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 Vmean) 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 (P = 0.60). Systemic vasoconstriction with a constant cardiac blood flow increased the MCA Vmean (17%, P = 0.001) while the cO2Hb declined (-2.2%, P = 0.005). Augmenting cardiac blood flow increased both MCA Vmean and cO2Hb (17%, P < 0.001, and 2.5%, P = 0.02) concordantly. Cerebral perfusion seemed to be dependent on cardiac blood flow, since both cO2Hb and MCA Vmean 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 Vmean but a discordant effect on cO2Hb, presumably due to both cutaneous and cerebral vasoconstriction.NEW & NOTEWORTHY 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].