Cardiorespiratory mechanisms underlying the impaired oxygen uptake kinetics at exercise onset after stroke

Background: Oxygen uptake (V˙O₂) kinetics at the onset of exercise is slower in patients with stroke than in healthy adults. However, little is known about the cardiorespiratory mechanisms underlying the impaired V˙O₂ kinetics. Objectives: This study aimed to investigate the relative effect of impaired oxygen delivery and utilisation on V˙O₂ kinetics at the onset of submaximal exercise in patients with stroke by comparing the time constants of cardiac output (τCO) and V˙O₂ (τV˙O₂). In addition, we aimed to examine the association between the kinetics of cardiorespiratory variables and functional outcomes. Methods: We included 21 patients with stroke (15 males, mean [SD] age 58.7 [9.5] years, mean days post-stroke 67.9 [30.9]). A submaximal constant-load exercise test was performed to measure τV˙O₂, τCO, and the time constant of arterialvenous oxygen difference (τAVO₂diff). The ratio of τCO to τV˙O₂ was calculated to assess the matching of oxygen delivery and consumption. Fugl–Meyer lower-extremity motor scores, comfortable gait speeds, and Functional Independence Measure motor scores were used as functional variables. Results: Mean (SD) τAVO₂diff was markedly shorter than τV˙O₂ and τCO (26.1 [7.1] vs. 38.7 [10.2] and 46.6 [23.2 s], P < 0.05), with no significant difference between τV˙O₂ and τCO (P = 0.444). The greater ratio of τCO to τV˙O₂ was related to poorer motor function (rho = −0.484, P = 0.026) and slower comfortable gait speed (r = −0.482, P = 0.027). Conclusions: An increase in CO was slower than that in AVO₂diff in patients with stroke. Therefore, V˙O₂ kinetics in patients with stroke appears to be affected by a delayed increase in CO rather than AVO₂diff. Furthermore, these patients with motor and gait impairments may have a poor matching of oxygen delivery and consumption during exercise onset.