How to measure blood pressure with a mechanical BP monitoring device


Blood pressure is the pressure at which blood is flowing in the arteries. Blood pressure is written as systolic/diastolic. For example, 130/80 means systolic blood pressure is 130 mm of mercury and diastolic blood pressure is 80 mm of mercury. Normal average blood pressure is around 120/80. Systolic blood pressure depends on the cardiac output. This is the blood pressure in vessels when the heart pumps and diastolic blood pressure is due to the resistance or vascular tone in peripheral blood vessels.

Mercury blood pressure monitors are still the best. They are being removed from clinical practice because of environmental reasons and are replaced by hybrids and digital blood pressure measuring devices. Accurate reading with a mechanical blood pressure device depends on the person taking a reading, proper training, use of an accurate, well-maintained device, correct selection ,and positioning of the cuff. To take accurate blood pressure with a mechanical device one should take care of the following:

  • The patient should avoid smoke, alcohol or caffeine for at least 30 minutes before taking blood pressure. Taking alcohol, caffeine, and smoking causes an increase in blood pressure reading.
  • The patient should be seated comfortably for at least 5-10 minutes before taking blood pressure with his back supported and feet uncrossed on the floor. Diastolic blood pressure increases in a seated position, and crossing legs increase systolic blood pressure. Systolic blood pressure also increased in the laying down position.
  • Remove tight clothing from the arm on which blood pressure has to be taken. Tight clothing may cause a tourniquet effect.
  • Wrap the cuff on the upper arm one inch above the elbow joint at the level of heart. If the arm is below the level of the heart the reading will be higher. If above the heart level then the reading will be too low. Raise the arm on the pillow to bring it to the heart level while laying on the bed. (that is halfway between the bed and the chest)
  • Neither the patient nor the person taking blood pressure should talk and move unnecessarily while taking blood pressure.
  • The first and the last audible sound should be recorded as systolic and diastolic blood pressure respectively. Inflate the cuff till the radial pulse is impalpable. Note the pressure, this is a rough estimate of systolic blood pressure. Now inflate 20-30 mm more, and deflate slowly, 2-3 mm of mercury/second until regular heart sounds (called knockoff sounds) can be heard again. This is systolic blood pressure which should be measured to the nearest 2mm of Hg. A deflation rate more than this causes systolic blood pressure to appear lower and diastolic blood pressure to appear higher. Continue deflating, the sounds get muffled and then slowly disappear. The point at which sounds disappear is diastolic blood pressure.
  • Two readings should be taken with at least one minute interval in between them.
  • Blood pressure should be taken on both arms at least once. Normally there is a difference of 5mm of Hg, sometimes up to 10 mm. The difference of more than 15 mm of mercury is not normal and should be investigated further. Subsequent measurements should be taken on the arm that gave a higher reading.

Categories of Blood Pressure in Adults

According to American college of cardiology /American Heart Association guidelines for Hypertension 2017

BP Category Systolic BP(mmHg) Diastolic BP(mmHg)
NormalLess than120 andLess than 80
Elevated blood pressure 120-129 andLess than 80
High blood pressure
stage 1 Hypertension
130-139 or 80-89
High blood pressure
stage 2 Hypertension
140 or Higher or 90 or Higher
Hypertensive crisis
(seek medical attention immediately)
Higher than 180 and/orHigher than 120

Note: If systolic and diastolic blood pressure falls in two different categories then a higher blood pressure category is recorded.

July 19, 2022

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