Test-retest Reliability of Cardiorespiratory Variables Measured with the Metamax 3b During the Six-minute Walking Test after Stroke

Polese JC^1,2*, Ada L², Parreira VF¹, Faria GS¹, Avelino P¹, Teixeira-Salmela LF¹

¹Department of Physical Therapy, Universidade Federal de Minas Gerais, Brazil

²Discipline of Physiotherapy, The University of Sydney, Australia

*Corresponding author: Polese JC, Department of Physical Therapy, Universidad Federal de Minas Gerais, Brazil, Discipline of Physiotherapy, The University of Sydney, Australia

Received: December 12, 2014; Accepted: February 01, 2015 Published: February 03, 2015

Abstract

Introduction: Subjects after stroke expend more energy, when performing the same activity as healthy controls, but it is not known whether subjects after stroke expend the same amount of energy during daily life activities. However, to allow the measurement during daily activities is necessary to use portable equipment. Thus, the aim of this study was determine the test-retest reliability of cardiorespiratory variables measured with the Metamax 3B and polar heart rate monitor during the 6-minute Walking Test (6MWT) after stroke.

Methods: Twenty-one individuals with chronic stroke (13 men, mean age of 59 years, mean time since the onset of stroke of 30 months) underwent two 6MWT within an interval of 7 days wearing a Metamax 3B and a polar heart rate monitor. Intra-class correlations coefficients (ICC_2,1), within-participant t-tests, standard errors of measurement and Bland-Altman plots were calculated for cardiorespiratory variables (relative and absolute oxygen consumption, carbon dioxide production, respiratory exchange ratio, minute ventilation and heart rate) during the 6MWT. Results: The ICC_2,1 values ranged from 0.76-0.97. Mean differences between test 1 and 2 ranged from 0-4%.The SEM% ranged from 1-12%.

Conclusions: The present findings support the test-retest reliability of the Metamax 3B during overground walking after stroke.

Keywords: Stroke; Walking; Portable gas analysis; Oxygen consumption; Measurement; Reliability

Introduction

Stroke is the leading cause of disability worldwide, which lead to serious disabilities [1]. According to the latest statement by the American Heart Association, the central goal for the current decade is a sufficient level of physical activity, with the aim of improving cardiovascular health and reducing deaths caused by stroke [2]. This goal is based on the finding that physical activity has a neuro protective effect against recurrent stroke [3] and decreasing recurrent stroke will decrease the incidence of stroke [4,5]. However, subjects after stroke have low levels of physical activity, compared with healthy individuals and very rarely meet the American Guidelines for Stroke which recommend at least 150 minutes of moderate-intensity physical activity per week [2,3]. A recent systematic review on physical activity after stroke found that subjects with stroke took 53% steps/ day of healthy controls [6]. On the other hand, the findings from two studies [7,8] suggested that although there is a decrease in frequency of activity, subjects after stroke spend much the same amount of time being active as normal. Given that subjects after stroke expend more energy when performing the same standardised activity as healthy controls [9], the question raised as to whether subjects after stroke would expend the same amount of energy during everyday life as healthy controls, in spite of their reduced frequency of activity.

Gas analysis is the gold standard measure to investigate energy expenditure [10]. However, most gas analysis in subjects with stroke has been done during incremental tests on a treadmill or cycle ergometer in order to measure maximum oxygen consumption [10]. To investigate energy consumption during everyday life, it is necessary to use portable monitoring equipment, which allows the person to move freely around the environment.

The aim of this study was, therefore, to investigate the test-retest reliability of portable monitor’s equipment during an everyday activity – overground walking. The Metamax 3B (Cortex, Germany) is a portable, gas analysis system which is light (1.4 kg) and allows the transmission “online” of respiratory variables for a distance of up to 800 meters. With the addition of a heart rate monitor, this allows the collection of both cardiac and respiratory variables during everyday activities such as walking. The cardiorespiratory variables are adjusted in real time, according to the environmental conditions, by means of temperature sensors, internal pressure sensors and an electronic barometer. The Metamax 3B is valid and reliable when used to measure cardio respiratory variables during everyday activities in healthy individuals [11]. However, no studies have thus been conducted to assess cardiorespiratory variables during overground walking in stroke individuals using the Metamax 3B. To investigate test-retest reliability, subjects with chronic stroke (time since stroke >1 year) were studied. The specific research question for this study was:

What is the test-retest reliability of cardiorespiratory variables measured with a Metamax 3B and a polar heart rate monitor during the 6-minute Walk Test (6MWT)?

The establishment of reliability of portable monitoring equipment under stable conditions (i.e., in chronic stroke) is the first step in investigating energy expenditure during everyday activities after stroke.

Methods

Design

Subjects with chronic stroke were recruited from the general community in a metropolitan city in Brazil. Individuals came to a University laboratory twice within 7 days. On each occasion, they performed a 6MWT while metabolic monitoring equipment collected their cardiorespiratory variables. The 6MWT was performed at the same time of day and at least 500 ml of water was provided prior to each test, to guarantee normal hydration. First, the participants walked the course of 30 meters without monitoring equipment to familiarise themselves with the 6MWT. Then, after familiarisation, they rested for 3 minutes to stabilise the Metamax 3B. After, they performed the 6MWT, following the procedures and recommendations of the American Thoracic Society [12,13]. This study was approved by the Research Review Board of the Universidad Federal de Minas Gerais and all participants provided written consent prior to data collection.

Participants

Volunteers were eligible if they: were = 20 years old; had a mean time since the onset of a unilateral stroke between one and five years; were able to walk independently with or without assistive devices; had no cognitive deficits, as determined by the Mini-Mental State Examination Brazilian cut-off scores, which were adjusted for levels of education [14]; were not undergoing rehabilitation, and had no other neurological or orthopaedic disorders.

Characteristics, such as age, gender, body mass index, side of hemiparesis, time since onset of stroke, number of medications and co-morbidities, levels of physical activity (adjusted activity scores of the Human Activity Profile) [15], levels of independence (Barthel Index) [16], walking speed (10-m Walk Test) [17] and walkingcapacity (distance covered during the 6MWT) [17] were collected for characterization purposes.

Measurement of cardiorespiratory variables

Participants wore a Metamax 3B for the collection of the following respiratory variables: Relative and absolute oxygen consumption [Vo₂] in ml/min/kg and L/min), carbon dioxide production [VCo₂] in L/min, respiratory exchange ratio [RER], minute ventilation [VE] in L/min). In addition, the polar heart rate monitor provided the cardiac variables (heart rate [HR], in bpm). The Metamax 3B was calibrated in three ways before each test: barometric, gas, and volume calibration. The means of the final three minutes of the 6MWT (steady state condition) were used for analyses [18].

Analysis of test-retest reliability

Test-retest reliability was assessed in four ways. Firstly, intra-class correlation coefficients (ICC_2,1) and 95% confidence intervals (CI) were calculated for all cardiorespiratory variables obtained at Tests 1 and 2. An ICC value < 0.4 was considered poor, 0.4 to 0.75 as fair to good, and > 0.75 as excellent [19]. Secondly, within-participant t-tests (mean and 95% CI) were calculated between the tests 1 and 2. Third, the standard errors of measurement (SEM) were calculated, as follows: SEM= s v(1.00 – r), where s is the standard deviation and r is the test-retest ICC [20,21]. The SEM was expressed as a percentage of the average values of tests 1 and 2. SEM values < 15% were considered acceptable [21]. Lastly, Bland & Altman plots were generated for relative Vo² to assess the agreement between the tests 1 and 2 [22]. All analyses were carried out with the SPSS for Windows software (release 17.0) with a significance level of 5%.

Results

Twenty-one individuals with stroke (13 men), with a mean age of 59 (SD 15) years, a mean time since the onset of stroke of 30 (SD 17) months participated. All participants were taking oral medications, primarily anti-hypertensive drugs. Eight individuals required assistive devices (cane = 4 individuals; walker = 4 individuals) to walk during the 6MWT. The distance covered during the 6MWT was very similar between the tests, illustrating that the participants’ walking was stable. Their characteristics are summarized in Table 1.

Table 1: Characteristics of participants.




  
    Characteristic
    n=21
  
  
    Age (years), mean (SD)
    59 (15)
  
  
    Gender, n male (%)
    13 (62)
  
  
    Body mass index (kg/m2), mean (SD)
    26 (4)
  
  
    Paretic side, n right (%)
    10 (48)
  
  
    Time since the onset of stroke (months), mean (SD)
    30 (17)
  
  
    Medications (number), mean (SD)
    5 (2)
  
  
    Associated pathologies (number), mean (SD)
    2 (1)
  
  
    Human Activity Profile (0-94), mean (SD)
    58 (16)
  
  
    Barthel Index (0-20), mean (SD)
    19 (1)
  
  
    Walking speed (m/s), mean (SD)
    0.89 (0.30)
  
  
    Walking distance (m), mean (SD)
    Test 1
    359 (140)
  
  
     
    Test 2
    366 (139)
  
  
    SD: standard    deviation, AAS: adjusted activity scores.



Table 1:  Characteristics of participants.

The cardiorespiratory variables for both tests along with the correlation coefficients are given in Table 2. The ICC_2,1 values ranged from 0.76 to 0.97 the mean differences between the tests 1 and 2 ranged from 0-3%.The SEM% ranged from 1 to 12%.

Table 2: Cardiorespiratory variables for each 6-min Walk Test and the relation between tests (n=21).




  
    Cardiorespiratory variables
    Test 1
    Test 2
    Average of Tests
     
    Test-retest reliability
  
  
     
    Mean (SD)
    Mean (SD)
    Mean (SD)
     
    ICC2,1 (95% CI)
    MD (95%IC)
    SEM (%)
  
  
    Relative VO2 (ml/min/kg)
    11.0 (2.4)
    10.9 (3.1)
    10.9 (2.5)
     
    0.80 (0.51 to 0.92)
    -0.1 (-1.1 to 1.0)
    1.1 (10)
  
  
    Absolut VO2 (L/min)
    0.76 (0.18)
    0.78 (0.18)
    0.77 (0.17)
     
    0.85 (0.62 to 0.94)
    0.01 (-0.04 to 0.07)
    0.07 (9)
  
  
    VCO2 (L/min)
    0.70 (0.18)
    0.76 (0.18)
    0.73 (0.16)
     
    0.76 (0.42 to 0.90)
    0.06 (-0.01 to 0.12)
    0.09 (12)
  
  
    RER
    0.91 (0.04)
    0.91 (0.04)
    0.91 (0.04)
     
    0.97 (0.92 to 0.99)
    0.00 (-0.01 to 0.00)
    0.01 (1)
  
  
    VE (L/min)
    27 (6)
    28 (9)
    28 (7)
     
    0.81 (0.52 to 0.92)
    1 (-2 to 3)
    3 (9)
  
  
    HR (bpm)
    101 (18)
    104 (20)
    103 (18)
     
    0.92 (0.80 to 0.97)
    3 (-2 to 7)
    5 (5)
  
  
    ICC = Intraclass Correlation    Coefficient; CI = Confidence Interval; VO₂=, Oxygen Consumption;    VCO₂= carbon dioxide production; RER = Respiratory Exchange Ratio; VE = Ventilation; HR = Heart Rate; MD = Mean Difference; SEM = Standard Error of Measurement.



Table 2:  Cardiorespiratory variables for each 6-min Walk Test and the relation between tests (n=21).

The Bland and Altman plot in Figure 1 showed the withinparticipant change across tests 1 and 2 in relative Vo₂ as a function of the individual mean on tests 1 and 2 relative to the Vo₂. The mean difference between tests 1 and 2 was approximately 1%. Second, the SD (2.3) was also small compared with the relative Vo₂(11.0). Third, there appeared to be no relations between the differences in the two tests and the mean of the two tests.

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Citation: Polese JC, Ada L, Parreira VF, Faria GS, Avelino P, Teixeira-Salmela LF. Test-retest Reliability of Cardiorespiratory Variables Measured with the Metamax 3b During the Six-minute Walking Test after Stroke. Phys Med Rehabil Int. 2015;2(1): 1028. ISSN:2471-0377

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