bypass brain
n. Memory loss and reduced mental functioning after coronary bypass surgery.
Aides to Bill Clinton last week vehemently denied speculation that the former president's intemperate remarks on the campaign trail were due to mild cognitive damage from his quadruple-bypass surgery in 2004.

"This theory is false and is flatly rejected by President Clinton's doctors, who say he is in excellent shape. . . ." the statement said.

But the condition dubbed "pump head" or "bypass brain" has long been recognized by doctors, even if they seldom warn patients about it.

Symptoms include short-term memory loss, slowed responses, trouble concentrating and emotional instability. In a landmark study published in the New England Journal of Medicine in 2001, researchers at Duke University Medical Center tested 261 patients before and after bypass surgery and found that 53% of them had significant cognitive decline when they were discharged — and 42% still suffered from it five years later.
—Melinda Beck, “'Bypass Brain': How Surgery May Affect Mental Acuity,” The Wall Street Journal, June 10, 2008
More than half of heart bypass patients awoke with cognitive problems, ranging from a stroke to the inability to remember phone numbers, according to a 2001 study in the New England Journal of Medicine.

Though surgeons have worked to reduce the risks of brain damage, the expanding baby-boomer population leaves Galloway and others concerned that more operations on older patients will lead to increased numbers of people with "bypass brain."
—Robert Davis, “Doctors debate merits of brain-oxygen monitor,” USA Today, January 30, 2008
2006 (earliest)
And then there is the condition known as "bypass brain." That's where brain cells are killed due to the lack of oxygen to the brain during a critical part of the surgery.
—Wiliam Kelley Eidem, “Rebuttal to 'Quackwatch',”, September 04, 2006
As the Wall Street Journal citation mentions, bypass brain is also called pump head (a term that dates to 1996), because during a bypass the patient's blood is pumped through a heart-lung machine. It's suspected that this causes the brain to receive less oxygen than normal, thus causing the damage. Another theory is that the pump creates air bubbles, fat cells, and other particles that travel to the brain and clog the capillaries, essentially creating mini-strokes.