Blood Pressure and Cardiac Output - the influence on cerebral perfusion during cardiopulmonary bypass.

A jeopardized cerebral blood flow (CBF) for a few minutes* results in
irreversible cerebral tissue damage. Therefore, the CBF is autoregulated
implying that CBF remains constant despite changes in cerebral perfusion
pressure. Since the 1970s, Near Infra-Red spectroscopy (NIRS) derived frontal
lobe oxygenation (rSO2) and flow velocity of the middle cerebral artery (Vmca)
measured by transcranial doppler (TCD) can be used continuously and
non-invasively, as a derivative for CBF. These monitoring techniques are used
extensively and in the case of a decreasing rSO2, various algorithms have been
proposed to optimize rSO2. One of the first steps in the algorithm is to
increase blood pressure by administrating an α1-receptor mediated vasopressor
like phenylephrine. However, several studies recently described that the rSO2
decreased after phenylephrine administration. It remains unknown why rSO2
decreased, but it could be a result of the α1-receptor mediated decline in
cardiac output (CO). This hypothesis is supported by the finding that in
patients on CPB, i.e. with constant CO, the rSO2 only decreased with 3% after
administering phenylephrine in comparison to a 10% decline in rSO2 in
non-cardiac surgery. It therefore still remains unclear whether rSO2 is (more)
dependent on blood pressure or on CO. Therefore our aim is to determine whether
cerebral perfusion is most dependent on cardiac output or on mean arterial
pressure in patients undergoing cardiac surgery with the use of CPB.

To study the influence of systemic blood flow and mean arterial pressure on
cerebral blood flow in patients undergoing on-pump cardiac surgery.

Cross-over design

5 interventions which will include raising mean arterial pressure (MAP) by
administering 100µg phenylephrine, increasing and decreasing cardiac output by
adjusting roller pump flow or a combination of the previous.

Interventions pre-cardiopulmonary bypass
(1) An increase in MAP of approximately 20 mmHg by administrating 100µg
phenylephrine.

Interventions during cardiopulmonary bypass
Modifications in blood pressure:
(2) An increase in MAP by administrating 100µg phenylephrine while maintaining
roller pump flow constant.

Modifications in systemic flow:
(3) A modification of roller pump flow to achieve the CO measured at
intervention (1) meaning: preCPB after the bolus of 100µg phenylephrine.
(4) A modification of roller pump flow as in (3) together with a bolus of 100µg
phenylephrine to increase MAP.
(5) An increase in roller pump flow aiming for a 20 mmHg increase in MAP.

The burden and risks of participating in this study are neglible because all
modifications will be in the physiological range. Phenylephrine used in this
study will be used within its indication and is approved by the Food and Drug
Administration (FDA).
There will be no time burden since all the investigations can be done during
cardiopulmonary bypass time. There will be no follow up.