Monitoring of Blood Pressure: Types of Self-Monitors
Mercury
Although the mercury sphygmomanometer is still considered the most accurate device for clinical use, it is generally not practical for home use. The mercury sphygmomanometer works by gravity to give consistent and accurate readings. It has a long, tubular gauge made of glass or plastic connected to a reservoir of mercury. The reservoir is linked by rubber tubing to a compression cuff. Pressure from the cuff exerts a force on the mercury causing it to rise. These monitors are typically equipped with a cuff that is manually inflated by the user. Korotkoff sounds are auscultated by the patient using a stethoscope. Portable mercury sphygmomanometers may be cumbersome and require a good deal of skill and dexterity. In addition, there are environmental concerns about mercury leakage and waste.
Aneroid
These dial-type monitors have been widely used by patients but also require skill because blood pressure is auscultated with a stethoscope and inflation of the cuff is done manually. Aneroid sphygmomanometers register pressure through a bellows and lever system, which can become uncalibrated with the jolts and bumps of everyday use. Aneroid monitors are less expensivebut less accurate than mercury sphygmomanometers.
Digital
Available with automatic inflation or manual inflation cuffs, nearly all digital devices deflate automatically. Automatic-inflation devices are especially useful for patients with arthritis. Digital monitors run on batteries and some come with or have optional AC adapters. The devices’ cost tends to be related to their degree of automation. Nearly all digital self-monitors measure blood pressure oscillometrically rather than by auscultation. Oscillometric measurement uses small oscillations, or changes, in cuff pressure to identify the systolic, mean, and diastolic pressures. The mean blood pressure is determined at the peak of the amplitude of the oscillations; the systolic blood pressure, approximately 55% prior to the peak; and the diastolic blood pressure, approximately 85% after the peak. The exact points are proprietary to each manufacturer. There is high correlation between auscultatory and oscillometric devices and simultaneous physician readings. Movement tends to influence the oscillometric method, whereas unrelated noise influences the auscultatory method. Rounding of the terminal digit of the blood pressure reading, also known as terminal digit preference, is a more common problem with auscultatory technique compared with the oscillometric technique used with automatic monitors. The oscillometric technique permits faster measurement and is less expensive to manufacture.
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