A sphygmomanometer is a blood pressure machine/monitor. Considering the inherent risks associated with wrong pressure readings from a faulty monitor, a sphygmomanometer may be classified as a class II medical device.
The non-invasive measurement of blood pressure (which will be the focus of this blog post) is accomplished by occluding an artery, in the upper arm, with an inflatable cuff that is connected to a manometer.
As a result, such equipment as a sphygmomanometer must be used in full working condition, with permissibility of only the simplest maintenance to be carried out by hospital staff with specialized training – a biomedical engineer.
With the use of a stethoscope to listen for the Korotkoff sounds of blood flow, as the cuff is deflated pressure measurements are obtained. The first sound heard is the systolic pressure and the last sound heard is the diastolic pressure.
The ideal pressure is 120 mmHg systolic and 80 mmHg diastolic. Systolic pressures above 140 or diastolic pressures above 100 are of clinical concern.
Complexities surrounding the different types of Sphygmomanometer.
Funny it is, that despite the Environmental Protection Agency’s (EPA) prohibition of continual use of Mercury Manometers in hospitals beyond the year 2005, it remains to this date the most popular blood pressure monitor in healthcare facilities across the world. Mercury is a hazardous material and its vapours are toxic to developing nervous systems. The more reason for which it was recommended to be phased out.
It consists of a reservoir of mercury which can be pumped into a manometer tube. This tube lies on a plastic or glass column with graduations from 0 to 300 mmHg and is connected to the cuff via latex or rubber tubing.
At the top of the tube, under the cap is a calfskin diaphragm that allows air to move in both directions of the calibrated tube. If this diaphragm is dirty the mercury in the column will not move smoothly. The blood pressure cuff is wrapped round the patient’s arm just above the elbow, the machine connected up, and air pumped in using the bulb provided. When the pumping of air into the cuff is stopped, the reading on the scale will indicate the pressure in the cuff.
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Zero Error of Mercury: The machine will give falsely high reading if the initial mercury level is higher than Zero and a falsely low reading when the initial mercury level is lower than Zero.
The Solution is to correct the initial mercury level to Zero by undoing the reservoir cap and thereafter add or remove mercury, as the situation requires, using a needle and syringe.
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Faulty Seal: Mercury will continue to rise in the tube for an extended period upon cessation of inflation of the cuff giving higher than normal readings. Deflating the cuff displays an opposite effect as the mercury descends further to the nadir upon cessation of deflation, also giving lower than normal reading.
The higher than normal readings imply that air above the mercury continues to pass out of the tube, past the seal and vice versa for lower readings. You should confirm your diagnosis by removing the seal and repeating the test. The ideal is that the mercury stops movement in the column immediately pumping is stopped.
The Solution is to change the seal.
Dirty and Poisinuous Mercury: Mercury over time is oxidized to Mercuric Oxide which is a dirty black powder. Aspirate the mercury using needle and syringe and filter until it is clean.
Finally, mercury is accumulative poison and should be handled with care.