Yet, after wrapping a flexible cuff around your arm and pumping it up, your doctor frowns and gives you the bad news:you are one of the roughly 60 million Americans who have high blood pressure. Because it can lurk in the body without your knowing it, high blood pressure has been called the silent killler. (47) (48) Depending on how high your pressure is, how you live and what changes you're willing to make in your lifestyle, treatment can be tailored specifically to you. (49) The first, or top, number is the systolic reading, the pressure of blood against artery walls when the heart beats—the period of highest tension. The second, or bottom, number is the diastolic—the minimum pressure, between beats, when the heart is filling with blood and artery walls are relaxed. If the diastolic goes over 105 and stays there through several readings, that's the beginning of the danger zone, moderate hypertension, and continued monitoring is essential. Once the diastolic rises above 115, the condition is severe. A diastolic between 90 and 104 falls into the gray area of 'mild' hypertension. (50)