Enhancing Heart Rate-Based Estimation of Energy Expenditure and Exercise Intensity in Patients Post Stroke

Indirect calorimetry is the gold standard field-testing technique for measuring energy expenditure and exercise intensity based on the volume of oxygen consumed (VO₂, mL O₂/min). Although heart rate is often used as a proxy for VO₂, heart rate-based estimates of VO₂ may be inaccurate after stroke due to changes in the heart rate-VO₂ relationship. Our objective was to evaluate in people post stroke the accuracy of using heart rate to estimate relative walking VO₂ (wVO₂) and classify exercise intensity. Moreover, we sought to determine if estimation accuracy could be improved by including clinical variables related to patients' function and health in the estimation.

Sixteen individuals post stroke completed treadmill walking exercises with concurrent indirect calorimetry and heart rate monitoring. Using 70% of the data, forward selection regression with repeated k-fold cross-validation was used to build wVO₂ estimation equations that use heart rate alone and together with clinical variables available at the point-of-care (i.e., BMI, age, sex, and comfortable walking speed). The remaining 30% of the data were used to evaluate accuracy by comparing (1) the estimated and actual wVO₂ measurements and (2) the exercise intensity classifications based on metabolic equivalents (METs) calculated using the estimated and actual wVO₂ measurements.

Heart rate-based wVO₂ estimates were inaccurate (MAE = 3.11 mL O₂/kg/min) and unreliable (ICC = 0.68). Incorporating BMI, age, and sex in the estimation resulted in improvements in accuracy (MAE Δ: -36.01%, MAE = 1.99 mL O₂/kg/min) and reliability (ICC Δ: +20, ICC = 0.88). Improved exercise intensity classifications were also observed, with higher accuracy (Δ: +29.85%, from 0.67 to 0.87), kappa (Δ: +108.33%, from 0.36 to 0.75), sensitivity (Δ: +30.43%, from 0.46 to 0.60), and specificity (Δ: +17.95%, from 0.78 to 0.92).

In people post stroke, heart rate-based wVO₂ estimations are inaccurate but can be substantially improved by incorporating clinical variables readily available at the point of care.