Clinical Science and Molecular Medicine (1978), 54, 313-321
The interpretation of different measurements of airways
obstruction in the presence of lung volume changes in
bronchial asthma
K. B. SAUNDERS AND M. RUDOLF
Department of Medicine, The Middlesex Hospital Medical School, London
(Received 7 October 1976; accepted 10 October 1977)
Summary
1. We measured changes in peak expiratory
flow rate (PEFR), forced expiratory volume in 1 s
(FEVj.0), airways resistance (.Raw), specific con­
ductance (sC7aw), residual volume (RV), functional
residual capacity (FRC) and total lung capacity
(TLC) in 44 patients with asthma.
2. When asthma was induced by exercise in five
patients there were large changes in volumes, but
these did not obscure changes in PEFR, which
adequately defined the time course of the response.
3. In 70 comparisons before and after inhalation
of bronchodilator drug in 33 asthmatic subjects,
the responses were classified by the size of the
change in lung volumes, which showed a con­
cordant improvement, or no change, in 61 com­
parisons. Despite these lung volume changes,
measurement of both PEFR and FEVi.0 would
have detected a bronchodilator response in all but
two cases.
4. In 81 comparisons in 23 subjects over time
intervals varying from I day to 11 months, lung
volumes changed in concordance with PEFR and
FEV,.0 in 59. In eight of these comparisons,
measurement of lung volumes would have altered
our interpretation of the changes in PEFR and
FEV^.
5. In the same 81 comparisons changes in
airways resistance were concordant with changes
in PEFR and FEVj.0 on 44 occasions, with minor
discordant changes in 19. We could not explain the
Correspondence: Dr Kenneth B. Saunders, Department of
Medicine, The Middlesex Hospital Medical School, Mortimer
Street, London Wl.
313
remaining 18 cases showing major discordance be­
tween these two types of measurement of airway
calibre.
6. We conclude that both FEV,.,, and PEFR
should be used for detection of a bronchodilator
response, and that measurement of lung volumes
will rarely contribute to the interpretation. Over
longer periods, lung volumes should be measured if
possible. We found no practical use for routine
measurement of airways resistance in patients with
asthma.
Key words: asthma, airways resistance, airways
obstruction,
hyperinflation,
lung
function
laboratory.
Abbreviations: EPP, equal pressure point; FEV,.,,,
forced expiratory volume in 1 s; FRC, function­
al residual capacity; PEFR, peak expiratory
flow rate; Haw, airways resistance; RV, residual
volume; sGaw, specific airways conductance;
TLC, total lung capacity; VC, vital capacity.
Introduction
Patients with variable airways obstruction may
require frequent estimation of respiratory function,
especially for assessing the effect of therapy.
Several indices of airways resistance may be used,
the commonest of which are peak expiratory flow
rate (PEFR) and forced expiratory volume in 1 s
(FEV^j). In some laboratories airways resistance
(flaw) may also be measured by body plethysmography.
314
K. S. Sounders and M. Rudolf
In many patients with airways obstruction an
increase in Raw and decrease in PEFR and FEV,. 0
is accompanied by hyperinflation with increases in
residual volume (RV), functional residual capacity
(FRC), total lung capacity (TLC) and the
RV/TLC ratio. Hyperinflation of the lungs tends to
dilate the intrapulmonary airways, a mechanism
which may be regarded as compensatory for an
increase in Raw. If a patient with asthma improves
clinically, either spontaneously or after treatment,
intrinsic changes in the bronchial wall will be
expected to dilate the airways, and a reduction in
lung volume will tend to narrow them. Woolcock &
Read (1965) found that there was little change in
FEV,. 0 but a considerable fall in lung volumes in
two out of 30 patients recovering from severe
attacks of asthma. Thus FEV,.0, a simple index of
Raw, did not detect clinical changes in those
patients.
The hypothesis that lung volume changes might
affect interpretation of changes in PEFR and
FEV,. 0 in asthmatic subjects who were not severely
ill prompted this study. We measured PEFR,
FEV,. 0 Raw, RV, FRC, TLC and RV/TLC in 44
asthmatic subjects: (i) during an attack of exerciseinduced asthma, and after bronchodilatation pro­
duced by isoprenaline; (ii) before and after
inhalation of isoprenaline or salbutamol without
preceding exercise; (iii) at intervals of 1 day to 11
months, with changes either spontaneous or after
therapy (usually involving sodium cromoglycate).
Subjects
Series 1: exercise-induced asthma
Five extrinsic asthmatic subjects aged 20-35
years performed lung-function tests at rest, after 6
min free running and again after inhaling 800 ßg of
isoprenaline, on four occasions. Some of these
results have been previously reported in outline
(Rudolf, Grant, Saunders, Brostoff, Salt & Walker,
1975).
Series 2: immediate bronchodilator response
Lung-function tests were performed at rest,
before and 5 min after inhaling 800 μ% of iso­
prenaline (or 20 min after inhaling 20 μ% of
salbutamol, in three subjects with ischaemic heart
disease). Thirty-three patients were studied on up to
five occasions for a total of 70 comparisons.
Twenty had intrinsic asthma diagnosed by history
and negative skin-prick tests (mean age 58 ± SD 8
years), 10 had extrinsic asthma (mean age 36
+ 13 years), and in three the atopic status was not
clear.
Series 3: changes over long periods
Twenty-three patients were studied on up to
eight occasions, providing 81 pairs of consecutive
measurements, separated by intervals of 1 day to
11 months. Twenty of these patients were intrinsic
asthmatic subjects (mean age 58 ± 8 years), and
three extrinsic (aged 28, 56 and 62 years).
Seventeen of the 20 intrinsic asthmatic subjects
were included in both series 2 and series 3.
Methods
We followed our normal laboratory procedure by
first measuring PEFR with the Wright peak flow
meter (Clement Clarke International, London), and
FEVj.0 and vital capacity (VC) with a dry spirometer. Airways resistance and lung volumes were
then measured by plethysmography (Dubois,
Botelho, Bedell, Marshall & Comroe, 1956; Dubois
Botelho & Comroe, 1956). Flow at the mouth was
recorded by a Fleisch no. 4 pneumotachograph, the
signal being integrated to give a continuous record
of changes in lung volume. The thoracic gas
volume measured while the patient panted against a
shutter was related to TLC by a full inspiration
after the shutter was released and to FRC by a pre­
ceding record of lung volume changes during tidal
breathing. RV was obtained by subtracting the
previously measured VC from TLC. When a
bronchodilator was given, plethysmography was
repeated before the final measurements of PEFR,
FEV,. 0 andVC.
Measurements for inspiratory Raw were made
when the subject panted at 1-2 Hz as shallowly
as possible, from 0 to 0-9 litre/s. Two to three
loops from consecutive breaths were superimposed
on a Tektronix 564 B Oscilloscope. The best fit to
the flow-pressure slope was selected by eye. The
mean of three measurements was taken, except in
patients who had exceptional difficulty with the
manoeuvre, when the mean of their two best efforts
was obtained. Division of the reciprocal of Raw by
the lung volume at which it was measured gave
specific airways conductance (sGaw).
The mean (of 2-3 recordings) and coefficient of
variation were calculated for each set of measure­
ments of FEV,.0, PEFR, RV, FRC, TLC and Raw.
The mean coefficients of variation for each variable
were compared by unpaired /-tests. Paired i-tests
315
Airways resistance and lung volume
were used to test for difference between mean
values of consecutive measurements in the same set
of patients.
Results
In describing the results of multiple tests on the
same patient (e.g. lung volumes, PEFR and .Raw),
if all change towards normal values, or all change
away from normal values, we refer to 'concor­
dant' changes, whereas if some change towards
and some away from normal, we describe the
results as 'discordant'.
Variability
Mean coefficients of variation for within-patient
measurements on a single occasion were: for
FEV,.0, 3·9%; PEFR, 6-0%; RV, 7-2%; FRC,
4-8%; TLC, 2-2%; Raw, 7-6%. The mean
coefficient of variation was significantly larger for
Raw than for FEVV0 (P < 0-001), or PEFR (P <
0-02).
Series 1: exercise-induced asthma
In 120 comparisons of FRC, RV and TLC,
there were changes >0-5 litre in all but eight (Table
1). The changes were invariably concordant, as
FRC, RV, TLC and RV/TLC all rose after
exercise and fell after isoprenaline. PEFR always
changed inversely to lung volume, the smallest
change being 55 litres/min.
Series 2: immediate bronchodilator response
Lung volume changes were classified in three
groups. Group A: 'definite changes', with con­
cordant falls of >0·5 litre in FRC, RV and TLC.
Group B: 'probable changes', with concordant
falls in FRC, RV, TLC and RV/TLC, and one or
two volumes falling by >0·5 litre. Group C: 'no
change', with concordant or discordant changes,
all <0·5 litre for lung volumes, and <10% for
RV/TLC.
In group A RV/TLC fell >10% in all com­
parisons and in group B RV/TLC fell by 1-10%,
whereas in group C there were small changes of
<10% in either direction. Changes in all com­
parisons in group A and group B were concordant
for FRC, RV, TLC and RV/TLC.
Patients with the larger lung volume changes
tended to have the larger changes in PEFR and
FEVJ.J (Fig. 1). There is no indication from the
TABLE 1. Exercise-induced asthma
Changes in lung volumes and peak expiratory flow rate
(PEFR) after exercise, and again after an inhalation of
isoprenaline, in five extrinsic asthmatic patients. Mean changes
are given with 1 SD in parentheses, n = 20 for all observations,
since five subjects performed the exercise test followed by
isoprenaline inhalation each on four occasions. FRC, Functional
residual capacity; RV, residual volume; TLC, total lung
capacity.
FRC (1)
RV(i)
TLC0)
RV/TLC (%)
PEFR (1/min)
After exercise
After isoprenaline
+2-4(1-6)
+2-4 (1-8)
+ 1-3(1-0)
+ 18-8(12-9)
-212(102)
-2-8(1-5)
-2-8(1-8)
-1-7(1-0)
-21-6(11-7)
+ 175(92)
mean data that large falls in lung volumes had
offset the effect of bronchodilatation to the extent
that PEFR and FEV^ were unchanged. The
greater rise in PEFR and FEVi.0 in group A than
in group B may only reflect the significantly lower
initial values in group A (P < 0-05), thus leaving
more scope for improvement.
The changes in individual patients showed dis­
crepancies from this average pattern. We ar­
bitrarily define a change of 10% in PEFR or
FEV[,0 as 'significant'. The trend is for the
patients with larger lung volume changes to show
more consistently a 'significant' increase in PEFR,
FEVj.0 or both, and for these concordant changes
to occur more frequently for FEVj.p than for
PEFR (Table 2). All patients in group A showed a
significant response in either PEFR or FEV,.0 or
both, whereas on two occasions in group B and five
occasions in group C these values did not change.
The responses so far described account for 61 of
the 70 comparisons. Onfiveoccasions there was an
increase of FRC, RV, TLC and RV/TLC after the
bronchodilator, this occurring three times in one
patient. This increase in lung volumes was never
accompanied by a 'significant' fall in PEFR or
FEVj.0, but 'significant' rises occurred on three
occasions in PEFR and four occasions in FEVj.,,.
The remaining four comparisons showed changes in
the measured volumes, at least one of which was
>0-5 litre, which varied in direction, allowing no
general statement as to whether hyperinflation was
increasing or subsiding.
Raw fell on 65 occasions, twice showed no
change, and rose on three occasions. On the five
occasions where Raw did not fall FEVj.,, rose in
two and PEFR rose in one case, with no 'signifi­
cant' falls in either measurement. sGaw changed in
the opposite direction to R aw in 69 out of 70 com­
parisons, as expected. On one occasion .Raw fell
Κ. Β. Sounders andM. Rudolf
316
450
400
350
^300
3r
Ja 250
Si
2 200
s2-
I,
V. 150
100
50
%m
A B C
4PEFR
A
B
C
Mean PEFR
Ufa
A
B
C
^FEV,.„
A
B
C
Mean FEV,.„
FIG. 1. Changes in lung volumes, PEFR and FEV,.,, after a bronohodilator, grouped according to magnitude of lung
volume changes (group A, definite; group 6, probable; group C, no change). Results are expressed as mean change
with 1 SD. Mean values for PEFR and FEV,.,, before bronohodilator are also given. Group A, n = 14; group B,« = 18;
group C, n = 29. *P < 0.05.
TABLE 2. Bronohodilator effect
Occurrence of changes in peak expiratory flow rate
(PEFR) and (FEV,.0) greater than 10%.
Change in
lung volumes
Group A
('definite')
Group B
('probable')
Group C
('none')
No. in group
> 10% change in
PEFR
FEV,.„
Neither
14
18
29
12
13
0
12
14
2
17
21
5
from 0-43 to 0·34 kPa 1_1 s, accompanied by a rise
in lung volume so that sGaw remained constant.
Series 3: long-term changes
Since these changes were the result of spon­
taneous variation and of initiation or withdrawal of
treatment, changes in either direction were to be
expected. We selected first the 65/81 comparisons
with concordant changes in lung volumes and
RV/TLC. To avoid further subdivision into groups
showing improvement and deterioration, and since
we are concerned with the magnitude of the
changes rather than direction, we took the absolute
values of the changes in group A and group B (Fig.
2).
As in series 2 the subjects with the largest lung
volume changes showed largest changes in PEFR
and FEV,.0. The long-term mean changes in PEFR
and FEV,.0 in group B subjects were much smaller
than those observed after bronchodilatation, and
these changes were not significant, in the presence
of significant changes in lung volumes (Fig. 1 and
Fig. 2). This may reflect the conflicting interaction
of primary changes in bronchial calibre versus lung
volume changes.
The individual results showed greater discrepan­
cies than were observed in series 2 (Table 3). In
group A when lung volumes changed either FEV,.0
or PEFR or both changed concordantly in all but
Airways resistance and lung volume
317
A
B
C
MeanFEV,.
FIG. 2. Long-term changes in lung volumes, PEFR and FEV,.0. Conventions as for Fig. 1. Changes in group A and
group B are absolute values. Group A, n = 21; group B, n = 26; group C, n = 18.
TABLE 3. Changes over long periods
Occurrence of >10% changes in peak expiratory flow rate
(PEFR) and forced expiratory volume in 1 s (FEVli0).
Change in
lung volumes
Group A
('definite')
GroupB
('probable')
Group C ('none')
n
21
26
18
Concordant
10% change in
PEFR
FEV,.0
Neither
16
17
0
13
14
6
13
11
1
Discordant
10% change in
PEFR alone
FEV,.,, alone
Both
1
0
0
4
2
2
one patient, where there was a discordant change in
PEFR and no change in FEVj.0. In group B the
changes were much less consistent; for example, in
26 comparisons with significant lung volume
changes, seven showed no 'significant' change in
PEFR, and six showed a discordant change.
In the remaining 16/81 comparisons the
measured lung volumes changed discordantly with
at least one change of >0·5 litre so that there was
no consistent picture of hyperinflation or deflation,
in contrast to series 2 where only four out of 70
comparisons were similarly confusing.
A aw changed concordantly with PEFR and
FEV,.0 in only 44 of the 81 comparisons, the
change being discordant to both indices in nine
cases, to PEFR alone in 15 and to FEV^ alone in
13. These confusing combinations usually arose
when all changes were small or when one variable
changed markedly with little change in the other
(Fig. 3). sGaw changed in opposite direction to .Raw
in 60 of the 81 comparisons (cf. series 2, where this
occurred on 69 out of 70 occasions). In the
remaining 21, lung volumes were sufficiently
different on the two occasions so that sGaw did not
Κ. Β. Sounders and M. Rudolf
318
(a)
(ft)
02
-
ΙΛ
7
o
s
"-' 0 1
s
a. oi
51
OS
00
°···
°.
-100
1
JFEV,.0(1)
•
•
*
"o
•
O
• o
.e
,
100
200
dPEFR (1/min)
0
—0-1
-0-1
•So
-0 2
O
o --0 2
FIG. 3. Discordant simultaneous changes in FEV I0 (a) and PEFR (b) plotted against simultaneous changes in Äaw,
from series 3. Changes in PEFR and FEV,.0 are marked as less than 10% (O) or more than 10% ( · ) of the initial
value.
change at all in 13 comparisons, and changed by
small amounts in the same direction as Äaw in the
remaining eight (mean of absolute value change
0-2 + 0-25 s- 1 kPa" 1 ).
Discussion
In a hospital service laboratory, lung-function tests
are used to identify and quantify abnormality and
then in serial assessment of the individual patient to
assess disease progress and the effect of therapy
(Saunders, 1975). If more than one measurement is
made interpretation is easy if all change towards, or
away from, normal values, and we have used the
word 'concordant' for changes which occur in this
sense and 'discordant' for changes which are con­
flicting in direction. If changes are discordant
assessment of overall improvement or deterioration
is obviously difficult (and the more the measure­
ments made, the more likely this is to occur). Two
obvious reasons for discordance are: 1, that some
of the measurements are directionally wrong due to
a technical error, which is especially likely if the
recorded changes are small; 2, that different
measurements may reflect different aspects of the
pathophysiology. The main purpose of this study is
to explore the opposing effects of bronchodilatation and parenchymal deflation on airway
calibre.
It is not practical to subject every asthmatic
patient frequently to the battery of tests used in this
study. Do asthmatic patients need any measure­
ments at all? How often will simple measurements
such as PEFR or FEV,.0 suffice? How often are
changes in those simple measurements mis­
leading? More information is not necessarily more
useful, and we have critically examined the use of
Raw and sGaw in this respect.
The patients studied are not typical of the
asthmatic population, for there were many older
intrinsic and few young extrinsic asthmatic
patients. We only considered laboratory records
where the measurements had been made by one of
us, since we wished to exclude as far as possible
variation between observers.
It was particularly important to define our
confidence that lung volume changes were not due
to technical error. We did not have enough data on
multiple determinations of each measured volume
on each occasion to compare each measurement
statistically. Normally we make three measure­
ments and take the mean value, which does not
allow sufficient degrees of freedom for effective
statistics. If the patient finds the necessary
manoeuvres difficult we may accept two or even
one technically satisfactory measurement. We have
therefore grouped the patients according to the
magnitude and concordance of the lung volume
changes, assuming that concordant changes,
especially if large, give credence that the measured
changes were real. Thus in series 1, and group A of
series 2 and series 3, we are confident that large
concordant changes in volume occurred. In group
B of series 2 and series 3 we have moderate con­
fidence in the smaller lung volume changes. In
group C of series 2 and series 3 we assume that no
important volume changes occurred (Fig. 1 and
Fig. 2).
We have used the actual changes in lung
volume rather than expressing them as percentage
Airways resistance and lung volume
changes of the initial measurement, or as percen­
tages of a 'predicted normal value', as the range of
such normal values is wide and we do not know the
normal values of our subjects when healthy. We
have arbitrarily defined a 10% change in PEFR
and FEVj.,, as 'significant', but taking limits of
±20 litres/min for PEFR and ±0-2 litre for FEVj.,,
did not change our general conclusions.
In series 1 (Table 1) exercise caused large
changes in lung volumes and PEFR which were
reversed by a bronchodilator. An obvious change
in airway calibre could have been detected by the
use of PEFR alone, without measurements of lung
volumes (Rudolf et al., 1975), as concluded by
Haydu, Empey & Hughes (1974) from bronchial
provocation studies, and by Ellul-Micallef,
Borthwick & McHardy (1974) from the changes
after a single dose of prednisolone in asthma.
In series 2 we consider the detection of a
bronchodilator effect. In group A, with large lung
volume changes, "significant" changes were seen in
either PEFR or FEV^ in all patients (Table 2),
though the effect would have been missed in two
for PEFR and in one for FEV,.„ if these tests had
been used alone. In group B, with moderate lung
volume changes, both PEFR and FEVj.p were
unchanged in two cases, and here the effect of
diminished lung volume on airway calibre may be
of importance. In group C the changes in lung
volume were small and unlikely to affect airway
calibre. We interpret the additional absence of
change in both PEFR and FEV^,, in five cases as
indicating no bronchodilator effect. Our criteria for
a positive bronchodilator effect are therefore as
follows.
1. Concordant fall in all lung volumes as defined
for group A or group B with or without a 10%
increase in PEFR or FEV^ or both. (Only two of
32 comparisons did not show such a change in
PEFR or FEV,.0.)
2. Increase of 10% in PEFR, FEV,.0 or both in
patients where FRC, RV and TLC changed <0·5
litre. By these criteria, FEV^ showed a positive
bronchodilator effect more frequently than PEFR
(Table 2), and the use of both gave a positive result
in 59 out of 61 cases.
In series 3 we consider the clinical problem
of objective assessment in patients after treatment,
or when symptoms have changed spontaneously.
Discordant changes in lung volumes were seen in
16 of 81 comparisons as compared with four
of 70 comparisons in series 2 and none of 120 in
series 1. On six occasions (Table 3) in group B
patients there was no change in either PEFR or
319
TABLE 4. Changes in PEFR andFEV,.0
Number of comparisons where measurement of lung volumes
changed interpretation, in series 2, because of a missed
bronchodilator effect and in series 3 because of significant
lung volume changes with absent or paradoxical change in
peak expiratory flow rate (PEFR) and forced expiratory
volume in 1 s (FEV10).
Series 2 (61 comparisons)
Series 3 (81 comparisons)
PEFR
alone
FEV,.0
alone
PEFR
and
FEV,.0
8
18
5
16
2
8
FEV,.0 when lung volumes changed. On one
occasion both PEFR and FEV^ fell when lung
volumes also fell, and once the reverse occurred.
In these eight comparisons (out of 81) the direct
effect of changing lung volume on airway calibre
may have been dominant.
Examining series 2 and series 3 for occasions
when lung volumes 'definitely' or 'probably'
changed in the presence of no change, or a dis­
cordant change, in PEFR and FEVi.,, (Table 4), we
find that FEV,.„ gives positive results more often
than PEFR, and with both, lung volume measure­
ment would have changed interpretation only twice
in series 2, and eight times (10%) in series 3.
In series 2, Raw changed concordantly with
PEFR and FEV,.,, in 65/70 comparisons. In five
cases in Table 2 with no 'significant' broncho­
dilator effect by our criteria flaw increased in two,
did not change in one, and decreased by 0-06 and
0-03 kPa 1_1 s in two. This additional information
is not sufficient to recommend the routine measure­
ment of Raw before and after a bronchodilator
drug in patients with asthma.
In series 3 flaw frequently changed dis­
cordantly with respect to changes in PEFR, FEV1>0
or both, but when this occurred the changes in at
least one variable were small and possibly due to
technical error (Fig. 3). Six subjects had a change
in Äaw of >0· 1 kPa 1_1 s despite small discordant
changes in FEV,.0, whereas eight had a change in
flaw of >0-l kPa l -1 s despite small discordant
changes in PEFR. Two patients had a decrease in
FEV,.0 of >0·8 litre and two had a decrease in
PEFR of >90 litre/min with small discordant
changes in R aw.
Can we explain these results in terms of
differential behaviour of 'large' and 'small'
airways? Despas, Leroux & Macklem (1972) and
Antic & Macklem (1976), defining small airways
as those within which a laminar flow pattern pre­
vails, have suggested that some asthmatic subjects
320
Κ. Β. Sounders andM. Rudolf
have obstruction predominantly in small airways
and some in large.
It is sometimes assumed that Raw reflects
mainly changes in large airway calibre. The
FEV,.0, on the other hand, includes part of the
downslope of the maximal expiratory flow volume
loop (Pride, 1971), whenflowis limited by dynamic
compression. If small airways are defined as air­
ways peripheral to the equal pressure point or EPP
(Mead, Turner, Macklem & Little, 1967), F E y ^
might depend upon changes in small airway
calibre. These two assumptions, in patients, require
examination.
In normal subjects most of the resistance is in
airways of diameter >2 mm. If we define small
airways as airways of diameter <2 mm, and assign
representative resistance of 0-02 kPa 1_1 s to small
and 0-1 kPa l -1 s to large airways, small airway
resistance can be multiplied by 5, yet total
resistance will be still at the upper limit of normal
(0·2 kPa l·-1 s), whereas large airway resistance
need only be doubled to make resistance abnormal
at 0-22 kPa 1_1 s. Thus in normal subjects Raw is
more sensitive to changes in resistance of large
airways. In patients with abnormally high Raw this
may arise from a relatively small change in the
large airway resistance, or a relatively large change
in the small airway resistance. Therefore changes in
Raw, once Raw is abnormal, do not necessarily
reflect predominantly changes in the resistance of
large airways.
In the second assumption, the advantage of the
Mead et al. (1967) concept of the EPP was the
simple relation between upstream resistance (Rus),
elastic recoil pressure (fel.), and maximum flow
(Kmax.) once flow was limited by dynamic com­
pression. At a given lung volume (Pel. constant)
maximum flow was directly related to upstream
resistance, and this relation was thought to be
independent of events downstream of the EPP. If
small airways are defined as airways distal to the
EPP, some small airways become large airways as
the EPP moves peripherally during expiration.
Moreover, if small airways resistance changes, the
EPP at a given lung volume moves, and the
airways defined as small are not the same, a
confusing concept. Parallel work (Pride, Permutt,
Riley & Bromberg-Barnea, 1967) emphasized also
the importance of bronchial collapsibility during
dynamic compression and Jones, Fräser & Nadel
(1975a, b) showed in dogs that at a given lung
volume Kmax is not a simple function of upstream
resistance. Rather upstream resistance determines
the length of airway over which Pel. is dissipated,
thus defining the site of the EPP, but the maximum
flow is then dependent on the compliance of the
segment downstream of that particular EPP. We
therefore discard the hypothesis that changes in
FEV,.0 reflect changes in calibre of 'small'
airways because: 1, not all of the FEV,.0 takes
place during flow-limiting conditions; 2, the air­
ways defined as small according to the EPP
concept change within a single breath, and if
upstream resistance changes from breath to breath;
3, maximum flow may not be a simple function of
upstream resistance even at constant lung volume;
4, maximum flow is affected by bronchial com­
pliance, which should not be assumed to be normal
in patients with diseased airways.
We conclude that we cannot reasonably in­
terpret discordant changes in flaw and FEV,.0 in
terms of differing behaviour of small and large
airways. Indeed, we cannot interpret them in any
useful terms.
Is one of the measurements intrinsically prefer­
able to the others? Since PEFR and FEVj.,, are
described as indices of Raw, it might be preferable
to take Raw itself as a basis for clinical decisions.
Unfortunately there are technical and theoretical
difficulties in making and interpreting the measure­
ment in patients. In normal subjects measurement
of Raw is performed at low flows during gentle
panting so that airway calibre is quasistatic. Thus
Raw certainly measures a different airway function
from FEVj.0, during part of which dynamic com­
pression occurs. In normal subjects Raw may be
more relevant to real life since dynamic com­
pression is negligible during tidal breathing, but in
many patients with moderate or severe airways
obstruction, the tidal flow-volume loop may
coincide with the descending limb of the midexpiratory flow—volume curve, and quasistatic
conditions for airway calibre never occur. This is
presumably more common in exercise. Such
patients often cannot pant at the high frequencies
required for accurate measurements in normal
subjects, nor with maximum flows <0·5 litre/s
(Cotes, 1975). The plethysmogtaph pressure-flow
relation is often widely looped in expiration, due to
dynamic compression, to temperature changes, and
perhaps to variation in the glottal aperture which
cannot be assumed to be held widely open as it is
during shallow panting in normal subjects
(Jackson, Gulesian & Mead, 1975). Finally some
measurement of 'slope' must be taken from the
inspiratory portion of a loop and a single number,
Raw, taken to describe the magnitude of a highly
alinear phenomenon. For these reasons, we do not
Airways resistance and lung volume
321
take flaw as an intrinsically preferable measure­
Acknowledgments
ment in asthmatic patients.
During this work M. R. was in receipt of a Sir Jules
In series 3 the change in /Jaw in 44/81
Thorn Research Fellowship.
comparisons merely confirmed the result from
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24