Cerebrovascular regulation during increases in systemic blood flow and systemic vascular resistance

It is unknown whether, under anesthesia, an increase in blood pressure by augmenting cardiac output would exert a different effect on brain perfusion compared with a rise in blood pressure by elevating systemic vascular resistance. In this study, we disentangled these two parameters by monitoring changes in middle cerebral artery blood velocity (MCA V_mean) and frontal cerebral lobe oxygenation (cO₂Hb) in 15 patients on cardiopulmonary bypass (CPB) during two experiments. First, we increased systemic vascular resistance, using 100 μg phenylephrine, while CPB flow remained clamped. Second, we increased CPB flow while s-stemic vascular resistance remained unmodified. Mean arterial pressure increased 12 ± 4 mmHg and 11 ± 5 mmHg after raising systemic vascular resistance and cardiac blood flow, respectively (P ¼ 0.60). Systemic vasoconstriction with a constant cardiac blood flow increased the MCA V_mean (17%, P ¼ 0.001) while the cO₂Hb declined (-2.2%, P ¼ 0.005). Augmenting cardiac blood flow increased both MCA V_mean and cO₂Hb (17%, P < 0.001, and 2.5%, P ¼ 0.02) concordantly. Cerebral perfusion seemed to be dependent on cardiac blood flow, since both cO₂Hb and MCA V_mean changed concordantly with an increase in cardiopulmonary bypass flow. A systemic vascular resistance-mediated blood pressure increase with the a₁-adrenergic agonist phenylephrine had a comparable rise in MCA V_mean but a discordant effect on cO₂Hb, presumably due to both cutaneous and cerebral vasoconstriction. NEW & NOTEWORTHY Elevating blood pressure by increasing cardiac blood flow (Q) ^_ or by increasing systemic vascular resistance (SVR) results in a comparable increase in middle cerebral artery blood velocity. In contrast, although frontal cerebral lobe oxygenation increased with increasing Q, ^_ it decreased with increasing SVR. This difference was less pronounced when Q^_ was kept constant, suggesting that part of the decrease may be attributed to an SVR-induced decrease in Q.