Abstract 18113: O 2 Pulse is Not Stroke Volume: Changes in Oxygen Pulse Do Not Reliably Track Changes in Stroke Volume During Exercise in Health or Heart Failure

Abstract

Background: Oxygen Pulse (O2P) is often used to index peak stroke volume (SV) during cardiopulmonary exercise testing (CPET) when measures of cardiac output are not possible. O2P is calculated as Oxygen uptake (V˙O 2 ) / heart rate (HR) and based upon rearrangement of the Fick equation (V˙O 2 = [HR x SV] x arterial-venous O 2 difference [Δa-vO2]; Δa-vO2 is not included in the O2P calculation). As little data exist to validate the use of O2P, we assessed whether O2P tracks changes in SV during exercise in health and disease. Methods: We studied 401 individuals: 232 healthy adults (healthy; 51% males; age: 61 ± 11 [28-88] years); 119 adults at high-risk for future development of heart failure (high-risk; 47% males; age: 51 ± 6 [40-64] years); and 50 adults with heart failure with preserved ejection fraction (HFpEF; 36% males; age: 71 ± 7 [56-86] years). Participants were studied at rest, during light (~30% V˙O 2 max), moderate (Mod; ~60% V˙O 2 max) and maximal exercise (Max). V˙O 2 (indirect calorimetry) and cardiac output (Q˙c; acetylene rebreathing) were measured, enabling calculation of SV (Q˙c/HR) and O2P. Data were compared via two-way (measure*stage) ANOVA. Results: We observed a significant measure*stage interaction for all three groups (all, P< 0.001; Figure), suggesting different responses to exercise between measures. Specifically, SV and O2P increased from rest to light exercise in all groups (all, P <0.001; Figure). Thereafter, O2P continued to increase from light-Mod (all, P<0.001) and Mod-Max (all, P <0.001; other than for Mod-Max in HFpEF, P =0.124). However, actual SV did not increase from light-Mod exercise (all groups, P>0.917) or from Mod-Max exercise in high-risk (Δ0±13 ml, P =0.999) and HFpEF (Δ-3±14 ml, P =0.564), but fell slightly in healthy individuals from Mod-Max (Δ-5±13 ml, P <0.001). Conclusions: Changes in O2P do not reflect changes in SV between exercise stages during CPET in health or disease and should therefore be used with caution.