When you go on bypass(needed if the heart is stopped to replace a valve) air emboli (very small air bubbles) go into the brain and can get lodged in capillaries

When you go on bypass(needed if the heart is stopped to replace a valve) air emboli (very small air bubbles) go into the brain and can get lodged in capillaries cutting of blood flow. Personality changes are documented to occur after bypass and correlate with areas of the frontal cortex that die. Typically though this kind of injury is not progressive.