Partial body plethysmograph for measuring human respiratory system impedance

A partial body plethysmograph is provided to enable accurate modeling of a human's respiratory system for input frequencies of up to about 96 Hz using transfer impedance measurements. One preferred embodiment comprises a container for sealingly enclosing a portion of the subject's body, a pressure source for generating pressure signals over a predetermined frequency range, an air flow sensor for measuring the flow of air supplied to the subject, a pressure sensor for measuring the pressure applied to the cavity of the subject and a processor for controlling the pressure source, for storing the measured air flow and pressure and for determining the respiratory system impedance of the subject. An eight element model is used by the processor to inversely model the transfer impedance data and thereby determined characteristics of the respiratory system of the subject.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to plethysmography and forced oscillation 
respiratory impedance testing of humans to determine respiratory 
characteristics. 
2. Description of the Prior Art 
Measurements of human respiratory system impedance using a forced 
oscillation technique can provide quantitative insight into the mechanical 
properties of the respiratory system. Typically, the impedance spectra are 
interpreted using an electromechanical model with proposed physiological 
parameters. The most widely used electromechanical model is the 
six-element model proposed by DuBois et al. in 1956 which allows a lumped 
separation of airways and tissues. DuBois et al. introduced two methods of 
non-invasively measuring the mechanical properties of the human 
respiratory system. In one method, small amplitude pressure oscillations 
are applied at the airway opening (mouth) and the resulting air pressure 
and air flow at the airway opening are measured. The ratio of the pressure 
to flow at the airway opening (P.sub.ao /V.sub.ao) is termed input 
impedance (Z.sub.in). In the second method, the pressure oscillations are 
applied at the chest wall (P.sub.cw). The pressure applied to the chest 
and the air flow at the airway opening (V.sub.ao) are measured. The ratio 
(V.sub.ao) is termed transfer impedance (Z.sub.tr). 
To analyze respiratory system impedance data, DuBois et al. proposed a 
three compartment, six element model based on the following assumptions: 
1) the lung is a monoalveolar compartment that can be represented by a 
simple gas compression, 2) the model parameters are frequency independent, 
and 3) the airways are noncompliant structures. As shown in FIG. 1, this 
model comprises an airway impedance compartment (Z.sub.aw) comprising an 
airways resistance (R.sub.aw) in series with an airways inertance 
(I.sub.aw). The tissue impedance compartment (Z.sub.ti) is modeled as a 
tissue resistance (R.sub.ti) in series with a tissue inertance (I.sub.ti) 
and a tissue compliance (C.sub.ti). These two compartments are separated 
by a shunt gas compression compartment (Z.sub.g) which is modeled as a 
simple gas compression term (C.sub.g). 
From this model the transfer impedance (Z.sub.tr) for the DuBois model is 
given by Equation 1 below: 
##EQU1## 
As Peslin et al. pointed out, though, in using this DuBois model to analyze 
Z.sub.tr, it is necessary to independently measure one of the six element 
parameter values. The most common practice has been to measure functional 
residual capacity (FRC) and then calculate C.sub.g from Equation 2 below: 
##EQU2## 
wherein P.sub.ATM =atmospheric pressure (1033 cm H.sub.2 O) and P.sub.H2O 
=partial pressure of water vapor at 100% saturation (64 cmH.sub.2 O). 
In order to perform transfer impedance testing, Peslin et al. disclosed a 
body box which completely encloses the subject and provides a tube for 
connecting the air supply and flow measurement devices to the mouth of the 
subject inside of the box. Peslin et al. also disclosed the use of a 
signal generator connected to a loudspeaker to provide the pressure to the 
subject and a computer to analyze the data collected using a six element 
model. Because the box entirely encloses the patient, however, some 
patients were apprehensive about this type of testing. 
Most studies of human transfer impedance have been limited to frequency 
ranges of 4-30 Hz. Others have used frequencies up to 64 Hz. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a system for accurately 
measuring transfer impedance in humans at frequencies greater than 64 Hz. 
It is another object of this invention to develop a structure for measuring 
transfer impedance in humans which is comfortable, adjustable to users of 
different sizes and reduces noise due to vibration. 
It is a further object of this invention to provide a more accurate model 
for deriving information from the measurements. 
It is still another object of the present invention to provide a method and 
system for estimating airway properties separate from tissue properties as 
well as separating airway resistance into central and peripheral 
components. 
It is another object of the present invention to increase the lowest 
frequency at which standing waves will occur and thereby increase the 
range of frequencies over which a homogeneous pressure distribution around 
the chest wall may be determined. 
Various embodiments of the present invention are provided. One preferred 
embodiment comprises a container for sealingly enclosing a portion of the 
subject's body, a pressure source for generating pressure signals over a 
predetermined frequency range, an air flow sensor for measuring the amount 
of air transmitted to the subject, and a pressure sensor for measuring the 
pressure applied to the thorax of the subject. 
A head-out, legs-out plethysmograph according to the present invention 
allows data to be reliably collected and used for frequency values of up 
to 96 Hz. Thereby, forced oscillation data may be accurately used with an 
eight element model of the respiratory system to allow for separation of 
airway and tissue information, as well as separation of central and 
peripheral airway information. By providing a system which accurately 
separates central and peripheral airway information, doctors and 
technicians may more accurately diagnose the condition of the respiratory 
function of patients without the need for invasive testing procedures. 
Other objects and advantages of the present invention will be apparent when 
the preferred embodiments of the present invention and the drawings are 
considered.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIGS. 3-8, head-out, legs-out plethysmograph 10 will be 
described. Without limiting the preferred embodiments, some of the 
elements in the plethysmograph have been described as "front," "back," and 
"side." As best shown in FIGS. 3-4, the outer surface of the box comprises 
back wall 34, side walls 45a and 45b, front base 44 and door 28. Back wall 
34, side walls 45a and 45b and front base 44 are preferably made of 
plywood which is preferably about 1 inch thick. The 1 inch thick plywood 
provides a strong structure and absorbs sound with little vibration. 
Alternatively, other material may be used, e.g., plexiglass, heavy plastic 
or the like. Plethysmograph 10 preferably has a bottom portion 35 which 
attaches between back wall 34 and front base 44. The front edge of bottom 
portion 35 abuts door 28 when door 28 is closed. To reduce sound loss and 
to seal plethysmograph 10, tubing 32 is preferably installed in a groove 
formed along the edge of bottom portion 35. Tubing 32 is preferably 
seamless latex tubing having a diameter significantly less than the 
thickness of bottom portion 35, such as a 1/4 inch diameter for a 1 inch 
thick bottom portion. Braces 27, 37, 40, 41, 42, 43 and 46 (FIGS. 3 and 6) 
may be added to stabilize the structure. These braces are preferably made 
of 1 inch plywood or similar material suitable for providing stability. In 
each of side walls 45a and 45b, a speaker hole 39 is preferably disposed 
for inserting a speaker therethrough. Speaker hole 39 may be any size 
suitable to fit speakers 6 and 7. Preferably, speaker 6 is mounted to side 
wall 45a and speaker 7 is mounted to side wall 45b to provide a more 
uniform sound distribution throughout plethysmograph 10. 
Within plethysmograph 10, a plurality of brackets 36 may be provided. For 
example, brackets 36a, 36b, and 36c may be provided on side wall 45a 
whereas brackets 36d, 36e and 36f may be provided on side wall 45b. 
Additionally, brackets 36a and 36d may be positioned substantially 
parallel to form a lowest seat setting. Similarly, brackets 36b and 36d 
may be positioned to form a medium setting and brackets 36c and 36f (FIG. 
6) may be positioned to form a highest seat setting. A seat (not shown) 
may then be placed on appropriate brackets so that the distance between 
the seat and the top of plethysmograph 10 substantially corresponds to the 
torso height of the subject to be tested. For example, by placing the seat 
on brackets 36c and 36f a subject having a relatively short torso may be 
accommodated; whereas, by placing the seat on brackets 36a and 36d, a 
subject having a relatively tall torso may be accommodated. The seat used 
in plethysmograph 10 preferably includes a layer of vibration insulation, 
e.g., foam rubber or the like, to assist in reducing noise. The seat may 
be constructed of, for example, plywood or plastic or the like. 
Preferably, the distances between the various brackets and the top of the 
plethysmograph are selected to accommodate a wide range of torso heights. 
For example, the plethysmograph may be designed to accommodate 90 percent 
of all adult humans. By using anthropometric data regarding both male and 
female humans, the distance from the brackets to the top of the 
plethysmograph may be selected to accommodate the desired range of adult 
humans. Sample shoulder to seat height dimensions measured for the 5th 
percentile and 95th percentile adult humans are depicted in Table 1 below 
for both males and females. 
______________________________________ 
SEX 5th percentile 
95th percentile 
______________________________________ 
Male 22.5 inches 
26.6 inches 
Female 21.2 inches 
24.6 inches 
______________________________________ 
Table 1: Shoulder to seat height data for adult humans 
Using three brackets, for example, the distance between the brackets and 
the top of the plethysmograph may be selected to be as depicted in Table 2 
below. Other distances may alternatively be selected. Additionally, the 
number of brackets used may be varied to provide either fewer or more seat 
positions. It should also be understood that a plethysmograph which 
accommodates a wider range of adult humans may also be desired to take 
into consideration even the 100th percentile adult human. 
______________________________________ 
Distance from brackets to 
Brackets top of plethysmograph 
______________________________________ 
36a-36c 28.00 inches 
36b-36d 25.50 inches 
36c-36e 22.75 inches 
______________________________________ 
Table 2: Bracket to top of plethysmograph seat distance options 
Attached to door 28 and front base 44 are a plurality of fasteners 30a-30j. 
Fasteners 30a-30j may be any type of fastening device suitable for 
securing leg plate 31 to door 28 and front base 44. Fasteners 30a-30j 
preferably are nuts and bolts arranged so that the nuts (not shown) are 
affixed outside of leg plate 31. Other types of fasteners 30a-30j may also 
be employed. Additionally, the number of fasteners may be increased or 
decreased as desired to suit the particular door and front base 
arrangement. 
As best seen in FIGS. 3 and 5a, the top of plethysmograph 10 comprises back 
top portion 25 and front top portion 26. The top may alternatively be 
formed by a single top portion. Affixed to top portions 25, 26 are 
fasteners 21a-21h (FIG. 5a) which may comprise bolts or other fastening 
devices. Fasteners 21a-21h cooperate with holes 71a-71h on head plate 20 
to secure head plate 20 to top portions 25, 26. Tubing 23 may be 
positioned within shallow grooves formed in top portions 25, 26 to seal 
the mating surfaces between top portions 25, 26 and head plate 20. The top 
of plethysmograph 10 is preferably disposed at an angle with respect to 
bottom portion 35. Normal forward inclination of the axis through the 
human head from a vertical axis which respect to the ground for most 
humans is about 10.degree.-15.degree.. Therefore, the top is preferably 
inclined at an angle of about 12.5.degree. from front to back as depicted 
in FIG. 3. This configuration is more comfortable for a subject when the 
subject is positioned in plethysmograph 10 with his or her head extending 
through head opening 60. Head opening 60 is preferably large enough to 
allow a human head to pass therethrough, for example, about 9 inches in 
diameter. 
Hinges (not shown) may be installed whereby top portions 25 and 26 may each 
be swung upward and away from plethysmograph 10. Alternatively, front top 
portion 26 may be attached to door 28 using brace 27 (FIG. 3) or some 
other mechanism so that when door 28 swings open, front top portion 26 
also opens. This arrangement of front top portion 26 and door 28 allows 
the subject to place his or her neck into an enlarged head opening 60. As 
such, top portions 25 and 26 operate as a yoke so that head opening 60 
need only be large enough to permit a human neck to fit therethrough. 
After the subject places his head in the enlarged head opening 60, door 28 
may be closed and correspondingly front top portion 26 closes around the 
subject's neck. Head opening 60 need then only have a diameter of about 6 
inches, or approximately the diameter of the 95th percentile adult human 
neck, when top portions 25 and 26 operate as a yoke. 
With reference to FIG. 5b, head plate 20 is preferably comprised of two 
semicircular portions which form a yoke to fit around a human neck. 
Opening 72 in head plate 20 is formed by the two semicircle portions. 
Surrounding the inside of the two semicircle portions is cushion material 
70 which is preferably soft foam rubber or the like which may be glued or 
otherwise affixed to the semicircle portions of the two halves of head 
plate 20. Other soft materials may additionally or alternatively be used. 
A plurality of head plates 20 may be provided, each head plate having a 
different inner diameter of opening 72. For example, one head plate 20 may 
have an opening 72 with an inner diameter of 5 inches while another may 
have an inner diameter of 6 inches. The inner diameter of head plate 20 
may be then selected to fit the size of the neck of the subject to be 
placed in plethysmograph 10 for testing. Head plate 20 is also provided 
with a plurality of fastener holes 71a-71h which cooperate with fasteners 
31a-31h on top portions 25 and 26. Nuts or other securing devices may be 
placed over the head plate 20 for securing head plate 20 to top portions 
25 and 26. 
Turning to FIG. 4, the front of plethysmograph 10 is depicted. Door 28 
includes a top door portion 50 and a bottom door portion 49 attached via 
hinges 29a-29c. Hinges 29a-29c allow bottom door portion 49 to swing open 
in a direction away from plethysmograph 10. Bottom door portion 49 
comprises two leg openings 47 through which the subject's legs may pass 
when seated in plethysmograph 10. Alternatively, plethysmograph 10 may 
have a solid front, whereby the patient's legs may be contained within 
plethysmograph 10. If leg openings are used, tubing 48 is provided around 
the leg openings 47 to seal the mating surfaces between leg plate 31 
(FIGS. 7 and 8) and bottom door portion 49. Tubing 48 may additionally 
extend into front base 44 since leg plate 31 preferably fits over front 
base 44 as well. Further, as seen in FIG. 6, tubing 32 may be provided in 
shallow grooves formed in braces 46a and 46b to form a seal when door 28 
is closed and thereby abuts bottom portion 35. Door 28 is attached to side 
wall 45a by hinges 51a, 51b and may be secured to side wall 45b by a latch 
(not shown) or any other type of device for temporarily securing door 28 
to side wall 45b. 
FIGS. 7 and 8 depict two embodiments of leg plate 31. FIG. 7 depicts a leg 
plate 31 for use when the seat is set at a high setting such as on 
brackets 36c and 36f. FIG. 8 depicts a leg plate 31 for use when the seat 
is set for a medium setting such as on brackets 36b and 36e. Each of leg 
plates 31 is provided with a plurality of bolt holes 80 each corresponding 
to a bolt 30 on door 28. Additionally, two leg openings are formed by two 
semicircles in the two sections of leg plate 31. The leg openings may be 
provided with cushion material 32 which is preferably foam rubber or the 
like for providing a comfortable, yet snug fit of the leg plates around 
the legs of the subject. 
As will be readily appreciated, the components of plethysmograph 10 secure 
the torso of a subject within a substantially airtight environment. By 
providing a plurality of braces, the stability of plethysmograph 10 is 
increased. Consequently, plethysmograph 10 is stable, vibration is 
minimized, and the purity of the sound directed to the subject in 
plethysmograph 10 is maximized, which in turn increases the accuracy of 
the data collected during testing. Further, by reducing the volume of the 
chamber which surrounds the subject's chest during testing, a wider range 
of pressure signal frequencies is permitted. The reduced volume increases 
the frequency at which standing waves first occur. Because standing waves 
occur at a higher frequency, vibration correspondingly does not occur 
until a much higher frequency. Therefore, the range of frequencies over 
which homogeneous pressure distribution around the chest may be claimed 
without undue interference from vibration is increased. The range of 
reliable frequencies is extended to at least 96 Hz according to the 
preferred embodiments. 
With reference to FIG. 2, there is shown a system for measuring transfer 
impedance comprising head-out, legs-out plethysmograph 10 and a 
differential pressure transducer 8 which is inserted into the subject's 
mouth. Differential pressure transducer 8 is connected via a 
pneumotachometer (not shown) to signal amplifier 11. Signal amplifier 11 
is connected via band-pass filter 12 and A/D converter 13 to computer 1. 
Also attached to plethysmograph 10 is a pressure transducer 9, which, in 
turn, is connected to pressure amplifier 15. Pressure amplifier 15 is 
connected to computer 1 via band-pass filter 17 and A/D converter 18. 
Pressure input to the subject may be achieved via speakers 6 and 7. 
Computer 1 is connected via D/A converter 2 and band-pass filter 3 to a 
dual-channel stereo amplifier 4. Stereo amplifier 4 transmits output 
signals to speakers 6 and 7. 
Signal input to the subject is achieved via computer 1, amplifier 4 and 
speakers 6 and 7. A pseudo random noise signal (PRN) is generated by 
computer 1. Computer 1 may be any computer or processor capable of 
transmitting data at a rate sufficient to provide digital information to 
D/A converter 2 such that D/A converter 2 may generate analog signals from 
about 0.1 to 128 Hz in appropriate intervals, for example, 2 Hz intervals 
and analyzing data which is collected. A Gateway 2000 386/25 computer, for 
example, may be used. Computer 1 preferably provides data at a rate of at 
least twice the highest frequency analog signal to be generated. Hence, a 
data rate of about 512 Hz, for example may be used, which is well above 
twice 128 Hz. A slower data rate may be used down to a minimum of 256 Hz. 
Digital data is transmitted from computer 1 to D/A converter 2 and then to 
band-pass filter 3. Band-pass filter 3 preferably eliminates the DC (0 Hz) 
component and high frequency harmonics such as through a band-pass range 
of about 0.5 Hz to 160 Hz. Band-pass filter 3 may be any type of analog 
band pass filter, such as an Ithaco model 4113, for example. The output of 
band-pass filter 3 is sent to the two channels of stereo amplifier 4. To 
ensure that the two channels of stereo amplifier 4 are evenly matched, an 
oscilloscope 5 may be used. Such signal matching techniques using 
oscilloscopes are well known. Each signal output from band-pass filter 3 
is then amplified and provided to one of speakers 6 and 7. Amplifier 4 may 
be any type of amplifier, such as a Crown, model D150A, for example. 
Speakers 6 and 7 may be any type of speaker having a frequency response 
sufficient to distinguish between frequencies in the low range (2 to 128 
Hz) being transmitted, such as a Pyle 15 inch Professional High Fidelity 
Woofer, part number W 1560, for example, which has a frequency response of 
20-3000 Hz. 
As an alternative embodiment, instead of two speakers 6 and 7, only one 
speaker could be used. When using one speaker, a one-channel amplifier may 
be used in place of stereo amplifier 4. When two speakers are used, 
testing is preferably performed to ensure that the speaker cones of 
speakers 6 and 7 move synchronously. If each of the two cones of speakers 
6 and 7 do not move into and out of plethysmograph 10 simultaneously, the 
net volume change in the chamber will be less than expected, and thus the 
test results will be inaccurate. To ensure synchronicity of speakers 6 and 
7, an accelerometer (not shown) may be used. It should be recognized that 
methods for ensuring synchronicity of two speakers are within the skill of 
one of ordinary skill in the art and many variations on performing this 
process may be used. 
The data collection systems will hereinafter be described. Air is supplied 
to the subject through differential pressure transducer 8. Differential 
pressure transducer 8 may be any type of differential pressure transducer 
such as a Celsco, model LCVR, 0-2 cm H.sub.2 O, for example. Differential 
pressure transducer 8 is mounted across a pneumotachometer (not shown) 
such as a Fleisch No. 2, for example. The pneumotachometer produces analog 
signals representative of changes in air flow supplied to the subject's 
mouth for transmission to amplifier 11. Other devices and methods of 
gathering air flow data may also be used. 
Amplifier 11 receives the analog signals from the pneumotachometer and 
amplifies those signals. Amplifier 11 may be any type of analog amplifier, 
such as a Phillips PM 5171 amplifier, for example. Amplifier 11 transmits 
this amplified signal to band pass filter 12 which preferably has a band 
pass range of between about 0.8 Hz and about 160 Hz. An Ithaco, model 
4113, for example may be used as band pass filter 12. Band-pass filter 12 
preferably filters frequencies above 0.5 Hz, which is approximately the 
frequency of the breathing cycle, and below 160 Hz to filter out high 
frequency noise. A/D converter 13 samples the analog output of band-pass 
filter 15 at about 512 Hz and transmits this digital data to computer 1 
for processing and subsequent analysis. 
Pressure data is collected primarily via pressure transducer 9. Pressure 
transducer 9 may be any type of pressure transducer such as a Microswitch 
brand, for example. Pressure transducer 9 is preferably placed on a 
heavily braced portion of plethysmograph 10 to reduce vibrations 
transmitted to the transducer. The location of pressure transducer 9, 
however, with respect to plethysmograph 10 is irrelevant due to the 
homogeneity of pressure within plethysmograph 10 created by utilizing two 
speakers 6 and 7. The pressure signal from pressure transducer 9 is 
transmitted to pressure amplifier 15 which amplifies the signal before 
passing it to band-pass filter 17. Band-pass filter 17, like band-pass 
filter 12, filters out high frequencies and passes a frequency band from 
about 0.8 Hz to about 160 Hz. Band-pass filter 17 may be any analog 
band-pass filter, such as an Ithaco, model 4113, for example. A/D 
converter 18 samples the data at about 512 Hz and transmits this data to 
computer 1 for storage and subsequent analysis. Differential pressure 
transducer 8 and pressure transducer 9 are preferably digitally matched 
according to known techniques. Air flow and pressure amplifier gains may 
be checked via oscilloscopes 14 and 16 before each subject is tested to 
ensure consistency with previous settings. Also, oscilloscope 5 may be 
used to ensure matching of the amplitudes of the output signals from 
amplifier 4 to speakers 6 and 7. The gain of the power amplifier is 
preferably adjusted so that the highest pressure delivery to 
plethysmograph 10 is obtained while ensuring that the speaker cones still 
move in approximately a sinusoidal fashion at even the lowest frequency. 
Adjustments are also preferably made to ensure that the sound level is 
comfortable for the subject being tested, that the box vibrations caused 
by the high volume are minimized and that the pressure inside the box does 
not exceed the measurement range of pressure transducers 8 and 9. 
Operation of the data collection and analysis method will now be described. 
Due to the structural stability and noise reduction provided by the 
preferred embodiments, data from about 2 to about 128 Hz may be 
transmitted and collected. A pseudo random noise (PRN) signal with a 
frequency content from about 2 to about 128 Hz in about 2 Hz increments is 
used and sent by computer 1 to D/A converter 2. As noted above, the 
frequency response of speakers 6 and 7 as used in this example, is only 
about 20-3000 Hz. Therefore, the low end of the signal (2-32 Hz) is 
preferably enhanced to improve the performance of the speakers through 
amplifier 4. The high end of the signal (80-128 Hz) may also be enhanced 
to improve the signal to noise ratio by increasing the applied pressure 
and resulting flow at those frequencies. Techniques for enhancing the 
lower frequencies are known in the art. A technique for minimizing the 
crest factor (difference between the maximum and minimum peaks) of the PRN 
signal divided by a term related to the total energy in the signal is also 
preferably performed. Such techniques are also well known in the art from 
the teaching of Van der Ouderaa et al. 
After collection of data generated at these frequency ranges, a gradient 
optimization technique, which minimizes the square of the difference 
between measured data and model impedance values, is used. The performance 
index of the gradient optimization technique is given by Equation 3 below: 
##EQU3## 
where Z.sub.trd (i) is the i.sup.th measured impedance data point and 
Z.sub.trm is the i.sup.th predicted impedance data point. 
The quality of the model fit to the data (S.sup.2) is determined by 
Equation 4 below: 
##EQU4## 
where n is the number of data points fit and P is the number of 
parameters. As S decreases, data more accurately fits the expected value 
for the subject. 
With reference to FIG. 9, an eight element model is employed to inversely 
model the data collected for Z.sub.tr into the various components of the 
model. Simple gas compression, C.sub.g, may be estimated according to one 
of the many techniques such as open-circuit multi-breath nitrogen washout, 
closed-circuit multibreath helium dilution, and body plethysmography, for 
example. C.sub.g is set as a constant according to one of the measurement 
techniques as described. C.sub.b represents a shunt pathway corresponding 
to the lumped bronchial airway wall compliance of the subject and 
separates the airway compartment into central and peripheral resistance. 
Since the vast majority of inertance of humans occurs in the trachea, all 
airway inertance is modeled in the central airway branch where the trachea 
is located. R.sub.caw represents the resistance of the central airways and 
R.sub.paw represents the resistance of the peripheral airways. I.sub.aw, 
R.sub.t, I.sub.t, and C.sub.t correspond to inertance of the airways, 
resistance of the tissue, inertance of the tissue and compliance of the 
tissue, respectively. 
This eight element model is used for interpreting the Z.sub.tr data 
collected. By inversely modeling the Z.sub.tr data gathered over the 
frequency range from about 2 Hz to about 96 Hz, and comparing the 
determined model values with expected model values, potential sources of 
disease or malfunction of the respiratory system may be determined. For 
example, an increase in the inversely modeled data collected for R.sub.aw 
(either R.sub.caw or R.sub.paw) as compared to a predetermined expected 
value for a subject having similar physical conditions may indicate an 
airflow obstruction. Such an obstruction may be indicative of an 
obstructive disease such as asthma, chronic bronchitis or emphysema, for 
example. This method of comparison of data collected and fit into the 
eight element model with an expected eight element model enables fast and 
non-intrusive means of determining patient respiratory characteristics. 
Because of the design of the head-out, legs-out plethysmograph of the 
present invention, data may be reliably collected and used at frequencies 
up to about 96 Hz. This range of frequencies is even larger than necessary 
to effectively utilize the eight element model. Therefore, if desired, a 
smaller range of frequencies may be employed. The structure of the 
plethysmograph provides an increase in the reliable frequency range for 
Z.sub.tr forced oscillation data and facilitates the use of an eight 
element model which provides more insight into the operation of the 
respiratory system. This eight element model allows for separation of 
airway and tissue information as well as separation of data regarding the 
airways into its central and peripheral airway components. The increased 
accuracy and specificity of the data gathered enables doctors and 
technicians to diagnose patients without the use of invasive procedures on 
the patient. 
Further, the time required to perform these tests is usually only about 6 
minutes. The relatively short time of the test helps to reduce the stress 
often caused by other methods of respiratory testing. Also, because the 
present invention allows for at least the head and potentially also the 
legs of the patient to be outside of the plethysmograph, the patient will 
likely feel less confined. The head-out, legs-out structure also allows 
the doctor to more easily communicate with the patient and observe the 
patient during the testing. The adjustability of the present invention 
increases the comfort of this box and allows one device to be used for a 
wide range of adult patients. 
Although a detailed description of the invention has been provided, it 
should be understood that the scope of the invention is not to be limited 
thereby, but is to be determined by the claims which follow. Various 
modifications and alternatives will be readily apparent to one of ordinary 
skill in the art.