The relationship between cardiac output and dynamic cerebral autoregulation in 1 humans

32 Cerebral autoregulation adjusts cerebrovascular resistance in the face of changing 33 perfusion pressures to maintain relatively constant flow. Results from several studies 34 suggest that cardiac output may also play a role. We tested the hypothesis that cerebral 35 blood flow would autoregulate independent of changes in cardiac output. Transient 36 systemic hypotension was induced by thigh cuff deflation in 19 healthy volunteers (7 37 females) in both supine and seated positions. Mean arterial pressure (Finapres), cerebral 38 blood flow (TCD) in the anterior (ACA) and middle cerebral artery (MCA), beat-by-beat 39 cardiac output (echocardiography) and end-tidal CO2 were measured. Autoregulation was 40 assessed using the Autoregulatory Index (ARI) defined by Tiecks et al. Cerebral 41 autoregulation was better in the supine position in both the ACA (Supine ARI: 5.0 ± 0.21 42 (mean±SEM), Seated ARI: 3.9 ± 0.4, p=0.01) and MCA (Supine ARI: 5.0 ± 0.2, Seated 43 ARI: 3.8 ± 0.3, p=0.004). In contrast, cardiac output responses were not different 44 between positions and did not correlate with cerebral blood flow autoregulatory indices. 45 In addition, females had better autoregulation in the ACA (p=0.046) but not the MCA, 46 despite having the same cardiac output response. These data demonstrate cardiac output 47 does not appear to affect the dynamic cerebral autoregulatory response to sudden 48 hypotension in healthy controls, regardless of posture. These results also highlight the 49 importance of considering gender when studying cerebral autoregulation. 50 51