James L. Rudolph, acting clinical chief of the division of aging at Brigham and Women's Hospital, provides an answer:
To date, no study has adequately examined whether heart surgery can change a person's personality, mainly because personality is difficult to define and measure. When recovering from heart surgery, some patients report trouble remembering, slower mental processing and difficulty focusing. Although this condition, often referred to as “pumphead,” is usually short-lived, one study of bypass patients has suggested that the associated cognitive changes might worsen over time. Related research, however, indicates it is unlikely that cardiac surgery significantly alters how the brain works.
Coronary artery bypass surgery, the most common heart operation in adults, helps to increase blood flow to the cardiac muscle when the heart arteries have become too narrow. During the surgery, a patient's brain is subjected to many stressors, which may include medications, sleep deprivation, inflammation and blood clots. Normally the brain is protected from such assaults by the blood-brain barrier, a membrane that walls off the organ from the bloodstream. Yet when our arteries narrow, the function of the blood-brain barrier may be disrupted, allowing circulating substances to enter the brain. The brain's reaction to such influences is as complex and individualized as the brain itself.
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Still, many studies that have measured brain function, by evaluating such variables as cognitive performance and mood, before and after heart surgery have not found significant changes. For instance, in general, patients do as well or slightly better on cognitive tests one to three months after cardiac surgery, although any cognitive benefit is short-lived. When such patients are retested three years later, their cognitive test performance tends to be similar to that of patients with narrowed heart arteries.
A recent analysis of studies that measured depression before and after heart surgery found that the number of patients with depression decreased after surgery. This benefit, however, very likely is related to patients overreporting their depression symptoms just before surgery.
The current evidence does not to seem to support the idea that changes in cognitive ability or mood occur after cardiac surgery. Yet because no research directly looks at postoperative personality changes, we cannot say for certain that such alterations do not take place on a small scale. With further research we may develop a more nuanced understanding of how the brain responds under pressure.