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Respiratory Medicine (2005) 99, 1292–1296

KEYWORDAsthma;Exhaled camonoxide;Bronchial r

0954-6111/$ - sdoi:10.1016/j.r

�CorrespondiE-mail addr

(P. Pearson).

Exhaled carbon monoxide levels in atopic asthma:A longitudinal study

Philip Pearson�, Sarah Lewis, John Britton, Andrew Fogarty

Division of Respiratory Medicine, University of Nottingham, Clinical Sciences Building, City HospitalHucknall Road, Nottingham NG5 1PB, UK

Received 21 July 2004

S

rbon

eactivity

ee front matter & 2005med.2005.02.042

ng author. Tel.: +44 115ess: philip.pearson@no

Summary Exhaled carbon monoxide (eCO) is a potential non-invasive marker ofairway inflammation. We have investigated the cross-sectional and longitudinalrelationship between eCO and lung function and bronchial reactivity in 69 adultswith atopic asthma, in the course of participation in a 6-week randomised placebo-controlled trial of vitamin E supplementation. At baseline, there was no cross-sectional association between absolute eCO levels and either forced expiratoryvolume (FEV1), forced vital capacity (FVC) or bronchial reactivity. However, in thelongitudinal analysis within the placebo group, a rise in mean eCO was significantlyassociated with improvement in bronchial reactivity (change in eCO (parts permillion) per natural log unit change in bronchial hyperreactivity 0.498, 95%confidence interval 0.071 to 0.924, P ¼ 0:024). These findings suggest that, contraryto previous data, there is no cross-sectional relationship between eCO and lungfunction or bronchial reactivity, but that there may be a longitudinal trend withbronchial reactivity that is worth further investigation.& 2005 Elsevier Ltd. All rights reserved.

Introduction

Exhaled carbon monoxide (eCO) has been identifiedas a potential non-invasive marker of airwayinflammation.1,2 Carbon monoxide is produced byheme oxygenase-1 from heme,3,4 particularly inresponse to oxidative stress, and eCO levels areraised in adults with asthma.5,6 To date, however,studies have found no clear evidence of association

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between eCO and lung function, as assessed byboth spirometry in adults and children,7–10 orbronchial reactivity to methacholine (MCh) inchildren.9 There are no studies of the associationof eCO and bronchial reactivity in adults withasthma, and no longitudinal assessments of therelationship between change in eCO and change inforced expiratory volume in 1 s (FEV1) or bronchialreactivity have been reported in participants withasthma in the stable state.

We have therefore used data from a randomisedplacebo-controlled trial of vitamin E supplementa-tion in participants with stable asthma11 to explore

ed.

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Exhaled carbon monoxide levels in atopic asthma 1293

the relationship between eCO and bronchial hyper-reactivity.

Methods and materials

The analysis was carried out using data from arandomised placebo-controlled trial of vitamin Esupplementation in participants with stable asth-ma, which has been reported elsewhere.11 Briefly,we enrolled patients aged 18–60 with physician-diagnosed asthma, who were using at least onedose of inhaled corticosteroid (ICS) per day; hadbronchial reactivity to o12.25 mmol MCh sufficientto cause a 20% fall in FEV1 (PD20);

12 and were atopic(defined as at least one allergen skin test response(Dermatophagoides pteronyssinus, cat fur and grasspollen) 2mm greater than control). All were self-reported non-smokers with less than a 10 pack-yearhistory of smoking. Participants were studied atrandomisation and after 6 weeks, when spirometry,bronchial reactivity, skin prick testing and eCOwere measured. Bronchial reactivity to MCh wasmeasured by the Yan method,12 and the PD20 valueslog transformed for all analyses; change in PD20

being expressed in terms of doubling doses (DD).FEV1 and forced vital capacity (FVC) were mea-sured in litres using a Vitalograph 2120 spirometer(Vitalograph, Buckingham, UK), taking the higher oftwo successive measurements within 200ml forFEV1, and the best of three technically satisfactorymeasures for FVC. eCO was measured with aBedfont Micro Smokerlyzer (Bedfont Scientific Ltd,

Table 1 Baseline characteristics of participants.

Number of participantsAge in years: mean (SD)Sex: number (%)Weight (kg): mean (SD)Body mass index: mean (SD)Duration of asthma in years: mean (SD)Daily inhaled steroid dose (beclomethasone equivalent)�: m(range)Number on long-acting b2-agonists (number on salmeterolnumber on formoterol)FEV1 (l); mean (SD)Predicted FEV1 (%): mean (SD)FVC (l): mean (SD)Baseline PD20: geometric mean (geometric SD)

FEV1: forced expiratory volume in 1 s.FVC: forced vital capacity�Beclomethasone equivalence defined as 1mcg beclomethason

Rochester, England), following a previously de-scribed method13 to allow alveolar air sampling,the result being expressed in parts per million(ppm). The test was repeated to ensure consistencyand because previous research has reported thatthe second reading can be significantly higher.14 Allstudy visits took place at the same time of the dayin each individual if was possible, after abstainingfrom short-acting bronchodilator inhalers for 4 h,long-acting bronchodilators for 12 h, and antihista-mines for 24 h. Participants were asked to avoidenvironmental smoke for 12 h before assessment.All regular asthma medications were continuedunchanged. Participants experiencing clinical ex-acerbation of asthma were treated as consideredappropriate by their supervising physician, return-ing to their regular pre-exacerbation medicationuse as soon as possible. Ethics approval was grantedby the Local Research Ethics Committee at Notting-ham City Hospital.

Since it has been noted previously that eCOincreases on repeated measurements,13 we con-ducted a cross-sectional analysis of the mean oftwo eCO measurements at visit 215 against bron-chial reactivity, FEV1 and FVC; the mean beingassumed to allow for inter-observation variability.Longitudinal assessments within the placebo groupinvestigated the relationship between change inmean eCO15 and change in bronchial reactivity andFEV1 and FVC. A paired-samples t-test was used toanalyse change in eCO from first to secondmeasurement. For the longitudinal analysis, weconsidered only the placebo group participantscompleting the study per protocol (i.e. with no

All participants Placebo group

72 3747.9 (8.9) 47.5 (9.4)33 male (45.8%) 18 male (48.6%)74.2 (17.0) 76.7 (19.5)26.8 (7.6) 28.0 (9.6)26.3 (14.1) 28.0 (13.7)

edian 400 (100–2000) 400 (200–1000)

; 19 (14; 5) 12 (7; 5)

2.51 (0.66) 2.53 (0.71)77.1 (16.3) 76.5 (15.5)3.67 (0.88) 3.71 (0.94)0.77 (4.55) 0.69 (4.53)

e ¼ 1mcg budesonide ¼ 0.5mcg fluticasone.

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Table

2Re

gression

coefficien

tsbetwee

nlung

func

tion

andbronc

hial

reac

tivity

tometha

choline,

andca

rbon

mon

oxide.

Assoc

iation

swithmea

neC

Omea

suremen

tat

visit2N¼

72Assoc

iation

swithmea

neC

Omea

suremen

tat

visit3N¼

29Assoc

iation

swithch

ange

ineC

ON¼

29

Regression

coefficien

t95

%CI

PRe

gression

coefficien

t95

%CI

PRe

gression

coefficien

t95

%CI

P

FEV1

�0.03

8�0.16

9,0.09

40.56

9�0.12

9�0.40

4,0.14

60.34

5FV

C0.01

4�0.13

8,0.16

60.85

5�0.00

9�0.40

8,0.38

90.96

3

LogPD20

�0.02

6�0.40

7,0.35

40.89

1�0.53

3�1.28

5,0.21

80.15

6

DFE

V1

�0.03

94�0.11

1,0.03

20.26

5DFV

C�0.05

07�0.16

1,0.06

00.35

3DLo

gPD20(DD)

0.49

80.07

1,0.92

40.02

4

eCO:ex

haledca

rbon

mon

oxide.

FEV1:forced

expiratory

volumein

1s,

inlitres.

FVC:forced

vitalca

pac

ityin

litres.

PD20:doseof

metha

cholineto

cause20

%fallin

FEV1.

DFE

V1,DFV

C,DLo

gPD20:ch

ange

inFE

V1,FV

C,Lo

gPD20.

DD:dou

bling

doses

ofmetha

choline.

95%CI:95

%co

nfiden

ceintervals.

P. Pearson et al.1294

increase or decrease in asthma treatment). Linearregression was used to investigate the magnitude ofchange in eCO against DD change, adjusting forage, sex, height, and pack years smoked as a prioriconfounders.

Statistical assessment was carried out with theStatistical Package for the Social Sciences (SPSSInc., Chicago, USA).

Results

The treatment and placebo groups were similar atbaseline in terms of age, regular drug use, PD20 andlung function; baseline data are summarised inTable 1. Of the 72 participants, 69 provided fulldata at baseline. The second eCO measurementwas significantly higher than the first (mean (SD):first reading 1.8 (1.1) ppm; second reading 2.1 (1.3)ppm; P ¼ 0:028).

Of the 37 participants in the placebo group, 29had paired data available to calculate change inmean eCO. For these participants, the mean FEV1and FVC at baseline were 2.40 (SD 0.63) litres and3.64 (SD 0.94) litres, and at completion of the studythese values were 2.38 (SD 0.61) litres and 3.59 (SD0.91) litres respectively.

At baseline, there was no cross-sectional associa-tion between absolute eCO levels and either FEV1,FVC, bronchial reactivity or peak flow variability.However, in the longitudinal analysis of the placebogroup, a rise in mean eCO was significantlyassociated with improvement in bronchial reactiv-ity (change in eCO per natural log unit change inbronchial hyperreactivity 0.498 (95% confidence

Change in exhaled carbon monoxide

210-1-2-3-4

Cha

nge

in b

ronc

hial

rea

ctiv

ity

3

2

1

0

-1

-2

-3

-4

Figure 1 Scatter plot of log change in MCh dose againstchange in exhaled carbon monoxide. Change in bronchialreactivity is expressed as MCh doubling dose change.Carbon monoxide is measured in parts per million.

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Exhaled carbon monoxide levels in atopic asthma 1295

interval 0.071–0.924, P ¼ 0:024), although not witha change in FEV1 or FVC (Table 2; Fig. 1).

Discussion

This study has assessed the cross-sectional relation-ship between exhaled carbon monoxide (eCO) andbronchial reactivity in adults with asthma, andmeasured eCO longitudinally in stable asthma. Thecross-sectional analysis was consistent with aprevious study in hospitalised patients8 demon-strating that eCO was not correlated with FEV1. Arecent cross-sectional study in asthmatic patients16

found that eCO rose after histamine challenge andwas inversely associated with histamine PD20. Thisconflicts with our findings and requires explanation.It may be that differences in the participants,equipment or environment may have contributedto differences in outcome. For example, all ourparticipants were taking at least one dose of ICS,which may have blunted any response. In addition,no measurement of cotinine was made, so we areunable to determine the extent to which ourobserved carbon monoxide levels might have beeninfluenced by exposure to environmental tobaccosmoke.

However, our study also suggests that over time,a decrease in bronchial reactivity (i.e. an improve-ment) was associated with an increase in meaneCO, which is unexpected. We know of no otherlongitudinal data looking at this relationship.Previous data have suggested an inverse associationbetween change in lung function and eCO, but havebeen limited to recovery from exacerbations (serialpeak expiratory flow measurements over 21 days)7

or in response to antigen challenge17 (serial FEV1over 20 h), rather than measurements of bronchialreactivity. One possible explanation for our data isthat eCO, produced by inducible heme oxygenase,is acting as an immunomodulatory agent, asdemonstrated in an animal model,18 or alterna-tively as an antioxidant,3 which is consumed duringperiods of oxidative stress, the level rising onimprovement due to reduced consumption in theairways.

These findings are a secondary outcome of aclinical trial, and could, therefore, representeither a false negative (cross-sectional analysis)or a false positive (longitudinal analysis). Inparticular, it is common for participants in theplacebo arm of a clinical trial to improve due tobetter adherence or closer monitoring, whichmight explain an improvement in bronchial respon-siveness in the placebo group. However, this does

not explain the observed inverse relationshipbetween eCO and bronchial responsiveness, whichis novel. However, their findings are potentiallyimportant primarily because they suggest that,while a single eCO measurement may be of littleclinical use in asthma or as an indirect marker forbronchial activity, a change in eCO may be used as asurrogate marker of change in bronchial hyperreac-tivity. If replicated elsewhere, this latter findingmay in turn be useful in clinical studies where it isdesirable to have frequent measurements or whereit is not feasible to bring participants to a centralunit for formal testing. Future work in this areashould include a larger, longitudinal study to look atvariability in eCO and include measurements ofeCO, carbon dioxide and nitric oxide simulta-neously, which has been performed in response toallergen challenge,19 but not repeatedly over time.

In summary, eCO was not cross-sectionallyassociated with bronchial reactivity or lung func-tion in this study, but increased eCO was associatedwith improved bronchial reactivity. eCO thereforeshows potential as a non-invasive marker of changein bronchial reactivity.

Acknowledgement

This study was funded by the National AsthmaCampaign (now Asthma UK).

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