Archive for the ‘Blood pressure machines’ Category

The proper name for a blood pressure machine is the SPHYGMOMANOMETER, rather a mouthful under any circumstances so normally called a BP machine, or sphygmo (pronounced Sfigmo).
There are three main types in use,
a. The mercury type.
b. The aneroid type.
c. The electronic type.

(Just as a little matter of interest, mercury is named after the Roman god Mercury, son of Maia and Jupiter, to whom he acted as a messenger. He was also the god of science, commerce and patron of traders, rogues, vagabonds and thieves.)
This type is perhaps the most common and the most reliable. Note; these are now being phased out because of the toxicity of mercury. (2019)
It consists of a reservoir of mercury which can be pumped into a calibrated tube, this tube lies in a graduated scale which is most often marked from 0 to 300mm Hg or 0 to 270 (Hg is the chemical symbol for mercury).

The blood pressure cuff is most often wrapped around the arm just above the elbow, the machine connected up and air pumped in with the pump provided, the cuff is also connected to the top of the reservoir by a rubber tube.
The air that you have pumped into the cuff will now be under pressure and this same cuff pressure will be transmitted to the top of the reservoir thus pushing the mercury up the calibrated tube.
When you stop pumping air into the cuff the reading on the scale will indicate the pressure in the cuff.
The cuff is inflated until the pressure showing on the scale is above the expected blood pressure, a stethoscope is placed on the artery inside the elbow, now because the pressure in the cuff is above the expected blood pressure no sounds will be heard as the blood cannot get under the cuff.
The cuff is now slowly let down by opening the valve on the inflation bulb, the pressure in the cuff will now slowly fall as the air escapes, at some point the pressure in the cuff will be below the blood pressure and so the blood will start to flow under the cuff and you will now be able to hear the sounds of blood pumping past.
As soon as these sounds are heard the reading on the scale is noted, this reading is called the systolic pressure, as the pressure in the cuff falls further there is a point when the type of noise changes, at this point the reading on the scale is again noted, this is called the diastolic pressure.
The air is pumped into the cuff with a rubber inflation bulb, this has a one-way valve in so that air is drawn in and when squeezed will only go out and into the cuff, it goes in via the release valve, this also has a one-way valve in it which is there to stop the air leaving the cuff the wrong way, this valve has on the top most often a finger operated valve which when unscrewed will regulate the speed at which the air is released from the cuff.
When the cuff is inflated the mercury should rise in the tube and stop when you stop inflating the cuff and when the release valve is just opened the mercury should fall slowly. Below a photo of a mercury blood pressure machine.


ANEROID Machines. (aneroid means without liquid).
These are used in the same way as the mercury type, but instead of having mercury in a tube they have a small corrugated brass bellows. See my photo below.

This is a flat corrugated container into which the air can pass so keeping it at the same pressure as the cuff, as the pressure inside increases so the bellows expands upwards, this expansion is transmitted by a series of levers and a rack and pinion arrangement to the pointer to change the up and down movement to a rotary one.
There is in it a coiled return spring that brings the pointer back to zero when the pressure has left the bellows.
This type is much more likely to suffer damage than the mercury type and to become inaccurate.
Its accuracy should only be checked against a mercury machine or against an electronic pressure gauge of known accuracy.
At the bottom of the face is printed an ellipse or sometimes an oblong, the pointer should always return to rest inside this when there is no pressure in the cuff. This is the zero point.

See the section on the maintenance of blood pressure machines for maintenance details.

The simplest types of these machines have the same cuff and inflation bulb as all the others and the same sort of dial as the aneroid type, the difference is that in the cuff there is a transducer that picks up the sound of the blood passing, this is turned in to a note that sounds each time the pulse is heard.
They are fairly reliable but you may get problems with the connections between the transducer and the instrument, this can be a problem as the units are often sealed so that you can’t get at them.
If you really do have to get inside then you will have to very carefully open them up, even then you may have problems doing anything to them.
My opinion is that this simple type is of no real improvement on the mercury ones and will, in the long run, give you much more trouble.
There are more complicated types that will automatically inflate the cuff at regular set times and will display the blood pressures in windows with LCDs or LEDs.
This type of machine does not have a transducer in the cuff but detects the pulsation’s in the cuff with a very sensitive detector in the body of the machine.
This sort of machine is really out of my field as far as maintenance is concerned, all that I would look at is the air pump inside that inflates the cuff.
This pump should be fairly easy to repair if it goes wrong, also the tubing between the cuff and the pump and the cuff itself.
Most other repairs to these machines would require someone who was good at electronics.


Some other BP machines.

Von Recklinghausen’s oscillitonometer, used by an anaesthetist for measuring very low blood pressures.



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