Improving exercise cardiac reserve assessment using a novel cardiopulmonary and echocardiography testing protocol (CPET-ESE) based on individualised exercise intensity domains
Authors: Dorobantu D , Gibb J , Smith M , Williams M , Haywood A , Emirza C , Wadey C A , Stuart A G , Pieles G E , Williams C A 
Published: October 2024

Abstract

Background
Functional exercise stress echocardiography (ESE) is used to quantify changes in cardiac reserve and can improve early cardiac dysfunction diagnosis and follow-up.[1.2] Current ESE protocols do not account for individual variations in fitness and heart rate response, leading to difficulties in defining normal reference values.[2,3] Physiologically defined exercise intensity domains could be implemented as part of combined cardiopulmonary and ESE testing (CPET-ESE) to address this problem.

Purpose
We propose that using exercise intensity domains as part of CPET-ESE will result in cardiac reserve values not influenced by individual cardiorespiratory fitness or physical activity (PA) levels. We also hypothesise by matching work-rates and heart rates (HR), respectively, cardiac reserve will be higher in those exercising at the higher exercise intensity domain.

Methods
A total of 45 healthy adults (mean age 43.8 ± 14.2, range 18-65 y) underwent a CPET followed by a CPET-ESE consisting of two 6-minute stages: 1. Moderate intensity (MOD) at 90% of the work-rate at the gas exchange threshold (GET) and 2. High intensity (HIGH) at 40% of the difference between GET and peak. Global left ventricular and right ventricular free wall (LV and RV) longitudinal strain (Sl), RV fractional area change (RV-FAC), peak LV and RV TDI derived systolic (S’) and diastolic velocities (E’) were measured at each step, including rest. HIGH steps were matched 1-to-n with MOD steps from other participants based on work-rate and HR, respectively. Questionnaire derived PA was classified as low, moderate, and high active. Cardiorespiratory fitness was measured as peak oxygen uptake relative to body mass (mL·kg-1·min-1).

Results
After adjusting for age and sex, neither higher PA nor peak VO2 were associated with higher cardiac function parameters at either intensity (Table 1). Higher peak VO2 was associated with higher work-rate for HIGH only, while higher PA was associated with lower HR for MOD only (interaction term p&lt;0.05). When paired by matching HR and work-rate, respectively, HIGH was associated with higher cardiac function parameters compared to MOD (all except for LV-S’ and RV-FAC when HR matched, Table 2), despite those matched from HIGH being older, less physically active and having lower peak VO2 (p&lt;0.05 for all).

Conclusions
Current ESE protocols, which define submaximal exercise intensity using arbitrary work-rate or HR targets,[3] do not account for the complex differences in exercise response between healthy individuals. Cardiac function augmentation during exercise is likely a response to VO2 kinetics and this is not accurately described by either HR or work-rate alone. We derived normal reference values for cardiac reserve from implementing physiologically defined exercise intensity domains within the combined CPET-ESE testing frameworks. These values could be more appropriately used when comparing diverse clinical populations using CPET-ESE.