Sunil Dr. Sharma
Senior Resident
Dept of Pulmonary Medicine
lung volumes measured by spirometry are useful
for detecting, characterising & quantifying the
severity
severity of lung disease
Measurements of absolute lung volumes, RV, FRC
& TLC are technically more challenging
limiting use in clinical practice
Precise role of lung volume measurements in the
assessment of disease severity, functional
disability, course of disease and response to
treatment remains to be determined
Lung volume are necessary for a correct
physiological diagnosis in certain clinical
conditions
Contrast to the relative simplicity of spirometric
volumes
volumes variety of disparate techniques have
been developed for the measurement of
absolute lung volumes
Various methodologies of body plethysmography,
nitrogen washout, gas dilution, and radiographic
imaging methods
Eur Respir J 2005; 26: 511522
lung volume usually refers to the volume of
th
gas within the lungs, as measured by body
plethysmography, gas dilution or washout
Lung volumes derived from conventional chest
radiographs are usually based on the volumes
within the outlines of the thoracic cage &
include
volume of tissue (normal and abnormal)
lung gas volume
Lung volumes derived from CT scans can also
include estimates of abnormal lung tissue
volumes
There are four volume subdivisions which
do not overlap
can not be further divided
when added together equal total lung capacity
Lung capacities are subdivisions of total
volume that include two or more of the 4
basic
basic lung volumes
Tidal Volume
Inspiratory Reserve Volume
Expiratory Reserve Volume
Residual Volume
Tidal volume
The amount of gas inspired or expired with each
breath
Inspiratory Reserve Volume
Maximum amount of additional air that can be
inspired from the end of a normal inspiration
inspired
Expiratory Reserve Volume
The maximum volume of additional air that can
be
be expired from the end of a normal expiration
Residual Volume
The volume of air remaining in the lung after a
maximal expiration
This is the only lung volume which cannot be
meas
measured with a spirometer
Total Lung Capacity
Vital Capacity
Functional Residual Capacity
Inspiratory Capacity
Total Lung Capacity
volume of air contained in the lungs at the end of a
maximal inspiration
Sum of all four basic lung volumes
TLC = RV + IRV + TV + ERV
Vital Capacity
The maximum volume of air that can be forcefully
expelled from the lungs following a maximal
inspiration
Largest volume that can be measured with a
th
spirometer
VC = IRV + TV + ERV = TLC - RV
Functional Residual Capacity
The volume of air remaining in the lung at the
end of a normal expiration
FRC = RV + ERV
Inspiratory Capacity
Maximum volume of air that can be inspired from
Maximum
end expiratory position
This capacity is of less clinical significance than
the other three
IC = TV + IRV
Use a spirometer
IRV
IC
VC
TV
TLC
Can Use
Spiromenter
ERV
FRC
RV
RV
Cant Use a Spirometer
Cannot use spirometry
Measure FRC, then use:
FRC
RV = FRC ERV
Residual Volume is determined by one of 3
techniques
techniques
Gas Dilution Techniques
Nitrogen washout
Helium dilution
Whole Body Plethysmography
Radiography
Two most commonly used gas dilution
methods for measuring lung volume
open circuit nitrogen (N2) method
closed-circuit helium (He) method
Both methods take advantage of
physiologically inert gas that is poorly soluble in
alveolar blood and lung tissues
both
both are most often used to measure functional
residual capacity
In the open-circuit method, all exhaled gas is
collected while the subject inhales pure oxygen
Initial concentration of nitrogen in the lungs is
assumed to be about 0.81
rate of nitrogen elimination from blood and
tissues about 30 mL/min
measurement of the total amount of nitrogen
washed out from the lungs permits the
calculation of the volume of nitrogen-containing
gas present at the beginning of the manoeuvre
Mass Balance:
Balance:
0.80 FRC = Vspirometer x FN2
FRC
N2 Start
N2 Finish
V S p iro m e ter F N ( sp io m eter) 4 0 , 0 0 0 m l 0 . 0 5
FRC =
=
F N (lu n g )
0 .8
2
2
An advantage of the open-circuit method is that
th
permits an assessment of the uniformity of
ventilation
ventilation of the lungs by
analyzing the slope of the change in nitrogen
concentration over consecutive exhalations
measuring the end-expiratory concentration of
measuring
nitrogen after 7 minutes of washout
by measuring the total ventilation required to reduce
end-expiratory nitrogen to less than 2%
Am Rev Respir Dis 1980; 121:789-794
The open-circuit method is sensitive to
Leaks anywhere in the system mouthpiece
Errors in measurement of nitrogen
concentration & exhaled volume
If a pneumotachygraph is used attention
must be paid to the effects of the change
in viscosity of the gas exhaled, because it
contains a progressively decreasing
concentration of nitrogen
Disadvantages
Does not measure the volume of gas in poor
communication with the airways e.g. lung bullae
Assumes that the volume at which the measurement
was made corresponds to the end-expiratory point
requires a long period of reequilibration with room air
before the test can be repeated
Measuring spirometric volumes immediately before
measuring FRC can eliminate the assumption of a
constant
constant or reproducible end-expiratory volume
Subject rebreathe a gas mixture containing
helium in a closed system until equilibriation
is achieved
Volume and concentration of helium in the
gas mixture rebreathed are measured
Final equilibrium concentration of helium
permits calculation of the volume of gas in
the lungs at the start of the manoeuvre
Start: known ml of 10% He in Spirometer
Rebreath for 10 min (until He evenly distributed)
FHe
in itia l
V S p ir o m ete r = F H e
FVC =
( FHe
in itia l
fin a l
( V S p iro m eter + F R C )
FHe
fin a l
FHe
fin a l
) V S p iro m eter
Thermal-conductivity meter measures the helium
th
concentration continuously, permitting return of the
sampled gas to the system
Because the meter is sensitive to carbon dioxide it is
removed from the system by adding carbon dioxide
absorber
Removal of CO2 & O2 consumption results in a
constant fall in the volume of gas in the closed circuit
An equivalent amount of oxygen is to be introduced
as an initial bolus or as a continuous flow
Closed-circuit method is sensitive to
errors from leakage of gas and alinearity
of the gas analyzer
Fails to measure the volume of gas in lung
bullae & cannot be repeated at short
cannot
repeated
s ho r t
intervals
Test results are reproducible
Scand J Clin Lab Invest 1973; 32:271-277
Three types of plethysmograph
pressure
Volume
pressure-volume/flow
Has a closed chamber with a
fixed volume in which the
subject breathes
Volume changes associated
with compression or expansion
of gas within the thorax are
measured as pressure changes
in gas surrounding the subject
within the box
ithi th
Volume exchange between
lung and box does not directly
cause pressure changes
Thermal, humidity, & CO2- O2
exchange differences between
inspired and expired gas do
cause
cause pressure changes
Thoracic gas volume and resistance are
measured during rapid manoeuvres
Small leaks are tolerated or are introduced to
vent to slow thermal-pressure drift
Best suited for measuring small volume changes
because of its high sensitivity & excellent
frequency response
Measurements are usually brief and are used to
study rapid events it need not be leak-free,
absolutely rigid, or refrigerated
Has constant pressure and
variable
variable volume
When thoracic volume changes,
gas is displaced through a hole in
the box wall and is measured
spirometer or
integrating the flow through a
pneumotachygraph
Suitable for measuring small or
large
large volume changes
To attain good frequency response, the
impedance to gas displacement must be very
small
ll
Requires a
low-resistance pneumotachygraph
sensitive transducer
fast, drift-free integrator, or
meticulous utilization of special spirometers
Difficult to be used for routine studies
Combines features of both
types
As the subject breathes from
the room, changes in thoracic
gas volume compress or expand
the air around the subject in
the box and also displace it
di
it
through a hole in the box wall
Compression or decompression
of gas is measured as a
pressure change
displacement of gas is
measured
spirometer connected to the box
or
integrating airflow through a
pneumotachygraph
pneumotachygraph in the opening
All of the change in thoracic gas volume is accounted
for by adding the two components (pressure change
and volume displacement)
This combined approach has
wide range of sensitivities
permitting all types of measurements to be made with
the same instrument (i.e., thoracic gas volume and
airway resistance, spirometry, and flow-volume curves)
Box has excellent frequency response and relatively
modest
modest requirements for the spirometer
The integrated-flow version dispenses with waterfilled spirometers and is tolerant of leaks
Compressible gas in the thorax, whether or not it
th th
is in free communication with airways
By Boyle's law, pressure times the volume of the
gas in the thorax is constant if its temperature
remains
remains constant (PV = P'V')
At end-expiration, alveolar pressure (Palv)
equals atmospheric pressure (P) because there is
no airflow & V (thoracic gas volume) is unknown
Airway is occluded and the subject makes small
inspiratory and expiratory efforts against the
occluded
occluded airway
During inspiratory efforts, the thorax enlarge
th th
(V) and decompresses intrathoracic gas,
creating a new thoracic gas volume (V' = V + V)
and a new pressure (P' = P + P)
A pressure transducer between the and subject's
mouth the occluded airway measures the
new pressure (P)
Assumed - Pmouth = Palv during compressional
changes while there is no airflow at the mouth
pressure changes are equal throughout a static
fl
fluid system (Pascal's principle)
(P
Boyle Mariottes Law : P x V = constant under isothermal
conditions
PA x TGV = (PA - PA)(TGV + V)
Expanding and rearranging equation
TGV =( V / PA)(PA - PA)
Since PA is very small compared to PA (<2%) it is usually
omitted in the differential term
TGV ~ ( V / PA) x PA with PA = Pbar - PH2O,sat
TGV ~ ( V / PA) x (Pbar - PH2O,sat)
The measured TGV additionally includes any
apparatus dead spaces (Vd,app) as well as any
volume inspired above resting end-expiratory
lung volume at the moment of occlusion (Vt,occ)
FRCpleth can be derived from TGV by subtraction
of these two volume components
of
FRCpleth = TGV - Vd,app - Vt,occ
The thoracic gas volume usually measured is
th
slightly larger than FRC unless the shutter is
closed precisely after a normal tidal volume is
exhaled
Connecting
the mouth-piece assembly to a valve and spirometer
(or pneumotachygraph and integrator)
using a pressure-volume plethysmograph
makes it possible to measure TLC and all its
subdivisions in conjunction with the measurement of
thoracic gas volume
Problems
Effects of Heat, Humidity, and Respiratory
Gas Exchange Ratio
Changes in Outside Pressure
Cooling
Underestimation of Mouth Pressure
Compression Volume
In uncooperative subjects radiographic lung
volumes may be more feasible than physiological
measurements
The definition of the position of lung inflation at
the time of image acquisition is clearly essential
Volumes measured carry their own assumptions
and limitations, and cannot be directly
and
compared with volumes measured by the other
techniques
techniques
The principle is to outline the lungs in both A-P & lateral
chest radiographs, and determine the outlined areas
assuming a given geometry or
using
using planimeters in order to derive the confined volume
Adjustments are made for
magnification factors
volumes of the heart
intrathoracic tissue and blood
infradiaphragmatic spaces
In the determination of TLC, 625% of subjects differed by
>10% from plethysmographic measurements in adult subjects
Academic
Academic Press Inc., New York, 1982; pp. 155163
New
1982;
155
In addition to thoracic cage volumes, CTs can provide
estimates of
lung tissue and air volumes
volume of lung occupied by
Increased density (e.g. In patchy infiltrates) or
Decreased density (e.g. in emphysema or bullae)
In a study of children, comparable correlations were
observed for CT and radiographic measurements as
compared with plethysmographic TLC
Am J Respir Crit Care Med 1997; 155: 1649 1656
155 1649
Disadvantage
high radiation dose
MRI offers the advantage of a large number of
images within a short period of time, so that
volumes
volumes can be measured within a single breath
Potential for scanning specific regions of the
lung, as well as the ability to adjust for lung
water and tissue
water
despite the advantages of an absence of
radiation exposure its use for measuring thoracic
gas volume is limited by its considerable cost
Resistive Forces
Inertia of the respiratory system
(negligible)
Friction
lung & chest wall tissue surfaces gliding past
each other
lung tissue past itself during expansion
frictional resistance to flow of air through the
airways (80%)
Airflow in the Airways Exists in Three Patterns
Laminar
Turbulent
Transitional [distributed laminar]
Reynolds number = X Ve X D
= density
Ve= linear velocity of fluid
D = diameter of tube
= viscosity of fluid
Turbulent flow tends to take place when gas density, linear
velocity & tube radius are large
velocity
Linear velocity (cm/sec) of gas in the tube is calculated by
di
dividing the flow rate (L/sec) by tube area (cm2)
th fl
(L/
Tube area refers to total cross sectional area of the
airways of a given generation
Airflow is transitional throughout most
of tracheobronchial tree
Energy required to produce this flow is
thi fl
intermediate between laminar and
turbulent
Many bifurcations in tracheobronchial
tree, flow becomes laminar at very low
tree flow
Reynolds number in small airways distal
to the terminal bronchioles
Flow is turbulent only in the trachea
where the radius is large and linear
velocities reach high values [during
exercise, during a cough]
Airway resistance is easy to measure
repeatedly & is always related to the
lung volume at which it is measured
Measurements of RAW useful in differential
diagnosis of
type of airflow obstruction
localization of the major site of obstruction
Measured during airflow & represents the ratio of
the
the driving pressure and instantaneous airflow
RAW is determined by
measuring the slope ()
of a curve of
plethysmograph pressure
(x-axis) displayed
against airflow (y-axis)
on an oscilloscope during
rapid, shallow breathing
ap
through a
pneumotachygraph
within the
plethysmograph
Shutter is closed across the mouth-piece, and
the slope () of plethysmographic pressure (xaxis)
axis) displayed against mouth pressure (y-axis) is
measured during panting under static conditions
Because Pmouth equals Palv in a static system it
serves two purposes
serves
Relates changes in plethysmographic pressure
to changes in Palv in each subject
Relates RAW to a particular thoracic gas volume
Physiologic factors affecting plethysmographic measurement
of RAW
Airflow
RAW pertains to a particular flow rate during continuous
pressure-flow curves, so the slope may be read at any
desired airflow rate
RAW is measured at low flows, at which transmural
compressive pressures across the airways are small and the
relation to Palv is linear
Airway dynamics measured during forced respiratory
maneuvers is associated with
large transmural compressive pressures across the airways
maximal dynamic airway compression limiting airflow rates and
possible alterations in airway smooth muscle tone
under such circumstances, RAW may be increased markedly
be
Volume
Near TLC, resistance is small, but near RV,
TLC
RV
resistance is large
Lung volume may be changed voluntarily to
evaluate RAW at larger or smaller volumes in
health and disease
As a first approximation, airway conductance
(GAW), the reciprocal of RAW, is proportional to
lung
lung volume
Transpulmonary
Transpulmonary Pressure
RAW is related more directly to lung elastic recoil pressure
th
than to lung volume
Subjects with increased lung elastic recoil have a higher
GAW at a given lung volume because of increased tissue
tension pulling outward on airway walls
Loss of elastic recoil results in loss of tissue tension and
decreased traction on airway walls, so GAW is decreased
This relationship may be used to analyze the mechanism of
airflow limitation in various obstructive ventilatory defects
(e.g., bullous lung disease)
Airway
Airway Smooth Muscle Tone.
Airways affected markedly by smooth muscle tone,
depending on the state of inflation and volume
hi
history
Relationships are relevant to diseases in which
elat
smooth muscle tone is increased (e.g., asthma)
low lung volumes are encountered (e.g., during cough,
when pneumothorax is present)
when
Bronchoconstriction is not demonstrable temporarily
after
after a deep breath or at TLC in healthy subjects
RAW in healthy subjects may be greater when a given
lung volume is reached from RV than from TLC
Panting
Panting
Panting minimizes changes in the plethysmograph caused
by thermal, water saturation, and carbon dioxide-oxygen
exchange
exchange differences during inspiration and expiration
Improves the signal-to-drift ratio, because each respiratory
cycle
cycle is completed in a fraction of a second
gradual thermal changes and small leaks in the box
become insignificant compared with volume changes
attributable to compression and decompression of alveolar
gas
Glottis stays open, rather than partly closing and varying
position, as it does during tidal breathing
DuBois and colleagues described an oscillatory
method to measure the mechanical properties of
the
the lung and thorax
Eur Respir J 1996; 9:1747-1750
Use an external loudspeaker or similar device to
generate and impose flow oscillations on
spontaneous breathing
Impulse oscillometry measures RAW and lung
compliance independently of respiratory muscle
strength and patient cooperation
Sound waves at various frequencies (3 - 20 Hz)
(3 20
are applied to the entire respiratory system
piston pump can be used to apply pressure waves
around the body in a whole-body respirator
Slow frequency changes in pressure, flow, and
volume generated by the respiratory muscles
during normal breathing are subtracted from the
Raw
Raw data
permitting analysis of the pressure-flow-volume
relationships imposed by the oscillation device
The elastic forces of the lungs and chest wall oppose
the volume changes induced by the applied pressure
& decrease as the frequency of oscillation increases
The total force or pressure that opposes the driving
pressure applied by the loudspeaker can be measured
as peak-to-peak pressure difference divided by peakto-peak flow
combination of the resistance and
reactance
This resistance is proportional to the RAW in healthy
subjects and patients, although it does include a
small component of lung tissue and chest wall
resistance
resistance as well as the resistance of the airways
High frequency oscillating air flow is applied to
fl
the airways
Resultant pressure & airflow changes are
measured
Applying a/c theory Raw can be measured
contineously
J Appl Physol 1970; 28: 113-16
Measures total respiratory resistance through out
the vital capacity displaying resistance as
function
function of lung volume