Impact of Cardiopulmonary Bypass Flow on Cerebral Autoregulation

Cerebral autoregulation is defined by the capacity of the brain to maintain a constant cerebral blood flow (CBF) despite variations of arterial pressure. However, when the arterial pressure is below a critical threshold, cerebral blood decreases.

This critical threshold is called the lower limit of cerebral autoregulation (LLA).

Cardiopulmonary bypass is a unique environment wherein systemic blood flow is totally controlled by the cardiopulmonary bypass pump. High pump flows combined with low arterial pressures has been shown to not compromise neurologic postoperative outcomes.

Our hypothesis is that that LLA may depend on the cardiopulmonary bypass flow, ie the LLA may decrease when the cardiopulmonary bypass flow increases, explaining why low arterial pressure may be well tolerated.

Cerebral tissue perfusion is partly ensured by cardiac output and arterial pressure. In cardiac surgery, and especially during bypass surgery, one of the main objectives is to maintain sufficient blood flow and an optimal mean arterial pressure (MAP) to limit the deleterious consequences of organ hypoperfusion. To have a physiological regional organ blood flow, some organs, such as the brain, need a minimum MAP threshold (the lower limit of autoregulation, LLA) to ensure a constant cerebral blood flow (CBF). This is known as the cerebral autoregulation (CA). As the brain is at high risk of ischemia and definitive functional consequences in case of cerebral hypoperfusion, maintaining MAP above the LLA of the brain limit the deleterious postoperative neurological outcome.

CA can be determined by continuously calculating the correlation between MAP and CBF. CBF is assessed by continuous monitoring of the middle cerebral artery velocity (mV). moving Pearson correlation coefficient between 30 consecutive, paired MAP and cerebral blood flow velocity values will be calculated to generate the mean velocity index (Mx). The Blood pressure in the autoregulation range is indicated by an Mx value that approaches zero (there is no correlation between flow velocity and MAP), whereas an Mx approaching 1 indicates dysregulated cerebral blood flow (flow velocity and MAP are correlated). A value of 0.4 is accepted as the threshold of CA corresponding to the LLA.

During cardiopulmonary bypass (CPB), MAP is physiologically related to bypass flow and systemic vascular resistance. The adjustment of MAP during CPB is therefore achieved by the administration of vasoconstrictors or vasodilators as well as by the variations of the pump flow.

This research assumes that a change in the pump flow rate will lead to a change in the LLA. Therefore, if the flow rate is higher, it is possible that the LLA will be lower.

In this randomized trial, two different rates of pump flow will be compared with respect to LLA testing.

Regional brain oxygen saturation, assessed by near-infrared spectroscopy will also be monitored during surgery.

Eligibility Criteria:
Inclusion Criteria:

* Patient, male or female, over the age of 18
* Patient scheduled to undergo valvular heart surgery with planned CPB
* Patient with a Euroscore below 7%
* Patient affiliated or entitled to a social security scheme
* Patient having received informed information about the study and having signed a free and informed consent to participate in the study

Exclusion Criteria:

* Patient with untreated or uncontrolled severe hypertension despite treatment
* Patient with chronic renal failure, with glomerular filtration \< 30 mL/min/1.73m² or requiring a kidney transplant
* Patient with left ventricular ejection fraction \< 40%
* Patient with a history of ischemic stroke
* Patient having or about to benefit from renal vascular surgery
* Patient with preoperative sepsis
* Patient who required a norepinephrine infusion within 24 hours before surgery
* Patient presenting with an inaccessible temporal Doppler window
* Patient candidate for emergency surgery
* Pregnant, parturient or breastfeeding woman
* Patient with preoperative uni or bilateral carotid stenosis \> 50%
* Protected patient: adult under guardianship, curators or other legal protection, deprived of liberty by judicial or administrative decision