

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• Chronic obstructive disease, particularly emphysema, leads to an increase

in RV. This is due to loss of the alveolar walls and resultant gas trapping.

Obstructed individuals have higher FRC values measured by body

plethysmography than by nitrogen washout or helium dilution because the

former method includes trapped air. The difference represents volume of

“non-ventilated” lung.

• Restrictive diseases due to stiffening of the lungs or chest wall decrease

RV. Restrictive diseases resulting from muscular weakness have less

effect.

Activity 5: Lung capacities and respiratory diseases Using examples, compare and contrast between restrictive and obstructive lung diseases. What are the effects of the lung diseases on lung volumes and capacities?

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PULMONARY FUNCTION TEST DECISION TREE

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iv. Summary

In this Module, we discussed the lung volumes and capacities that would help us explore

in greater details about lung functions in health and diseases.

v. Conclusion

Have we met the objectives of Module 2?

Objective Achievement

1. Define the 4 basic lung volumes identify and describe

how these volumes are measured using a spirometer

2. LLiisstt tthhee vvoolluummeess tthhaatt ccoommpprriissee eeaacchh ooff tthhee ffoouurr

ccaappaacciittiieess..

3. Describe methods of measuring or evaluating

residual volume and explain how the techniques

differ in terms of volume actually measured.

4. DDiiffffeerreennttiiaattee bbeettwweeeenn ““rreessttrriiccttiivvee ddiisseeaassee”” aanndd

““oobbssttrruuccttiivvee ddiisseeaassee”” aanndd ssttaattee hhooww eeaacchh wwoouulldd aaffffeecctt

TTLLCC,, FFRRCC,, RRVV,, aanndd VVCC..

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Appendix 1

Tests of pulmonary function Tests of pulmonary function are used in:

Diagnosis of lung disease

Monitoring disease progression

Assessing patient response to treatment.

Pulmonary function tests can seem confusing, but there are just three basic questions that most tests aim to answer:

1. Are the airways narrowed (PEFR, FEV1, FEV1:FVC, flow volume loops)? 2. Are the lungs a normal size (TLC, RV, and FRC)? 3. Is gas uptake normal (DL(CO) and DL(CO)/VA)?

So, as a minimum, make sure you have a good understanding of peak flow monitoring and spirometry and know how you would measure RV, FRC, and gas transfer.

Tests of ventilation Ventilation can be impaired in two basic ways:

The airways become narrowed (obstructive disorders)

Expansion of the lungs is reduced (restrictive disorders).

These two types of disorder have characteristic patterns of lung function which can be measured using the tests below. Forced expiration

Peak expiratory flow rate (PEFR) is a simple and cheap test that uses a peak flow meter (Fig.

10.6) to measure the maximum expiratory rate in the first 10 ms of expiration. Peak flow meters can be issued on prescription and used at home by patients to monitor their lung function.

Before measuring PEFR (Fig. 10.7), the practitioner should instruct the patient to:

Take a full inspiration to maximum lung capacity

Seal the lips tightly around the mouthpiece

Blow out forcefully into the peak flow meter, which is held horizontally.

The best of three measurements is recorded and plotted on the appropriate graph. At least two recordings per day are required to obtain an accurate pattern. Normal PEFR is 400-650 L/min in healthy adults.

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Figure 10.6 Peak flow meter. Figure 10.7 Patient performing peak expiratory flow rate test.

PEFR is reduced in conditions that cause airway obstruction:

Asthma, in which there is wide diurnal variation in PEFR known as "morning dipping" (Fig. 10.8)

Chronic obstructive pulmonary disease

Upper airway tumors.

Other causes of reduced PEFR include expiratory muscle weakness, inadequate effort, and poor technique. PEFR is not a good measure of air flow limitation because it measures only initial expiration; it is best used to monitor progression of disease and response to treatment. Forced expiratory volume and forced vital capacity The forced expiratory volume in one second (FEV1) and the forced vital capacity (FVC) are measured using a spirometer. The spirometer works by converting volumes of inspiration and expiration into a single line trace. The subject is connected by a mouthpiece to a sealed chamber (Fig. 10.9). Each time the subject breathes, the volume inspired or expired is converted into the vertical position of a float. The position of the float is recorded on a rotating drum by means of a pen attachment. Electronic devices are becoming increasingly available. FEV1 and FVC FEV1 and FVC are related to height, age, and sex of the patient. FEV1 is the volume of air expelled in the first second of a forced expiration, starting from full inspiration. FVC is a measure of total lung volume exhaled. The patient is asked to exhale with maximal effort after a full inspiration.

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Figure 10.8 Typical peak expiratory flow rate graph for an asthmatic patient.

FEV1:FVC ratio The FEV1:FVC ratio is a more useful measurement than FEV1 or FVC alone. FEV1 is 80% of FVC in normal subjects. The FEV1:FVC ratio is an excellent measure of airway limitation and allows us to differentiate obstructive from restrictive lung disease. In restrictive disease:

Both FEV1 and FVC are reduced, often in proportion to each other

FEV1:FVC ratio is normal or increased (>80%).

Whereas in obstructive diseases:

High intrathoracic pressures generated by forced expiration cause premature closure of the airways with trapping of air in the chest

FEV1 is reduced much more than FVC

FEV1:FVC ratio is reduced (<80%). Flow-volume loops Flow-volume loops are graphs constructed from maximal expiratory and inspiratory maneuvers performed on a spirometer. The loop shape can identify the type and distribution of airway obstruction. After a small amount of gas has been exhaled, flow is limited by:

Elastic recoil force of the lung

Resistance of airways upstream of collapse.

Flow-volume loops are useful in diagnosing upper airway obstruction (Fig. 10.10). In restrictive diseases:

Maximum flow rate is reduced

Total volume exhaled is reduced

Flow rate is high during latter part of expiration because of increased lung recoil.

In obstructive diseases:

Flow rate is low in relation to lung volume

Expiration ends prematurely because of early airway closure

Scooped-out appearance is often seen after point of maximum flow.

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Figure 10.10 Typical flow-volume loops. (A) Normal; (B) restrictive defect (phrenic palsy); (C) volume-dependent obstruction (e.g., asthma); (D) pressure-dependent obstruction (e.g., severe emphysema); (E) rigid obstruction (e.g., tracheal stenosis).

Tests of lung volumes The amount of gas in the lungs can be thought of as being split into subdivisions (Fig. 4.3), with

disease processes altering these volumes in specific ways. In measuring tidal volume and vital capacity, we use spirometry; alternative techniques are needed for the other volumes. Residual volume (RV) and functional residual capacity (FRC) One important lung volume, residual volume (RV), cannot be measured in simple spirometry, because gas remains in the lungs at the end of each breath (otherwise the lungs would collapse). Without a measure for RV, we cannot calculate functional residual capacity (FRC) or total lung capacity (TLC). Remember that FRC is the volume of gas remaining in the lung at the end of a quiet expiration. RV is the volume remaining at the end of a maximal expiration. Look back at the subdivisions of lung volumes on p. 47 (Fig. 4.3) if you are unsure as to how FRC, RV, and TLC relate to each other.

RV is a useful measure in assessing obstructive disease. In a healthy subject, residual volume is approximately 30% of total lung capacity. In obstructive diseases, the lungs are hyperinflated with "air trapping" so that RV is greatly increased and the ratio of RV:TLC is also increased. There are three methods of measuring RV: helium dilution, plethysmography, and nitrogen washout.

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Helium dilution The patient is connected to a spirometer containing a mixture of 10% helium in air. Helium is used because it is an insoluble, inert gas that does not cross the alveolar-capillary membrane. At the end of an expiration, the patient begins to breathe from the closed spirometer; after several breaths, the helium concentration in the spirometer and lung becomes equal. The helium concentration is known at the start of the test and is measured when equilibrium has occurred. The dilution of helium is related to total lung capacity. Residual volume can be calculated by subtracting vital capacity from total lung capacity. The helium dilution method measures only gas that is in communication with the airways. Body plethysmography Plethysmography determines changes in lung volume by recording changes in pressure. The patient sits in a large air-tight box and breathes through a mouthpiece (Fig. 10.11). At the end of a normal expiration, a shutter closes the mouthpiece and the patient is asked to make respiratory efforts. As the patient tries to inhale, box pressure increases. Using Boyle's law, lung volume can be calculated. This method measures all intrathoracic gas including cysts, bullae, and pneumothoraces. In contrast to the helium dilution method, body plethysmography defines the extent of noncommunicating airspace within the lung; this is important in subjects with chronic obstructive pulmonary disease (e.g., emphysema). Nitrogen washout

Following a normal expiration, the patient breathes 100% oxygen. This "washes out" the nitrogen in the lungs. The gas exhaled subsequently is collected and its total volume and the concentration of nitrogen are measured. The concentration of nitrogen in the lung before washout is 80%. The concentration of nitrogen left in the lung can be measured by a nitrogen meter at the lips measuring end expiration gas. Assuming no net change in the amount of nitrogen (it does not participate in gas exchange) it is possible to estimate the FRC. Anatomic dead space The volume of anatomic dead space (i.e., areas of the airway not involved in gaseous exchange) is usually about 150 ml, or 2 ml/kg of body weight. In a healthy person, the physiologic and anatomic dead spaces are nearly equal; however, in patients with alveolar disease and nonfunctioning alveoli (e.g., in emphysema), physiologic dead space may be up to ten times that of the anatomic deadspace. Fowler's dead space Fowler's dead space method uses the single-breath nitrogen test to measure anatomic dead space. The patient makes a single inhalation of 100% O2. On expiration, the nitrogen concentration rises as the dead space gas (100% O2) is washed out by alveolar gas (a mixture of nitrogen and oxygen). If there were no mixing of alveolar and dead space gas during expiration there would be a stepwise increase in nitrogen concentration when alveolar gas is exhaled (Fig. 10.12A). In reality, mixing does occur which means that the nitrogen concentration increases slowly, then rises sharply. As pure alveolar gas is expired, nitrogen concentration reaches a plateau (the alveolar plateau). Nitrogen concentration is plotted against expired volume; dead space is the volume at which the two areas under the plot are equal (Fig. 10.12B).

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Tests of diffusion Oxygen and carbon dioxide pass by diffusion between the alveoli and pulmonary capillary blood. The diffusing capacity of carbon monoxide (DL(CO) known as the Transfer Factor or TLCO in Europe) measures the ability of gas to diffuse from inspired air to capillary blood, and also reflects the uptake of oxygen from the alveolus into the red blood cells. Carbon monoxide is used because:

It is highly soluble

It combines rapidly with hemoglobin.

The single-breath test is the test most commonly used to determine diffusing capacity.

Figure 10.12 Measurement of anatomic dead space. (A) Using Fowler's method, it would be expected that the gas expired from those areas not undergoing gas exchange (anatomic dead space) would contain no nitrogen and thus a stepwise change would occur to the nitrogen concentration of expired gas. The volume at which this occurs would be equal to the anatomic dead space volume. (B) On a graph showing the real-world results, a dotted line has been drawn to approximate the step change in nitrogen concentration.

Single-breath test The patient takes a single breath from residual volume to total lung capacity. The inhaled gas contains 0.28% carbon monoxide and 13.5% helium. The patient is instructed to hold his or her breath for 10 seconds before expiring. The concentration of helium and carbon monoxide in the final part of the expired gas mixture is measured and the diffusing capacity of carbon monoxide is calculated. You need to know the hemoglobin level before the test. In the normal lung, DLCO accurately measures the diffusing capacity of the lungs whereas, in diseased lung, diffusing capacity also depends on:

Area and thickness of alveolar membrane

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Ventilation:perfusion relationship. Diffusing capacity Diffusing capacity (DLCO) is defined as the amount of carbon monoxide transferred per minute, corrected for the concentration gradient of carbon monoxide across the alveolar capillary membrane (Fig. 10.13). DL(CO) is reduced in conditions where there are:

Fewer alveolar capillaries

Ventilation:perfusion mismatches

Reduced accessible lung volumes.

Gas transfer is a relatively sensitive but nonspecific test, useful at detecting early disease in lung parenchyma; DLCO/VA ratio is a better test. The DLCO is corrected for alveolar volume (VA) and is useful in distinguishing causes of low DLCO due to loss of lung volume:

DLCO and DLCO/VA are low in emphysema and fibrosing alveolitis

DLCO is low, but DLCO/VA is normal in pleural effusions and consolidation.

Figure 10.13 Conditions that affect diffusing capacity.

Tests of blood flow It might help to think of the difference between DLCO and DLCO/VA in terms of a patient who has had a lung removed. Clearly, lung volumes are reduced and therefore so is DLCO. But DLCO/VA corrects for the lost volume, and if the remaining lung is normal, DLCO/VA is also completely normal. Pulmonary blood flow can be measured by two methods: the Fick method and the indicator dilution technique. Fick method The amount of oxygen taken up by the blood passing through the lungs is related to pulmonary blood flow and the difference in oxygen content between arterial and mixed venous blood. Oxygen consumption is measured by collecting expired gas in a large spirometer and measuring its oxygen concentration. Indicator dilution technique Dye is injected into the venous circulation; the concentration and time of appearance of the dye in the arterial blood are recorded.

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Similarly, in the thermal solution method ice-cold saline is injected into the venous system (usually the right heart). The change in temperature in blood is measured in the pulmonary artery over time and is the basis for calculation of blood flow.

Appendix 2

Reference Values for Lung Function Tests

Normal va lues depend on age, gender, height , weight , and ethnic or igin . Th is is a

complex subject; for a deta i l ed discussion, see pages 445–513 of Cotes JE. Lung

Funct ion. 5 th ed. Oxford: Blackwel l , 1993. Reference va lues for some common tests

are shown in Table A-1. References 50 and 51 conta in addi t ional usefu l d iscuss ions of

normal values. There is evidence that people are becoming health ier and that lung

funct ion is improving.

TABLE A-1 Example of Reference Values for Common Pulmonary Function Tests in White

Nonsmoking Adults in the United States

Men Women

TLC (l) 7.95 St* + 0.003 A† - 7.33

(0.79)‡

5.90 St - 4.54 (0.54)

FVC (l) 7.74 St - 0.021 A - 7.75 (0.51) 4.14 St - 0.023 A - 2.20 (0.44)

RV (l) 2.16 St + 0.021 A - 2.84 (0.37) 1.97 St + 0.020 A - 2.42 (0.38)

FRC (l) 4.72 St + 0.009 A - 5.29 (0.72) 3.60 St + 0.003 A - 3.18 (0.52)

RV/TLC (%) 0.309 A + 14.1 (4.38) 0.416 A + 14.35 (5.46)

FEV1 (l) 5.66 St - 0.023 A - 4.91 (0.41) 2.68 St - 0.025 A - 0.38 (0.33)

FEV1/FVC (%) 110.2 - 13.1 St - 0.15 A (5.58) 124.4 - 21.4 St - 0.15 A (6.75)

FEF25–75% (ls–1

) 5.79 St - 0.036 A - 4.52 (1.08) 3.00 St - 0.031 A - 0.41 (0.85)

MEF50% FVC (ls–1

) 6.84 St - 0.037 A - 5.54 (1.29) 3.21 St - 0.024 A - 0.44 (0.98)

MEF25% FVC (ls–1

) 3.10 St - 0.023 A - 2.48 (0.69) 1.74 St - 0.025 A - 0.18 (0.66)

Dl (ml min–1

16.4 St - 0.229 A + 12.9 (4.84) 16.0 St - 0.111 A + 2.24 (3.95) mmHg–1

)

Dl/VA 10.09 - 2.24 St - 0.031 A (0.73) 8.33 - 1.81 St -n

*St is stature (height) (m),

† A is age (years).

‡ Standard deviation is in parentheses. From Cotes JE. Lung Function. 5th ed. Oxford:

Blackwell, 1993.