349
Clinical Science (1985)69,349-359
Ventilatory saving by external chest wall compression or oral
high-frequency oscillation in normal subjects and those with
chronic airflow obstruction
R . J . D. GEORGE*, R . J . D . WINTER*, S. J . F L O C K T O N T A N D D. M. GEDDES*
*The London Chest Hospital, London, and TDepartment of Applied Acoustics, Chelsea College, London
(Received 14 December 198415 March 1985; accepted 20 March 1985)
Summary
1. Oscillation of the air within the lungs at
high frequency is associated with an increased
clearance of COz. Because of the high frequency
and low volume of these oscillations, spontaneous
breathing is unhindered and the technique has
potential value as a supplement to ventilation.
2. High-frequency oscillations were superimposed upon tidal breathing by using a loudspeaker attached to a mouthpiece (oral highfrequency oscillation, OHFO) or by external chest
wall compression (ECWC). We set out (a) to
compare the changes in ventilation and breathlessness by using OHFO and ECWC in normal subjects
with those in patients with chronic airflow
obstruction (CAO), and (b) to relate the pattern
of saving to the resonant frequencies of the
respiratory system as a whole Cfor, 5-10 Hz in
normal subjects, 16-26 Hz in CAO) and those of
the ribcage dfoc, 70 Hz).
3. OHFO reduced minute ventilation (VE) by
up to 46% in normal subjects (P< 0.01) and 29%
in CAO (P<O.O)) without any rise in COz.
ECWC reduced VE by 27% in normal subjects
(P< 0.01) and 16% in CAO (P< 0.01) without a
rise in coz.
4. High-frequency oscillation by either method
relieved breathlessness in those with CAO and was
comfortable and well tolerated.
5 . In normal subjects for was discrete and
varied little with respiration. Maximum savings
occurred around f,, (5 -1 0 Hz).
6. In CAO, there was no obvious single
resonant frequency and flow and pressure signals
Correspondence: Dr R. J. D. George, The
London Chest Hospital, Bonner Road, London
E2 9JX.
were intermittently in phase over a band of about
10 Hz. Thus the reductions in minute ventilation
were only loosely related to for (13-26 Hz).
Neither group reduced
at foc (65-75 Hz).
7. OHFO has considerable potential in the
management of patients with CAO, where it may
be of value as an assistance to breathing and in the
relief of breathlessness. ECWC, although effective
in principle, is impractical by our methods and
awaits the development of an acceptable delivery
system.
v~
Key words: breathlessness, chronic airflow obstruction, high-frequency oscillation, resonance, respiratory failure.
Abbreviations: CAO, chronic airflow obstruction;
ECWC, external chest wall compression; foc, ribcage resonant frequency; for, resonant frequency
of the respiratory system; FRC, functional residual
capacity; HFV, high-frequency ventilation; OHFO,
oral high-frequency oscillation; t E , expiratory
time; tI, inspiratory time; ttot., total breath time;
VE, minute ventilation; V,, tidal volume.
Introduction
It is possible to ventilate an apnoeic subject with
volumes less than conventionally measured Fowler
dead space provided that they are delivered at
frequencies above 1 Hz [1, 21. When delivered by
mouthpiece, suchhigh-frequency ventilation (HFV)
has been shown also to enhance C02 elimination
in conscious subjects during breath-holding with
an open glottis [3]. However, as HFV does not
interfere mechanically with spontaneous breathing
high-frequency flow oscillations may be superimposed upon tidal breathing by using an adapted
350
R. J. D.George et al.
mouthpiece [4]. This is called oral high-frequency
oscillation (OHFO).
In a recent study using OHFO in normal subjects, we have shown that spontaneous minute
ventilation (VE) was reduced by 20-5076 compared with control without a rise in transcutaneous
COz [4]. The effect varied with frequency. Work
in animals and man has shown that the clearance
of COz from the lungs by HFV is related to both
oscillatory flow and volume [3,5,6]. The relationships between oscillatory flow, volume and
pressure depend upon the frequency response of
the system being oscillated. Flow resonance in a
linear system may be defined as the frequency at
which oscillatory flow is maximum for a given
input pressure and pressure and flow waves are in
phase. OHFO is uncomfortable at high mouth
pressures and for this reason the root mean square
input pressure was restricted to 126Pa (1.3
cm water). Thus the resonant frequency of the
respiratory system (for) may be an important
variable in defining the optimum frequency for
OHFO when there are restrictions on pressure but
a need to maintain oscillatory flow and volume.
This may be especially important in disease as the
resonant properties of the respiratory system in
patients with chronic airflow obstruction are
substantially different from normal in that their
for usually occurs above 15 Hz. for in normal
subjects lies between 5 and 10 Hz [7-91.
Adequate warming and humidification during
OHFO is difficult and cumbersome. If oscillation
of the column of gas in the airways can be
achieved by external chest wall compression
(ECWC), then it may also be possible to achieve
a reduction in ventilation by this method without
oral discomfort or drying effects. ECWC by use of
a pneumatic cuff at frequencies up to 10Hz
reduces VE in animals and man [lo, 111, and one
study in man with vibrations applied loca.!ly to
the chest also produced reductions in V, at
40 Hz [12].
We have therefore examined the effects of HFV
in two groups of subjects, with two techniques, in
order to (1) compare the ventilatory saving by
using OHFO in patients having chronic airflow
obstruction (CAO) with that in normal subjects;
(2) relate the pattern of saving to a resonance of
the respiratory system; (3) explore ECWC as an
alternative method of oscillating air within the
lungs.
Materials and methods
Physiological monitoring methods were common
to both studies. Ventilation was measured with an
inductance plethysmograph (Respitrace). The
chest and abdominal bands were attached firmly
to the skin with adhesive tape to prevent slipping.
The chest and abdominal signals were then
balanced by using the isovolume manoeuvre [13]
with the subject seated as for the experiment. The
summed signal was calibrated at the beginning and
end of the study by the subject exhaling and then
inhaling air from a bag of known volume. The
Respitrace had previously been validated against
a pneumotachograph with and without OHFO.
There was no significant difference in the estimation of tidal volume (paired t-test, 129 d.f.) with
a mean difference in estimated volume (mean 668
ml) of 23 ml (SEM 2.8 ml). Transcutaneous partial
pressures of C 0 2 and O2 were measured by
miniaturized electrochemical electrodes (Radiometer, TCM20, TCM2). Previous studies had
shown these to be reliable indices of changes in
arterialized capillary blood (unpublished work).
The electrodes were calibrated at the beginning
and end of each study according to the manufacturer’s instructions, with standard 5% and 10%
COz used as reference gas and room air.
Subjects
The group of normal subjects were seven
healthy males (mean age 33 years). None were
smokers. All had normal lung function assessed by
spirometry and carbon monoxide transfer.
Eight male subjects (mean age 57 years) with
CAO acted as the other group. All were previous
smokers. Details of their pulmonary function are
given in Table 1.
Experimental set-ups
Oral high-frequency oscillation. The experimental set-up and measurements are shown in Fig.
1. Sine wave oscillations were generated by using a
20 cm loudspeaker, which delivered sine wave flow
oscillations through a 2.5 cm internal diameter
tube. The instantaneous flow signal was measured
by using a Fleisch pneumotachograph and a
Gaeltec B873 differential pressure transducer. The
flow signal was passed through a 1 Hz high pass
filter to eliminate any influence of flow from tidal
breathing, amplified (SE4910) and recorded on
paper (SE 3006/DL). The input pressure of the
oscillations was maintained constant (mean
pressure zero, root mean square input pressure
126 Pa [1.3 cm water]) and measured by an
additional Gaeltec transducer (D361) between the
pneumotachograph and subject. Delivered oscillator volume (Vex.) was calculated by integration
of the instantaneous flow signal trace.
Supplementary ventilation technique
351
TABLE1. Lung function o f the group with chronic airflow obstruction
KCO, Transfer coefficient.
~~
No.
Age
(wars)
FEVl. 0
FVC
(ml)
(ml)
KCO
(mmol min-'
kPa-')
PaCO,
~
~-
PaCO,
(mmHg)
(mmHg)
~~
1
2
3
4
5
6
7
8
55
51
66
72
45
62
49
51
800
1800
550
900
900
900
640
500
3100
3800
1750
2400
1700
2900
3460
1300
0.95
1.80
1.17
0.34
1.17
0.63
0.33
0.52
58
89
48
46
45
68
I1
68
~
51
39
53
41
50
36
41
46
??
Radiometer TCM
FIG. 1. Experimental set-up for OHFO. See the text for details.
Connecting the subject to the apparatus was a
50cm length of semi-rigid tubing, which
corresponded exactly to the tubing used in the
external oscillation limb of the experiment. This
ensured that conditions in the two experiments
were comparable. Normal tidal breathing was
possible through a side port lying between the
loudspeaker and the pneumotachograph. The
352
R. J. D. George el al.
Amplifier
generator
Compressor
B & K 2603
dead space of the apparatus was reduced by
a 4litres/min suction bias flow at the mouth.
This did not influence measurements of flow or
pressure.
External chest wall compression. The experimental set-up is shown in Fig. 2 . The subject
was seated inside a sealed body box.
Within the walls of the box were four 60cm
loudspeakers (Fane). These were driven by a sine
wave generator (B&K 2010) and amplifier. Within
the box a microphone (B&K 4145) monitored
the root mean square sound pressure level. This
was maintained constant at 76 Pa (0.8 cm water)
via a feedback loop incorporating a microphone
amplifier and compressing amplifier (B&K 2603),
which in turn adjusted the output of the sine wave
generator. For reasons of safety, the mean pressure
level within the box at ribcage resonance was
restricted to 20 Pa (0.2 cm water). The subject
wore ear protection and breathed through a 50 cm
semi-rigid tube connected to the exterior. Previous
studies had verified that this tube did not transmit
oscillations by wall compression. Therefore any
high-frequency air column oscillation was due to
the effect of external chest wall compression
alone. Air column flow oscillations were monitored as in the OHFO study. The same bias flow
system was used to reduce dead space in both
experiments.
Calibrations
The frequency responses of both pressure and
flow transducers were measured by a technique
similar to that of Jackson & Vinegar [14]. Pressure
oscillations were produced in a closed tube (length
20cm, diameter 2.5 cm) by the 20 cm speaker.
The root mean square sound pressure was
monitored with a standard, independently calibrated transducer (B&K 4145) and acoustic
compressing amplifier (B&K 2603). By connecting
this to the speaker amplifier, its output was
controlled to maintain the pressure in the tube
constant over a frequency range 5-120 Hz. Both
transducers had a linear frequency response
(coefficient of variation <5%) beyond 120 Hz,
compared with the standard at 30 Pa, 70 Pa and
120 Pa. Both transducers were subsequently
calibrated against an inclined water manometer
at the beginning and end of each study.
The frequency response of the pneumotachograph and equipment as a whole was tested by
attaching an Helmholz resonator (volume 5.61
litres) to the mouthpiece. Thus the performance
Supplementary ventilation technique
of the equipment could be tested against a known
impedance. Measured flow impedance (peak
input pressurelpeak input flow) was then compared with calculated impedance. The system was
found to be linear to 30Hz (mean measured
impedance 101%calculated impedance, coefficient
of variation 11%).
Measurements o f resonant frequencies
AU structures have multiple resonant frequencies, determined by the characteristics of their
component parts. The dominant resonant
frequency will therefore be determined by the
component with the greatest response, although
all parts will oscillate to some extent at any given
frequency. In the respiratory system, airflow
oscillations within the lungs may peak about one
or more frequencies and may depend also upon
the nature in which the pressure and flow
oscillations were produced. Two resonances were
sought: (1) the dominant respiratory system
flow resonance
and (2) the velocity resonant
frequency of the ribcage Cf,,), as this may be
important in ECWC.
Respiratory system resonance (for). Flow and
pressure signals were displayed initially on an
X-Y oscilloscope. This allowed resonance to be
identified and the phase relationship between flow
and pressure around the for to be examined during
tidal breathing. In the normal subjects for lay
between 5 and 10Hz. For each normal subject
pressure and flow were consistently in phase over
a narrow band of about 2 Hz. However, in the
CAO group, flow and pressure were found to be in
phase intermittently over a frequency band of
approximately 10 Hz. To give a better idea of the
frequency characteristics of the respiratory
system, resonant frequency and impedance were
also estimated by the technique of forced oscillation described by Michaelson et aZ. [15]. This
provided a rapid plot of impedance when a band
of pseudo-random noise (1-25 Hz in the present
experiment) was delivered into the airways by an
arrangement identical to the set-up for OHFO.
Pressure and flow were monitored and analysed by
a Fourier spectrum analyser (HP3582A) to give a
continuous measurement of flow impedance as a
function of frequency [15]. Because of the change
in impedance with tidal breathing, we were
prepared to accept a coherence function of 0.9
to give a visual plot of the change across this
frequency band. Subjects were also asked to
indicate at what frequency they experienced the
sensation of maximum flow in and out of the
chest. This was variously described as shaking or
trembling. Both normal subjects and, surprisingly,
v,)
353
those with CAO were able to identify a frequency
repeatedly within about 2 Hz (preferred frequency).
This subjective frequency was taken to represent
resonance in the normal subjects. In those with
CAO a single frequency was inadequate to define
this resonant band. Two appropriate frequencies,
which differed by at least 5 Hz, were therefore
chosen in each patient. One of these was always
the preferred frequency. These all lay within the
range 13-26 Hz. In the normal subjects 15 Hz and
20 Hz were taken as comparable frequencies.
Ribcage resonance (f,,). This was measured in
the soundbox. An accelerometer (B&K 4332) was
held firmly against the xiphisternum by a rubber
strap and the measured instantaneous acceleration
was recorded continuously as the sine wave
generator automatically swept from 5 Hz to
100 Hz at a constant mean sound pressure of
20 Pa (0.2 cm water) within the box. The accelerometer signal was then integrated to give an
estimate of the velocity of the ribcage versus
frequency. The peak was taken as the resonant
frequency of the ribcage. The scan was repeated
with the accelerometer in 12 positions over the
chest to confirm that rib compression was in
phase.
Selection of frequencies
Normal subjects: 5 Hz, for (mean 7.5 Hz),
10 Hz, 15 Hz and 20 Hz (these were taken to
represent the frequencies of CAO resonance),
f,, (mean 70 Hz).
COA patients: 5 Hz, 10 Hz, for 1 (mean 14.3
Hz),fOr2(mean 20.1 Hz).f,, (mean 70 Hz).
Experimental procedure
The degree of disability of the patients meant
that they were unable to forego medication up to
the morning of the study day. The patients with
CAO had not taken inhaled bronchodilators for at
least 2 h before each study. Their other medication was otherwise unchanged. For each study, the
protocol was identical. Each subject was seated
comfortably and listened to music by headphone.
The subject acclimatized to the equipment while
measurements of resonance and impedance were
made. After a further 10 min period of acclimatization, OHFO for 10 min was alternated with 5 min
control periods. Frequencies were delivered in
random order. Five minutes control was found
sufficient to allow transcutaneous Pcoz to return
to control levels and prevented the experiment
being unacceptably long. Subjects were allowed to
remove the mouthpiece only for brief periods
during the control periods. The subjects with CAO
354
R. J. D. George et al.
were asked to score breathlessness at the end of
each oscillatory period and control period by using
a visual analogue scale in answer to the question
‘How breathless are you now?’. The extremes of
the 1 0 0 m m line were marked ‘No desire to
breathe’ and ‘Extremely breathless’. The patient
marked the line at a point that he considered
representative of his level of breathlessness [16].
Measurements of resonant frequency were the
same before and after each full study, and VE
was the same during the periods of control breathing throughout each study.
100
80
60
40
0
5 f, 10
15
20
7b
brequency (Hz)
Results
(b)
Data analysis
Tidal volume (VT), minute ventilation (pE),
inspiratory time ( t ~ ) expiratory
,
time ( t ~ )and
total breath time (ttot,)and transcutaneous gases
were measured during the final 4 min of each
period of HFV. The control was taken as the mean
of the last minute of every control period. The
acclimatization period was not analysed. Data
were compared by using Wilcoxon’s rank sum
tests.
-10
n
-20-30-
Minute ventilation
OHFO. Fig. 3 summarizes the results from both
groups of subjects. There were savings a t all
frequencies except a t 70 Hz, the ribcage
resonance, in normal subject and in CAO (P<
0.05). In those with CAO, reductions in VE were
similar between 5 Hz and f,, (24-2976). However,
in the normal subjects savings were greatest at
for and exceeded those at 15 Hz or 20 Hz (P<
0.05). Because of the wide variation in baseline
transcutaneous Pco,, absolute changes have been
plotted for each frequency. The reduction in VE
in the CAO patients was accompanied by a mean
fall in transcutaneous Pco2 of 2-4mmHg (P<
0.01). Four of these subjects had baseline C02
retention (Table 1). In the normal subjects, transcutaneous Pcoz was unchanged. This suggests that
alveolar ventilation was being supplemented. One
may therefore estimate the additional C02 elimination by OHFO and compare data with that of
other workers. Our normal subjects had a mean
transcutaneous Pco, of 42 mmHg, which according to our calculation data would represent a
Paco, of about 33 mmHg, and predicts an alveolar
ventilation of 5.2 litreslmin at a C02 production
of 200 ml/min. At 10 Hz, V, fell by between 0.3
litre/min and 4.3 litre/min (mean 2.6 litre/min). If
one assumes this reduction to be entirely a t the
-40
-50
--
0
5 f 10
15
20
h
70
Frequency (Hz)
FIG. 3. ( a ) Changes in ventilation, transcutaneous
PCO, and dyspnoea during OHFO: ventilatory
responses of seven normal subjects (c- - -0) and
The fall in
qf seven patients with CAO (u).
V, has been expressed as a percentage of control
to allow easy comparison between subjects with
widely differing baseline VE values. Stippled areas
show the resonant frequency bands for both
groups (for). f,, Resonant frequency of the normal
subjects. *P < 0.01; t P < 0.05. ( b ) Change in
transcutaneous (tc) PCo, from baseline in the
CAO group. The reductions at all but 70 Hz were
significant ( P < 0.05). (c) Change in breathlessness
during OHFO. Patients were asked to mark a 100
mm line, at the extremes of which the answers ‘No
desire to breathe’ and ‘Extremely breathless’ were
marked in response to the question ‘How breathless are you?’. Breathlessness varied between
patients during the control periods, i.e. their mark
was placed x mm from the end ‘No desire to
breathe’. For comparison between patients, this
measurement was taken as their baseline and
called zero. Thus a fall in breathlessness becomes
a negative value and a rise becomes positive. Hence
the arrow showing the direction in which breathlessness increased. Vertical bars denote SEM.
355
Supplementary ventilation technique
expense of alveolar ventilation, then OHFO
supplemented COz elimination by a mean of 100
ml/min (range 11-163 ml/min).
Oscillatory volume at 10 Hz was 30 ml. Goldstein et al. [ 3 ] looked at COz clearance with
OHFO at 1OHz during breath-hold in normal
subjects, and they also found a wide variation
between individuals. From their raw data COz
clearance at this oscillatory volume ranged from
about 10 to about 160ml/min, with a mean of
77.5 ml/min. Allowing for changes in lung volume
during our study and the other assumptions these
data are in reasonable agreement.
ECWC. Fig. 4 summarizes the results of ECWC.
This shows that oscillations may be induced in the
air column by ECWC. Although comparisons
between the savings in OHFO and ECWC cannot
be made with confidence, the pattern of savings
were similar. This confirms that the effect of these
oscillations is the same as those delivered orally.
Patients with CAO showed savings in VE at 5 HZ
(P< 0.05) and at the two respiratory resonant
frequencies (P<O.Ol).
The maximum mean
savings were 16%. In the normal subjects significant savings occurred at 5 Hz, lOHz and for
(P<0.01) only.
0
5 f, 10
15
20
700
Frequency (Hz)
(b)
TI
21
0
I
4
--I-; , : \ &
Sensations of breathlessness
To allow a comparison of breathlessness
between subjects, the distance of the mark from
the end 'No desire to breathe' during the control
periods was taken as zero. Any increase or decrease in breathlessness was then expressed as a
positive or negative change (in mm) from zero.
Breathlessness was reduced at all frequencies
(P< 0.01) except foc, the ribcage resonance, and
was independent of frequency (Figs. 3 and 4).
Breathing patterns
The changes in breathing patterns are shown in
Fig. 5 for resonant frequency in normal subjects
(mean 7 . 5 H z ) and the CAO patients (mean
17.5 Hz), the frequencies at which savings were
greatest in each group. In both groups there was a
fall in VT/tI (P< 0.05). In the CAO group there
was also a fall in VT (P < 0.05). Both groups had
a fall in respiratory frequency with prolongation
of t~ and breathholding at FRC (P< 0.01).
Resonant characteristics of both groups (Fig. 6 )
The resonant characteristics of both groups, as
measured by forced oscillation at the mouth, are
shown with flow impedence in Fig. 6 (upper
panel; the changes in flow and volume with
-'"1 1
J
0
5
10
15
20
Frequency (Hz)
'
7 0 0
FIG. 4. Ventilatory responses of both normal
(n = 7) and CAO (n = 8) groups to ECWC plotted
in the same manner as Fig. 3 to allow comparison.
For symbols etc. see Fig. 3.
frequency at a constant oscillatory pressure are
shown in the lower panel).
In normal subjects there was no change in
impedance between 5 and 10 Hz. The impedance
had doubled from the resonant frequency when
measured at 20 Hz (P<0.01). In contrast, those
with CAO had a reduction in flow impedance
from 5 H z (13.8 cms-ll-') to a mean of 7.5
cm s-' I-' at the resonant frequencies (P< 0.01).
Although there are substantial changes in the flow
impedance in both groups, the effect of this
particularly upon oscillatory volume is striking.
Although volume in the normal subjects falls off
R. J. D. George et al.
356
-
600
-
800 *-• Control
0------0
h
s
CAO patients OHFO (mean 19 Hz)
Normal subjects OHFO (mean 7.5 Hz)
(a)
800
Control
-m
HFO
n=6
n=6
400200*-•
I
0
1
2
a
0.._.____.....
0
3
4
Cycle time (s)
5
i
I
6
0
0-0
1
1
2
3
4
5
Cycle time (s)
(b)
900
IT
900
h
%v
700
c
3
k
500
*'
0
o
u
FIG. 5 . Breathing patterns during OHFO in both normal subjects and CAO groups
( n = 6) at their respective resonant frequencies. ( a ) Spirogram: mean VT plotted
against respiratory cycle time. End expiratory pause at FRC has been marked as a
horizontal line at zero VT. ( b ) Changes in V T / t l and absolute reduction in v~ are
plotted. HFO, High-frequency oscillation.
dramatically with frequency, in CAO the rise in
flow was sufficient to maintain volume.
Discussion
In these studies comparisons have been made in
two groups of subjects by using two systems to
produce flow oscillations in the air column. Some
workers have shown that ECWC does influence
ventilation and their results are similar to ours
[lo-121. Owing t o limitations imposed by the
equipment used, the study by Zidulka et al. [lo]
was confined t o frequencies below lOHz, and
Barach & Dulfano [12] examined only one frequency (40 Hz). We chose an external oscillation
system with loudspeakers because there would be
no restrictions in frequency, and by using sine
wave oscillations it was possible to relate the
effects of high-frequency oscillation to the
resonant characteristics of the respiratory system.
Resonance
The measurements of impedance in both groups
is in broad agreement with the literature [7-9,
17-19]. Resonance occurred over a wider
frequency range in CAO than in normal subjects.
The changes in for during tidal breathing were
related to lung volume, in that the frequency at
which pressure and flow were in phase vaned by
about 10 Hz, with a fall towards TLC and a rise
towards FRC. This is consistent with the respiratory system acting as a resonating cavity, where
resonant frequency is proportional to 1/volume
and where regional variations in time constants are
small. In subjects with widespread, increased airways resistance, regional variation in time
constants is likely t o partition one large cavity into
several smaller, parallel cavities with different
acoustic properties. These time constants will vary
with airway calibre, and although they may lead
to favourable peripheral gas mixing by Pendeluft
Supplementary ventilation technique
T
i'...
357
in frequency characteristics of the lungs with
volume. The repeated choice of a single preferred
frequency in the resonant band may be related to
the expiratory pause, when oscillations are felt
more easily.
Flow resonance
Ribcage resonance
I-
5
10
15
20
60
0.6
40
0.4
Y
5:
2
0
6
2
3
9
20
0
0.2
L
k
0
r
5
#
10
15
20
Any real system has multiple resonances and
different methods of excitation frequency
emphasize different resonances. Studies upon the
somatic effects of vertical vibration, produced by
shaking the subject up and down, have confirmed
a respiratory resonance between 5 and 12 Hz
in normal subjects [22, 231. However, none of
these studies has measured ribcage resonance
alone. Brown 1241, with the experimental sound
box employed in these studies, used an accelerometer to demonstrate pure ribcage resonance at
around 70 Hz. This was confirmed by altering the
acoustic characteristics of the cavity with inhaled
gases of density different from air. Because of the
frequency limitations of our measuring system,
oscillatory flow at the mouth was unquantifiable
at 70 Hz,although there was no sensation felt by
the subjects at this frequency. However, Peslin
et al., using a system similar to this, showed
impedance to continue to rise up to 70Hz [17].
Thus it seems that ribcage resonance has no major
influence upon oscillatory flow at the mouth.
Frequency (Hz)
FIG. 6. Flow impedance, oscillatory flow ( 0 , 0 )
and delivered oscillatory volume (0,m) in normal
subjects (filled-in points) and CAO patients (open
points), showing the differences in resonant
characteristics of the two groups and the variation
between delivered flow and volume with
frequency. Results are plotted as means ?1 SEM. See
the text for further details.
[2, 201, small changes in airway resistance (and
time constant) may lead to substantial, rapid
variation in the frequency characteristics of the
system as a whole throughout a respiratory cycle.
Bronchodilatation, for example, is well known to
reduce respiratory system resonance [18, 191.
Recent work in animals with cinebronchography
during high-frequency oscillation has shown that
under control conditions the main airways change
calibre slightly, but that all volume oscillations
occur in the peripheral lung. However, after the
administration of histamine, oscillation is seen in
the central airways rather than the periphery
[21]. The response of the CAO group to oscillations may therefore be influenced by this variation
Changes in ventilation
The reductions in t& were related loosely to
oscillatory flow and volume and this accounts for
the differences seen between normal subjects and
CAO patients, as flow and volume varied according
to the resonant characteristics of the respiratory
system. Fig. 6 shows frequency versus flow impedance, comparing normal subjects with the CAO
patients. The shapes of the plots show some sir+
larity to those detailing the changes in VE.
However, the relationship is not as impressive as
one might expect. The absence of a clear
resonance in the respiratory system shows it to be
very damped. In the normal subjects, beyondf,,
the fall in volume is substantial and follows more
closely changes in pE.In CAO the rise in oscillatory flow as resonance is approached is sufficient
to preserve the delivered volume between 5 Hz
and 20 Hz. Within the limitations of this study, we
have detailed the contributions of flow and
volume to C02 clearance, and our predictions in
normal subjects (mean 100 ml/min, range 11-163
mllmin) agree fairly well with the measurements
of Goldstein et al. (mean 78 ml/min, range 10-160
ml/min) [3]. In that study measurements were
358
R. J. D. George et al.
made at a constant lung volume. Applying the
technique practically it seems that, with a constant
input pressure, the effect of frequency is less
important in maintaining oscillatory volume in
CAO than in normal subjects. However, there may
be additional losses of flow and volume within the
mouth and hypopharynx at higher frequencies, as
the resonant frequency of the upper airways is in
the region of 30Hz. The local impedance may
then fall to 10 cm water s-' 1-' [15]. From these
studies we do not know how much flow or volume
has been lost to these structures.
The preservation of adequate gas exchange
despite a fall in VE, and the relationship between
VE, oscillatory flow and volume, makes improved
C02 elimination the most important mechanism
underlying these observations. In addition, however, changes in the breathing patterns in both
groups confirm a reduction in respiratory drive,
witnessed by the fall in V T / ~The
~ .central control
of breathing is extremely complex [25], and the
may also have a neurological basis, as
fall in f i ~
animal studies have demonstrated that highfrequency oscillation may prolong tE and even
cause apnoea independent of changes in blood
gases or lung volume [26]. These reflexes are
abolished by vagotomy, and are probably
mediated via the pulmonary stretch receptors
[27]. It is also possible that the reduction in
breathlessness in the CAO patients seen in both
studies has been produced by the change in
afferent traffic altering the perception of breathlessness, in addition to a reduction in VE.
Clinical application of HFV in breathing subjects
Oral. OHFO was comfortable and well tolerated,
and the patients were less breathless than during
control periods without OHFO. In no patient was
there a deterioration in blood gases. Reduction in
VE was not dependent upon frequency in CAO,
but all expressed a preference for their chosen
resonant for, where they were more aware of the
sensation of oscillation and the high frequency
was less intrusive upon swallowing. Other work
also suggests that OHFO reduces v~ and breathlessness during steady-state exercise [28]. The
mouth pressures used in this study are much lower
than those used in conventional HFV, as the
intention of this technique is to supplement and
not replace spontaneous breathing. We are not able
to comment upon the relationship between
ventilatory saving and input pressure, but in a
system such as this there must be a trade-off
between the desired effect and the local drying
and irritation of oscillating the upper airways.
This would be of particular importance in
providing support for those with respiratory
failure where intubation was undesirable.
External chest wall compression. The physiological response to external vibration depends
upon its nature. There is a substantial literature
dating from the 1960s showing whole body
vertical vibration to cause hyperventilation [22,
23, 291. The mechanism seems unclear, but the
response has the characteristics of a Pavlovian
conditioned reflex and is abolished by anaesthesia.
There is no enhanced C 0 2 clearance, although
air column oscillations were noted [23]. Other
workers have demonstrated differences in intercostal muscle activity in response to vibration
applied locally to the chest. They ascribe this to
excitatory or inhibitory tonic reflexes [30]. Data
on circumferential or compression oscillation agree
with our findings of a reduced VE without COz
retention [lo, 111. It therefore seems that ECWC
of the right type is capable of assisting ventilation.
There may be some advantages over an oral system
as there is no need for a mouthpiece and there
may also be less drying and cooling of the air. This
may be suitable for nocturnal use. In addition the
technique developed by King et al. improves
tracheal mucus velocity in dogs and may be an aid
to physiotherapy [31].
Although the use of HFV in conscious, breathing subjects, either as an intermediate between
drug treatment and formal ventilation in those
with acute respiratory failure or as a domiciliary
aid to those with chronic respiratory difficulties,
is an exciting prospect. The need remains to define
its therapeutic potential and to develop practical
ways of oscillating the air column.
Acknowledgments
We thank Dr R. C . Schroter for his assistance in
the preparation of this manuscript. This work was
supported by the Medical Research Council.
References
1. Bohn, D.J., Mijasaka, K., Marchac, B.E., Thompson,
W.K., Froese, A.B. & Bryan, A.C. (1980) Ventilation
by high frequency oscillation. Journal of Applied
Physiology: Respiratory, Environmental and Exercise
Physiology, 48,710-716.
2. Drazen, J.M., Kamm, R.D. & Slutsky, A.S. (1984)
High frequency ventilation. Physiological Reviews, 6 4 ,
505-543.
3. Goldstein, D., Slutsky, A S . , Ingram, R.H., Westerman, P., Venegas, J. & Drazen, J . (1981) CO, elimination by high frequency ventilation (4-10 Hz) in
normal subjects. American Review of Respiratory
Disease, 123, 251-255.
4. George, R.J.D., Winter, R.J.D., Johnson, M.A., Slee,
1.P. & Geddes, D.M. (1984) Oral high frequency
ventilation by jet or oscillator reduces minute ventilation in normal subjects. Thorax, 39,234-235.
Supplementary ventilation technique
5. Slutsky, A.S., Kamm, R.D., Rossing, T.H., Loring,
S.A., Lehr, J., Shapiro, A.H. & Drazen, J.M. (1981)
Effect of frequency, tidal volume and lung volume
on CO, clearance in dogs by high frequency oscillation (2-30 Hz) low tidal volume ventilation. Journal
o f Clinical Investigation, 68, 1475-1484.
6. Solway, J., Gavriely, N., Kamm, R.D., Drazen, J.M.,
Ingram, F.H., Jr, Khoo, M.C.K., Brown, R. & Slutsky,
A S . (1984) Intra-airway gas mixing during high
frequency ventilation. Journal of Applied Physiology:
Respiratory, Environmental and Exercise Physiology,
56,343-354.
7. Dubois, A.B., Brody, A.W., Lewis, D.H. & Burgess,
B.F. (1956) Oscillation mechanics of the lungs and
chest in man. Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology, 8,
587-594.
8. Grimby, G., Takishima, T., Graham, W., Macklem, P.
& Mead, J. (1968) Frequency dependence of flow
resistance in patients with obstructive lung disease.
Journal of Clinical Investigation, 47, 1455-1465.
9. Clement, J., Landser, F. J. & Van de Woestijne, K.P.
(1983) Total resistance and reactance in patients
with respiratory complaints with and without airway obstruction. Chest, 83, 215-220.
10. Zidulka, A., Gross, D., Minami, H., Vartian, V. &
Chang, H.K. (1983) Ventilation by high frequency
chest wall compression in dogs with normal lungs.
American Review o f Respiratory Disease, 127, 709713.
11. Calverley, P.M.A., Chang, H.K., White, D. & Zidulka,
A. (1984) Effects of high frequency chest wall compression in normal subjects and COPD patients
(Abstract). Thorax, 39, 235.
12. Barach, A.L. & Dulfano, M.J. (1968) The effect of
chest vibration in pulmonary emphysema: a preliminary report. Annals ofAllergy, 26, 10-17.
13.Konn0, K. & Mead, J. (1967) Measurement of the
separate volume changes of the ribcage and abdomen
during breathing. Journal of Applied Physiology:
Respiratory, Environmental and Exercise Physiology,
22,481-522.
14. Jackson, A.C. & Vinegar, A. (1979) A technique for
measuring the frequency response of pressure, volume
and flow transducers. Journal of Applied Physiology
(H), 47,462-467.
15. Michaelson, E.D., Grassman, E.D. & Peters, W.R.
(1975) Pulmonary mechanics by spectral analysis of
forced random noise. Journal of Clinical Investigation,
56,1210-1230.
16.Bond, A. & Lader, M. (1974) The use of visual
analogue scales in rating subjective feelings. British
Journal of Medical Psychology, 47, 211-218.
17.Peslin, R., Papon, J., Duvivier, C. & Richalet, J.
(1975) Frequency response of the chest: modeling
and parameter estimation. Journal of Applied Physiology: Respiratory, Environmental and Exercise
Physiology, 39,523-534.
18. Landser, F.J., Nagels, J., Demedts, M., Billiet, L. &
van de Woestijne, K.P. (1976) A new method to
359
determine the frequency characteristics of the respiratory system. Journal of Applied Physiology: Respiratory, Environmental and Exercise Physiology, 41,
101-106.
19. Rozen, D., Bracamonte, M. & Sergysels, R. (1983)
Comparison between plethysmographic and forced
oscillation techniques in the assessment of airflow
obstruction. Respiration, 44, 197-203.
20. Lehr, J. (1980) Circulating currents during high
frequency ventilation (Abstract). Federation Proceedings, 39,5 76.
21. Gavriely, N., Solway, J., Loring, S., Ingram, R.H., Jr,
Westerman, P., Venegas, J. & Drazen, J. (1982)
Airway dynamics during high frequency ventilation
(HFV): a cineradiographic study (Abstract). Physiologist, 25, 282.
22. Duffner, L.R., Hamilton, L.H. & Schmitz, M.A.
(1962) The effect of whole-body vertical vibration on
respiration in human subjects. Journal of Applied
Physiology: Respiratory, Environmental and Exercise
Physiology, 17,913-916.
23. Zeckman, F.W., Peck, D. & Luce, E. (1965) Effect of
vertical vibration on respiratory airflow and transpulmonary pressure. Journal of Applied Physiology:
Respiratory, Environmental and Exercise Physiology,
20,849-854.
24. Brown, F.D. (1976) M.Sc. Project report. Chelsea
College, London.
25. Von Euler, C. (1983) On the central pattern generator
for the basic breathing rhythmicity. Journal of
Applied Physiology: Respiratory, Environmental and
Exercise Physiology, 55, 1647-1659.
26. Banzett, R., Lehr, J. & Geffroy, B. (1983) High
frequency ventilation lengthens expiration in the
anesthetised dog. Journal of Applied Physiology:
Respiratory, Environmental and Exercise Physiology,
55,329-334.
27. Man, G.C.W., Man, F.P. & Kappagoda, C.T. (1983)
Effect of high frequency oscillating ventilation on
vagal and phrenic nerve activities. Journal of Applied
Physiology: Respiratory, Environmental and Exercise
Physiology, 54,502-507.
28. George, R.J.D., Howard, R.S., Stearn, A. & Geddes,
D.M. (1985) The effect of oral high frequency oscillation on steady state exercise in chronic lung disease
(Abstract). Thorax, 40,213-214.
29. Lamb, T.W. & Tenney, S.M. (1966) Nature of vibration hyperventilation. Journal of Applied Physiology:
Respiratory, Environmental and Exercise Physiology,
21,404-410.
30. Homma, I., Nagai, T., Sakai, M., Ohashi, M., Beppu,
M. & Yonemoto, K. (1981) Effect of chest wall
vibration on ventilation in patients with spinal cord
lesions. Journal of Applied Physiology: Respiratory,
Environmental and Exercise Physiology, 50, 107-1 11.
31. King, M., Phillips, D.M., Gross, V., Vartian, V., Chang,
H.K. & Zidulka, A. (1983) Enhanced tracheal mucus
clearance with b h frequency chest wall compression.
American Review of Respiratory Disease, 128, 51 1515.