Oxygen Uptake Efficiency Slope Across the Disability Spectrum in Persons with Multiple Sclerosis

Background:  Cardiorespiratory fitness (CRF) is considered an important indicator of health status, and low CRF is associated with increased risk for morbidity and mortality. Cardiopulmonary exercise testing (CPET), the current gold standard for assessing CRF, is highly limited by patient effort when the test involves maximal exertion. The oxygen uptake efficiency slope (OUES) has been proposed as an alternative, objective and effort-independent submaximal measure of cardiorespiratory exercise capacity. This may be particularly appropriate for clinical populations such as multiple sclerosis (MS) who present with a broad range of disability that can influence CPET performance.

Objectives: (i) Examine differences in the OUES between persons with MS and controls matched on sex, age, height and weight; (ii) examine differences in the OUES across the MS disability spectrum; and (iii) examine the relationship between the OUES and other CPET variables.

Methods:  62 participants  with MS (EDSS = 1.5–6.5) and 21 non-MS controls matched on age, sex, height and weight completed a symptom-limited CPET using a recumbent stepper and an open-circuit spirometry system. 

Results:  The OUES was significantly lower in persons with MS (mean=1708.5 (SD=503.7)) compared to non-MS controls (mean=2074.2 (SD=823.2)). A significant, negative correlation was found between OUES and disability (EDSS) (r=-.45). Among those with MS, OUES was significantly correlated with VO_2peak, WR_peak, HR_peak and VE_peak (p<.05) and submaximal OUES (i.e., 50% and 75%) was significantly correlated with VO_2peak, WR_peak, HR_peak and VE_peak (p<.05).  

Conclusions: Overall, the OUES was lower in individuals with MS compared to controls, and decreased as a function of increasing disability level in persons with MS. There was a strong relationship observed between the OUES and other CPET variables in persons with MS. Since the OUES can be determined from submaximal exercise, it represents a promising alternative for expressing CRF in persons with MS.