A major head injury can cause an increase in intracranial pressure (ICP), which can lead to an increase in systolic blood pressure. This is because the body is trying to maintain adequate blood flow to the brain despite the increased pressure. The body does this by increasing the force with which the heart pumps blood, which increases the systolic blood pressure. Therefore, an increase in systolic blood pressure can be a sign of increased intracranial pressure and should be carefully monitored in patients with major head injuries.