A pheochromocytoma can cause large changes in blood pressure. When that happens, it's called a spell. Blood pressure may return to a healthy range between spells. That can make it harder to diagnose a pheochromocytoma. The graph shows a nine-day period of short, irregular bursts in blood pressure due to a pheochromocytoma. The lower points show the bottom number of the reading, called diastolic pressure. The higher points show the top number of the reading, called systolic pressure. For example, the first burst in blood pressure is seen on day two with a reading of 250/110 millimeters of mercury.