Validity and reliability of VO₂-max measurements in persons with multiple sclerosis

Highlights

• MS subjects achieve VO₂-max validity criteria to the same extent as healthy controls.

• A valid VO₂-max test can be performed in PwMS.

• The proportion of valid VO₂-max tests in PwMS depends on the applied validity criteria.

• VO₂-max variability in PwMS exhibits a 95% prediction interval of − 9.8 to 8.1%.

• In interventions, a change of > 10% is required to be interpreted as a real change.

Abstract

Background

Direct measurement of whole body maximal oxygen consumption (VO₂-max test) is considered the gold standard when assessing cardiorespiratory fitness. Nonetheless, the validity and reliability of the test have not been examined in persons with multiple sclerosis (PwMS).

Objective

To investigate the validity and reliability of VO₂-max measurements in PwMS, and additionally to compare these measures to those of healthy controls.

Methods

Twenty PwMS completed two incremental VO₂-max tests on a leg cycling ergometer. Test validity was analyzed based on the first VO₂-max test in the total sample and in patient subgroups based on Expanded Disability Status Scale (EDSS) scores (EDSS ≤ 2.5, n = 10 and EDSS ≥ 3.0, n = 10) by evaluation of the primary VO₂ plateau criterion and four common secondary validity criteria. Data from 20 age- and gender-matched healthy controls were used for comparison. The second VO₂-max test was used to establish day-to-day reliability.

Results

In PwMS 40% were able to achieve the primary validity criterion for VO₂-max measurements, while 65–95% were able to achieve the secondary criteria. This corresponded to the age-matched healthy controls. Strong correlations were found between measurements of VO₂-max and between the validity criteria from test 1 compared to test 2 in PwMS. MS disability level did not influence criteria attainment. The variability analysis exhibited a 95% prediction interval of − 238 to 201 mL·min^− 1 (− 9.8 to 8.1%) for the difference between the two measurements of VO₂-max.

Conclusion

In mild to moderately impaired PwMS less than half achieve the primary validity criterion when performing a VO₂-max test, but the high reliability and the better achievement of the secondary criteria implies that a valid test of VO₂-max can be performed, at a level corresponding to that of healthy controls. The day-to-day variation implies that a change of more than 10% in VO₂-max is required to be interpreted as a real change.

Introduction

Multiple sclerosis (MS) is an immune-mediated chronic inflammatory and neurodegenerative disorder of the central nervous system (CNS) with an elusive etiology [1], [2]. The progressively disabling nature of the disease manifests as sensory, motor, cerebellar, and cognitive dysfunctions that may limit physical activity in persons with MS (PwMS) with subsequent deconditioning [3]. The cardiorespiratory fitness is regarded as a crucial health indicator for PwMS, since a good physical fitness is associated with a lower mortality [4]. Direct measurement of the whole body maximal oxygen consumption during incremental exercise to exhaustion (VO₂-max test) is considered the gold standard when assessing cardiorespiratory fitness [5]. The VO₂-max is both an important health [6], [7] and performance [8] marker. Moreover, higher VO₂-max is associated with better walking performance [9], improved cognitive processing speed [10], and a possible prophylactic influence on the structural decline of brain tissue in PwMS [11], while an impaired VO₂-max is related to functional limitations that may hinder independent living in healthy populations [12].

VO₂-max is measured directly by respiratory gas exchange measurements during incremental exercise. Several criteria have been proposed that should be attained before a measurement is regarded as valid [13], [14]. The primary validity criterion describes a plateau in oxygen uptake despite an increase in work rate, while the secondary criteria include: 1) an elevated respiratory exchange ratio (RER) above 1.15; 2) achievement of a fixed percentage of the age-predicted maximal heart rate (HR-max); 3) a high level of lactic acid in the blood in the minutes following the exercise test; and 4) subjective ratings of perceived exertion (RPE) indicating exhaustion [13], [14]. However, no study has yet assessed if all of these criteria are achieved in PwMS following a VO₂-max test. This is a problem since obtaining an accurate and valid VO₂-max value is of importance when evaluating patients' health and performance status in clinical practice and research [15]. Recent findings have questioned the ability of healthy populations to attain the commonly used primary validity criterion, by reporting that only 37–59% were able to do this [16], [17]. Consequently, it is possible that PwMS will not be able to attain the plateau criterion, but may be able to attain the secondary criteria.

Another important property of a test is the reliability of the test. In healthy people studies have shown a high test–retest reliability of the VO₂-max test [18], [19], [20], [21]. However, day-to-day variation of VO₂-max measurements has not yet been evaluated in PwMS.

Consequently, the objectives of the present study were: 1) to investigate the validity of the VO₂-max test in ambulatory PwMS by evaluating primary and secondary criteria attainment and 2) to investigate the test–retest (day-to-day) reliability of the VO₂-max test in PwMS. In addition the criteria attainment and the VO₂-max measurements were analyzed in MS subgroups based on disability level, and were compared to those of healthy age-matched controls from our lab database.

It was hypothesized: 1) that most PwMS would not attain the primary VO₂-max criterion but to a higher degree would be able to meet the secondary criteria, and 2) that the VO₂-max test would show a low day-to-day variation in PwMS.