Generally, doctors do not diagnose a patient with high blood pressure based on one set of readings, because there are many factors that can artificially raise a patient's blood pressure. A patient may have rushed to the doctor's office and may still have high adrenaline levels. Seeing a new health care provider for the first time can make a patient nervous and raise blood pressure. "White coat hypertension" is experienced by some patients whose blood pressure is higher when measured by the doctor than when measured by another health care provider.
Measuring Blood Pressure
The most precise way to measure blood pressure is to place a small tube or catheter into an artery and directly measure the pressures. Usually, a much simpler method is employed. The health care provider places a blood pressure cuff around the upper arm and inflates the cuff with air. As the cuff becomes filled with air, the pressure in the cuff increases, eventually cutting off the flow of blood through the arteries in the arm. The health care provider slowly lets air leak out of the blood pressure cuff, causing the pressure in the cuff to gradually decrease. When the pressure in the cuff falls below the systolic blood pressure in the arteries, the provider begins to hear a characteristic thumping sound as blood starts to flow again in the arm. The blood pressure cuff continues to deflate, and when the pressure in the cuff falls below the diastolic blood pressure in the arteries, the characteristic thumping sound disappears. By listening for the beginning and termination of this sound and simultaneously watching the pressure gauge in the cuff when these events occur, the health care provider determines the systolic and diastolic blood pressure.
If the blood pressure is not measured correctly, the readings obtained may be artificially high. Several steps ensure that the measured blood pressure truly represents the patient's blood pressure:
- Patients should sit with their arms supported at heart level.
- Patients should not smoke or ingest caffeine for 30 minutes prior to blood pressure measurement.
- Patients should sit down for at least 5 minutes before blood pressure is measured.
- The bladder (inflatable part) of the blood pressure cuff should encircle at least 80% of the arm. A large cuff should be used for patients with thick arms.
- Two or more readings should be taken at least 2 minutes apart.