(bx•Qb)in
PPL
Ubx
(AWOx •QAWO)in
VA
(a)
PA
QAWO
AWO
(bx•Qb)out
VL
(b)
Figure 9.1 Models of the lungs (a) basic gas-transport unit of the pulmonary system. Here (x  Q) is
the molar flow of X through the airway opening, AWO, and the pulmonary capillary blood network, b.
Ubx is the net rate of molar uptake –that is, the net rate of diffusion of X into the blood. VD and VA are
the dead-space volume and alveolar volume, respectively. (b) A basic mechanical unit of the
pulmonary system. PA is the pressure inside the lung – that is, in the alveolar compartment. PPL and
PAWO are the pressures on the pleural surface of the lungs and at the airway opening, respectively. VL
is the volume of the gas space within the lungs, including the airways; QAWO is the volume flow of gas
into the lungs measured at the airway opening.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
pAWO
CstW
RAW
RAW
L  qAWO
pA
CstL
L
CstL
qAWO
pPL
pPL
pAWO
pA
CstW
pBS
 pMUS
+
pMUS
pBS
(a)
(b)
Figure 9.2 Models of normal ventilatory mechanics for small-amplitude, low-frequency
(normal lungs, resting) breathing (a) Lung mechanical unit enclosed by chest wall. (b)
Equivalent circuit for model in Figure 9.2(a).
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 9.3 Pneumotachometer flow-resistance elements (a) Screen. (b) Capillary tubes or
channels.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 9.4 Pneumotachometer for measurements at the mouth (a) Diameter adapter that
acts as a diffuser. (b) An application in which a constant flow is used to clear the dead space.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 9.5 Volume ranges of the intact ventilatory system (with no external loads applied).
TLC, FRC, and RV are measured as absolute volumes. VC, IC, ERV and VT are volume
changes. Closing volume (CV) and closing capacity (CC) are obtained from a single-breath
washout experiment.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Rotational
displacement
sensor
Other signal
processing
Strip-chart
recorder
Counterweight
Kymograph
Bell
Water seal
One-way
valves
PS
TS
FS x
VS
Uabs
Mouthpiece
Soda-lime
canister
Thermometer for
spirometer gas
temperature
TL
PA
QAWO
Spirometer
system
Blood
flow
VL
Ubs
FA x
Pulmonary
system
Figure 9.6 A water-sealed spirometer set up to measure slow lung-volume changes. The
soda-lime and one-way-valve arrangement prevent buildup of CO2 during rebreathing.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
100% O2
One-way
valves
TS
TL
VL
FSN2
Spirometer
FAN2
VS
O2
+
N2
+
CO2
Nitrogen
analyzer
Figure 9.7 Diagram of an N2 washout experiment The expired gas can be collected in a
spirometer, as shown here, or in a rubberized-canvas or plastic Douglas bag. N2 content is then
determined off-line. An alternative is to measure expiratory flow and nitrogen concentration
continuously to determine the volume flow of expired nitrogen, which can be integrated to
yield an estimate of the volume of nitrogen expired.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
(dPM)0
(dPB)0
(PM  Patm )
PM
Shutter
closed
PB
Shutter
Figure 9.8 A pressure-type total-body
plethysmography is used with the
shutter closed to determine lung
volume and with the shutter open to
determine changes in alveolar
pressure. Airway resistance can also
be computed if volume flow of gas is
measured at the airway opening.
Because atmospheric pressure is
constant, changes in the pressures of
interest can be obtained from
measurements made relative to
atmospheric pressure.
QAWO
QAWO
VL
PA
TL
NL
-QAWO
dQAWO
dPB
Shutter
open
VB
PB
TB
NB
Pump
VP
PB
(PB –Patm)
Calibration
VP VP
=
PB BPB
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
PB
V
L
TLC
Less stiff
Normal
TLC
Normal
VC
Normal
VC
FRC
RV
Slope of linear approximation
to curve (static compliance)
VT
Normal
FRC
TLC
VC
FRC
Normal
RV
Stiffer lung
RV
PL = PAWO –PPL
Figure 9.9 Idealized statically determined expiratory pressure-volume relations for the lung.
The positions and slopes for lungs with different elastic properties are shown relative to
scales of absolute volume and pressure difference.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
-QAWO
(Expiration)
VL  TLC
VL < 0.8 TLC
(Inspiration)
(PAWO –PA)
Figure 9.10 Idealized isovolume pressure-flow curves for two lung volumes for a normal
respiratory system. Each curve represents a composite from numerous inspiratory-expiratory
cycles, each with successively increased efforts. The pressure and flow values measured as the
lungs passed through the respective volumes of interest are plotted and connected to yield the
corresponding curves.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
QAWO
Maximal expiratory
Flowvolume (MEFV) curves
(Expiration)
Effort independent
TLC
(FVC  QAWOdt)
TLC
Reduced FVC
Normal FVC
Effort
independent
0
1
2
3
4
Time vital capacity (TVC) spirograms
Time, s
Figure 9.11 Alternative methods of displaying data produced during a forced vital capacity
expiration. Equivalent information can be obtained from each type of curve; however,
reductions in expiratory flow are subjectively more apparent on the MEFV curve than on the
timed spirogram.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 9.12 Essential elements of a medical mass spectrometer.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 9.13 General arrangements of the components of an infrared spectroscopy system.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Figure 9.14 N2 analyzer employing emission spectroscopy.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Readout
scale
Light
source
Sample
in
A
Pressure
sensor
D
F
J
E
Magnets
Dumbbell-shaped
test body
Point of
suspension
(a)
B
(b)
Figure 9.15 Oxygen analyzers (a) Diagram of the top view of a balance-type paramagnetic
oxygen analyzer. The test body either is allowed to rotate (as shown) or is held in place by
counter torque, which is measured to determine the oxygen concentration in the gas mixture.
(b) Diagram of a differential pressure and a magneto-acoustic oxygen analyzer (see text for
descriptions).
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
C
Figure 9.16 Distributions of
volume and gas species at RV and
TLC for a vital-capacity
inspiration of air or pure oxygen.
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.
Conducting airway
filled with 100% O2
Well-mixed alveolar
compartment
(a)
Figure 9.17 single-breath
nitrogen-washout maneuver
(a) An idealized model of a
FEN2
lung at the end of a vitalcapacity inspiration of pure O2,
preceded by breathing of
normal air. (b) Single-breath
N2-washout curves for
idealized lung, normal lung,
and abnormal lung. Parameters
of these curves include
anatomical dead space, slope
of phase III, and closing
volume.
Normal slope
 0.02/500 ml
Ideal lung
Abnormal slope
>0.02/500 ml
Normal lung
I
II
III
Abnormal lung
0
750
1250
IV
CV
Expired volume, vS
(ml)
Anatomical dead space
volume, V' D
TLC
Lung volume, vL
RV
(b)
© From J. G. Webster (ed.), Medical instrumentation: application and design. 3rd ed. New York: John Wiley & Sons, 1998.