Method for measurement of nonradioisotopic aerosol retained in the lungs with re-breathing

A subject rebreathes an inert aerosol from a closed system and, during each of a plurality of breaths, the aerosol concentration in the closed system is determined and compared to determine differences with a predetermined concentration value. Identified differences indicating enhanced aerosol deposition signify airway narrowing and/or an increase in accumulated airway secretions.

TECHNICAL FIELD 
This invention pertains to a method for analyzing airway function by 
re-breathing an inert aerosol. 
BACKGROUND ART 
The prior art discloses numerous techniques for analyzing a subject's 
pulmonary function. In one such technique, the subject takes a single 
breath of an inert aerosol, and the aerosol concentration is determined 
upon inspiration and expiration. The resulting difference is attributed to 
aerosol deposition in the subject's airways. Differing levels of aerosol 
deposition as between subjects may be used, for example, as an indication 
of increased airway obstruction, which is known to cause enhanced aerosol 
deposition. This technique, however, is relatively insensitive to small 
variations in pulmonary function, and hence has limited applications. In 
addition, it appears that the accuracy of the data derived from the single 
breath method may be rendered inaccurate due to effects of aerosol 
dilution, as described hereinafter. The prior art single aerosol 
deposition technique is disclosed, for example, in Distribution of Aerosol 
Particles in Exhaled Air, Muir, Journal of Applied Physiology, Vol. 23, 
No. 2, 1967, The Effect of Airways Obstruction on the Single Breath 
Aerosol Curve, Muir, as appearing in Airway Dynamics by Bouhys, 1970 
Edition, pp. 319-325, and Aerosol Transport in the Human Lung From 
Analysis of Single Breaths, Taulbee et al., Journal of Applied Physiology, 
Vol. 44, No. 5, pp. 803-812, 1978. 
Evaluation of pulmonary function by analysis of aerosol deposition data has 
also been carried out for single breaths by scanning the lungs after 
inhalation of a radioactive aerosol. Such studies are described, for 
example, in Early Detection of Chronic Obstructive Pulmonary Disease Using 
Radionuclide Lung-Imaging Procedures, Taplin, et al., Chest, Vol. 71, pp. 
567-575, 1977, and Imaging Sites of Airway Obstruction and Measuring the 
Functional Responses to Bronchodilator Treatment in Asthma, Chopra, et 
al., Thorax, Vol. 34, pp. 493-500, 1979. The obvious disadvantage of this 
technique is that due to effects of radiation exposure, it cannot be 
repeated to follow serial changes after therapeutic intervention. 
The prior art also recognizes that accumulated secretions in the airways 
may result in a two phase gas-liquid flow. Resistance to Two-Phase 
Gas-Liquid Flow in Airways, Clarke et al., Journal of Applied Physiology, 
Vol. 29, No. 4, 1970. However, the effect of this phenomenon on aerosol 
deposition has not heretofore been studied. 
DISCLOSURE OF THE INVENTION 
In accordance with the method of the present invention, airway function 
analysis is based on interpretation of aerosol concentration data 
generated during re-breathing of aerosol from a closed system. The method 
provides a highly sensitive indicator of airway function. The data 
generated by the method may be compared in several different ways. In one 
technique, the number of breaths required to deposit 90% of the initial 
aerosol concentration in the subject's airway is used. In another, the 
slope of the aerosol disappearance curve over a plurality of breaths is 
used. In either event, by comparing the results among different subjects, 
clinically useful information may be obtained. For example, the method may 
be used to evaluate the re-distribution of secretions with various types 
of therapeutic intervention, such as chest physiotherapy, expectorant 
agents, and mucolytic agents. Consequently, the method may be used to 
evaluate the efficacy of such therapeutic interventions. 
Further aspects and advantages of the method in accordance with the present 
invention will be more fully apparent from the following detailed 
description and annexed drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Apparatus for practicing the method of the present invention is 
diagrammatically illustrated in FIG. 1 and generally designated by the 
reference numeral 10. In practicing the method, monodisperse aerosol 
droplets are used. Aerosols of di(2-ethyl hexyl)sebacate as generated from 
a LaMer-Sinclair apparatus have been employed. Di(2-ethyl hexyl)sebacate 
is a bland, non-hygroscopic, oily liquid with a low vapor pressure at body 
temperature, and its use in aerosol deposition studies is well known. The 
generation of such aerosols by employing the LaMer-Sinclair apparatus is 
also well known, and described, for example, in Light Scattering as a 
Measure of Particle Size in Aerosols, Sinclair and LaMer, Chemical 
Reviews, 1949, Vo. 44, pp. 245-257. In lieu of Di(2-ethyl hexyl)sebacate, 
aerosols formed from polystyrene latex or Teflon particles may be used. 
Again, the formation of aerosols from such particles is known to the 
skilled art worker. 
Typically, aerosol sizes of 1.0 and 2.5 micron mass median aerodynamic 
diameter (MMAD) having a geometric standard deviation less than 1.2 are 
used. Once generated, they are diluted with filtered air to a final 
concentration of approximately 10.sup.4 particles per cubic centimeter. 
The aerosol is then introduced, as by a syringe, into a 0.5 liter 
collapsible plastic reservoir bag 12, this volume being chosen based on 
the observation that subjects with chronic obstructive bronchitis cannot 
perform the required re-breathing manuever with high volumes at the 
respiratory rates of interest. 
As shown in FIG. 1, an aerosol detection module 14 is connected on one side 
to the opening of bag 12 and at the other side to a mouthpiece 16 through 
which a subject 18 inhales and exhales the aerosol from bag 12. 
Consequently, as the subject inspires, the contents of bag 12 pass through 
the module 14 into mouthpiece 16, and as the subject expires, the expirate 
passes through the module 14 back toward the bag. The detection module 14 
is included for continuously measuring the aerosol concentration in the 
module in order that the drop in aerosol concentration with each breath 
may be determined. The use of such a detection module 14 is known in the 
art in connection with single breath aerosol deposition studies and is 
disclosed, for example, in The Deposition of 0.4 .mu.m Diameter Aerosols 
in the Lungs of Man, Muir and Davies, Annals of Occupational Hygiene, Vol. 
10, pp. 161-174. Suffice it to say that in the detection module 14, the 
aerosol passes through a light scattering cell thereby interrupting a 
light beam from a source 20. A portion of the scattered light impinges on 
a photomultiplier tube 22 positioned at a right angle to the light beam, 
the amount of light reaching the photomultiplier being dependent on the 
aerosol concentration in the chamber 14. In one study, a high intensity 
tungsten lamp, General Electric #2331, Cleveland, Ohio, was used for the 
light source, and a photomultiplier tube, Model 9798B as manufactured by 
EMI-Gencom, Inc., Plainview, N.Y., was used as the detector. The output 
from the photomultiplier is preferably amplified by a picoammeter 24 with 
a high level output, driving an integrated circuit DC amplifer 26. The 
response of the foregoing aerosol concentration detector is flow 
independent and linear with aerosol concentrations ranging from 0 to 
10.sup.5 particles per cubic centimeter. The aerosol concentration as 
represented at the output of the DC amplifier 26 was continuously recorded 
both on a strip chart recorder 28 and a digital computer 30, the latter 
comprising the Model S110 as manufactured by the Harris Corporation of 
Melbourne, Fla. 
To insure that the subject 18 breathes through the mouthpiece 16, the 
subject may be fitted with a nose clip (not shown). To insure uniformity 
between breaths, subjects may be trained to match their breathing pattern 
to the sounds of a Satter Respiration Simulator, such as the Model 700 as 
manufactured by Somanetics of LaJolla, Calif. The subjects may also be 
instructed to feel the bag as an aid to perceiving the filling and 
emptying thereof. Also, to insure uniformity between the first breath and 
subsequent breaths, the initial inhalation is preferably carried out from 
the FRC position, i.e. at the lung volume after normal expiration. In 
actual studies, the subjects were capable of performing the required 
re-breathing manuever with ease after approximately five to ten minutes of 
training. 
Several studies have been performed in accordance with the method of the 
present invention as described above. In one study, seventeen subjects 
were involved. Nine were life time non-smokers with a negative pulmonary 
history and normal spirometry, normal body plethysmography and normal 
single breath nitrogen test. The remaining eight subjects had chronic 
bronchitis and a history of chronic productive cough. Pulmonary function 
tests among these eight subjects were variable with FEV.sub.1.0 ranging 
from 0.82 to 3.19 liters and 44% to 107% predicted normal. Two of the 
eight subjects with chronic bronchitis were classified as chronic simple 
bronchitis (normal FEV.sub.1.0 and normal airway resistance) and six were 
classified as chronic obstructive bronchitis (low FEV.sub.1.0 and/or 
increased airway resistance). 
For each subject 18, the aerosol concentration in the chamber 14 during 
each inhalation was determined and expressed as a fraction of the initial 
inhaled concentration, i.e. the concentration during the first breath. 
This data was then plotted semi-logarithmically as a function of the 
breath number, and a least-squares linear fit was obtained. The slope of 
this line, which corresponded to the constant K in the equation 
Y=e.sup.-KX was calculated. In determining the slope, two satisfactory 
runs for each subject were averaged. To simplify the foregoing 
calculations, a computer program for the computer 30 was written in 
Fortran 66 to perform the collection, analysis and storage of the data. 
For each subject, the computer sampled the aerosol concentration 
approximately twenty times per second through an analog to digital 
converter. The data was then digitally filtered using a Gaussian low-pass 
function having a cut off frequency of 200 Hz with a window size of eight 
sample points. The resulting filtered data were displayed graphically on a 
cathode ray oscilloscope. The program was written such that aerosol 
concentrations could be recalled from the memory and displayed on the 
graphics terminal for analysis. The computer was also programmed to 
display the normalized concentrations, i.e. aerosol concentration as a 
fraction of initial inhaled concentration, and also to compute the least 
squares fit and plot the result. Once this description is known, any 
skilled art worker can write a suitable program for accomplishing these 
tasks, and the program per se forms no part of the present invention. 
The graph of FIG. 2 is a semilogarithmic plot of aerosol disappearance 
versus breath number for a normal subject breathing 1.0 .mu.m MMAD aerosol 
at a tidal volume of 500 ml and a frequency of 30 breaths per minute. FIG. 
3 is a plot of corresponding data for a subject with chronic simple 
bronchitis. As is apparent from a comparison of FIGS. 2 and 3, in the 
normal subject aerosol concentration was reduced by 90% after about 12 
breaths, whereas in the bronchitic patient, the same level was reached 
after about five breaths. This was so despite comparable airway 
resistance, as indicated by comparable values of FEV.sub.1.0 which, for 
the normal and bronchitic subjects studied, were 84% and 83% predicted 
normal, respectively. In the course of the studies, it was found that the 
number of breaths required to reduce the initial aerosol concentration by 
90%, which is referred to as N.sub.90, is a convenient way of expressing 
aerosol disappearance for purposes of comparison. Another useful indicator 
is the slope of the aerosol disappearance curves. A cumulative loss of 
aerosol after a specific number of breaths as a percentage of initial 
aerosol concentration can also be a useful indicator. 
Table 1 below illustrates the aerosol disappearance slopes for all trials 
for the 17 subjects, at breath rates of both 10 and 30 breaths per minute, 
and for 1.0 and 2.5 .mu.m MMAD aerosol sizes. 
TABLE 1 
______________________________________ 
Aerosol Disappearance Slopes 
(Mean .+-. Standard Deviation) 
Difference 
between 10 and 
10 30 30 breaths/min 
breaths/min 
breaths/min 
p value 
______________________________________ 
1.0 .mu.m MMAD 
Normals .267 .+-. .059 
.128 .+-. .032 
&lt;.001 
Bronchitics 
.484 .+-. .142 
.284 .+-. .105 
&lt;.01 
Difference &lt;.001 &lt;.001 
Between Normals 
& Bronchitics 
p value 
2.5 .mu.m MMAD 
Normals .753 .+-. .202 
.494 .+-. .167 
&lt;.001 
Bronchitics 
1.390 .+-. .352 
1.020 .+-. .287 
&lt;.01 
Difference &lt;.001 &lt;.001 
Between Normals 
& Bronchitics 
p value 
______________________________________ 
As is apparent from Table 1, in general there was minimal scatter in the 
data for the two groups, normals and bronchitics, as evidenced by the low 
standard deviation relative to the means. The aerosol disappearance slopes 
for patients with chronic bronchitis evidenced greater variation when the 
subjects rebreathed the 2.5 .mu.m MMAD aerosol particles at 10 breaths per 
minute. However, this is possibly because only three or four breaths could 
be analyzed due to the rapid aerosol disappearance in these subjects. In 
general, there was a highly significant increase in the rate of aerosol 
disappearance in the chronic bronchitics as compared with the normal 
subjects for both particle sizes and both breathing rates. For both the 
normal subjects and the bronchitics, the steepest disappearance slopes, 
i.e., fastest rate of aerosol disappearance, occurred with the larger 
aerosol particles and slower breathing rates. 
Referring now to FIG. 4, the aerosol disappearance slopes for all subjects 
during breathing of 1.0 .mu.m MMAD at a 500 ml tidal volume and a 
frequency of 30 breaths per minute are plotted versus FEV.sub.1.0 percent 
predicted normal. FIG. 5 is a similar plot showing the aerosol 
disappearance slopes for all subjects during breathing of 1.0 .mu.m MMAD 
at a 500 ml tidal volume and a frequency of 30 breaths per minute plotted 
versus airway resistance. Both FEV.sub.1.0 and airway resistance 
measurements are used to evaluate the extent of airway obstruction. 
Consequently, the graphs of FIGS. 4 and 5 were generated to show the 
correlation, if any, between aerosol disappearance and airway obstruction. 
In FIGS. 4 and 5, the hatched area encloses the data of the normal 
subjects, which are depicted as solid circles. The bronchitic subjects are 
depicted by the open circles. As is apparent from FIGS. 4 and 5, only one 
bronchitic subject fell within the data of the normal group. Furthermore, 
the graphs show a wider scatter of aerosol disappearance slopes in 
bronchitics as a function of airway size. The graphs in FIGS. 6 and 7 
correspond to the graphs in FIGS. 4 and 5, respectively, except that the 
data was accumulated during breathing of 2.5 .mu.m MMAD aerosol. In FIGS. 
6 and 7 the significance of the solid and open circles and of the hatched 
area is the same as in FIGS. 4 and 5. As is apparent from FIGS. 6 and 7, 
when the larger aerosol particles were used, two of the bronchitic 
subjects had values within the normal range of aerosol disappearance 
slopes. The lowest correlation between airway obstruction and aerosol 
disappearance slopes occurred during breathing of 1.0 .mu.m MMAD aerosol 
at 500 ml tidal volume and a frequency of 30 breaths per minute. 
From Table 1 and FIGS. 2-7 it is clear that aerosol retention within the 
lungs during rebreathing differs both quantitatively and qualitatively in 
subjects with chronic bronchitis as compared with normals. Specifically, 
more aerosol per breath is deposited in the airways of the bronchitics as 
compared with normal subjects. This result is not surprising and is 
consistent with findings made by others utilizing a single breath 
technique with subjects having bronchial asthma, chronic bronchitis and 
pumonary emphysema. 
However, the data described above as recorded in accordance with the method 
of the present invention demonstrates a lack of correlation for the 
bronchitic subjects between aerosol disappearance and airway obstruction 
as determined both by FEV.sub.1.0 and airway resistance. This indicates 
that a reduction in airway size is not the sole factor contributing to the 
difference in the aerosol disappearance data for normal subjects and 
bronchitics. 
It is generally known that altered aerosol deposition in subjects with 
pulmonary disease is attributable to two factors, namely, altered 
breathing patterns or variations in airways size and geometry. In the 
study described above, tidal volume and respiratory rate were controlled 
to eliminate variations in breathing patterns as a factor. It appears, 
therefore, that the different aerosol deposition data observed for the 
bronchitic subjects, as compared with the normals, was due to changes in 
airway dimensions in the bronchitics. 
One possible explanation for the lack of correlation between aerosol 
disappearance and airway obstruction in the bronchitics is the 
accumulation of secretions in the airways. In one form, secretions may 
occur as focal accumulations. The effect of this was studied 
experimentally with glass tubes simulating the airways for different types 
of focal constrictions. This produced marked amplification of aerosol 
deposition just beyond the focal constriction resulting from turbulent 
impaction of the aerosol particles. However, the focal constriction also 
produced a proportionally greater increase in airway resistance than 
aerosol deposition. Accordingly, the phenomenon of focal accumulation of 
secretions does not explain the large increase in aerosol disappearance 
for the bronchitics which were observed in the study despite only nominal 
increases in airway resistance. The effect of oscillatory motion of airway 
wall on aerosol deposition was also studied experimentally using a 
vibrating compliant tube wall fabricated from a glass tube having a 
flexible section of thin rubber latex. Our experimental study did indicate 
that with oscillatory motion of airway wall, the aerosol deposition rate 
shows a proportionally greater increase than mean airway resistance. 
However, a low frequency oscillatory motion of airway walls due to the 
periodic nature of breathing and cardiac oscillations would not differ 
among subjects, and hence this too does not explain the lack of 
correlation between aerosol deposition and airway obstruction in the 
bronchitics. 
Based on experimental models, it now appears that the lack of correlation 
between aerosol disappearance and airway obstruction in bronchitics may be 
caused by accumulation of secretions lining the walls of the airways in 
the bronchitics. Thus, it appears that when the airways are lined with 
secretions, the interaction between the secretions and the inhaled air may 
result in a form of two-phase gas-liquid pumping which produces marked 
wave-like and slug motion of the secretions, localized turbulence of air 
flow, and enhanced aerosol deposition to a much greater extent than the 
rise in mean airway resistance resulting from the secretions. This is to 
be contrasted with a focal airway constriction as discussed above. As 
noted, such focal constrictions produce a greater rise in airway 
resistance than aerosol deposition. 
The conclusion that secretions lining the airways result in a 
proportionally greater increase in aerosol deposition than airway 
obstruction has been verified in connection with experiments performed on 
sheep. During these experiments, human airway secretions and viscoelastic 
polymer solutions having similar rheologic properties to human sputum were 
transferred to the airways of sheep. Both aerosol disappearance rates and 
airway obstruction were monitored. The results confirmed the conclusion 
that the secretions produce a proportionally greater increase in aerosol 
deposition than airway obstruction. 
Since secretions lining the airways increase the aerosol deposition rate to 
a greater extent than would be expected by virtue of the increased airway 
resistance caused by the reduction in airway diameter resulting from such 
secretions, then the method of the present invention may be utilized to 
evaluate the redistribution (i.e. removal or lessening) of secretions 
resulting from therapeutic intervention. For example, various procedures 
and drugs, such as chest physiotherapy, mucolytic agents and expectorant 
agents have little or no effect on pulmonary mechanics, i.e., they do not 
significantly alter airway resistance. Accordingly, if a subject exhibits 
an abnormally steep aerosol disappearance curve which is converted to a 
slower, more normal curve after therapeutic intervention, then it may be 
assumed that the resulting decrease in the aerosol disappearance rate is 
due to a redistribution of secretions within the airways. Clearly, this 
provides the method of the present invention with significant applications 
for evaluating the efficacy of such therapies. 
A study was also conducted to specifically determine whether chest 
physiotherapy causes a shift to more normal aerosol deposition rates in 
chronic bronchitics with productive cough. This study was conducted with 
fourteen subjects and using an aerosol of 1.0 m MMAD at 30 breaths per 
minute from a 500 ml reservoir. The chest physiotherapy consisted of a 
postural drainage with manual or mechanical percussion in six body 
positions followed by vibration during purse lip breathing on exhalation 
three times and one augmented cough. The resulting data indicated a rise 
of N.sub.90 after chest physiotherapy without any corresponding change in 
pulmonary mechanics. This indicates that chest physiotherapy results in a 
redistribution of secretions. In another study, the aerosol disappearance 
data generated in accordance with the present invention was found 
sufficient to distinguish between normal subjects and smokers without any 
evidence of small airway disease as determined by conventional pulmonary 
function tests. In fact, the data was sufficient to make this distinction 
after only four breaths. 
In general, the rebreathing technique in accordance with the method of the 
present invention exhibits superior sensitivity over the single breath 
technique of the prior art. Thus, if analysis is based on a single breath, 
variations in the aerosol deposition rate as between normal and abnormals 
may be masked. For example, if analysis is based on a single breath, less 
recovery of aerosol from that breath would be expected in subjects with 
larger functional residual capacities by virtue of aerosol dilution. 
However, when the rebreathing method of the present invention is employed, 
the disappearance slope and N.sub.90 is calculated over several breaths, 
and dilution of aerosol becomes less important. This is true so long as 
there is no significant trapping of aerosol within poorly ventilated 
spaces in the lung, and the absence of such trapping has been supported by 
the simultaneous analysis of helium wash-in curves in accordance with 
techniques known to those skilled in the art. Consequently, by using the 
rebreathing technique of the present invention, differences in aerosol 
deposition between different subjects may be more accurately determined. 
In the single breath method data analysis is cumbersome because it 
requires an accurate integration of aerosol the concentration curve to 
determine the total aerosol recovery in an expired volume. The 
re-breathing method is straightforward, requires no calibration, no 
corrective factors or any other unproven assumptions so that reliable 
quantitative data can be obtained. Considering that the method of the 
present invention is highly sensitive to minor variations in pulmonary 
function, once this description is known those skilled in the art will 
undoubtedly conceive numerous applications for the inventive method other 
than those described above. 
While we have herein shown and described the preferred method in accordance 
with the present invention, and have suggested certain changes and 
modifications thereto, the above description should be construed as 
illustrative and not in a limiting sense, the scope of the invention being 
defined by the following claims.