2. LungVolume
-
Upload
shalini-soorya -
Category
Documents
-
view
566 -
download
1
Transcript of 2. LungVolume
1
Respiratory
System
Module 2
Lung Volumes and
Lung Capacities
Prof. Dr. Hamdan Noor
August, 2010
2
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
3
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).
4
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.
5
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.
6
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?
7
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.
8
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.
9
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.
10
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.
11
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?
12
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
13
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.
14
• 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?
15
PULMONARY FUNCTION TEST DECISION TREE
16
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..
17
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.
18
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.
19
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.
20
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.
21
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).
22
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
23
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.
24
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.