To measure one’s blood pressure, doctors use a stethoscope (an instrument used for listening to sounds in the body). To hear the sounds of your heart pumping blood through the main artery in your arm (the brachial artery), which are heard best through a stethoscope placed on the crook of your elbow. The pressure required first to start and then to stop these pumping sounds is used as the measure of your blood pressure.
When your blood pressure is taken, the cuff wrapped around your upper arm is inflated until the pulse in your wrist can no longer be felt; this indicates that the flow of blood through your brachial artery has been stopped. The cuff is then inflated a little more, until the sphygmomanometer reading is about 20 mmHg higher than the point at which your pulse could no longer be felt. This is the point at which your doctor or nurse (or yourself if you are taking your own measurements) starts listening through the stethoscope. The air is then slowly released from the cuff until clear, regular tapping sounds are heard through the stethoscope. The level pressure at which the sounds are heard is your systolic pressure and the figure shown on the sphygmomanometer scale is recorded.
The pressure in the blood pressure machine is then released further. First the tapping sounds disappear and then, at a pressure about 50-100 mmHg lower, soft, regular whooshing noises are heard through the stethoscope. When these sounds disappear, it indicates that the blood is once again flowing smoothly through your brachial artery. The level of pressure at which the sounds disappear is your diastolic pressure and the figure on the sphygmomanometer scale is again recorded.
In an electronic sphygmomanometer, a sensor in the cuff replaces the stethoscope. The sensor detects the appearance and disappearance of pulsatile sounds or movements, rather than someone’s ears and the machine notes the systolic and diastolic pressures for you automatically.
Mercury sphygmomanometers are accurate to the nearest 2 mmHg if they are well maintained and used carefully. They have a great advantage over the other types in that when they go wrong, it is usually obvious and can be easily corrected. Aneroid sphygmomanometers are small and are more convenient to carry than the larger mercury sphygmomanometers. Modern machines developed in the past 5 years or so are accurate and reliable; older machines are not. Because of the way they work, electronic sphygmomanometers have the potential to eliminate potential sources of error. They are also easy to use, particularly by people measuring their own blood pressure at home. Unfortunately, their futuristic appearance is no guarantee for accuracy and this is especially true for the machines on sale to the public at an affordable price. Also, unlike the traditional mercury machines, it may not be obvious if anything goes wrong with an electronic sphygmomanometer, so you may go on recording systematically incorrect and misleading readings. However, blood pressure machine are constantly improving and, once their accuracy is beyond doubt, they will certainly replace mercury sphygmomanometers.