Influence of Variations in Systemic Blood Flow and Blood Pressure on Cerebral Oxygenation Measured by Cerebral Oximetry

Cerebral autoregulation is defined as the whole of regulatory mechanisms that maintains a constant cerebral blood flow (CBF) during changes in cerebral perfusion pressure (CPP). Cerebrovascular resistance adjusts when CPP changes in order to keep CBF constant. In contrast with this concept of pressure-mediated autoregulation, it is suggested that cerebral autoregulation is focused on maintaining homeostasis of the cerebral metabolic rate of oxygen (CMRO2). We assume that both flow and pressure contribute to the regulation of CMRO2.

Assessment of cerebral oxygenation - Cerebral oxygen saturation will be monitored with near-infrared spectroscopy (NIRS). NIRS allows simple, continuous and non-invasive measurement of cerebral oxygen saturation (ScO2)and primarily cerebral venous saturation. Cerebral oxygen saturation will be monitored with a FDA-approved devices: INVOS 5100 (Somanetics Corporation, Troy, MI, USA).

To study the effects of changes in pressure and in flow, we need a condition where we can alter these variables separately and in a controlled manner. Therefore this study will be performed during cardiopulmonary bypass (CPB). Pressure will be varied with the administration of routinely used vasoactive substances, while flow will be varied by altering the pump flow manually. We hypothesize that if we change one parameter (pressure or flow), a compensatory mechanism will preserve the CMRO2 homeostasis, with no change in ScO2. On the other hand, if we change both pressure and flow, we expect a significant effect on cerebral oxygen saturation. With 20 % changes in pressure and/or flow, we expect a change in NIRS values of approximately 5 %. Previous studies showed that this kind of reduction is well tolerated by the brain. This means that the proposed changes are within the normal physiological range, and will have no adverse effects.