Measuring Pressure in Medicine (Blood Pressure)


Measuring blood pressure can either be direct(invasive) or indirect.

1 ) Direct methods: Mostly used to measure the pressure within the heart.

  • A thin flexible probe is inserted directly into the blood vessel. Its opposite end is usually connected to a transducer (capacity or piezoelectric), but it is also possible to introduce the transducer directly into the vessel (rarely used because it is dangerous).

2 ) Indirect methods:

The Riva-Rocci Method1 is a method in which an inflatable rubber cuff placed around the patient’s arm, 3-5 cm from the elbow (around A. Brachialis). One end of the cuff is attached to a mercury manometer (Bourdon tube gauge).

220px-Sphygmomanometer&Cuff

  • The Riva-Rocci Method is based on hearing Korotkoff’s sounds with a stethoscope.

Korotkoff’s sounds 2.

  • The Riva-Rocci Method can be automised – systolic and diastolic pressures are measured by a machine, stored in memory, and analysed later. This is called the Halter Monitoring.

220px-HolterAFT1000

  • Oscillotonometry isa method that measures the oscillations in the cuff directly.

Schiotz tonometer 3 is a device used in ocular tonometry, a non-invasive method of measuring the blood pressure in the eye. Similar, but more simplistic, are applanation tonometers – which also rely on interpreting the mechanical deformation of the eye.

  • Catherters have been used to measure intrapulmonary pressure (by monitoring oscillations at atmospheric pressure, as the patients performs breathing. They have also been used experimentally to measure intrapleural pressure.
  • Neurologists measure liquor pressure (the pressure of the cerebrospinal fluid in the brain hemispheres and the spinal canal) by inserting a hollow puncture needle – connected to a manometer – into the spinal canal (It is inserted either suboccipitally or between L3 and L4).

Notes compiled by Andrei Cociug.


  1. A sphygmomanometer (/ˌsfɪɡmməˈnɒmɨtər/ SFIG-moh-mə-NOM-i-tər), blood pressure meter, blood pressure monitor or blood pressure gauge (also referred to as a sphygmometer[1]) is a device used to measure blood pressure, composed of an inflatable cuff to collapse and then release the artery under the cuff in a controlled manner,[2] and a mercury or mechanical manometer to measure the pressure. It is always used in conjunction with a means to determine at what pressure blood flow is just starting, and at what pressure it is unimpeded. Manual sphygmomanometers are used in conjunction with a stethoscope
  2. If a stethoscope is placed over the brachial artery in the antecubital fossa in a normal person (without arterial disease), no sound should be audible. As the heart beats, these pulses are transmitted smoothly via laminar (non-turbulent) blood flow throughout the arteries, and no sound is produced. Similarly, if the cuff of a sphygmomanometer is placed around a patient’s upper arm and inflated to a pressure above the patient’s systolic blood pressure, there will be no sound audible. This is because the pressure in the cuff is high enough such that it completely occludes the blood flow. This is similar to a flexible tube or pipe with fluid in it that is being pinched shut. If the pressure is dropped to a level equal to that of the patient’s systolic blood pressure, the first Korotkoff sound will be heard. As the pressure in the cuff is the same as the pressure produced by the heart, some blood will be able to pass through the upper arm when the pressure in the artery rises during systole. This blood flows in spurts as the pressure in the artery rises above the pressure in the cuff and then drops back down beyond the cuffed region, resulting in turbulence that produces an audible sound. 
  3. Schiøtz tonometer is an indentation tonometer, used to measure the intraocular pressure (IOP) by measuring the depth produced on the surface of the cornea by a load of a known weight. The indentation of corneal surface is indirectly proportional to the IOP. 
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