血壓計原理

Current blood pressure monitors use manual and digital meters, with different trade-offs in accuracy and convenience.

manual

A stethoscope is required for auscultation. Manual meters are best used by a trained practitioner, and while it is possible to obtain a basic reading by palpation alone, this will only produce systolic blood pressure.

  • Mercury sphygmomanometers are considered the gold standard. They indicate pressure as a column of mercury and do not require recalibration. Due to their accuracy, they are frequently used in drug clinical trials and clinical evaluation of high-risk patients, including pregnant women. A commonly used wall-mounted mercury sphygmomanometer is also called a Baumanometer .
  • Aneroid sphygmomanometers (mechanical ones with a dial) are common; unlike mercury manometers, they may require calibration checks. Aneroid sphygmomanometers are considered safer than mercury sphygmomanometers, although cheaper sphygmomanometers are less accurate. A major cause of deviations from calibration is mechanical shock. Aneroids mounted on walls or brackets are less susceptible to this particular problem.

digital

Recent advances in medical technology have paved the way for affordable, easy-to-use digital blood pressure monitors that anyone can use in the comfort of their own home, without the need for specialized knowledge or training.

Digital meters use oscillometric measurements and electronic calculations rather than auscultation. They may use manual or automatic inflation, but both types are electronic, can be operated easily without training, and can be used in noisy environments.

Digital blood pressure monitors use an inflatable balloon cuff, a battery-operated air pump, and a pressure sensor to sense artery wall vibrations to measure arterial blood pressure. This is called the oscillometric method .

They measure systolic and diastolic blood pressure by oscillometric detection, using a deformable membrane, using differential capacitance or differential piezoresistive measurements, and they include a microprocessor. They measure average blood pressure and pulse rate, and obtaining systolic and diastolic blood pressure is less accurate than manual meters, and calibration is an issue. In certain patients, such as those with arteriosclerosis, cardiac arrhythmias, preeclampsia , alternans , and paradoxical pulses , the use of digital oscillometric monitors may not be recommended because their calculations may not be corrected for these conditions. When used by trained personnel, it is best to use an analog sphygmomanometer.

Digital instruments may use cuffs, in order of accuracy and in reverse order of portability and convenience, placed around the upper arm, wrist, or fingers. Recently, a team of researchers at Michigan State University developed a smartphone-based device that uses the oscillometric method to estimate blood pressure. The oscillometric testing methods used give blood pressure readings that differ from those determined by auscultation and vary based on a number of factors such as pulse pressure, heart rate and arterial stiffness, although some instruments are said to also measure arterial stiffness, and some Detect irregular heartbeats.

Measurement technology

Digital blood pressure monitors use an air pump to inflate a cuff around the upper arm or wrist with enough pressure to prevent blood flow in the local aorta. This pressure is then gradually released using a digitally controlled solenoid valve until blood begins to flow through the artery.

The blood pressure measured by the pressure sensor at this time determines the systolic blood pressure. The pulse rate is also sensed at this time. Diastolic blood pressure is determined by measurements taken when blood flow is no longer restricted. This complete measurement cycle is automatically controlled by a microcontroller.

The signal from the pressure sensor is conditioned by an instrumentation amplifier before data conversion by an analog-to-digital converter (ADC). Systolic blood pressure, diastolic blood pressure, and pulse rate are then calculated in the digital domain using algorithms appropriate for the type of monitor and sensor used. The resulting systolic blood pressure, diastolic blood pressure, and pulse rate measurements are displayed on a liquid crystal display (LCD), time-stamped, and stored in non-volatile memory.

operate

Medical student measures brachial artery blood pressure

In humans, the cuff is typically placed smoothly and snugly around the upper arm, at about the same vertical height as the heart, while the subject is seated and supporting the arm. Other placement locations depend on the species and may include flippers or tails. Choosing the correct size cuff for the patient is critical. A cuff that is too small will result in too high a pressure, while a cuff that is too large will result in too low a pressure. For clinical measurements, both arms are typically measured and recorded during the initial consultation to determine if the pressure in one arm is significantly higher than the other. A difference of 10 mm Hg may be a sign of aortic coarctation. If the arms have different readings, the higher reading arm will be used for subsequent readings. The cuff is inflated until the artery is completely occluded.

Using a hand tool, with a stethoscope to the brachial artery, the examiner slowly releases pressure on the cuff at a rate of approximately 2 mm per heart beat. As the pressure in the cuff drops, a "whooshing" or popping sound is heard as blood flow starts flowing again in the artery. The pressure at which this sound begins is recorded and recorded as systolic pressure. Release the cuff pressure further until the sound is no longer heard. This is recorded as diastolic blood pressure. In noisy environments where auscultation is not possible, systolic blood pressure can be read alone by releasing pressure until a radial pulse is palpated. In veterinary medicine, auscultation is rarely useful and palpation or visualization of the pulse distal to the sphygmomanometer is used to detect systolic blood pressure.

Digital instruments use a cuff, which, depending on the instrument, can be placed around the upper arm, wrist, or fingers, and in all cases is raised to the same height as the heart. They inflated the cuff and gradually lowered the pressure in the same way as a manual meter, and measured blood pressure by the oscillometric method.

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