High Blood Pressure By The Numbers

Glenn Gandelman, MD, MPH Health Pro November 13, 2006
  • Most visits to the doctor’s office include a measurement of blood pressure. But what do the blood pressure numbers really mean?

    Blood pressure is usually measured in the upper arm with a cuff wrapped tightly around the arm and inflated with air. The amount of air pressure inside the cuff is related to the pressure of blood inside the arms’ arteries. When the heart beats it sends a wave of pressure through the body’s arteries called a pulse. This pulse can be felt in your neck, wrist, and many other spots. The pulse can also be heard with a stethoscope.

    As the blood pressure cuff fills with air the cuff pressure eventually becomes higher than the blood pressure in the arm. At this point the pulse cannot make it past the tight cuff and nothing is heard with the stethoscope positioned below the cuff. As the pressure is slowly released a monitor indicates the pressure inside the cuff. When a pulse is heard the number on the monitor is recorded as “millimeters of mercury or mmHg.” This “top number” is called the systolic blood pressure. It corresponds to the pressure in the arteries when the heart is squeezing.

    If the systolic blood pressure is high, the heart has to squeeze harder to send the pulse through the body. In those with high blood pressure the cuff has to be inflated with more air to determine the top number.

    As the blood pressure cuff continues to deflate, the pressure inside falls and the arteries in the arm allow the pulse to flow freely. Eventually, the pressure in the cuff falls to a point where it becomes difficult to hear the pulse as it passes by. The pressure indicated on the monitor is call the “bottom number,” or diastolic blood pressure. This is the pressure in the arteries when the heart is relaxing, or filling with blood and getting ready for the next squeezing cycle.

    Both the systolic and diastolic blood pressure are important numbers. If either is too high, the heart can enlarge and thicken due to the extra strain. Recent publications indicate that truly normal blood pressure is a top number less than 120 mmHg and a bottom number less than 80 mmHg. A slightly elevated blood pressure, top number between 120 and 140 and bottom number between 80 and 90, are considered pre-hypertensive.

    I recommend such patients watch their diet including lowering salt intake, losing weight, exercising and getting regular blood pressure checks. Those with blood pressure of 140/90 and higher will often require medication in addition to the lifestyle changes mentioned above. Of note, in those with diabetes or kidney disease the cut off number is lower at 130/80.

    The recommendations above should be discussed with your doctor prior to starting or changing any blood pressure treatment.