Test–Retest Reliability of Portable Metabolic Monitoring After Disabling Stroke

Introduction

Stroke is the most common neurological disability in older Americans and a leading cause of death in the United States. ¹ The majority of stroke survivors exhibit residual neurological deficits that impair function and mobility, predisposing them to a sedentary lifestyle. ² These deficits also result in a marked increase in energy expenditure during regular walking, contributing to a general intolerance for performing basic activities of daily living (ADLs). ³

Measurement of cardiopulmonary parameters during maximal and submaximal exercise is critical to the assessment of energy expenditure, cardiovascular fitness, and economy of gait. Research using stationary metabolic equipment demonstrates that submaximal and peak treadmill testing in chronic hemiparetic stroke survivors is highly reproducible ⁴ and that hemiparetic stroke survivors expend 1.5 to 2 times more energy during submaximal treadmill walking as compared to age-matched, sedentary, nonstroke controls. ⁵ Higher oxygen consumption in stroke survivors during treadmill walking reflects impaired economy of gait or gait inefficiency. However, the full extent of gait inefficiency after stroke may be partially masked by the artificial impact that a moving treadmill belt has on hemiparetic gait parameters and subsequently oxygen consumption. For this reason, it may be far more relevant to capture cardiopulmonary parameters reflective of gait economy during overground locomotion, based on the more natural gait dynamics that accompany floor walking. To date, the energy cost of overground walking in chronic, hemiparetic stroke survivors has not been studied either cross-sectionally or across interventions due to the absence of reliability studies for portable metabolic monitoring equipment in this population.

The K4b² (COSMED USA; Chicago, IL) is a portable, metabolic monitoring system that has shown initial reliability for measuring energy expenditure and economy of gait during steady-state, submaximal exercise in healthy adults without neurologic disability.^6,7 However, no studies have thus far been conducted to assess overground movement efficiency in either healthy individuals or stroke survivors using this device. Establishing the reliability of such portable monitoring technology in chronic stroke would affect our ability to gain insight into the metabolic costs associated with overground walking and performance of ADLs in this population. The purpose of the current study was, therefore, to determine the test–retest reliability of the COSMED K4b² during a 6-minute walk test in older, chronic hemiparetic stroke participants. Based on previous results in nonstroke populations during treadmill walking,^6,7 we hypothesized that cardiopulmonary measures would demonstrate high reproducibility in overground walking after stroke, defined by an intraclass correlation coefficient (ICC) greater than or equal to 0.85.

Methods

Results

Subjects

A total of 23 hemiparetic stroke patients (12 women, 11 men; mean age 61.2 ± 8.4 years, range 41-80 years) completed this study. Of those, 65% were African American and 30% were White (remainder Hispanic). Fourteen of the participants had left-sided hemiparesis and 9 were diabetic. Fifty-seven percent required the use of an assistive device for ambulation (cane n = 12; walker n = 1), and 35% used an ankle–foot orthosis. Additional details related to the physical characteristics of these participants are summarized in Table 1. There were no adverse events during portable testing, and the equipment was generally well tolerated.

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Table 1.

Subject Demographics and Physical Performance Measures.

Variable	n	Mean ± SD	Range
Age (years)	23	61.2 ± 8.4	41-80
Height (cm)	23	170.0 ± 10.9	152-196
Weight (kg)	23	88.0 ± 20.5	58.0-134.1
Body mass index	23	30.5 ± 6.8	19.8-47.6
6-Minute walk distance (m)
Without portable monitor	23	278.0 ± 123.2	78 ± 510.5
Trial 1 portable monitor	23	307.7 ± 142.0	62.2-567.5
Trial 2 portable monitor	23	305.1 ± 138.3	61.0-559.3

Six-Minute Walk

The difference in mean distance covered between the first and second 6MW (as shown in Table 1) was less than 1%. This nearly identical distance enabled confidence in the gas exchange comparisons shown below (Table 2), eliminating total work as a potential confounder. There was no significant difference between Test 1 and Test 2 (−2.6 m; P = .43) for mean 6MW distance, and the R² value of .98 indicates that performance on Test 1 explained 98% of the variability in Test 2. Moreover, Bland–Altman plots of within-subject difference for distance walked versus the mean of Tests 1 and 2 (Figure 1) demonstrates excellent repeatability for this test. An ICC of 0.99 for distance covered further reflects an extremely high reliability for the 6MW test in this population.

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Table 2.

Reliability of 6-Minute Walk Measurements in Stroke Patients.

	n	Test 1	Test 2	Difference Mean	P	ICC	95% CI
6MW distance (m)	23	307.7 ± 142.0	305.1 ± 138.3	−2.6	.43	0.99*	0.99-1.0
Vo2 (mL kg−1 min−1)	23	12.8 ± 3.2	12.8 ± 3.1	0.02	.95	0.90*	0.79-0.95
Vo2 (L min−1)	23	1120.2 ± 368.0	1119.3 ± 351.7	−0.9	.97	0.93*	0.86-0.97
Vco2 (mL min−1)	23	1063.5 ± 353.0	1087.1 ± 362.4	23.6	.38	0.93*	0.86-0.97
VE (L min−1)	23	38.6 ± 12.8	38.8 ± 12.2	0.2	.86	0.95*	0.89-0.98
RQ	23	0.95 ± 0.07	0.96 ± 0.08	0.01	.37	0.64*	0.33-0.82
HR (bpm)	22	109.9 ± 21.6	105.8 ± 21.7	−4.1	.21	0.76*	0.53-0.89

Abbreviations: ICC, intraclass correlation coefficient; CI, confidence interval; 6MW, 6-minute walk; Vo₂, oxygen consumption; Vco₂, carbon dioxide production; VE, ventilation; RQ, respiratory quotient; HR, heart rate.

Difference mean is within person mean difference.

*

P < .001.

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Figure 1.

Bland–Altman plot of distance walked.

Portable Metabolic Testing

Test–retest results for all cardiopulmonary measures across the 2 separate 6MW tests with portable monitoring are also summarized in Table 2. As shown, there was no significant difference between tests for any of the cardiopulmonary measures taken. Bland–Altman plots of within-subject change in steady-state Vo₂ (mL.kg⁻¹.min⁻¹) as a function of mean steady-state Vo₂ (mL.kg⁻¹.min⁻¹) during Tests 1 and 2 (Figure 2) shows the high reliability of this measure. The gas exchange measures for Vo₂, Vco₂, and VE from test to test were also shown to be highly reliable as indicated by ICCs ≥ 0.90 (Table 2). Although there was no significant difference for HR or RER between Test 1 and Test 2, ICCs for these variables (0.78 and 0.66, respectively) fell below the previously defined reliability cutoff of >0.85.

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Figure 2.

Bland–Altman plot of steady-state relative Vo₂ (mL kg⁻¹ min⁻¹).

Discussion

This study is the first to establish test–retest reliability/repeatability of portable metabolic monitoring during overground walking in older chronic hemiparetic stroke survivors. The clinical significance is that disabled stroke participants may eventually be better evaluated for movement efficiency in the context of more natural settings compared to the artificial constraints imposed by laboratory treadmill testing. The reliability coefficients observed for Vo₂, Vco₂, and VE more than exceeded our preestablished reliability threshold, providing strong evidence for the validity of the Kb4² to measure overground gait economy and track intervention–induced gains in movement efficiency after disabling stroke. Importantly, this study also provides supportive evidence that the 6MW test is a highly reliable metric in the chronic phase of stroke recovery.

The 6MW was first established as a means of assessing the effectiveness of treatment in patients with cardiovascular and pulmonary disease. ⁸ It has since developed into a functional measure to characterize long-distance ambulation and disability in chronic stroke and other elderly populations.^9-19 Research demonstrates that 6MW distances for healthy individuals range from 400 to 700 meters,^{20 -22} whereas, on average, stroke survivors cover 49.8% the distance of their healthy counterparts, ⁹ with reported ranges between 116 and 400 meters.^4-19 Our cohort of 23 stroke survivors show a mean 6MW distance of 307.7 ± 142 m, placing them in the middle of the range reported by other stroke studies and establishing them as moderately impaired. Furthermore, our cohort of stroke survivors had a mean walking speed of 0.86 ± 0.39 m/s, which was 35% slower than that observed in age-matched healthy community dwelling adults (1.33 ± 0.21 m/s). ²³

There is limited research analyzing the test–retest reliability of the distance covered during the 6MW in chronic stroke, but the few studies that have been conducted to date do support our current reproducibility findings. Reliability coefficients (ICCs) previously ranged from 0.98 to 0.99,^11-13 which matches our finding (ICC = 0.99). Flansbjer et al ¹² reported a 3.5% difference in mean distance covered between Test 1 and Test 2 of the 6MW, whereas Lui et al ¹³ reported a 0.5% difference in stroke. Our investigation demonstrated a less than 1% difference during testing with the portable monitoring equipment, adding to the literature by showing that the addition of the Kb4² device does not interfere with the reliability of this important measure of walking endurance. This may have important future ramifications in the context of using this functional test with portable metabolic monitoring to assess training induced improvements in poststroke functional limitations.

Reliable portable metabolic monitoring technology enables quantification of cardiopulmonary parameters outside of the laboratory, in settings more representative of typical free-living conditions. In healthy adults, research has shown that normal overground gait mechanics are altered during treadmill walking.^24,25 This issue undoubtedly becomes even more prominent in the chonically disabled stroke population based on hemiparetic gait dynamics. This suggests that overground gait measures are more indicative of a patient’s functional status and should be favored over treadmill measures if possible. In fact, our current results demonstrate that walking economy numbers are 38% worse overground compared to a previously conducted treadmill gait economy study in stroke. ²⁶ Therefore, to fully understand overground economy of gait after stroke, it is imperative to first establish reliable technology able to capture the cardiopulmonary effects of free-living movement.

The COSMED K4b² has been shown to be valid and reliable for measuring energy expenditure and economy of gait during steady-state submaximal treadmill walking in healthy adults without neurological disability.^6,7 Schrack et al ⁷ compared Vo₂ and Vco₂ obtained from the K4b² to those obtained from a standard, stationary metabolic cart (Medgraphics D-Series Gas Exchange System) during submaximal treadmill walking (N = 19). ICCs between the 2 systems for Vo₂ (0.95 and 0.97 for mL.kg⁻¹.min⁻¹and L.min⁻¹, respectively) and Vco₂ (0.93) showed high accuracy. Additionally, there was no significant difference between tests using the portable and stationary systems for Vo₂ (P = .16), Vo₂ adjusted for body weight (P = .25), or Vco₂ (P = .09). Duffield et al ⁶ analyzed test–retest reliability of the K4b² during treadmill running in physically fit men. High reliability was reported for VE, Vo₂, and Vco₂, with ICC ranging from 0.7 to 0.9, P < .05. Mendelsohn et al. ²⁷ demonstrated moderate to high test–retest reliability of the K4b² in older adults (82.4 ± 5.4 years) during various intensities of submaximal, all-extremity semirecumbent exercise for heart rate and METs (ICCs ranging from 0.85 to 0.91). Thus, although previous research has established the validity and test–retest reliability of the K4b² during exercise modalities that do not incorporate overground walking,^6,7,26 there are no overground studies available to gauge the reliability of this portable technology in more natural, free-living settings, representing a large gap in the literature. Furthermore, no research has determined the reliability of the K4b² portable device in older hemiparetic stroke survivors, either on the treadmill or over ground. The current study is the first to have established this reliability in any population during overground walking, reporting ICCs quite similar to the portable unit reliability studies conducted in nonstroke adults on a treadmill, and comparing favorably with test–retest results obtained using stationary metabolic equipment during a treadmill reliability study after stroke. ⁴ Despite HR and RER falling below the previously defined reliability cutoff of >0.85, the most important cardiopulmonary measures in the context of overground walking economy assessment (Vo₂ in particular) showed very high reliability, demonstrating the utility of the K4b² for this purpose in older, hemiparetic stroke survivors. Further investigation is recommended prior to rendering final judgment on the utility of this device for capturing HR and RQ responses to physical activity after stroke.

Importantly, gait economy can improve with an exercise intervention after stroke according to a study using oxygen consumption during steady-state submaximal treadmill walking as the metric for economy. Specifically, Macko et al ²⁶ observed that the same absolute level of treadmill walking required 15% less oxygen consumption following a period of regular treadmill aerobic training. Future studies, supported by our current reliability results, are now well positioned to evaluate how varying rehabilitation interventions affect the economy of movement over ground. Improvement in such measures would be far more relevant to community function and would not be confounded by either treadmill belt movement or the handrail support typically associated with treadmill testing after stroke.

Conclusion

The high reliability of portable metabolic monitoring for assessing cardiopulmonary parameters (Vo₂, Vco₂, and VE) during overground locomotion in older, chronic hemiparetic stroke survivors could justify a line of productive research that uniquely captures functional progression and regression in this population. Additionally, our results suggest that the 6MW test, independent of portable monitoring, is an extremely reliable and reproducible functional test in chronic stroke. Having established the K4b² as reliable technology in the context of stroke walking economy could prove vital in gaining more insight into the energy cost of ADLs in this population. This study may have been limited by the fact that this was a heterogeneous cohort of medically stable outpatient stroke survivors. Therefore, the translation of these findings to an inpatient stroke cohort may be limited. Future research should focus on determining the extent to which economy of gait can be improved overground and understanding what types of exercise training produce the most impact on overground gait efficiency.