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PJMS- Volume 3 Number 1: January-June 2013
Review Article
Pulmonary function tests in clinical practice:
Importance, requirements and limitations
Choudhary Sumer1
Abstract:
1
Associate Professor,
Department of Pulmonary
Medicine, NKP SIMS&RC,
Digdoh Hills,Hingna Road,
Nagpur -440019.
[email protected]
Pulmonary function tests have progressed from initially used water seal types to modern era electronic
computerized versions. The newer software are comparatively easier to operate and less time consuming. They are
patient friendly and easier to understand. However there are limitations to pulmonary function tests, as the pattern
of abnormality indicates type of problem however they do not provide anatomic diagnosis. Battery of tests is
available which help in evaluation of different aspects of pulmonary function. It is possible to monitor the
progression of disease and effect of management. However no single test can evaluate all aspects of pulmonary
function. Pulmonary function test can be carried out at bedside in critically ill patients with the help of portable
spirometers in addition to the routine clinical tests. Commonly performed pulmonary function tests are dynamic
studies –pre and post bronchodilator tests, evaluation of lung volumes with body plethysmography, nitrogen
washout or helium dilution methods, diffusion capacity of the lung carbon monoxide by single breath analysis,
arterial blood gases and pulse oximetry. Other commonly performed tests are maximal expiratory and inspiratory
pressure, exercise induced(Treadmill)or allergen induced bronchoprovocative tests, shunt studies and Dead space
measurements. If the pulmonary function tests are done with quality assurance, validation of the equipment,
proper technique, reference values and applying right ethnic correction factors, the data generated are most of the
times accurate and reproducible.
Keywords: Spirometry, Static lung volume, Pulmonary function, Diffusion capacity, Plethysmography.
Objectives:
1)
To understand the need of pulmonary function test as a
routine in a pulmonary clinic.
2)
To review the methods to evaluate the lung function's.
3)
To appreciate that pulmonary function test
interpretation with relevant patient information has
greater clinical meaning.
4)
To recognize the limitation of pulmonary function tests.
5)
To know the mistakes and pitfalls during pulmonary
function test and how to avoid them.
Introduction:
7)
8)
Detection of the precipitating factors.
Smokers who are at higher risk of developing airway
disease.
9)
Those who are at occupational risk of developing airway
disease such as coal miners, farmers etc.
The results of pulmonary function test of a given
individual are compared with those obtained from a normal
population of comparable height, age and gender. The test is
considered abnormal if it falls outside the range based on
standard error of the estimate in which 95% lies (1).
Measuring the lung function is now a routine clinical
tool in OPD for a practicing pulmonologist as it is not possible
to detect conduction disorders in heart on skiagram, similarly
one can't measure functional/physiological changes of lung on
X-Ray for which pulmonary function test is mandatory. The
treating physician should know about the physiology and
pathology of lung in health and disease for proper
management of patients.
Instruments Used for Measuring Pulmonary Function
Peak Flow Meter: This is a handheld device routinely used in
pulmonary OPD. There are different scales used. The scale
closest to the ethnic and race of the region should be used. It
measures peak expiratory flow rate. Inspiratory flow rate can
be also measured in some devices.
Spirometry: Initially there were two types of spirometry used.
They were mainly:
1)
Closed:
a)
Wet-Classical bell spirometer with water seal which was
commonly used prior to electronic pulmonary test.
b)
Dry- They are bellow volume, rolling seal, electronic
spirometers.
Pulmonary Function Tests provide
1)
2)
3)
4)
5)
6)
Objective evidence of deranged lung function caused by
underlying disease involving the bronchial tree,
pulmonary vasculature and chest bellows.
Help to identify the underlying cause either obstructive
or restrictive.
Helpful in preoperative evaluation.
Helpful in assessing progress of the diseases.
Helpful in monitoring the efficacy of treatment.
Evaluation of disability.
A spirometer, including the waterless, rolling seal type
and Stead-Wells water seal type is an instrument that directly
measures the volume of air displaced or measures airflow by a
flow-sensing device, such as a pneumotachometer or a tube
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PJMS- Volume 3 Number 1: January-June 2013
Review Article
containing a fixed resistance to flow (2). Today, most clinical
pulmonary function testing laboratories use a microprocessordriven pneumotachometer to measure air flow directly and
then to mathematically derive volume.
P1-resting mouth pressure
V-FRC
P2-pressure on inspiration
rV-rise in lung volume on inspiration
Methods and Means to Evaluate Functions of lung
Types of Spirometers (3)
With the help of the above mentioned devices one can
measure static and dynamic lung volumes.
The static mechanical properties of the lung are:
l
Static compliance of the lungs.
l
Static compliance of the chest wall.
l
Elastic properties of the respiratory system as a whole.
l
Respiratory muscle strength.
The dynamic mechanical properties of the respiratory system
are:
l
Forced vital capacity.
l
Flow volume curves.
l
Maximal voluntary ventilation.
Volume
Bellows
Rolling seal
Water
Dry
Flow sensing (Pneumotach)
Fleisch
Screen
Hot- wire
Computerized Spirometry: Now-a-days most commonly used
method. It is comparatively easier to use than the older
versions, is doctor and patient friendly device with almost
accurate results.
Bedside Clinical Tests
There are certain bedside tests which can help a clinician
in assessing the lung function. These are:
Pnemotactograph: This is another instrument very popularly
used for testing airflow. This device measures instant
respiratory airflow. Differential pressure transducer or turbine
or volume transducer are used of which differential pressure
flow transducer is commonly used. In this a sensitive
manometer detects the pressure drop across the slight
resistance, in the air stream Ultrasonic flow meters and the hot
wire anemometer is the other types used (3,4).
Body Plethysmography: This instrument was first introduced
by DuBois and colleague's in 1956. The word plethysmography
is a Greek word means “enlargement”(5). It measures both
absolute lung volume and airway resistance. There are many
types available. Three different types of body
plethysmography are used.
l
Pressure Plethysmography
l
Volume Plethysmography
l
Pressure Corrected Flow Plethysmography
Most commonly used is constant-volume plethysmography.
This method provides more accurate measure of thoracic gas
volume in patients with lung disease associated with gas
trapping. Plethysmography is a complex test. Proper
calibration of transducers and frequency response, thermal
stability and leaks should be properly checked (6-8).
The patient is seated comfortably, told to relax remove
dentures and support cheek. The door is closed and the
thermal transients to stabilize. After establishing a stable
baseline representing Functional Residual Capacity (FRC),
airway is occluded by closing the valve at end expiration and
the patient is instructed gentle pants at a frequency of 1Hz.
FRC is calculated as: P1V=P2(v+rV)
8
1)
Snider Test: The ability to blow a match held at 15 cm from
mouth. This correlates with maximum voluntary ventilation
of 60 liter/min or FEV1 of 1.6 litres. Same test done with a
match held at 8 cm correlates with maximum voluntary
ventilation of 40 liters/min (9).
2)
Forced Expiratory Time: It is measured over the trachea. It is
measured from the beginning to the end of the maximal
expiration from total lung capacity. Most normal subjects
have a forced expiratory time of 4 seconds and patients
with obstruction have more than 6 seconds (10, 11).
Now-a-days portable spirometers are available by which
lung function test can be done at the bedside. Other clinical
tests are tracheal descent with inspiration, inspiratory
excavation of supraclavicular and suprasternal fossae and
paradoxical inward movement of the lower ribs during
inspiration. These tests are good to detect qualitative
abnormalities but lack precision of quantitative
measurement. Following measurements are done in lung
function:
1.
Airway Function: This includes following:
1)
2.
3.
4.
Spirometry : Vital Capacity (VC), Forced Expiratory Volume
in 1 sec (FEV1), Forced Vital Capacity (FVC), FEV1/FVC,
Forced expiratory flow 25%-75%.
Flow Volume loops.
Peak Expiratory Flow Rate.
Airway resistance by body plethysmography.
2)
Lung Volumes: Lung volumes are commonly measured by:
PJMS- Volume 3 Number 1: January-June 2013
Review Article
1.
2.
3.
Spirometry
Gas Dilution Technique
Plethysmography
are highest around noon and lowest during early morning. In
normal persons FEV1 increase by 0.15l in the morning and
decrease by 0.05l in the afternoon. In asthmatics, lung
function parameters are lower in the morning (Morning Dip).
Circadian variations have also been documented for airway
resistance, functional residual capacity, total lung capacity and
residual volume.
Lung volumes commonly measured are tidal volume, total lung
capacity, functional residual capacity and residual volume.
3)
Diffusion: It is the ability to transfer oxygen across the
alveolar –capillary interface is diffusion capacity.
4)
Respiratory Muscle Function: Peak inspiratory and peak
expiratory maximal mouth pressure is simple test to
evaluate respiratory muscle strength.
Variation in measured lung function between
individuals can largely be attributed to height, weight, sex, age,
health statesman to some extent, race. Around 27%of interindividual variation remains unexplained (16).Young men have
continuous increase in lung function till reaching adult height
as compared to young women. After age of 30 years, a non
smoking individual usually loses 25 to 30 ml of FEV1 every year.
Measured spirometric values also differ between races.
Spirometry
The American Thoracic Society standardization
guidelines for acceptability and reproducibility criteria are
shown in (12). A well-trained pulmonary function technician
usually coaches the patient through the session until the
demonstrated reproducibility of key parameters suggests the
results represent the best possible measure of lung function at
that time.
Spirometry is the most commonly used test to measure
lung functions. Modern spirometer utilizing computer
technology offer distinct advantages over earlier generation
instruments. Spirometers can be programmed to detect and
flag technique related factors that effect results, including
cough, late peak flows, premature effort termination and
variation in maneuvers (12, 13). Before starting the tests,
technician should ascertain that the subject has understood
the instructions. One should observe following precautions:
l
l
l
l
l
l
Acceptability and Reproducibility Criteria for Spirograms
Acceptability Criteria(12):
Patient should avoid wearing tight clothes which may
restrict the chest movements and abdominal expansion
and be instructed against smoking, alcohol consumption,
vigorous exercises, or eating large meals 2 to 4 hours prior
to test.
Data gathered prior to testing include patient age, height,
weight, and time of day.
Transmission of infection should be avoided by strictly
adhering to hygiene and infection control measures.
Unobstructed mouth piece: Remove prosthetic loose
dentures if any, put mouth piece over the tongue.
Maximum inspiration.
Smooth continuous expiration with maximal effort.
Body position has a significant impact on spirometry,
especially FVC and vital capacity. The values are 8% and
2% lower, respectively, in supine and sitting position,
compared to standing. This is important in comparison
studies and obese persons. Increased peak expiratory
flow is seen in hyper extension of the neck due to
elongation and stiffening of the trachea. Flexion of the
neck decreases peak flow and increases airway resistance
(14, 15).
1.
Free from artifacts
l
Cough or glottis closure during the first second of
exhalation
Early termination or cutoff
Variable effort
Leak
Obstructed mouthpiece
l
l
l
l
2.
l
l
3.
l
l
l
Good start
Extrapolated volume is <5% of FVC or 0.15 L,
whichever is greater Or
Time to PEF is <120 ms (optional until further
information is available)
Satisfactory exhalation
6sec of exhalation and/or a plateau in the volume-time
curve or
Reasonable duration or a plateau in the volume-time
curve or
The subject cannot or should not continue to exhale
Repeatability Criteria
In certain patients, repeated force tests may trigger
bronchospasm resulting in decline of FVC and FEV1. A
diagnosis of reactive airway disease should be considered,
technical factors should not be assumed to explain this
observation. Lung function has a diurnal pattern. Peak flows
After three acceptable spirograms have been obtained, apply
the following tests.
l
l
9
Are the two largest FVCs within 0.2L of each other?
Are the two largest FEV1s within 0.2L of each other?
PJMS- Volume 3 Number 1: January-June 2013
Review Article
l
l
l
If both of these criteria are met, the test session may be
concluded.
If both of these criteria are not met, continue testing
until:
Both of the criteria are met with analysis of additional
acceptable spirograms or
A total of eight tests have been performed or
Save a minimum of three best maneuvers
Static Lung Volumes
Static lung volumes are determined using methods in
which airflow velocity does not play a role. The sums of two or
more lung volume subdivision constitute a lung capacity and
are expressed in liters at body temperature. Functional
Residual Capacity (FRC), the volume of gas present in the lung
at end expiration during tidal breathing, is a key component in
the measurement of lung volumes. FRC can be assessed by
following methods:
alveolus to the hemoglobin binding site, hence the term
transfer factor. It is commonly determined by the use of CO, as
its uptake is easy to measure and it follows the same diffusion
pathways that of oxygen. The basic equation used to calculate
DLCO is:
DLCO = Vco/PACO-PCCO
Where:
Vco is rate of disappearance of CO
PACO is alveolar concentration of CO
PCCO partial pressure of CO in blood
To ensure consistency following precaution should be taken.
1. The patient should be comfortably seated upright and
fully co operative.
2. The patient should not have recent strenuous exercise or
meal within 2 hours of prior to test.
3. The patient should refrain from smoking and alcohol
intake.
Nitrogen Washout Method (open- circuit method using
N2) This technique was introduced in 1940 by Darling,
Cournand, and Richards. In this method the patient breathes
100% oxygen for 7 minutes and during this period Nitrogen
concentration in expired air is measured. When the N2 level
falls to zero all N2 present in the lungs at the beginning of the
test has been washed out. The total volume of gas expired and
N2 in the expired gas is measured (17).
FRC= (volume of N2washed out)-(N2 tissue extraction)
/initial –finalN2 concentration.
N2 tissue extraction is calculated as (Body Surface Area x 96.5)
+35/ 0.8
l
Helium Dilution Method (closed – circuit method u s i n g
helium)
This is a closed circuit method. The system is readied by adding
2L of air and sufficient helium to achieve concentration of
approximately 10% helium. The patient rebreathes from the
closed circuit. The CO2 is absorbed by absorbent while O2 is
added through valve. As the helium mixes with the air in the
lungs its concentration falls. Stabilization of the helium
concentration indicated by a rate of change of less than 0.02
percent over 30 sec interval, indicates helium concentration
has equilibrated through the lung breathing circuit system. The
test usually ends in 7 minutes.
FRC=Initial He %-Final He %/Final He % (5, 18,19).
l
Body Plethysmography
As described previously.
Imaging techniques and sulphur hexafluoride
washout are not routinely used.
The maneuver begins with unforced exhalation to
residual volume. A good effort is when the inspiration is rapid
to ensure instantaneous lung filling and the inspired volume is
85 to 90% of vital capacity. Slower lung fillings reduces CO
uptake and a sub maximal effort from residual volume reduces
alveolar volume and produces low DLCO. A breath hold time of
10 to 12 s is standard (2,20-24).
Measurement of Arterial Blood Gasses
Oximetry arterial blood gasses and pH analysis are done
to evaluate the ventilatory, acid base status, oxygen carrying
capacity, and intrapulmonary shunt (25).
Factors Affecting Outcome of Pulmonary Function Test
The factors affecting lung functions can be broadly
described as analytical and non-analytical factors (20, 26-27).
Non-Analytical Factors
Diffusion Capacity (2)
The non-analytical factors are mostly patient related
factors. Anxiety related to the procedure, smoking just before
the test, underlying pain, wearing tight clothing, certain
medication, time of the test, having meals before the test may
alter the actual findings in a particular patient.
Diffusion Capacity:
It implies the maximum transfer ability of the lung and is
governed by its structural and functional properties. The gas
has to travel through several barriers as it moves from the
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PJMS- Volume 3 Number 1: January-June 2013
Review Article
The major non-analytical factor leading to
misinterpretation is ethnic correction scale used. Most of the
earlier versions of electronic spirometers have correction scale
according to European standard, whose height weight and age
factor doesn't correspond to our region. Now-a-days
Spirometers are available in which we can manually enter
ethnic correction factor according to the place where the test
is being carried out.
7.
8.
9.
Analytical Factors
There are many opportunities of error in doing lung
function test. This may be equipment related, technique
employed or calculations done. In the beginning there was no
proper standardization of the different test carried out. In the
recent era a global standardization of the method, calculation
technique and precaution to be taken have been formulized
which if followed would certainly give more accurate results.
10.
11)
Conclusion :
12)
There is no doubt that pulmonary function tests play an
important role in diagnosis, management, and followup in
patients at risk and those who are having lung disease. Since
the invention of spirometry, when water seal types spirometer
where used to the recent times, has passed through various
generation to the present computerized electronic
spirometer. The presently used spirometers have ethnic
correction factor according to the region and can also be fed
manually by the treating pulmonologist. The newer
generation spirometer are simpler to use and both patient and
doctor friendly. The new software used are self explanatory
and the patient either adult or children can easily understand
the type of test to be done. Though the tests are subjective has
got clinical importance in assessing the patient disease and
management. All such functional laboratories should have
quality control data's so the tests are accurate and
reproducible. Quality control depends upon equipment
performance validation technique, reference values and
uniformity in interpretation methods.
13)
14)
15)
16)
17)
18)
References :
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2.
3.
4.
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