Ann. occup. Hyg., Vol. 41, Supplement 1, pp. 659-666, 1997
© 1997 British Occupational Hygiene Society
Published by Elsevier Science Ltd. All rights reserved
Printed in Great Britain
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ACUTE EXPOSURE OF HUMANS TO OZONE IMPAIRS SMALL
AIRWAY FUNCTION
W. M. Foster, G. G. Weinmann, E. Menkes and K. Macri
Department of Environmental Health Sciences, School of Hygiene and Public Health,
The Johns Hopkins University, Baltimore, MD 21205, U.S.A.
INTRODUCTION
Our approach has been to use noninvasive, regional analysis techniques and we
have separated central from peripheral mucus membrane responses to ozone
(Foster et al., 1987) and characterised the effects of ozone on regional ventilation
and particle dosimetry for the lung periphery (Foster et al., 1993). In these prior
studies we relied upon techniques that utilised radioisotopic markers to assess
O3-induced changes in regional lung function. The goal of the present study was to
expand upon the earlier results using nonradioisotopic methods and multibreath
washout of a resident lung gas (N2) to measure nonhomogeneous lung function
following exposure to O3.
METHODS
Fifteen healthy male volunteers were recruited for the study. All were nonsmokers and without history of lung disease nor receiving medications for any other
disease. The subjects had a mean age of 25.4 ± 2 years (± SD) and were free of
respiratory infection at the time of evaluation. The forced vital capacity (FVC),
forced expiratory volume at 1 s (FEVi) and mid-maximal expiratory flow rate
(FEF25_75) of the group averaged > 92 ± 11% of predicted (Crapo et al, 1981).
Informed consent was obtained from each individual; the study had the approval of
the University's Committee on Research of Human Subjects.
Experimental protocol
The treatment plan (two treatments; crossover design) was selected to reduce
within-subject variability and to facilitate comparisons between treatments to
filtered air (FA) and O 3 , with each subject used as his own control (FA). The order
of treatments was randomized and the mean washout time between treatments was
8 weeks.
Methods of exposure of human subjects have been utilised previously (Foster et
al., 1993). Briefly, exposures lasted 130 min and were accomplished in a 13.6 m3
chamber maintained at 19-23°C and 48-55% relative humidity. O 3 was generated
from a 100% oxygen source by a high-frequency electric field (model Gl-L, PCVI
Ozone, West Caldwell, NJ, U.S.A.) and mixed with FA before being added to the
chamber. Chamber O 3 was continuously monitored by an ultraviolet O 3 photo659
660
W. M. Foster et al.
meter (model 1003-AH, Dasibi, Glendale, CA, U.S.A.). Mean (± SD) O 3
concentrations during exposures were 351 ± 6 ppb and 1 ± 1 ppb for O3 and FA
exposures, respectively. Air transport to the chamber was prefiltered and had a
one-pass design with 24 changes of chamber air h" 1 . During the initial 120 min of
the exposure, the subjects alternated between 30 min periods of rest and treadmill
exercise (Model Q55, Quinton Instrument, Seattle, WA, U.S.A.) and attained a
cumulative ventilation per minute during exercise that was approximately ten times
the volume of the FVC. The exposure ended with a final 10 min rest period. The
treadmill speed and grade necessary to obtain the targeted minute ventilation were
predetermined for each subject before commencement of the exposures. Exercise
is frequently used during chamber exposures to increase minute ventilation to
mimic an individual performing light activity under ambient conditions.
During exposures the subjects could freely choose between nasal, oral, oro-nasal
breathing modes, except during assessment of minute ventilation when oral
breathing and expiration into a dry gas meter was obligatory. Before and
immediately following the exposures pulmonary function (at least three determinations of the FVC, FEVj and FEF25_75 by water-seal spirometer), the measurement
of multibreath N2 washout (Mauderly, 1977), and body plethysmographic measurement of thoracic gas volume (Vtg) were accomplished.
For the multibreath N2 washout technique the subjects cleared N2 gas from their
lungs by mouth breathing 100% oxygen in a seated and erect position with a
noseclip in place. Valving separated inspiratory and expiratory gases with samples
of expiratory gas continuously analyzed for N2 concentration using a mass
spectrometer (Medical Gas Analyzer, Perkin-Elmer Inc., Pomona, CA, U.S.A.).
Inspiratory and expiratory flow rates were measured with a heated pneumotachograph connected to the mouth piece and sealer tracings of flow were integrated to
provide an instantaneous recording of tidal volume. During washout the tidal
volume was presented visually to the subject on an oscilloscopic screen and
simultaneous with an audio signal (for pacing the frequency of the respiratory
cycle) to assist in the achievement of a targeted ventilatory pattern during the
washout of N2 from the lung. The natural logarithm of each end-tidal N2
concentration was plotted against cumulative minute ventilation (from the start of
the washout). These curves generally had two phases; an initial, steep slope
followed by a more shallow slope. We defined the latter phase as the washout from
20 to 9% end-tidal N2) concentration; flattening of the slope of this phase was used
to identify nonuniform ventilation following exposure to O 3 of FA. A washout
index, slope, was calculated from the best fit of the washout data to a curve
generated by regression analysis.
Measures of pulmonary function were corrected to BTPS; and the trials with the
highest sum of FVC and FEVx for both pre- and postexposure were utilised for
statistical analysis.
RESULTS
The effects of O 3 on washout of lung N2 are demonstrated in the plots presented
in Fig. 1 of end-tidal % N2 concentration v. cumulative expired volume measured
during the washout. These data were acquired in a single subject pre- and
Ozone impairs small airway function
661
4.4• pre-ozone
o post-ozone
4.03.6-
•
3.2
o
•
o
o
o
£
u
o
2.8-
o o
2.42.0-
2500
5000
7500
10000
12500
15000
17500
20000
Volume (ml)
3.2-
• pre-ozone ^ 2 == 0.994
o post-ozone RA2 = 0.985
•
3.02.8-
c
2.6>
2.42.2As
—i
8000
10000
1
1
12000
1
14000
16000
18000
20000
Volume (ml)
Fig. 1. Multibreath washout of lung N2 breathing 100% O2. Upper panel: relationship between endtidal
concentration of N2 (expressed as natural logarithm, In, of the % N2 concentration) and the cumulative
expired volume (ml) measured from the start of the washout; data are presented for a single subject
measured pre- and post-exposure to O3. Lower panel: data from washout of lung N2 presented in the
upper panel are fit to a linear regression (solid line, /?2 = coefficient) to characterize the slope of the
curves during the later part of the washout between the expired lung N2 concentrations of 25 and 9%.
post-exposure to O3. The mean pre-post exposure, washout slopes for all 15
subjects are presented in Fig. 2 for exposures to FA and O3. Washout slopes preand post-exposure to FA were similar; but the differences in washout pre-post
exposure to O 3 were significant (P < 0.05) and represented a 24% decrease
W. M. Foster et al.
662
FILTERED AIR
PRE
POST
Fig. 2. Slope of lung N2 washout pre- and post-exposure to filtered air and O3. Mean slopes (±SE) for
washouts between N2 concentrations of 25 and 9% are presented for 15 subjects. *indicates significant as
compared to mean preexposure value (P < 0.05).
(O3-induced) in the mean slope of the N2 washout, washout of lung nitrogen by
breathing 100% O 2 was delayed by preexposure to O3.
The corresponding measures of thoracic gas volume (Vtg) and minute ventilation
acquired either prior to, and during, the washouts are presented in Fig. 3 and listed
in Table 1, respectively. These factors were equivalent during the performance of
the pre- and post-exposure washouts. Although influential to the calculation of the
slope during the respective washouts, the compared values (Vtg and minute
0
1
2
3
4
5
POST EXPOSURE (litres)
Fig. 3. Thoracic gas volume during lung washout pre- and post-exposure to O3. Relationship between
the pre- and post-exposure values of the thoracic gas volume, functional residual capacity, that were
assessed in each subject at the time of the washout measurements. Data (N = 15) were fit to a linear
regression (solid line) and for comparison is included line of identity (dashed line).
Ozone impairs small airway function
663
Table 1. Minute ventilation during washout of lung N2*
Subject
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Mean
± SE
Ozone exposure
Pre (1)
Post (1)
Filter air exposure
Pre (1)
Post (1)
11.37
11.87
17.00
8.76
8.52
9.35
8.61
8.02
17.68
12.51
13.11
8.56
10.38
9.69
15.45
11.39
0.82
14.86
15.20
20.61
10.33
9.31
12.31
12.85
11.19
12.60
9.06
11.06
11.68
11.10
11.79
11.98
12.40
0.73
10.20
11.41
20.93
11.48
8.95
12.83
13.30
11.11
11.54
8.99
10.72
11.81
10.61
11.46
11.53
11.79
0.72
10.26
13.85
19.50
9.99
9.95
9.33
9.38
8.65
12.86
11.74
12.18
11.02
11.06
8.83
16.56
11.68
0.78
*Average minute ventilation (1.) measured for each subject during respective washouts of
lung N2 by breathing 100% O2.
volume) were not different and thus not factors in the effect of O 3 on the washout
of lung N2.
The effects of exposure to O 3 on spirometric indices of lung function are
presented in Fig. 4. The mean values of the FVC and the FEV1, pre- and
post-exposure, are included and the influence of O 3 exposure on these indices,
decrements post-exposure averaged 12 and 14%, respectively, and were significant
(P < 0.05). The O3-induced changes in FVC and FEVj (and the FEF25_75, however
PRE
POST
FVC
PRE
POST
FEV,
Fig. 4. Spirometric response to O3. The mean (±SE) response data of the 15 subjects are presented for
the forced vital capacity (FVC) and forced expiratory volume in 1 s (FEVj). indicates significantly
different than mean preexposure value (P < 0.05).
664
W. M. Foster et al.
these data are not presented) were not correlated to the observed O3-induced
changes in the slope of the N2 washout. Although the response data have not been
presented, changes in spirometric indices following exposure to FA were less than
± 2% of the pre-exposure values.
DISCUSSION
This study expands upon our prior demonstration that exposure to O3 leads to an
acute redistribution of regional ventilation (Foster et al., 1993). We used a
non-radioactive gas technique to evaluate the effects of O 3 on the washout kinetics
of lung N2 and small airway function. Following exposure to 350 ppb O3, the latter
phase of lung N2 washout became prolonged. This response developed acutely after
exposure to O3. Delays in washout postexposure averaged 24% of the washout
values observed preexposure and were not caused by the ventilatory pattern during
the washout, nor changes in the volume of the space (Vtg) being ventilated. We
suggest this response is attributed to O3-induced alterations in bronchial tone
and/or mucus secretions within smaller peripheral airways (Foster et al., 1987,
1993), similar to nonuniform ventilation associated with bronchitis (Seaton and
Ogilvie, 1978) and disease of the small airways (Ebert and Terracio, 1975; Wright et
al., 1984). The delay which occurred during the later phase of N2 washout, did not
correlate to the functional decrements observed in the FVC and FEVi. Acute
changes in lung function (FVC and FEVi) a r e believed to be related to irritant and
neural mechanisms (Hazucha et al., 1989) within the larger airways of the lung,
whereas the effect on N2 washout we observed may represent injury, increased
permeability and inflammation of more distal, smaller airways (Foster and
Stetkiewicz, 1996).
Regional absorption of O 3 seems to impair small airway function and adversely
affect ventilation to distal lung units, either separately or in combination with
constriction of the larger bronchi. We have previously observed using radiolabelled
gases that during the period immediately following exposure to O3, lung regions
with the highest ventilation per unit volume at baseline, have ventilation consistently reduced and redistributed to regions of lower ventilation (Foster et al., 1993). We
suggested that constriction of smooth muscle (Beckett et al., 1985) and hypersecretion of mucus in peripheral airways (Foster et al., 1987) within the lung regions
receiving the highest local doses of O 3 could lead to uneven time constants in these
airways and nonhomogeneous ventilation.
Of interest, was an additional observation that not all of the subjects exposed to
O 3 exhibited delays in washout as an acute response; however when washout was
remeasured 20-24 h postexposure, delays in washout of lung N2 were found to be
present. This demonstrated for a single subject in Fig. 5 and suggests that for some
individuals, following oxidant exposure injury to the smaller airways may develop
slowly and/or be related to the late onset of inflammatory changes known to occur
with exposure to O 3 (Koren et al., 1989; Foster and Stetkiewicz, 1996).
In summary the results suggest that in addition to decrements in spirometric
function apparent after an acute exposure to O3, tests of multibreath N2 washout
are also abnormal (delayed). The delays in washout are not related to changes in
ventilatory pattern, nor alteration of lung volume at FRC. Changes in the washout
Ozone impairs small airway function
665
•
pre-ozone
O
post-ozooe
+
post-ozone 24hr
•o*
o»
o«
+
o»
+ +.
+
Volume (ml)
Fig. 5. Multibreath washout of lung N2 breathing 100% O2. Relationship between natural logarithm
(In) of endtidal % N2 concentration and cumulative expired volume (ml) for a single subject. Washouts
of lung N2 measured pre-, immediately post- and 24 h post-exposure to O3.
slope were most notable for the later portion of the washout and did not correlate
to decreases observed in spirometric indices (FVC, FEVx, FEF25_75). Twelve of the
15 subjects were re-evaluated 24 h postexposure to O3, and for half of these
subjects the washout of lung N2 was delayed in comparison to preexposure washout
values (for two of these six subjects, the delay in washout developed at some time
point after exposure and the 24 h postexposure time point).
Acknowledgments—A summary of the results was presented in part at the annual meeting of the
American Thoracic Society held in San Francisco, CA, U.S.A. in 1993. The authors thank the Maryland
Department of the Environment for daily reporting of ambient oxidant levels during the course of the
investigation. This research was supported by awards from the National Heart, Lung and Blood
Institute, #RO1-HL-31429 and National Institute for Environmental Health Sciences, #ES-03819,
Washington, D.C., U.S.A..
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