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Review article
255
Components of respiratory function tests
Bahaa El-Din Ewees Hassan, Mai M. Abdel-Aziz
Correspondence to Mai M. Abdel-Aziz, MD,
Department of Anesthesiology, Intensive Care,
and Pain Management, Faculty of Medicine,
Ain Shams University, Cairo 11566, Egypt
e-mail: [email protected]
The respiratory system is composed of the lungs, the conducting airways, the parts of the
central nervous system concerned with the control of the muscles of respiration, and the
chest wall [1]. The main functions of the respiratory system are to obtain oxygen from the
external environment and supply it to the cells and to remove from the body the carbon
dioxide produced by cellular metabolism [2]. Pulmonary function tests provide valuable
clinical information. They are designed to identify and quantify defects in the respiratory
system [3].
Received 19 April 2014
Accepted 29 May 2014
Keywords:
Department of Anesthesiology, Intensive Care,
and Pain Management, Faculty of Medicine,
Ain Shams University, Cairo, Egypt
Ain-Shams Journal of Anesthesiology
2014, 07:255–258
lung volumes, pulmonary function, respiratory function, spirometry
Ain-Shams J Anesthesiol 07:255–258
© 2014 Department of Anesthesiology, Intensive Care and Pain Managment,
Faculty of Medicine, Ain-Shams University, Cairo, Egypt
1687-7934
Components of pulmonary function tests
The volume of air entering or leaving the lung
in a single breath (SB) is called the tidal volume.
It is usually 500 ml. The maximum volume of air
that can be inhaled beyond this value is called the
inspiratory reserve volume and is about 3000 ml.
After normal expiration, the lung still contains the
functional residual capacity (FRC) and it averages
2500 ml. The maximal volume of air, beyond the
tidal volume, that can be exhaled using maximal
expiratory effort is the expiratory reserve volume.
Even after maximal expiration, 1000 ml air remains
in the lungs and is termed the residual volume (RV )
[4]. The maximal volume of air expired after a
maximal inspiration is the vital capacity; the forced
expiratory volume in 1 s ( FEV 1) is a variant of this
method [5] (Fig. 1).
Figure 1
Spirometry
Spirometry is used to measure the rate at which the lung
changes volume during forced breathing maneuvers. It
is the simplest and the most common test; it provides
most of the information obtained from performing
pulmonary function tests [6].
Spirograms and flow-volume curves
There are two methods for recording the flow-vital
capacity (FVC). The first, called the classic spirogram,
is that the patient blows into a spirometer that records
the volume exhaled, which is plotted as a function of
time. The FVC can also be plotted as flow-volume
(FV ) curve, in which the patient exhales forcefully
and rapidly through a flow meter that measures
the flow rate (l/s) at which the patient exhales [7]
(Fig. 2). The FVC is the total volume of air expired
during forceful expiration after maximal inhalation.
Its normal value varies with age, sex, and height. The
causes of decreased FVC are due to problems in either
of the following:
Figure 2
a
b
Lung volumes and capacities recorded on a spirometer [5].
The two ways to record spirogram: volume recorded as a function of
time (a) and flow-volume curve (b) [7]. FEF, maximal forced expiratory
flow; FEV, forced expiratory volume.
1687-7934 © 2014 Department of Anesthesiology, Intensive Care and Pain Management,
Faculty of Medicine, Ain-Shams University, Cairo, Egypt
DOI: 10.4103/1687-7934.139533
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256
Ain-Shams Journal of Anesthesiology
(1) The lung itself — for example, resection,
pulmonary fibrosis, congestive heart failure.
(2) The pleural cavity — for example, effusion.
(3) The chest wall — for example, scleroderma,
obesity, kyphoscoliosis.
(4) The respiratory muscles — for example,
diaphragmatic paralysis and myasthenia gravis.
FEF25–75
Forced expiratory value in 1 s
FEF75
The FEV1 is perhaps the most useful measurement
obtained from spirometry. It is the volume of air
exhaled in the first second of the FVC test. Similar
to the FVC, its normal value varies with age, sex,
and height [6]. In an obstructive defect, the FEV1 is
decreased by an amount that reflects the severity of
the disease. The FVC may be also decreased but to a
lesser degree. In a restrictive defect, the FEV1 is also
decreased. The FVC is almost always decreased. The
FEV1/FVC ratio is used to differentiate obstructive
form restrictive patterns [7].
This is the flow rate after 75% of the FVC has been
exhaled.
FEV1/FVC ratio
It is generally expressed as a percentage. The FEV1 is a
constant fraction of the FVC irrespective of lung size
in the normal adult. The ratio normally ranges from
75 to 85%, but it decreases somewhat with aging [8].
The significance of this ratio is that it differentiates
between obstructive and restrictive defects when the
FVC is low. For example, in pulmonary restriction,
without any obstruction, the FEV1 and the FVC are
decreased proportionally; hence, the ratio remains
in the normal range. In severe obstructive disease,
the flow may be very low at the end of a forced
expiration. Continuation of a forced expiration can
be very tiring, and the FEV6 can be substituted for the
FVC [8] (Fig. 3).
Figure 3
a
b
c
Typical spirograms and flow-volume curves during forced expiration.
(a) Normal individuals of different sizes; (b) patient with severe airway
obstruction; (c) values typical of a pulmonary restrictive process [8].
This is the average maximal forced expiratory flow
(FEF) rate over the middle 50% of the FVC.
FEF50
This is the flow rate after 50% of the FVC has been
exhaled.
Maximal forced expiratory flow or peak expiratory flow
This occurs shortly after the onset of expiration and
can be calculated using hand-held devices, making this
measurement valuable for asthmatic patients at home
to monitor their status.
Maximum voluntary ventilation
The patient is instructed to breathe as hard and fast as
possible for 10–15 s. The results are extrapolated to 60 s
and reported in liters per minute. It correlates well with
a patient’s exercise capacity and with the complaint of
dyspnea [8].
Inspiratory flows
Some spirometers are capable of recording both
expiratory and inspiratory flows. The patient exhales
maximally (the FVC test) and then immediately
inhales as rapidly and completely as possible, producing
an inspiratory curve. The combined expiratory and
inspiratory FV curves form the FV loop. Increased
airway resistance decreases both maximal expiratory
flow and maximal inspiratory flow, detecting lesions
of the major airway [8]. Two major characteristics are
used to identify the obstruction as well as its site:
(1) According to the behavior of the lesion during
forced expiration and inspiration, the lesion
(obstruction) can be classified into:
(a) Variable: When narrowing occurs and flow
decreases, during one phase of respiration but
not the other.
(b) Fixed: When narrowing occurs and flow
decreases, equally during both expiration and
inspiration.
(2) The location of the lesion:
(a) Extrathoracic: This is when the lesion lies
outside the thoracic outlet.
(b) Intrathoracic: This is when the lesion lies
within the thoracic portion of the trachea
down to the carina but generally not
beyond [9] (Fig. 4).
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Pulmonary function tests Ewees Hassan and Abdel-Aziz
Figure 4
257
in the spirometer and lung. As essentially no helium is
absorbed, equations (1) and (2) can be combined and
solved for Vx, the FRC [10].
a
b
Plethysmography
c
d
e
f
g
h
Comparison of typical flow-volume loops in normal individuals: (a)
normal; (b) obstructive defect; (c) restrictive defect; (d) variable
extrathoracic obstruction; (e) variable intrathoracic obstruction; (f)
fixed airway obstruction [9].
Static (absolute) lung volumes
Measurement of the static lung volume is often useful.
The most important volumes are:
(1) The vital capacity or the slow vital capacity,
which is measured by having the patient inhale
maximally and then exhale slowly and completely.
(2) The RV with complete exhalation; the volume of
air remaining in the lungs is termed the RV.
(3) The total lung capacity.
The three most commonly used methods of
measuring the FRC (from which the RV is obtained)
are nitrogen (N2) washout, inert gas dilution, and
plethysmography.
Nitrogen washout method
At the end of a normal expiration, the patient is
connected to the system. The lung contains an
unknown volume (Vx) of air containing 80% N2.
With inspiration of N2-free oxygen and exhalation
into a separate bag, all N2 can be washed out of
the lung. The volume of the expired bag and its
N2 concentration are measured, and the unknown
volume is obtained with the simple mass balance
equation [10].
Inert gas dilution technique
Helium, argon, or neon can be used. In the helium
method, the spirometer system contains a known
volume of helium (V1) with a known concentration
(C1). At FRC, the patient is connected to the system
and rebreathes until the helium concentration reaches
a plateau, indicating equal concentration of helium (C2)
The theory is based on Boyle’s law, which states that the
product of the pressure (P) and volume (V) (PV) of a gas
is constant under constant temperature (isothermal)
conditions. The gas in the lungs is isothermal because
of its intimate contact with capillary blood. The
plethysmographic method measures essentially all the
gas in the lung, including that in poorly ventilated
areas [10].
Diffusing capacity of the lungs
As measuring the diffusing capacity of oxygen is
technically extremely difficult, the diffusing capacity
of carbon monoxide (DLCO) is much easier and
provides a valid reflection of the diffusion of oxygen.
The most widely used method to measure the
DLCO is the SB method. The patient exhales to RV
and then inhales a gas mixture containing a very low
concentration of carbon monoxide and an inert gas,
usually helium. After a maximal inhalation to total
lung capacity, the patient holds his or her breath
for 10 s and then exhales completely. A sample of
exhaled alveolar gas is collected and analyzed. By
measuring the concentration of the exhaled carbon
monoxide and helium, the value of the DLCO can
be computed.
Bronchodilator tests
Performing the spirometry test before and after
the administration of a broncholdilator is usually
carried out for patients undergoing spirometry for
the first time. A β-2 receptor agonist is usually
selected.
Tests for distribution of ventilation
There are many tests used to detect abnormal patterns
of ventilation distribution. The simplest method is the
SBN2 test.
The SBN2 test is performed as follows; the patient
exhales to RV and then inhales a full breath of
100% oxygen from the bag on the left (Fig. 5). A
slow, complete exhalation is directed by the one-way
valve through the orifice past the N2 meter into the
spirometer. The orifice ensures that expiratory flow will
be steady and slow (<0.5 l/s). N2 meter continuously
records the N2 concentration of the expired gas as it
enters the spirometer. With simultaneous plotting of
the expired N2 concentration against expired volume,
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258
Ain-Shams Journal of Anesthesiology
Figure 5
is said to reflect the onset of airway closure in the
dependent regions, and it is often called the closing
volume [11].
Acknowledgements
Conflicts of interest
None declared.
References
Equipment required to perform the single-breath nitrogen washout test.
A plot of exhaled nitrogen concentration (N2 conc) against exhaled
volume is shown at the lower right [11].
the normal graph is shown in Fig. 4. There are four
portions of the normal graph: phases I–IV. The
events during expiration in a normal individual are
as follows; the initial gas using the N2 meter comes
from the trachea and upper airway and contains 100%
oxygen. Thus, phase I shows 0% N2. As expiration
continues during phase II, alveolar gas begins washing
out the dead space oxygen and the N2 concentration
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of alveolar gas. During a slow expiration, initially gas
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regions, where the N2 concentration is the lowest.
As expiration continues, increasing amounts of gas
come from the more superior regions, where N2
concentrations are higher. This produces a gradually
increasing N2 concentration during phase III. An
abrupt increase in N2 concentration occurs at the onset
of phase IV. This reflects the decreased emptying of
the dependent regions of the lung. Most of the final
expiration comes from the apical regions, which have
higher concentration of N2. The onset of phase IV
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