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 <i>V</i>_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 systemic 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 (<i>P</i> = 0.60). Systemic vasoconstriction with a constant cardiac blood flow increased the MCA <i>V</i>_mean (17%, <i>P</i> = 0.001) while the cO₂Hb declined (-2.2%, <i>P</i> = 0.005). Augmenting cardiac blood flow increased both MCA <i>V</i>_mean and cO₂Hb (17%, <i>P</i> < 0.001, and 2.5%, <i>P</i> = 0.02) concordantly. Cerebral perfusion seemed to be dependent on cardiac blood flow, since both cO₂Hb and MCA <i>V</i>_mean changed concordantly with an increase in cardiopulmonary bypass flow. A systemic vascular resistance-mediated blood pressure increase with the α₁-adrenergic agonist phenylephrine had a comparable rise in MCA <i>V</i>_mean but a discordant effect on cO₂Hb, presumably due to both cutaneous and cerebral vasoconstriction.<b>NEW & NOTEWORTHY</b> Elevating blood pressure by increasing cardiac blood flow ([Formula: see text]) 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 [Formula: see text], it decreased with increasing SVR. This difference was less pronounced when [Formula: see text] was kept constant, suggesting that part of the decrease may be attributed to an SVR-induced decrease in [Formula: see text].