• Blood pressure BP is a frequently measured quantity, as it is a very good indicator of the state of the cardiovascular system.
• The contraction of the heart muscle creates a pressure force by which blood is pushed into the aorta and the pulmonary artery.
• Blood overcomes the resistance of the peripheral bloodstream.
• The farther the vessels are from the aorta, the less pressure there is in them.

Division of blood pressure


According to the place of measurement, we distinguish:

• Central BP - we measure it directly in the heart chambers. It can only be measured invasively. It is measured in both halves of the heart.
• Peripheral BP - we measure it on the limbs. Most often non-invasively using a cuff on the left upper limb. Here, the values are closest to arterial pressure.

Unit of blood pressure 


Blood pressure can be given as: 

• Absolute – defined as force acting per unit area
• Relative – defined as the difference between the absolute pressures in two environments.
• The internationally used unit of BP is mmHg (millimeter of mercury).
• BP is given relative to normal atmospheric pressure, which is 760 mmHg, i.e. 101.325 kPa.

  

Values of blood pressure 


Mean arterial pressure (MAP) is calculated using the empirical relationship: 

• Pressure of 120 / 80 mmHg is considered a normal value
• The pressure value depends on age 
• In children, 75 ÷ 80 mmHg
• At school age 110 ÷ 120 mmHg
• In men aged 65, it reaches 145 mmHg
• Diastolic pressure does not exceed 95 mmHg

  

Non-invasive methods of measurement of BP 

• It uses cuffs that restrict blood flow at the measurement site. Most often on the left arm (brachial artery).
• At the moment of suspension or restoration of blood flow, we compare the pressure in the cuff to the pressure in the bloodstream.
• The cuff including the entire device is outside the patient's body.
• The disadvantage of this method of measurement is that it cannot be measured continuously over time.
• Auscultation method - by listening with a stethoscope.
• Oscillometry - detection of oscillations of an inflated cuff.
• Sensing the movements of the arterial wall under the cuff.
• Plethysmography.
• Impedance rheography.
• Ultrasound detector using the Doppler effect.

Auscultation method 

• A sphygmomanometer and a stethoscope are needed for the measurement.
• In the cuff, the pressure is higher than the systolic pressure - the artery is choked.
• By gradually reducing the pressure in the cuff, sounds can be registered by listening to the artery - the so-called Korotkoff sounds

  

Korotkoff sounds  


They are divided into five phases:

• Phase I – First sound detected when cuff pressure drops just below SBP. The SBP value is relative to the first audible echo.
• Phase II – The sounds take on a murmuring character.
• Phase III – We hear an intensifying sound, which takes on the character of a thud as the pressure drops further.
• Phase IV – The sound suddenly becomes muffled and weak.
• Phase V – The DBP value is related to the complete disappearance of sounds.

  

Oscillometric method 


The volume pulsations in the arteries are transmitted through the cuff (closed system) to the device, where they are evaluated.

• To evaluate the amplitude of the oscillations, it is necessary to find the envelope of the corresponding signal:
  • Signal interpolation.
  • The first derivative of the signal.
• The most common use in electronic tonometers.

  

• The following are used as sensors:
  • Piezoresistive bridges. 
  • Capacitive pressure sensors.
• Automatic determination of the BP value (without the use of a stethoscope) by the oscillometric method.
• The measurement accuracy is less than that of classic tonometers.
• Suitable for home BP monitoring. 

• The pressure is measured using a suitable sensor (nowadays integrated semiconductor sensors) and an A/D converter.
• The pump is driven by a DC motor (tightness of the pump is important).
• An electromagnetic valve is used to quickly deflate the cuff.
• The control element is an integrated circuit with an 8-bit microcomputer.

  

Patient monitors – BP measurement 

• Pressure gauges are realized in the form of plug-in modules.
• Easy cuff replacement (newborn, adult…)
• Possibility of cycling in adjustable intervals.
• The cuff is permanently attached. To avoid slipping, it is constantly inflated to a certain low pressure (15 mmHg).

  

Mercury tonometers 

• The most reliable and widespread method of BP measurement.
• Hg used for its good physical properties.
• Pressure 120 mmHg ~ 155 cm of blood.
• Measurement by auscultation method using a stethoscope. 

Deformation manometers 

• Tonometers without the use of liquid.
• Elastic deformation of suitable pressure measuring elements occurs.
• The advantages are small dimensions, weight, sufficient precision and simplicity.
• They are less widespread than mercury tonometers.
• Measurement by auscultation method using a stethoscope.

  

Tonometers are important gauges 

• Tonometers are among the specified measuring devices that must be regularly verified (metrological operation).
• It is necessary to choose the cuff size correctly.
• As a result of use, tonometer components wear out and pressure leaks.
• Check of the tightness of the cuff is necessary - a common problem
• Tonometers are susceptible to mechanical damage (falls), beware of mercury (toxic material)
• The precision of electronic tonometers is affected by batteries (if battery-operated) 

Verification of tonometers 

• Classic and electronic tonometers are subject to a specification test (must meet EN 1060 – 1 and EN 1060 – 2 standards) and regular verification.
• Period for verification of mercury and deformation tonometers – 2 years.
• Period for verification of electronic tonometers – 1 year.
• Verification is carried out by accredited metrology labs. 

Invasive measurement of BP 

• The measurement takes place continuously, for a relatively long time.
• The measurement site is made accessible by inserting a catheter.
• The pressure sensor can be located:
  • outside the catheter and the patient's body (extravascular).
  • at the tip of the catheter in the bloodstream (intravascularí).
• Invasive measurement is most often performed:
  • In patients with circulatory disorders.
  • When examining haemodynamic parameters.

    

Swan-Ganz catheter 

• At the end of the catheter is an inflatable balloon. When introduced to the pulmonary artery, it is inflated using an injection and one of the channels. It is then carried away by the blood stream.
• One of the channels with an outlet at the end is intended for the measurement of blood pressure.
• The built-in thermistor is designed to measure cardiac output.
• An optical sensor for measuring SpO2 is also part of the equipment.

  

1. Ballon
2. Doppellumiger Katheter mit Rontgenmarkierungen
3. Druck-Messchip
4. Spritze fur Ballon
5. Anschlusse fur Messgerate
6. Anschlusse fur Messgerate  

Measurement using a liquid-filled catheter 

• The sensor is placed outside the patient's body.
• The liquid provides the transfer between the sensing point and the sensor.
• The sensor is placed in a special casing.
• A membrane is placed in the small chamber, which transmits the pressure to the semiconductor sensor itself.
• One inlet is for the supply of the catheter, the other for the container with the flushing solution. 
• Blood must not clot in the catheter and air must not enter the bloodstream.
• The catheter is continuously flushed with an intravascular solution that prevents blood clotting.
• The flow rate of the solution is very small (several ml/h).
• During measurement, the catheter and sensor must be at the same level as the measurement site, otherwise a measurement error occurs due to hydrostatic pressure.
• Equipment for this measurement is often part of vital signs monitors.