Sphygmomanometer: Sphygmomanometer is an instrument used to measure the blood pressure at the bedside. Till date most sphygmomanometers used a mercury column to measure the pressure. But aneroid sphygmomanometers are being used more often now because of the environmental issues associated with the use of mercury. Sphygmomanometer is more commonly known as BP apparatus (BP: blood pressure). The device has a cuff for application over the upper or lower limb, which can be inflated using a small bellow. Blood pressure is noted from the dial in aneroid devices and from the level of the mercury column in mercury based devices. Digital devices give an electronic digital display.
What are the different types of BP apparatus available in the market?
In many parts of the world, mercury based equipment is still being used, though they are gradually being phased out. New ones are aneroid sphygmomanometers. It uses a calibrated dial to show the blood pressure while the mercury BP apparatus uses the graduated mercury column to show the blood pressure. Automatic electronic devices with digital displays are also freely available online. Regular calibration with a reference standard is required for any type of instrument to maintain accuracy of measurement. While using the aneroid BP apparatus, it is worthwhile checking for zero error and correcting it for accurate readings. In some devices, the needle will be away from zero before inflation of the cuff, sometimes at minus 10 levels or more!
How is blood pressure measured?
Blood pressure can be measured by automatic and manual devices. Automatic devices have sensors in the cuff or device which senses the change in pressure over time. The pressure during contraction of the heart is called systolic pressure and that during relaxation is called diastolic pressure. The device automatically detects these and displays electronically.
While using manual instruments, the operator applies the cuff over the arm (encircles) for measurement of upper limb blood pressure. A stethoscope is placed over the inner aspect of the elbow to listen to sounds over the brachial artery (blood vessel supplying oxygenated blood to the arm). Initially the cuff is inflated till the pulse at the wrist disappears. Then the pressure in the cuff is slowly lowered till sounds are heard over the elbow. At this level pulse also re-appears. This is the systolic blood pressure. On further lowering of the cuff pressure, the sounds disappears. This level is noted as the diastolic pressure.
What are the potential mistakes which can cause false readings?
If the pressure in the cuff is brought down too rapidly, the blood pressure readings may not be accurate.
A loosely applied cuff can show a falsely elevated pressure and a too tight cuff the other way. A cuff which is too small for the arm may show a high pressure and a large one may show a lower pressure.
Ideally blood pressure has to be measured with the subject sitting relaxed in a chair for at least 20 minutes. This is because stress and exercise can increase blood pressure. Intake of caffeinated drinks a short while before the measurement can also elevate the blood pressure. But practical considerations may often limit these precautions.
Sitting position is meant for recording in the clinic. In the wards and intensive care units, BP is measured with the person lying down. Sometimes we may have to check standing BP if we think that giddiness due fall in standing BP is likely. This is more important in the elderly and those who are on medications to lower the blood pressure.