Elevated Cardiorespiratory Fitness is Associated with Blunted Cerebral Pressure‐Flow Relationships

There appears to be a “hemodynamic paradox” associated with elevated cardiorespiratory fitness; in that it has a differential impact on cerebral blood flow and orthostatic tolerance. A reduction in orthostatic tolerance with high cardiorespiratory fitness may be related to a blunted cerebral pressure‐flow relationship during rapid changes in blood pressure, and not only a consequence of inadequate cerebral perfusion. However, research on how cardiorespiratory fitness influences cerebral pressure‐flow relationships during rapid changes in blood pressure, and the consequences for orthostatic tolerance remain equivocal. Mean arterial pressure (MAP; finger photoplethysmography) and middle cerebral artery flow velocity (MCAv; transcranial Doppler ultrasound) were continuously monitored in 19 male endurance athletes (V̇O 2 max: 56±5 mL·kg −1 ·min −1 ) and 8 inactive/moderately active control participants (39±5 mL·kg −1 ·min −1 , P<0.01). Orthostatic tolerance was assessed by the prevalence of presyncope during a 20‐min 70° head‐up tilt. Cerebral pressure‐flow relationships were assessed by: [1] the time delay before onset of regulation (increase in the cerebrovascular conductance index (MCAv mean /MAP)) and [2] rate of regulation, after transient hypotension induced by bilateral thigh‐cuff release and sit‐to‐stand, and transfer function analysis (TFA) of blood pressure and MCAv responses during [3] spontaneous and [4] forced oscillations induced via 5‐min of repeated squat‐stand maneuvers at both 0.05 and 0.10 Hz. Prevalence of presyncope was not different between groups (p=0.41). The time delay before onset of regulation tended to be longer in athletes compared to controls after thigh‐cuff deflation (p=0.11) and prolonged after sit‐to‐stand (3.2 ± 1.6 vs. 1.4 ± 1.1 s; p=0.01), and the latter correlated with V̇O 2max (r=0.46, p=0.03). Rate of regulation was similar between groups irrespective of the technique employed. Athletes had lower TFA phase in the low frequency (p<0.01) range and higher TFA gain during 0.1 Hz squat‐stand compared to controls (p=0.01). Metrics describing cerebral pressure‐flow relationships did not correlate with orthostatic tolerance. These results indicate an attenuated ability of the cerebral vasculature to react to slow rhythmical oscillations as well as rapid and large changes in blood pressure in fit individuals. However, these functional cerebrovascular adaptations do not influence orthostatic tolerance. Support or Funding Information The present study was funded by the Ministère de l'Education, du Loisir et du Sport du Québec, and the Foundation of the Institut universitaire de cardiologie et de pneumologie de Québec