BC Biomedical NIBP-1010 Bedienungsanleitung PDF herunterladen (Seite 9)

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Apparatusforbloodpressuremeasurements

 The basic concept of blood pressure measurements has remained the same from

historical apparatus to the new automatic systems. The entire set consists of a sleeve to be

placed above the patient’s elbow, or eventually on the thigh or the wrist, of a pressure gauge

(a manometer or an electronic pressure sensor), of a balloon with air relief valve, or of a

compressor for inflating the sleeve; in the electronic systems eventually also of electronic

circuits guaranteeing the running of the gauge and the safety valves and escape valves, which

deflate the sleeve quickly after the measurement.

The original tonometers, or else the mercury tonometers, only contained the bare

elements for the measurement itself, namely: pressure sleeve (cuff), inflatable balloon with an

air relief valve, and a manometer in the form of a mercury column in a tube. Mercury was

selected rather intentionally, as it kept its physical properties over time (density, viscosity,

surface tension). The principle of such measurement is very simple. Think of an U-shaped

tube, which is partially filled with mercury in accordance with the function (1.1), where p is

hydrostatic pressure, h is height of the column, g is gravitation constant of normal

gravitational acceleration, and

is density of the substance.

ghp ..

(1.1)

Establishing this function gives us the result that 1 mm of mercury column

corresponds to the pressure of 133.322 Pa. Of course in the technical practice the unit Pa is

more common, or eventually its multiples kPa, MPa, but in case of blood pressure, the

description uses millimetres of mercury column, also marked as Torr. This mark was named

after the Italian physicist J. E. Torricelli. Yet let’s return to the tonometers. These mercury

tonometers were, and even today still are considered as the golden standard. They are stable

over time, they are easy to use, but mercury presents a problem due to its toxicity. They are

therefore replaced by other types of apparatus. Mercury tonometer is depicted in Figure 1.1.

Among other types of instruments, quite commonly used, there are the so called

aneroid tonometers. The arrangement is identical to the mercury tonometers, but mercury

column is not used for the pressure measurement. A manometer is used in this case, with a

deformation spring. There is a disadvantage compared to mercury, and it is the fact that the

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BC Biomedical NIBP-1010 Bedienungsanleitung Seite 9