Pediatric Pulmonology 44:222–230 (2009)

Non-Invasive Assessment of Exercise Performancein Children With Cystic Fibrosis (CF) and

Non-Cystic Fibrosis Bronchiectasis:Is There a CF Specific Muscle Defect?

Mark Rosenthal, MD, FRCP, FRCPCH,1 Indra Narang, MD, MRCPCH,1{,§

Liz Edwards, MD, MRCPCH,1{,{ and Andrew Bush, MD, FRCP2{

Summary. Introduction: Peripheral muscle dysfunction is increasingly recognized as complicat-

ing respiratory disease, but this is difficult to measure non-invasively. Research Question: Can

skeletal muscle function and efficiency be measured during exercise non-invasively using

respiratory mass spectrometry (RMS); and is the known exercise dysfunction in cystic fibrosis (CF)

children related in part to a disease specific defect of skeletal muscle, or a non-specific mani-

festation of chronic airway infection and inflammation. Methods: Calculations of effective pulmon-

ary blood flow and stroke volume, blood oxygen content and oxygen dispatch from the lungs,

skeletal muscle oxygen extraction and consumption, anerobic threshold and capacity, and gross,

net and work efficiency in 106 controls and 36 children (18 CF) with bronchiectasis, all aged from 8

to 17 years. Results: Normal values for control subjects are tabulated. CF and non-CF bronchi-

ectatic subjects had similar physiology, and skeletal muscle abnormalities could not be detected.

Reduced oxygen dispatch from the lungs, due to an inability to raise stroke volume, without an in-

crease in functional residual capacity was the major factor in reduced exercise ability. Conclusions:

Non-invasive RMS can be used to determine skeletal muscle function in children. The changes

observed in CF subjectswere very similar to non-CF bronchiectatic subjects and thus a CF specific

defect was not demonstrated. Pediatr Pulmonol. 2009; 44:222–230. � 2009 Wiley-Liss, Inc.

Key words: children; cystic fibrosis; exercise efficiency; skeletal muscle function.

INTRODUCTION

Many,1–7 but not all8 studies suggest that CF patientsmay have a disease specific defect in skeletal musclefunction leading to peripheral, as well as cardio-respiratory, exercise limitation. This suggested defectmay also vary with genotype, be independent of lungfunction9 and affect the sexes differently.10 However allthese studies compared CF patients with normal controlsonly and therefore could not reliably conclude that anyobserved differences were not simply a non-specificchronic disease effect. We hypothesized that CF patientsdid have a disease specific skeletal muscle problem, and sowe compared CF patients with normals, but also withchildren with non-CF bronchiectasis. We used non-invasive respiratory mass spectrometry (RMS) to inferskeletal muscle function in these three groups. Thedetailed rationale and tests employed to study musclefunction are described under Methods Section.

SUBJECTS AND METHODS

The study received ethical approval from the RoyalBrompton Hospital Trust ethical committee and writteninformed consent was obtained from patients and theircarers.

Additional Supporting Information may be found in the online version of

this article.

1Royal Brompton Hospital, Sydney Street, London, UK.

2Imperial School of Medicine at National Heart and Lung Institute; and

Honorary Consultant Paediatric Chest Physician, Royal Brompton

Hospital, Sydney Street, London, UK.

{Research Fellow.

{Professor of Paediatric Respirology.

§

The Hospital for Sick Children, Toronto, Canada.

{The Starship Children’s Hospital, Auckland, New Zealand.

*Correspondence to: Mark Rosenthal, Consultant Respiratory Paediatri-

cian, Royal Brompton Hospital, Sydney St, London SW3 6NP, UK.

E-mail: m.rosenthal@rbht.nhs.uk

Received 8 February 2008; Revised 9 May 2008; Accepted 22 June 2008.

DOI 10.1002/ppul.20899

Published online 10 February 2009 in Wiley InterScience

(www.interscience.wiley.com).

� 2009 Wiley-Liss, Inc.

Subjects

a. Controls: One hundred six healthy schoolchildren(M¼ 54), median age 12.8 years. These childrenwere previously studied and represent controls whounderwent an identical study in the past using thesame equipment as the patients described below.Hemoglobin (Hb) is assumed to be 140 g/L in allmales and females �12.5 years, and 120 g/L infemales >12.50 years and not to change withexercise.

b. Patientsi. Children with CF: Eighteen children (7 males,

15 with at least one class 2 gene mutation -inability to fold and process the abnormalprotein) diagnosed on conventional criteria,11

median age 13.0 years and known to havebronchiectasis on computed tomographic (CT)scanning.12 Hb measured during the previous12 months was used in this study.

ii. Children with non-CF bronchiectasis: Eighteen

children (11 males) with bronchiectasis12 from

other causes, CF having been excluded (6 with

primary ciliary dyskinesia, 1 post-pertussis, 1

severe gastroesophageal reflux, 9 idiopathic),

median age 13.3 years. Hb measured during the

previous 12 months was used in this study.

All children had to be >125 cm to be able to undertakethe exercise test. Any child whose birthweight was <2 kg,was chronically infected with Burkholderia Cepacia, hadcoexisting insulin dependent diabetes mellitus, arthritis orarthralgia or a resting oxygen saturation <90% was alsoexcluded.

Methods

Current methods used to study skeletal muscle tendto be invasive13 or test isolated muscles14 both ofwhich are impractical and unethical for use in children.To exercise, the body has to absorb oxygen across thealveolar-capillary membrane; dispatch it from the pulmo-nary veins and left heart and deliver it to the exercisingmuscle. Oxygen has then to be taken up by the muscle andappropriately utilized. In addition, carbon dioxide needsto be liberated from the muscle to the circulation anddischarged into the atmosphere via the lung.

RMS can measure many cardiopulmonary variablesat rest and on exercise using a simple rebreathingtechnique.15–19 Those relevant to this study are functionalresidual capacity (FRC), effective pulmonary blood flow(Qp.eff, defined as that part of pulmonary blood flowwhich takes part in gas exchange) measured by acetyleneuptake, and oxygen consumption (VO2). Arteriovenousoxygen content difference (AVO) can be derived fromthese measurements (VO2/Qp.eff). At rest, VO2 represents

the sum of the contributions from all body organs, andthe signal is dominated by those with relatively highmetabolic needs, such as the brain and the kidney.However, during progressive exercise, VO2 rises somesix- to sevenfold, so the VO2 signal is increasinglydominated by the energy requirement of exercisingskeletal muscle. The greater the intensity of exercise, themore valid is it to consider VO2 as being dominated byskeletal muscle metabolism; this assumption underpinsthe work described in this report. Oxygen dispatch/delivery (OD) is the product of effective pulmonaryblood flow and blood oxygen content; it should be notedthat dispatching oxygen from the lungs is not the samenecessarily as delivered oxygen to the metabolizingtissues, which may be bypassed by arteriovenousshunts, for example. OD is defined as Qp.eff�SaO2�Hb� 0.00134, where SaO2 is fractional arterialoxygen saturation, Hb in g/L and 0.00134 is the assumedblood oxygen carrying capacity expressed in l/g Hb. Theability of the muscle to remove and use oxygen is thearteriovenous oxygen content difference (AVO) dividedby the delivered arterial oxygen content expressed aspercentage—the oxygen extraction rate. It should be notedthat this method cannot distinguish an inability of themuscle to strip off oxygen from Hb due to blooddistribution problems from an inability to metabolizedelivered oxygen because of myocyte metabolic issues.AVO subtracted from the arterial oxygen content gives anestimate of the mixed venous oxygen content from whichmixed venous saturation can be calculated. These lasttwo at maximum workload provides an estimate of theability of muscles to remove oxygen from circulatinghemoglobin.

The efficiency of the exercising skeletal muscle can bedetermined most simply by the VO2 at a given workload; ifthis is higher than normal it is inefficient or and if lower,efficient. More sophisticated measures include exerciseefficiency, oxygen debt, anerobic threshold, and anerobiccapacity. There are at least eight potential definitions ofexercise efficiency.20 Gross exercise efficiency is the ratioof external work performed to the energy available fromthe total VO2 during the work. Net efficiency excludesresting VO2 whilst work efficiency excludes both restingVO2 and that used when moving against a zero workload.To determine external work performed and energyavailable for the work, see below.

As exercise progressively increases, the oxygenrequirements of exercising muscles begin to outstripsupply and anerobic metabolism increases. This oxygendebt is repaid during recovery at rest and the amountgenerated as a proportion of total additional oxygenconsumption due to exercise is another measure of theefficiency of the integrated cardiopulmonary-skeletalmuscle unit. A high oxygen debt for a given amount ofwork performed would signify a less efficient exercising

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Cystic Fibrosis Muscle Function 223

unit. The anerobic capacity21 is the amount of oxygenusage that can be deferred until after completion ofexercise, usually expressed per unit kg body weight. Analternative measure is the anerobic threshold;22 that pointwhere the linear relationship between VO2 and carbondioxide production changes, heralding the manifestationof anerobic metabolism although not its actual time ofonset.

Thus exercise efficiency cannot be interpretedwithout some measure of anerobic metabolism be itoxygen debt, capacity or anerobic threshold. To usean extreme example as illustration; if during exerciselittle or no oxygen was used to perform the externalwork, this would appear very efficient however if a verylarge oxygen debt was incurred, the overall efficiencywould be far less. Examination of data after exercisecessation is as important as data collected duringexercise. E-Figure 1 and E methods shows the time courseof a study for oxygen consumption during rest exerciseand recovery and the calculation of additional oxygenusage.

Mass Spectrometer and Exercise Protocol

The description of the principles of mass spectrometryand the exercise protocol has been described pre-viously,15–19 but is summarized below.

The study used an Innovision 2000 mass spectrometer(Innovision, Odense, Denmark) and the rebreathingtest gas was a zero error grade, medicinal quality (BOC,UK) mixture comprising 0.3% acetylene (the soluble gas),35% oxygen, 5% sulfur hexaflouride (the insoluble,inert gas), 0.3% carbon monoxide (of stable isotopeoxygen,18 data not utilized in this report), 35% oxygenand balance nitrogen. Subjects underwent two practicerebreathing maneuvers followed by 15 min rest, then5 resting maneuvers. After moving to the electromagneticcycle (SECA 100, Birmingham, UK), continuous meta-bolic monitoring measurements were commencedwhilst at rest and during exercise (3 min stages startingat 25 W/m2 increasing in 15 W/m2 steps until exhaustion).In addition, during the last 30 sec of each stage, afurther rebreathing maneuver was performed. At cessationof exercise the same measurements were performedat 3 min intervals for 9 min during recovery. To determinearterial oxygen saturation, a calibrated pulse oximeter(Nellcor, Hatwood, CA) recording heart rate and oxygensaturation was placed over the right supraorbital arteryfixed with a bandanna, this site being the least prone toexercise artifact. The whole study typically lasted 75 min.

Calculation of Work Performed

The work performed was the sum of the wattage of theelectronic cycle during exercise. Thus if a subject peddledthe 25, 40, and 55 W/m2 3 min (180 sec) stages, then

stopped, then work performed is ((25� 180)þ(40� 180)þ (55� 180))� surface area expressed asJoules of work.

Calculation of Energy Consumption From Oxygen

and Carbon Dioxide Values

Most studies assume that 1 L of oxygen consumptionprovides 20,160 J. To refine this and make use of bothoxygen consumption and carbon dioxide production, andhence respiratory quotient (RQ), Lusk’s data on animalcalorimetry was used.23 From this data, the number ofjoules consumed in each time interval is (4.686þ(((RQ� 0.707)/0.293)� 0.361))� 4,200�VO2 in litersfor the situation where the RQ was �1 and the measure-ment occurred before the anerobic threshold. As stressinduced hyperventilation can easily produce an RQ> 1even at rest then any value prior to the anerobic thresholdof an RQ> 1, the value of RQ used in the calculationabove was assumed to be 1. At or after the anerobicthreshold, consideration was given to the small contri-bution to energy usage from anerobic metabolism andthe joules consumed was calculated for each time intervalas 5.047� 4,200�VO2 in litersþ (carbon dioxideproduction�VO2)� 4,200� 5.047/9. 9 is used as anero-bic metabolism produces only 1/9th the number ofATP molecules as aerobic metabolism. This part of theargument assumes that Lusk’s findings apply equally tonormal controls and patients.

Normal Control Data

Basic cardiorespiratory data from the control subjectshave been published before.18,19 Normal data on the new,derived variables: oxygen saturation (arterial and venous);oxygen dispatch/delivery and extraction; gross, work andnet exercise efficiencies; anerobic capacity, and oxygendebt have not. Any reproduction of previous normal data isonly to serve as a comparison with patient data.

Statistical Methods

Healthy control results were divided by age group(<10.5, 10.51–12.50, 12.51–14.50, and >14.50 years),and gender. Healthy control data >3 interquartile rangesfrom the mean were excluded (never >2 results pervariable). No CF or non-CF data was excluded. Resultsare presented as mean and 95% confidence interval (CI)unless stated. Where there were age, gender or pubertalstage effects on a variable, results were converted to zscores ((result� age and gender predicted result)/age andgender predicted SD) so that for any variable in thecontrol group, the mean z score is always zero and SD 1.Comparisons were then made on the z scores. The resultsare however presented as raw data as they are moreintuitive. If no auxological effects were observed,

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224 Rosenthal et al.

comparisons were made on raw data. All comparisonswere made between groups using ANOVAwith TamhanesT2 correction for multiple contrasts, not assuming equalvariances (SPSS v 12, Chicago, IL). Spearman’s rankcorrelation and Mann–Whitney tests were used for othernon-parametric comparisons.

Study Power

The calculation of power in three groups with differentnumbers of subjects is complex. However 18 patients ineach patient group permits an 80% certainty of detecting a1 z-score difference at the 5% level between them. Forpulmonary blood flow as an example, this represents about2 L/min. Comparison between the control group (n¼ 106)and a study group (n¼ 18) detects a 0.5 SD change with80% certainty.

RESULTS

All patients completed all parts of the exercise test.Comparative data for all three groups (Controls, CF, non-CF bronchiectasis) for demographics and auxology areshown in Table 1. Percent body fat and lean body mass iscalculated from skinfold thickness.24

Healthy Controls

E-Tables 1–4 in the online supplement shows the resultsfor healthy controls by age group and gender for oxygendispatch and oxygen extraction (E-Table 1), gross, net andwork efficiency together with oxygen debt (E-Table 2) andanerobic threshold, time to anerobic threshold and anerobiccapacity (E-Table 3). E-Table 4 shows the correlationsbetween these various measures. Multiple regressiondemonstrated that age and gender were the significantinfluences on the results and pubertal stage was not with theexception of functional residual capacity.

Comparison Between Controls andthe Two Study Groups

The controls and the study groups were very similarfor height, weight, age, surface area and body mass index(Table 1). Despite having a very similar BMI, both diseasegroups had reduced body fat estimated from skinfoldthickness as a percentage of total weight compared withcontrols.

Table 2 shows all the physiological data comparingcontrols with each disease group at rest, after 9 min ofexercise having completed the 40 W/m2 exercise stage andat maximum exercise.

i. Effective pulmonary blood flow and stroke volume,heart rate.

At rest, Qp.eff is equal in all three groups;however heart rate in the patient groups issignificantly raised by about 10 bpm, thus strokevolume is proportionately reduced. During exer-cise, Qp.eff fails to rise as much in the patientgroups as controls even though heart rate remainssignificantly higher than controls by about 13 bpmresulting in effective stroke volume being about30% less in patients than controls. This patternremains at maximal exercise, although at max-imum exercise, heart rate for patients and controlsis very similar. The percent rise in effective strokevolume from rest to the end of the 40 W/m2 stage(9 min of exercise) was mean 25% (95% CI 20–31)for controls, 3% (�7 to 13) for CF and 1% (�14 to16) for non-CF patients (ANOVA P< 0.001 forControl vs. CF and P< 0.02 for control vs. non-CFcorrected for multiple contrasts and equal variancenot assumed). From Table 2 the zFRC scores aresignificantly lower in the patient groups comparedwith controls excluding dynamic hyperinflation asan explanation for the above changes.

TABLE 1— Summary of Demographics and Results Expressed as Mean (95% CI) Unless Stated

Control CF bronchiectasis Non-CF bronchiectasis

Male/female 54/50 7/11 11/7

Age (median, range) 12.75 (7.3, 17.7) 12.95 (10.7, 16.9) 13.25 (10.6, 17.1)

Percent predicted FEV1 median (range) 76 (40–98) 69 (35–90)

Height (cm) 1.54 (1.52–1.57) 1.54 (1.49–1.59) 1.58 (1.52–1.64)

Weight (kg) 47.5 (45.2–49.8) 46.6 (41.2–52.1) 47.7 (42.4–52.9)

Body mass index (wt/ht2) 19.6 (19.0–20.2) 19.4 (18.0–20.7) 18.9 (17.6–20.2)

Body mass index z score 0.45 (0.24–0.65) 0.12 (�0.31–0.56) 0.03 (�0.42–0.48)

Surface area (m2) 1.42 (1.38–1.47) 1.42 (1.32–1.51) 1.42 (1.31–1.52)

Mean triceps and subscapular skinfold z score 0.72 (0.56–0.88) �1.50 (�1.78 to �1.23)**** �1.13 (�1.38 to �0.88)****

Percent body fat estimated from skinfold thickness 26.4 (24.7–28.0) 16.7 (15.3–18.2)**** 15.9 (14.0–17.8)****

Estimated lean body wt (kg) 31.4 (30.1–32.7) 38.8 (934.3–43.4) 39.9 (35.8–44.1)

No statistical differences could be demonstrated between either study group.

****P< 0.001 compared with control using ANOVA with Tamhane’s T2 post hoc correction for multiple contrasts not assuming equal variances.

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Cystic Fibrosis Muscle Function 225

ii. Oxygen dispatch, oxygen extraction (Table 2).Because of the failure to augment stroke volume

and despite oxygen saturations being normal(median always >98% in controls and patientsduring both rest and exercise) both patient groupshad reduced OD compared with controls (Table 2)such that at peak exercise there is a significantdifference (ANOVA, P< 0.0001 for CF andP¼ 0.02 for non-CF) as controls more thandoubled their OD (mean % rise 126, 113–138)but the CF group could only raise theirs by 60%

(34–87, P< 0.001) and non-CF by 90% (49–132).The conclusion was unchanged after correction forsurface area. Table 2 shows that the mixed venoussaturation after 9 min of exercise was identicalbetween controls and either patient group, despitethe reduced oxygen delivery, suggesting that thisreduced delivery was not compensated for byincreased extraction.

iii. Gross, net and work efficiency.As the total amount of oxygen used to perform

the work was very similar between groups (Table 2)

TABLE 2— Summary of Unadjusted (for Age and Sex) Results (Mean, 95% CI) Unless Stated

Controls CF Non-CF

Qp.eff (L/min) rest 4.66 (4.45–4.87) 4.45 (3.99–4.92) 4.46 (3.99–4.92)

Heart rate (bpm) 85 (82–88) 96 (91–102)*** 94 (89–100)*

Stroke volume (ml) rest 55.6 (52.8–58.8) 47.0 (41.1–52.8)* 47.6 (42.7–52.6)*

Qp.eff (L/min) after 40 W/m2 9.49 (9.02–9.95) 7.29 (6.20–8.37)** 7.84 ( 6.35–9.33)

Heart rate 40 W/m2 (bpm) 139 (134–143) 151 (145–158)* 152 (142–162)*

Stroke volume (ml) 40 W/m2 69.2 (65.3–73.6) 48.8 (40.9–56.8)*** 53.0 (41.6–64.3)*

Qp.eff (L/min) max ex 11.3 (10.6–11.9) 8.25 (6.87–9.63)*** 8.82 (6.79–10.85)

Heart rate Max ex (bpm) 178 (174–183) 172 (166–178) 175 (168–180)

Stroke volume (ml) max ex 63.4 (59.8–66.2) 48.1 (40.1–56.2)** 50.2 (38.9–61.5)

OD at rest (L/min) 0.90 (0.85–0.96) 0.87 (0.74–1.00) 0.76 (0.62–0.89)

OD after 40 W/m2 (L/min) 1.67 (1.58–1.76) 1.28 (1.08–1.48)** 1.23 (1.03–1.43)

OD at Max exercise (L/min) 1.98 (1.86–2.11) 1.44 (1.18–1.70)** 1.54 (1.16–1.92)*

OE at rest (%) 29 (27–31) 25 (20–29) 26 (20–33)

OE after 40 W/m2 (%) 59 (57–62) 58 (50–67) 58 (50–66)

OE at max exercise (%) 68 (66–70) 61 (56–67) 64 (57–71)

Arteriovenous oxygen saturation difference after 40 W/m2 40 (38–42) 45 (39–51) 40 (32–48)

Arteriovenous oxygen saturation difference

at max exercise

32 (30–34) 38 (32–43) 36 (29–43)

Max alveolar ventilation (L/min) 47 (44–50) 43 (38–47) 40 (32–47)

Gross efficiency (%) 13.5 (12.8–14.3) 9.1 (8.2–10.0)*** 9.6 (7.8–11.4)***

Net efficiency (%) 21.3 (20.1–22.4) 14.6 (13.4–15.7)*** 15.5 (13.1–17.8)***

Work efficiency (%) 22.7 (21.3–24.1) 16.6 (15.3–17.9)*** 18.2 (15.7–20.8)***

Oxygen consumed above rest and zero work (l) 10.7 (9.6–11.8) 9.0 (7.1–10.9) 8.6 (5.8–11.3)

Resting oxygen consumption (L/min) 0.25 (0.24–0.27) 0.19 (0.18–0.21)**** 0.18 (0.16–0.21)****

Resting oxygen consumption ml/min/lean kg 7.3 (6.9–7.6) 5.3 (4.8–6.1)**** 4.7 (4.0–5.4)****

Oxygen consumption after 40 W/m2 ex

stage (9 min exercise) L/min

1.00 (0.95–1.06) 0.84 (0.78–0.90)** 0.76 (0.69–0.84)**

Oxygen consumption/lean kg (ml/kg) after

40 W/m2 ex stage

30 (28–31) 23 (19–27)*** 20 (17–23)***

Oxygen consumption at max exercise (L/min) 1.31 (1.20–1.42) 0.88 (0.72–1.03)**** 1.00 (0.70–1.32)****

Oxygen consumption/lean kg (ml/kg) at max exercise 38 (36–41) 22 (19–25) 24 (19–30)

Work performed kJ (median, range) 50.4 (13.5, 153) 31.1 (13.8, 59.2)***** 30.5 (13.8, 119)*****

Total energy expended during protocol (kJ) 235 (209–261) 199 (158–239) 211 (139–283)

Oxygen debt (%) 20.7 (19.7–21.7) 21.5 (19.4–23.6) 18.7 (15.6–21.7)

Oxygen debt %/10,000 J external work median (95% CI) 4.0 (3.4–4.5) 7.9 (5.5–8.8)* 6.7 (3.1–12.6)

Anerobic capacity (ml O2/kg) 49.5 (45.8–53.2) 45.6 (41.2–50.1) 40.6 (32.3–49.0)

Time to anerobic threshold from onset of exercise (min) 8.9 ( 8.3–9.5) 7.2 (6.0–8.4)* 6.6 ( 5.5–7.7)***

zFRC at rest 0.05 (�0.20–0.30) 0.28 (�0.48–1.04) 0.89 (0.20–1.54)

zFRC after 40 W/m2 ex stage 0.04 (�0.16–0.23) �1.55 (�2.13 to –0.97)**** �0.29 (�0.84–0.25)***

zFRC at max exercise �0.01 (�0.27–0.24) �1.83 (�2.45 to �1.21)**** �1.18 (�1.70 to �0.67)***

No statistical differences could be demonstrated between either study group. Qp.eff¼ effective pulmonary blood flow (L min), OD¼ oxygen

dispatch (L min), OE¼ percent oxygen extraction, zFRC¼ functional residual capacity z score.

*P¼ 0.035 compared with control using ANOVA with Tamhane’s T2 post hoc correction for multiple contrasts not assuming equal variances.**P¼ 0.002 compared with control using ANOVA with Tamhane’s T2 post hoc correction for multiple contrasts not assuming equal variances.***P< 0.003 compared with control using ANOVA with Tamhane’s T2 post hoc correction for multiple contrasts not assuming equal variances.****P< 0.001 compared with control using ANOVA with Tamhane’s T2 post hoc correction for multiple contrasts not assuming equal variances.*****P< 0.001 compared with controls using non-parametric Mann–Whitney U-test.

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226 Rosenthal et al.

but the amount of work performed by the diseasegroups was some 60% that of controls ((P< 0.001Kruskall–Wallis and Mann–Whitney U-test)),inevitably gross, net and work efficiency (Table 2)were significantly reduced in the disease groups.

iv. Oxygen consumption, oxygen debt, anerobiccapacity and anerobic time.

VO2 at rest was lower in the patient groups, andwas less both after the 40 W/m2 stage and atmaximum exercise (Table 2). However the totalVO2 during exercise and during recovery wassimilar between the 3 groups. Even though overallpercent oxygen debt between the groups wassimilar, the debt accumulated per unit workperformed was greater in the patient groups thancontrols by 50–100%. The anerobic capacity wassimilar between all three groups but the time whenanerobic metabolism became manifest occurredsignificantly earlier in the two patient groups bysome 2 min.

v. Specific putative effects of gender and genotype inCF (Table 3).

Using z scores to account for the general expectedgender differences for many variables, only gross effici-ency remained statistically different (P¼ 0.009) betweenthe sexes in CF with none of the other parameters different.As 15/18 CF subjects had class 2 mutations, comparisonsbetween mutation classes were not possible. There was nocorrelation between any parameter and FEV1.

DISCUSSION

Summary of Principal Findings of the Study

We have reported a novel use of well established RMSmeasurements of cardiorespiratory variables to study thephysiology of exercising skeletal muscle non-invasively.It must be emphasized that no direct invasive skeletalmuscle measurements were made and that all conclusionsare inferences from interlinked calculations. These are

based on a number of assumptions (see below) imposed bythe need to maintain the objective of an ethical andpractical, clinical assessment in children. Nevertheless,we believe it is possible to disentangle the variouscomponents and answer a logical hierarchy of questionsto localize the reason for exercise impairment. Firstly, it isnecessary to ask whether OD from the cardiopulmonaryunit is normal. The results demonstrate that OD issimilarly reduced in both CF and non-CF patientscompared with controls. This is not due to arterial oxygendesaturation, which excludes the possibility of a signifi-cant but unmeasured right to left shunt, or ventilation–perfusion mismatch in the patient population. Is this aprimary cardiac defect or a secondary response to dynamicpulmonary hyperinflation in the patient groups? Table 2shows that the functional residual capacities of the patientgroups were significantly lower than controls which doesnot support hyperinflation as the important factor.25 Thisstudy was not designed to elucidate the mechanism for thisfinding, and so we do not speculate further. Finally, thepatients compensated for their reduced OD by accumulat-ing a larger oxygen debt, and not by extracting moreoxygen from the delivered blood.

Assumptions Involved in the Calculations

It could be argued that measurement of Qp.eff does notapproximate to cardiac output in patient groups withairflow obstruction, as mass spectrometry measures onlythe flow and volume of blood in contact with gasexchanging alveoli, and therefore in diseased lungs theresults may be falsely low. This is unlikely in our study forseveral reasons. Firstly, the disease groups were onlymoderately affected, with a mean FEV1 of about 70%.Secondly, arterial oxygen saturation did not drop even atmaximum exercise. Thirdly, although from Table 2, therewas a trend for maximum alveolar ventilation (measuredfrom carbon dioxide production) in patients to be 10–15%lower than controls, it did not reach statistical significance(P¼ 0.15); this should be compared with the marked

TABLE 3— CF Subjects Only to Compare the Specific Exercise Issues Between the Genders (Ref. 20) z Scores for the Data(Mean, SD, 95% CI) Together with the Significance of the Non-Parametric Mann–Whitney U-Test Unadjusted for MultipleContrasts

z score Male Female P value

Oxygen dispatch rest 0.28, 1.6 (�1.74 to 2.29) �0.07, 1.31 (�0.95 to 0.81) 0.89

Oxygen dispatch 40 W/m2 �1.34, 0.94 (�2.52 to �0.16) �2.60, 3.41 (�4.9 to �0.3) 0.44

Oxygen dispatch at maximum exercise �1.49, 0.80 (�2.50 to �0.49) �1.72, 0.85 (�2.29 to �1.15) 0.19

Oxygen extraction rest �1.27, 0.92 (�2.41 to �0.13) 0.75, 2.40 (�0.86 to 2.36) 0.06

Oxygen extraction 40 W/m2 1.15, 2.17 (�1.54 to 3.83) 1.61, 2.61 (�0.84 to 4.0) 0.22

Oxygen extraction at maximum exercise 1.22, 1.56 (�0.71–3.17) 0.92, 0.90 (0.32–1.52) 0.69

Gross efficiency �1.63, 0.50 (�2.21 to �1.03) �2.79, 0.78 (�3.31 to �2.27) 0.009

Net efficiency �1.28, 0.52 (�1.94 to �0.63) �2.28, 0.84 (�2.84 to �1.71) 0.16

Work efficiency �1.26, 0.50 (�1.87 to �0.64) �1.69, 0.76 (�2.20 to �1.19) 0.11

Oxygen debt �1.52, 1.26 (�3.07 to 0.04) �1.19, 3.31 (�3.42 to 1.3) 0.96

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Cystic Fibrosis Muscle Function 227

inability to raise effective stroke volume with exercise inpatients. Fourthly, because in this study, gas mixing asdemonstrated by inspecting the plateau of the SF6 trace,was always complete within three breaths, then a falselylow Qp.eff due to a failure of speedy gas mixing for anyreason during a rebreathing maneuver is very unlikely.From the above, we therefore believe that the inability ofour patient groups to augment their stroke volume duringexercise is a real phenomenon. If Qp.eff had beenunderestimated in the patients, with VO2 being correctlymeasured, the true mixed venous saturation would behigher, making the extraction efficiency of the patientseven less than we suppose. Thus we believe we haveaccurately measured Qp.eff in patients. Nonetheless, itmust be acknowledged that our method cannot be usedin the presence of major gas mixing and ventilation–perfusion abnormalities, and thus in very severely affectedpatients.

There are other physiological and methodologicalassumptions. The physiological primarily refer to theassumptions about the calorific availability from 1 L ofoxygen, 20,160 J/L being typically used in studies. Ourmore elaborate method was compared with this figure andthe differences were <5%. It is also assumed that theoverwhelming majority of additional oxygen usage is byskeletal muscle. The lower the level of exercise studied,the more unreliable becomes that assumption. It remainsunknown whether CF or non-CF patients have the sameenergy availability from oxygen as each other or controlsand the same applies to the oxygen carrying capacity ofhemoglobin. Hemoglobin levels were assumed in controlsand measured in patients which may lead to a smalldiscrepancy. No attempt was made to allow for hemo-concentration with exercise as it could not be measurednon-invasively, and was assumed to be similar in eachgroup. The lack of simultaneous direct muscle measure-ments precludes us determining whether there is anyadditional muscle problem, in addition to the ODproblems described above.

The methodological assumptions centre aroundwhether all the oxygen used during exercise and recoverywas measured; whether all the work expended wasmeasured; whether 8.25 min of measured recovery wassufficient to return to baseline (it was); the interpolation ofoxygen usage whilst rebreathing measurements wereoccurring (E-Fig. 1); how to correctly divide oxygenusage into that occurring during exercise and earlyrecovery (E-Fig. 2) and an analysis of the assumptionthat work against a zero load remains constant throughoutexercise (debatable). Many of the errors that can beestimated tend to cancel each other out. This is fullydescribed in the E-discussion.

It could also be argued that a progressive exercise testmakes the calculation of exercise efficiency difficult as asteady state may not be reached. This is possible, and

emphasizes the need for a standardized protocol tobe followed for all groups. It is the relative differencesbetween the groups that are important not necessarilyabsolute values of efficiency.

Meaning of the Results

Are physiological changes a full explanation for thepatients’ poorer exercise performance? The resultssuggest that this is a reasonable conclusion. The lowerstroke volume is not sufficiently compensated for by araised heart rate and thus OD is reduced. This findingis consistent with that of Benson et al.26 who, usingradionuclide cardiac scans in 31 young CF adults showedthat 29% could only raise their right ventricular ejectionfraction by <5% during exercise. Doppler studiesconfirmed similar right ventricular abnormalities inmild CF disease.27 We have no explanation for their andour findings of a primary functional cardiac defect incomparatively mildly affected patients with differentrespiratory diseases.

If percent oxygen extraction remains the same in theface of reduced OD, then less oxygen is available to themuscle. In theory, reduced OD could be compensated byincreased extraction by the skeletal muscle. This failed tohappen, for reasons we cannot determine, hence the mixedvenous oxygen saturations during exercise were similar inall groups. We have no data on the behavior of normalskeletal muscle in the face of reduced OD, so we cannotdetermine whether the failure to increase oxygenextraction is a result of an intrinsic muscle abnormality,or simply reflects reduced flow. It should be noted thatin the lungs, capillary recruitment appears to be flowdependant,28 and it may be that to increase extraction, flowdependant recruitment of capillaries is also necessaryin exercising muscle; this is, however, speculative. It isunsurprising therefore that a reduced OD and similar OEresults in an earlier anerobic threshold and a raised oxygendebt. As less work was performed but the same amountof oxygen was used, their efficiency was inevitably less. Itshould be noted that had oxygen debt not been measured,we would have wrongly concluded that there was greaterskeletal muscle efficiency in the disease groups.

The most relevant finding however is that the CF andnon-CF bronchiectasis groups had very similar exercisephysiology, strongly implying that if there as any down-stream muscle defect, it is common to both diseases,and the putative CF metabolic defect in muscle is actuallya non-specific cardiopulmonary consequence of chronicbronchopulmonary sepsis. If a non-CF group had not beenincluded, and we had relied solely on normal controls,entirely different conclusions would have been reached.We did not measure bronchial or systemic inflammation,and it is possible that this may have been different betweenthe two groups. However, non-CF bronchiectasis is far

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228 Rosenthal et al.

from being benign, and may run a course similar to that ofCF,28 so significant systematic differences between thetwo groups seems unlikely.

Could motivation have accounted for differencesbetween the study groups and controls? One advantageof comparison at identical workloads, in this case after40 W/m2 exercise, rather than only at maximum exerciseis to minimize this problem, however measures ofefficiency and oxygen debt will inevitably be affected bymotivation. Controls and study groups were tested usingan identical protocol, methods of encouragement andequipment, however the controls were tested by differentpersonnel at an earlier time. Heart and respiratory ratesat the end of exercise were similar (Median control 44, CF43 and non-CF bronchiectasis 44 breaths/min). Given thatbreathlessness was easily the most common reason citedby subjects for stopping exercise, differences in motiva-tion are unlikely and would not account for the differencesin oxygen debt/unit work performed, oxygen dispatch oroxygen extraction as they were compared at an identicalexercise stage (40 W/m2). Furthermore, there were clearcut physiological abnormalities at the end of exercise inthe disease groups, implying that motivation was at worstonly a part of exercise limitation. Given the similarityof BMIs between all three groups but the qualitativereduction in body fat in the study groups compared withcontrols, it is possible that the exercise efficiency of thestudy groups may still be being overestimated. Howevergreat care should be taken in over-interpreting lean bodymass measurements derived from skinfolds as they areprone to error.

Comparing our efficiency findings in normal childrenwith those of others is problematical, firstly because whatis termed delta efficiency (the ratio of a change in workrate to its corresponding change in VO2) is confused withwork efficiency.29 Secondly, no recent study30–36 deter-mined VO2 after completion of exercise, without whichefficiency cannot realistically be calculated. In childrenthis led to the conclusion that the observed increasedefficiency in girls was artifactual29 whereas our findingssupport a true gender difference. In adults, failure tomeasure VO2 during recovery has led to the erroneousconclusion that exercise efficiency is enhanced in thosewith heart failure on the basis of a reduced VO2/Watt ofwork during exercise.37,38 However Mitchell et al.39

clearly demonstrated that when the recovery VO2 isconsidered as well, overall exercise efficiency is markedlyreduced. In the present study the same conclusionwould have been reached if recovery oxygen usage wasnot considered. Thirdly, studies in children have usedcalculated resting energy expenditure from otherstudies to determine resting energy expenditure through-out exercise32 again increasing the number of assump-tions. Despite the above, our work efficiencies appear

similar to the results of these authors, which were23–30%.

Anerobic capacity has been reported in a longitudinaltreadmill study in athletic boys from 11 to 15 years ofage.21 Age and maturational independent rises in anerobiccapacity were found, which closely matched the values inthe current study in unselected individuals, namely risingfrom about 45 ml O2/kg in the youngest to 75 ml O2/kg inthe oldest. The finding of increased muscle efficiencytogether with an increased percent oxygen debt in girlsimplies an increased effort on their part to pushphysiological boundaries.

The lack of correlation between variables describingmuscle efficiency is important (Table e4), as theimplication is that each measures a different aspect ofthe exercising cardiopulmonary-skeletal muscle unit, andthat all need to be measured to provide a complete picture.However, further work is needed to understand the precisemetabolic correlates of each function. Nonetheless, theability to measure several different parameters simulta-neously is a strength of this method, because it allowsseveral checks before pronouncing muscle function to benormal.

In conclusion, systematic calculation of oxygendispatch, peripheral muscle oxygen extraction, andmeasures of exercise efficiency during exercise is possibleusing an RMS. Measurement of only a few variables willgive a misleading picture of why exercise is limited, andcareful consideration should be given to all aspects of thecardiopulmonary skeletal axis. Exercise efficiency, oxy-gen debt, anerobic threshold, anerobic capacity, oxygendispatch and extraction all provide information aboutdifferent aspects of muscle function in the exercisingchild and it is essential that measurements continueinto recovery from exercise. These methods show thatreduced oxygen dispatch is the major determinationof exercise limitation in children with bronchiectasisand is not compensated for by increased muscle extractionin either patient group. This study refutes the suggestionthat there is a CF-specific defect of exercising skeletalmuscle.

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