Does increased oxygen extraction contribute to the improvement of VO2_peak observed in patients who exercise with increased LVAD speed?

We read with great interest the recent report of Jung et al.1 which contributes significantly to the understanding of exercise physiology in patients who are supported with LV support devices and provides encouraging information for the development of automated speed algorithms to increase exercise capacity. The main finding of this report is a 9.2% increase in VO2_peak in left ventricular assist device (LVAD) patients who exercised with the increased speed protocol. We may immediately conclude that this finding was due to an increase in cardiac output (CO). However, a previous publication from the same group2 using a similar protocol demonstrated that CO at maximal exercise was not different between the fixed speed (13.6 ± 2.5 L/min) and the increased speed (12.1 ± 3.6 L/min) protocols. The authors also noted a difference in CO during the initial phase of exercise (30 W, 8.1 ± 0.6 L/min vs. 8.7 ± 1.1 L/min). In addition, cerebral perfusion was higher in the increased speed protocol group. If CO was similar in both groups of the current investigation, the Fick equation (VO2_max = CO_max × O₂ extraction_max) would suggest an increase in oxygen extraction as the cause of the higher observed VO2_peak in the increased speed protocol and, if so, could this be related to the improved cerebral flow observed in this group?2 Could an increase in cerebral perfusion have favourably influenced autonomic reflexes and improved oxygen extraction?