This is a real concern with open heart surgery. You need to find a surgeon that will use retrograde perfusion; this keeps blood flowing to your brain during surgery and the risks of cognitive impairment are far less.
Even in the same practice, not all surgeons know how to do this. We believe that it is very very important to do this.
Go to pubmed and search for articles on "cerebral perfusion in aortic aneurysm surgery." This will provide you with a list of surgeons that use this technique and also will provide information on questions to ask.
I asked one of my earlier husband's potential surgeons "what happens if you don't get the repair done in the planned 20 minute cooling timefrane." He had no answer. He laughed actually. We looked for (and found) another surgeon although this guy had a great reputation.