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Original Research: Sleep Disorders |

Do Field Walking Tests Produce Similar Cardiopulmonary Demands to an Incremental Treadmill Test in Obese Individuals With Treated OSA?Walking Test Demands in Obese Individuals With OSA FREE TO VIEW

Rachael A. Evans, PhD; Thomas E. Dolmage, MSc; Priscila G. Robles, MSc; Roger S. Goldstein, MD, FCCP; Dina Brooks, PhD
Author and Funding Information

From the Department of Respiratory Medicine (Drs Evansand Goldstein, Mr Dolmage, Ms Robles, and Prof Brooks) and Department of Respiratory Diagnostic & Evaluation Services (Mr Dolmage and Dr Goldstein), West Park Healthcare Centre, Toronto, ON, Canada; Department of Infection, Immunity and Inflammation (Dr Evans), University of Leicester, Leicester, England; and Department of Medicine (Drs Evans and Goldstein) and Department of Physical Therapy (Ms Robles, Dr Goldstein, and Prof Brooks), University of Toronto,Toronto, ON, Canada.

CORRESPONDENCE TO: Rachael A. Evans, PhD, Department of Respiratory Medicine, Glenfield Hospital, Groby Rd, Leicester, LE3 9QP, England; e-mail: rachael.evans@uhl-tr.nhs.uk


FUNDING/SUPPORT: Dr Evans was supported by a postdoctoral fellowship for exercise rehabilitation supported by West Park Foundation and a National Institute for Health Research Clinical Lectureship in Respiratory Medicine (UK). Dr Goldstein is supported by the University of Toronto National Sanatorium Association Chair for Respiratory Rehabilitation Research, and Prof Brooks holds a Canada Research Chair for Pulmonary Rehabilitation.

Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details.


Chest. 2014;146(1):81-87. doi:10.1378/chest.13-2060
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BACKGROUND:  Cardiorespiratory fitness, assessed during cardiopulmonary exercise tests by peak oxygen uptake (V˙ o2pk), is an independent predictor of mortality in obesity. We investigated whether V˙ o2pk and systemic responses measured during field walking tests were similar to those measured during an incremental treadmill test (ITMT) in obese individuals with treated OSA.

METHODS:  Individuals with treated OSA and a BMI > 30 kg/m2 were recruited. Participants completed an ITMT, two 6-min walk tests (6MWTs), and two incremental shuttle walk tests (ISWTs) on three separate days in a randomized order. Expired gas analysis was performed during all tests.

RESULTS:  The study was completed by 16 patients (nine men) (mean [SD] age, 58 [12] y; BMI, 36.1 [7.6] kg/m2). There was no difference (P = .27) in V˙ o2pk assessed by the ITMT and the ISWT (2,266 [478] and 2,017 [561] mL/min, respectively). The V˙ o2pk measured by the 6MWT (1,778 [360] mL/min) was lower than that measured by the ITMT (P < .01). The limits of agreement for V˙ o2pk between the ISWT and the ITM were ± 730 mL/min. Cardiorespiratory responses during the ISWT and the ITMT reflected a graded response to a peak, whereas the 6MWT demonstrated a rapid rise to a plateau.

CONCLUSIONS:  The ISWT can be used instead of an ITMT and in preference to the 6MWT to assess cardiorespiratory fitness for a cohort of obese people with treated OSA. However, the imprecision of the agreement in V˙ o2pk between the ITMT and ISWT means they cannot be used interchangeably in an individual.

TRIAL REGISTRY:  ClinicalTrials.gov; No.: NCT01930513; www.clinicaltrials.gov

Figures in this Article

Obesity is increasing worldwide, with nearly one-tenth of the adult population being obese, and is associated with an increased mortality from cardiovascular disease.1,2 The prognosis is worsened by OSA,3,4 which commonly accompanies obesity.5

In the general population, improving physical fitness confers a reduction in mortality.6 Similarly, obese but fit individuals have a lower cardiovascular mortality than their unfit counterparts.7 Despite effective treatment of OSA in reducing daytime sleepiness with CPAP, inactivity persists8 such that lifestyle interventions are necessary to impact activity levels and fitness.9

Although the gold standard measure of cardiorespiratory fitness is the peak oxygen uptake (V˙ o2pk) derived from a maximal incremental cardiopulmonary exercise test, this requires specialized equipment and a qualified operator. Simple, less expensive, and less time-consuming field tests are attractive to assess lifestyle interventions in clinical settings. The 6-min walk test (6MWT) is the most commonly used test,10 and its reproducibility has been assessed in obese individuals.11 It has been suggested that the incremental shuttle walk test (ISWT)1214 could also be used as an outcome measure for obese individuals.15 In populations disabled by breathlessness due to COPD or congestive heart failure, the 6MWT and ISWT elicit a V˙ o2pk similar to a maximal incremental cardiopulmonary exercise test.16,17 Whether this also applies to obese individuals treated is unknown. It may be that the mechanical constraints of obesity that limit walking speed during an ISWT may limit V˙ o2pk to less than can be achieved on a treadmill in which the grade can also be increased to increase the metabolic demands of the exercise. Similarly, the 10-m course of the ISWT may lower the mechanical ceiling with the increased frequency of turns compared with the 30-m course of the 6MWT.

Nevertheless, in clinical practice it would be useful to accurately assess cardiorespiratory fitness using a field test rather than a laboratory exercise test. Our primary aim was to investigate whether either the 6MWT or the ISWT could adequately predict the V˙ o2pk achieved on an incremental treadmill test (ITMT). We also wanted to know whether these tests elicit patterns of cardiopulmonary responses similar to that of an ITMT in obese individuals treated with OSA. The null hypothesis was that the V˙ o2pk achieved during both the 6MWT and ISWT would not be different from that observed during an ITMT.

Study Design

This was a prospective cross-sectional study approved by the Joint Bridgepoint/West Park/Toronto Central CCAC/Toronto Grace Health Centre Research Ethics Board (JREB2010-10). After providing written informed consent, each participant attended three assessment visits in a randomized order, each separated by at least 48 h. The visits comprised either an ITMT, two 6MWTs, or two ISWTs. Height and weight were measured.

Participants

Individuals with BMI > 30 kg/m2 were recruited from a sleep clinic. All participants had OSA18 that was diagnosed by full polysomnography performed at a sleep clinic and that had been effectively treated with CPAP for at least 3 months. Significant nocturnal hypoventilation was excluded. Individuals with known chronic lung or heart disease or neurologic or orthopedic conditions that would interfere with their ability to exercise were excluded. All participants underwent a thorough clinical assessment by a respirologist to exclude previously undiagnosed, clinically significant cardiac or peripheral vascular disease. Spirometry was performed with a flow-volume loop19 to exclude significant airways disease and extrathoracic airway obstruction. If individuals had a history of joint or back pain but were mobile, they were invited to participate and undergo the first exercise test for assessment of the predominant limitation to exercise. If the latter was predominantly joint or back pain, they were excluded from the rest of the study.

Outcome Measurements

During all exercise tests, breath-by-breath expired gas analysis was performed using a portable gas analysis system (K4b2; Cosmed SRL) and smoothed using a rolling eight-breath average of variables that included oxygen uptake (V˙ o2), CO2 output, and minute ventilation. The anaerobic threshold (AT) was identified using the modified V slope method20 and confirmed by the ventilatory equivalents method.21 Heart rate (Polar AI heart rate monitor; Polar Electro) and pulse oximetry (Nellcor Max-Fast Forehead Sensor; Covidien plc) were measured continuously. Heart rate recovery was defined as the reduction in the heart rate from the rate at peak exercise to the rate 1 min after the cessation of exercise.

Incremental Treadmill Test:

Participants performed a cardiopulmonary exercise test on a treadmill (ITMT) with unsupported free walking. The test was continued to participant intolerance, and the American College of Sports Medicine guidelines for exercise testing were adhered to.22 An incremental step protocol was used, designed to achieve a linear response in V˙ o223 by combining a linear increase in speed and an exponential increase in grade for each 1-min step. The protocol was individualized by calculating the grade and speed increments according to the formulae by Porszasz et al,23 using the participant’s usual and fast walking speeds as well as their predicted peak power.24 Patients demonstrated their usual and fast speeds along a 30-m corridor. They were instructed to “walk at your usual pace until you reach the far pylon and return at a speed you consider being fast.” Using a stopwatch, the test administrator measured the time the participant traversed the middle 15 m of the 30 m, and speed was calculated.

6-Min Walk Test:

Two 6MWTs were performed on the same day with a ≥ 30 min rest in between. Each test was performed according to international guidelines10 along a 30-m corridor with standardized encouragement provided by the test administrator and considering the first test as familiarization.

Incremental Shuttle Walk Test:

The ISWT is an externally paced, symptom-limited test conducted along a 10-m course. It is reproducible after one practice test in patients with chronic cardiac and respiratory disease.13,17 The modified ISWT was used, which has an additional four levels (ie, speeds).25 Two ISWTs, separated by a ≥ 30 min rest, were performed on the same day, with the first test considered as familiarization.

Statistical Analysis

The difference between the peak parameters for the two field walking tests compared with the ITMT was assessed by an analysis of variance and the Dunnett test for multiple comparisons vs a control. The intraclass correlation coefficients for V˙ o2pk between the ITMT and ISWT and between the ITMT and 6MWT were determined. The limits of agreement and the precision for the V˙ o2pk between the ITMT and ISWT were calculated using the methods described by Bland and Altman.26 Linear regression was performed between the ISWT distance and the ITMT V˙ o2pk data. A visual comparison of the average cardiorespiratory response during the different protocols was produced by first dividing individual tests into epochs equivalent to 10% increments of the total test duration (ie, deciles) using a negative, exponential, two-dimensional data transformation and then calculating the group mean at each decile. Data points of V˙ o2 over the last 3 min were analyzed with linear regression to determine the presence of a steady state. We confirmed that steady state had occurred if, after a mono-exponential rise in the V˙ o2 in the first 3 min, there was a plateau (ie, without a significant slope) as determined by linear regression analysis.24

Sample-Size Calculation

The sample-size calculation was based on the provision of an acceptable precision of the limits of agreement between V˙ o2pk from the maximal treadmill test and either the V˙ o2pk from the ISWT or from the 6MWT. Using observations of the repeatability of maximal treadmill testing,27 in which the mean V˙ o2pk was 28 mL/kg/min and the mean difference (95% CI) in repeated measures was 0.1 mL/kg/min (−1.3-1.1 mL/kg/min), we estimated that 16 participants would be needed to provide precision of 1 mL/kg/min SE of the estimate of agreement.

Of the 17 patients recruited, one was excluded due to the finding of previously undiagnosed COPD. No participants had evidence of extrathoracic upper airway obstruction. Sixteen participants, described in Table 1, completed all five exercise tests.

Table Graphic Jump Location
TABLE 1  ] Demographics and Spirometry

Data given as mean (SD) unless otherwise indicated. AHI = apnea-hypopnea index; IQR = interquartile range.

Data Used for the Individualized ITMT Protocol

The predicted power (mean [SD]) was 134 (35) W; the usual walking and fast walking speeds were 79.7 (15.9) and 103.9 (23.4) m/min, respectively. The observed peak grade and speed for the treadmill tests were 8.8% (2.1) and 96.7 (23.3) m/min, respectively, which were not different to the targeted values of 9.0% (1.6) and 91.6 (13.3) m/min (P= .43 and P= .18, respectively).

Data for the Two Field Walking Tests

The 6MWT distances for test 1 (550 [95] m) and test 2 (565 [98] m) were not significantly different (P = .07), but there was a trend toward a higher value on the second test. The ISWT distances for test 1 (549 [210] m) and test 2 (580 [207] m) were significantly different (P = .03) and lower than the age-specific reference values.28

Comparison of the Peak Parameters and Responses for the Three Different Exercise Tests

The intraclass correlation coefficient between ITMT V˙ o2pk and ISWT V˙ o2pk was 0.87 (P < .001) and between the ITMT V˙ o2pk and 6MWT V˙ o2pk was 0.75 (P = .005). However, the 6MWT V˙ o2pk was significantly less than the ITMT V˙ o2pk (Table 2). Although the ISWT V˙ o2pk was not different from the ITMT V˙ o2pk (P = .24), the limits of agreement were wide at ± 730 mL (Fig 1), with no evidence of heteroscedascity (ie, no relationship between the difference between the two tests and the magnitude of V˙ o2pk) (P = .36). There was a trend toward a weak relationship between the ISWT distance and the ITMT V˙ o2pk (r2 = 0.24, P = .054) (Fig 2). The regression equation was ITMT V˙ o2pk (mL/min) = 1,590 + 1.14 (ISWT distance, m), where the 95% CI of the intercept was 1,252 to 1,928 and the slope was 0.60 to 1.68. However, the relationship between the ISWT distance and the ITMT V˙ o2pk was stronger if the ISWT result was < 800 m (r2 = 0.54; P = .008).

Table Graphic Jump Location
TABLE 2  ] A Comparison of the Peak Exercise Parameters Between the ITMT, ISWT, and 6MWT

Data given as mean (SD) unless otherwise indicated. 6MWT = 6-min walk test; AT = anaerobic threshold; BS = Borg score for breathlessness; ISWT = incremental shuttle walk test; ITMT = incremental treadmill test; LE = Borg score for leg effort; N/A = not applicable; RER = respiratory exchange ratio; Spo2 = oxygen saturation by pulse oximetry V˙ co2 = rate of CO2 output; V˙ e = minute ventilation; V˙ o2 = rate of oxygen uptake.

a 

P < .05.

b 

One patient, prescribed β-blockers, was omitted.

c 

P < .001.

Figure Jump LinkFigure 1  Limits of agreement between the ITM and the ISWTV˙ O2pk and ATV˙ O2. Hashed line is the mean difference between ISWT and ITM, dot-and-hash line is the limits of agreement, and dotted line is the precision of the limits of agreement. ATV˙ O2 = oxygen uptake at the anaerobic threshold; ISWT = incremental shuttle walk test; ITM = incremental treadmill test; V˙ O2pk = peak oxygen uptake.Grahic Jump Location
Figure Jump LinkFigure 2  The relationship between the ISWT distance and ITMV˙ O2pk. Broken lines are 95% CI. See Figure 1 legend for expansion of abbreviations.Grahic Jump Location

The cardiorespiratory responses during the ISWT closely matched those of the ITMT (ie, a graded cardiovascular response to a maximum) for V˙ o2, ventilation, respiratory exchange ratio (RER), and heart rate; whereas the 6MWT demonstrated an earlier, rapid rise to a plateau (Fig 3). The slope was not significantly different from 0 (slope = 18 mL/min2 [−18-55]). Only three of 16 participants demonstrated a significant progression of V˙ o2 after 3 min.

Figure Jump LinkFigure 3  A comparison of the exercise responses among ITM, ISWT, and 6MWT. 6MWT = 6-min walk test; V˙ E = minute ventilation; V˙ O2=oxygen uptake. See Figure 1 legend for expansion of other abbreviations.Grahic Jump Location

The AT was identified in all the ITMTs and ISWTs (Table 2), and V˙ o2 at AT was not different (P = .16). The mean RER at peak exercise was < 1.0 for the 6MWT.

We demonstrated that the cardiopulmonary demands were similar between an ISWT and an ITMT in obese individuals with treated OSA. However, the 6MWT did not elicit the same V˙ o2pk as the ISWT or ITMT in this population. Both the ISWT and ITMT demonstrated steadily rising metabolic and cardiovascular responses consistent with the steady rise in mechanical demand imposed. A different pattern of exercise response was seen with the 6MWT, whereby a more rapid increase in parameters occurred to either a plateau or a slow rise, as determined by the individual’s effort. Overall, participants did not increase their walking speed in the latter one-half of the test despite having reserve at the end. This supports the 6MWT as being a submaximal exercise test in this population. An AT was identifiable with both the ITMT and the ISWT. The V˙ o2pk for the 6MWT did not exceed the AT of the ITMT in all participants, demonstrating that the self-paced 6MWT does not ensure that the participant walks at a high intensity that will elicit a maximal response within the imposed 6-min time limit.

The properties of the 6MWT vary according to the population assessed. Despite instruction to walk “as far as possible,” with the implication of maximal effort, it is often referred to as a submaximal test. For example, many patients with COPD select a sustainable walking speed, achieving a metabolic steady state below their maximum.29 However, the 6MWT has also been shown to elicit similar peak cardiopulmonary demands to an incremental test16 performed on a cycle ergometer in patients with COPD, but not in patients with chronic heart failure (CHF) using a treadmill (which typically uses more muscles).17 We used a treadmill protocol that is individualized to produce a linear increase in V˙ o2 not seen in previous protocols, and this may also have been associated with eliciting a greater V˙ o2pk.23

In clinical practice, it would be unusual to measure portable expiratory gas analysis during an ISWT; typically, the outcome measured is the distance walked. Unlike other chronic conditions such as COPD and CHF,14,30 in obesity-related OSA, the weak relationship between ISWT distance and V˙ o2pk ITMT precludes the latter predicting the V˙ o2pk of an individual. The difference between these populations is likely to result from their underlying fitness (ie, whether walking on the flat provides enough of a cardiopulmonary demand to reflect their maximal exercise capacity). The patient populations included in the studies of COPD and CHF had moderate to severe disease, and the mean ISWT results were only 302 (133) m and 380 (190) m compared with 580 (207) m in the current study.13,17 The relationship between ISWT distance and V˙ o2pk ITMT in the current study was stronger when the ISWT was < 800 m. It is around this level that most people would need to jog to maintain the speed and the turns become more frequent. On the treadmill, the grade can be increased to provide the increased cardiopulmonary demand without the need to jog. Despite the weak overall relationship with ISWT distance and ITMT V˙ o2pk, as the mean ISWT V˙ o2pk and the ITMT V˙ o2pk were similar, the ISWT may be a reasonable alternative to compare maximal exercise capacity between groups of obese individuals. However, the imprecision in the agreement in V˙ o2pk between the ITMT and the ISWT means they cannot be used interchangeably in an individual.

Cardiorespiratory fitness is often used to assess surgical risk. For example, V˙ o2pk is used to qualify patients’ eligibility for cardiac transplantation and the ISWT has been used instead of a cardiopulmonary exercise test in this assessment.31 Obese individuals with a V˙ o2pk < 15.8 mL/min/kg are at greater risk and have a worse outcome after bariatric surgery.32 As the demands for bariatric surgery increase, it would be attractive to use a simple field test like the ISWT as part of the assessment. Only two of 16 participants had a V˙ o2pk < 16 mL/min/kg, but they were also the only two participants to have an ISWT distance < 400 m (the cutoff for cardiac transplantation). Further work would be needed to identify if the ISWT distance could be used preoperatively to assess and inform safety of an individual for the procedure.

The heart rate was elevated during the ISWT compared with the ITMT (which remained when heart rate was plotted vs V˙ o2). During exercise, cardiac output increases linearly as a function of V˙ o2, so we assume that the increased heart rate during the ISWT was accompanied by a proportional decrease in stroke volume. It is beyond the scope of this study to know the mechanisms, central or peripheral in origin, that could account for these observations. Peripheral factors include a reduced venous return characteristic of lower muscle volume contributing to the exercise, but it is hard to account for these large differences between two apparently similar activities such as shuttle and treadmill walking. An alternative physiologic explanation includes that shuttle walking, with its increasing rate of abrupt turns at high speeds, results in more rapid withdrawal of vagal discharge and elevated sympathetic activity as compared with the treadmill test, in which the V˙ o2 demands were provoked by also increasing grade, thereby lessening the reliance on mechanically challenging increases in speed. This is consistent with the alinear increase in heart rate near the end of exercise during the ISWT.

As the majority of the study population had a BMI within the class I and II range (< 40 kg/m), the results may not be applicable for individuals with class III obesity. We focused on patients with obesity and treated OSA, as they have a worse prognosis and exert a larger health-care burden than obesity alone and, therefore, there is a greater need for health-care-provided exercise interventions for which field testing would be an attractive outcome measure. We included obese individuals in whom OSA had been treated with CPAP for at least 3 months, to diminish the potential impact that OSA may have on exercise capacity.33,34 Although our observations may be relevant to obese individuals in the absence of OSA, this remains to be confirmed. A familiarization test was not performed for the ITMT, as overall studies show that peak V˙ o2 is resistant to a learning effect, especially when maximal values can be confirmed or indicated by other parameters (eg, RER ≥ 1.05 and HR ≥ 80% predicted).21,3537 Further work is needed to investigate whether the ISWT is responsive to clinical interventions or can predict prognosis in obese individuals with treated OSA.

The ISWT, with a similar V˙ o2pk and graded cardiovascular response to an ITMT, can be used as a measure of cardiorespiratory fitness for a cohort of obese people with treated OSA and in preference to the 6MWT. However, incremental cardiopulmonary exercise testing remains the measurement of choice for assessing V˙ o2pk in an individual.

Author contributions: R. A. E. had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. R. A. E., R. S. G., and D. B. contributed to the study concept and design; R. A. E., T. E. D., and P. G. R. contributed to data acquisition; R. A. E. and T. E. D. contributed to data analysis; R. A. E., T. E. D., R. S. G., and D. B. contributed to data interpretation; R. A. E. drafted the submitted article; R. A. E., T. E. D., P. G. R., R. S. G., and D. B. revised the submitted article; and R. A. E., T. E. D., P. G. R., R. S. G., and D. B. approved the submitted article.

Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.

Role of sponsors: The sponsors had no role in the design of the study, the collection and analysis of the data, or the preparation of the manuscript.

6MWT

6-min walk test

AT

anaerobic threshold

CHF

chronic heart failure

ISWT

incremental shuttle walk test

ITMT

incremental treadmill test

RER

respiratory exchange ratio

V˙ o2

oxygen uptake

V˙ o2pk

peak oxygen uptake

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McCullough PA, Gallagher MJ, Dejong AT, et al. Cardiorespiratory fitness and short-term complications after bariatric surgery. Chest. 2006;130(2):517-525. [CrossRef] [PubMed]
 
Vanhecke TE, Franklin BA, Zalesin KC, et al. Cardiorespiratory fitness and obstructive sleep apnea syndrome in morbidly obese patients. Chest. 2008;134(3):539-545. [CrossRef] [PubMed]
 
Rizzi CF, Cintra F, Mello-Fujita L, et al. Does obstructive sleep apnea impair the cardiopulmonary response to exercise? Sleep. 2013;36(4):547-553. [PubMed]
 
Dolmage TE, Goldstein RS. Repeatability of inspiratory capacity during incremental exercise in patients with severe COPD. Chest. 2002;121(3):708-714. [CrossRef] [PubMed]
 
Keteyian SJ, Brawner CA, Ehrman JK, Ivanhoe R, Boehmer JP, Abraham WT; PEERLESS-HF Trial Investigators. Reproducibility of peak oxygen uptake and other cardiopulmonary exercise parameters: implications for clinical trials and clinical practice. Chest. 2010;138(4):950-955. [CrossRef] [PubMed]
 
Skinner JS, Wilmore KM, Jaskolska A, et al. Reproducibility of maximal exercise test data in the HERITAGE family study. Med Sci Sports Exerc. 1999;31(11):1623-1628. [CrossRef] [PubMed]
 

Figures

Figure Jump LinkFigure 1  Limits of agreement between the ITM and the ISWTV˙ O2pk and ATV˙ O2. Hashed line is the mean difference between ISWT and ITM, dot-and-hash line is the limits of agreement, and dotted line is the precision of the limits of agreement. ATV˙ O2 = oxygen uptake at the anaerobic threshold; ISWT = incremental shuttle walk test; ITM = incremental treadmill test; V˙ O2pk = peak oxygen uptake.Grahic Jump Location
Figure Jump LinkFigure 2  The relationship between the ISWT distance and ITMV˙ O2pk. Broken lines are 95% CI. See Figure 1 legend for expansion of abbreviations.Grahic Jump Location
Figure Jump LinkFigure 3  A comparison of the exercise responses among ITM, ISWT, and 6MWT. 6MWT = 6-min walk test; V˙ E = minute ventilation; V˙ O2=oxygen uptake. See Figure 1 legend for expansion of other abbreviations.Grahic Jump Location

Tables

Table Graphic Jump Location
TABLE 1  ] Demographics and Spirometry

Data given as mean (SD) unless otherwise indicated. AHI = apnea-hypopnea index; IQR = interquartile range.

Table Graphic Jump Location
TABLE 2  ] A Comparison of the Peak Exercise Parameters Between the ITMT, ISWT, and 6MWT

Data given as mean (SD) unless otherwise indicated. 6MWT = 6-min walk test; AT = anaerobic threshold; BS = Borg score for breathlessness; ISWT = incremental shuttle walk test; ITMT = incremental treadmill test; LE = Borg score for leg effort; N/A = not applicable; RER = respiratory exchange ratio; Spo2 = oxygen saturation by pulse oximetry V˙ co2 = rate of CO2 output; V˙ e = minute ventilation; V˙ o2 = rate of oxygen uptake.

a 

P < .05.

b 

One patient, prescribed β-blockers, was omitted.

c 

P < .001.

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Vanhecke TE, Franklin BA, Zalesin KC, et al. Cardiorespiratory fitness and obstructive sleep apnea syndrome in morbidly obese patients. Chest. 2008;134(3):539-545. [CrossRef] [PubMed]
 
Rizzi CF, Cintra F, Mello-Fujita L, et al. Does obstructive sleep apnea impair the cardiopulmonary response to exercise? Sleep. 2013;36(4):547-553. [PubMed]
 
Dolmage TE, Goldstein RS. Repeatability of inspiratory capacity during incremental exercise in patients with severe COPD. Chest. 2002;121(3):708-714. [CrossRef] [PubMed]
 
Keteyian SJ, Brawner CA, Ehrman JK, Ivanhoe R, Boehmer JP, Abraham WT; PEERLESS-HF Trial Investigators. Reproducibility of peak oxygen uptake and other cardiopulmonary exercise parameters: implications for clinical trials and clinical practice. Chest. 2010;138(4):950-955. [CrossRef] [PubMed]
 
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