Measurement of blood pressure

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Measurement of blood pressure – preparative page for educational slides designed for medical practice, with some chunk of theory introduction.

This page is created paralell with this Czech one: cs: Měření krevního tlaku


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Units of pressure

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SI units

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The metric system: The International System of Units (French: "le Système international d'unités")

  • Pa = the main unit SI which should be used
  • Pa is pronouncated as "Pascal" = the name of French physician
  • definition:
    • 1 Pa = 1 N/m2 = 1 kg.m-1.s-2

where N is Newton as main unit of force:
1 N = 1 kg.m.s-2

Traditional units

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  • mmHg = the height of the mercury column which has this hydrostatic pressure on its bottom
  • 1 mmHg = 1 torr (It should be noted that , after 1 torr was redefined as 1/760 of atmospheric pressure, 1 mmHg differs from 1 torr by less than 0.000015%)
    (name of the unit becomes from the name of Italian physicist Toricelli)

Barometric pressure

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It is good idea to memorize: 750 mmHg = 750 torr = 100 kPa = 1 bar which is near of the normal barometric pressure

and then:

  • 1 torr = 100/750 kPa = 0.133 kPa
  • 1 kPa = 750/100 torr = 7.5 torr

The normal diastolic pressure is about 10-times lower: 75 mmHg = 75 torr = 10 kPa

Pressure of the atrial blood

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  • It has no constant value, but it is rather a function of time.
  • It changes it's value between two extremes during the cardiac cycle:
    • Diastolic pressure = the lower pressure
    • Systolic pressure = the higher pressure

An example for a healthy resting human, written as:

  • 110/75 torr (spoken as 110 over 75), means:
    • systolic pressure: 110 torr
    • diastolic pressure: 75 torr
  • Intervals for normal values:
    • systolic pressure: 100-145 torr
    • diastolic pressure: 60 – 85 torr
  • Lower values are symptoms of Hypotension
  • Higher values are sumptoms of Prehypertension or Hypertension
  • Pressure amplitude = the difference between systolic and diastolic pressure
 A = Psyst - Pdiast
  • Mean arterial pressure (MAP) may be estimated as:
 MAP ~ (Psyst/3) +  (2 . Pdiast/3)

Methods of measurement

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  • Invasive (bloody) methods – used in surgery, experimental medicine etc.:
    The canulla inserted into blood vessel is connected with the manometer
  • Non-invasive (indirect) methods – commonly used:
    • Manual methods:
      • Auscultatory method
      • Palpation method
      • Oscillometric method
    • Automatic and half-automatic (semi-automatic) methods

Auscultatory method

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  • Sphygmomanometer = clinical device consisting of:
    • mercury manometer
    • inflatable cuff to restrict blood flow
    • inflation bulb with a valve
  • stethoscope for auscultation


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  • the cuff is placed around the arm
  • the cuff is inflated by the air → the brachial artery under the cuff is pressed
  • the stetoscope is placed below the cuff
  • the next situations can distinctguished:
    • if the pressure in the cuff is higher than the systolic pressure then:
      the artery is totally occluded, no blood can flow thru – no sound is heard
    • if the pressure in the cuff is lower than the diastolic pressure then:
      the artery is not occluded, all blood can flow thru as normally, during the whole heart period – no sound is heard
    • if the value of the pressure in the cuff lies between the diastolic and the systolic pressures then:
      the artery is partially occluded → the blood flow is disturbed and periodically interrupted – the blood flows not during the whole cardiac cycle, but only during the systolic interval → some sounds (Korotkoff sounds) are heard


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  1. The subject of measurement is seated with the arm supported
  2. Place the cuff snugly but not too tightly around the upper arm.
  3. The valve on the bulb is closed
  4. Inflate the cuff rapidly by means of rubber bulb: the raising pressure is observed on the manometer
  5. Stop pumping the air when the pressure is a little higher then the supposed systolic pressure. (When supposing subject with normal blood pressure then the pressure of 150 torr in the cuff is sufficient.)
  6. Place the stetoscope ower the brachial artery at the elbow. No sound should be heard now.
  7. Release the valve on the bulb gently so the mercury column will fall slowly, observe it attentively.
  8. Simultaneously listen carefully until you hear the specific Korotkoff sounds (in the rhytm of heart beating); keep in mind the value of the appropriate pressure at the timepoint when this sounds was starting.
  9. Leave the mercury column fall further listening the sounds simulteneously.
  10. Carefully observing the mercury column you will see it falls down not quite smoothly but rather a little convulsively. (This phenomena can by used by oscillometric method.)
  11. Keep in mind the value of the appropriate pressure at the timepoint when the sounds vanished.
  12. You can release the valve wholly, blow off the air from the cuff.
  13. Write down both high (systolic) and low (diastolic) pressure keeping in your mind.
  14. The measurement is done but it should be repeated in doubt.

Half-automatic method

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  1. Principle: as oscillometric method mentioned above.
  2. Automatic methods use engine air-pumpe to inflating the cuff.
  3. The half-automatic methods use the standard rubber bulb.
  4. Read the manufacturer instruction carefuly before using such device! The basic principle may be similar but some operational steps may differ!
  1. Take the measurements of the blood pressure each other using the auscultatory method
  2. Take this measurement on both left and right arms; the difference should not be greater than 10 torr.
  3. Calculate the appropriate amplitudes and values of the mean pressure.
  4. Calculate the heart frequency palpating the pulse on a. radialis
  5. Verify your measurements with the digital sphygmomanometer
  6. The examined subject makes 20 squats rapidly in 1 minute.
  7. The new measurement of heart frequency and blood pressure will be made immediatelly after the exertion. (The measurement of heart frequency and blood pressure may be taken simultaneously using both arms by two examinators.)
  8. Discuss the results of measurement, compare it with the norm, compare values between the members of your group, make conclusion.
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