Accurate measurement of blood pressure is the basis for diagnosis, classification, risk stratification and treatment guidance of hypertension. Blood pressure measurement methods are divided into invasive direct measurements and non-invasive indirect measurements. Direct measurement is the blood pressure measured by a special catheter delivered to the aorta through a percutaneous puncture, while indirect measurement is the blood pressure measured indirectly using a cuff pressurization method, with a significant range of difference between the two, up to 25 mmHg in some patients.
Section I. Principles of indirect blood pressure measurement
Indirect measurement is performed by deflating and depressurizing the cuff after the cuff has been inflated and pressurized to close the artery and block the blood flow, during which the arterial blood pressure is estimated using the Koch sound auscultation method and the oscillometric method.
Korotkoff sound auscultation method (Korotkoff sound)
The cuff pressure method was discovered in 1905 by Nikolai Korotkoff, a Russian physician, and continues today.
Principle: Cuff pressure greater than systolic pressure can block arterial blood flow without sound, when it is equal to or lower than the highest intra-arterial pressure, blood flow begins to resume and cause turbulence and cause the arterial wall to vibrate, then you can hear the sound through the stethoscope and can touch the pulse, once the cuff pressure drops to the diastolic level, the blood vessel is completely open and no turbulence, then the sound disappears. The sounds heard above are generally classified into the Korotkoff 5 phases. The first loud clap heard is phase 1 (i.e., systolic pressure, which is lower than the systolic pressure measured by direct measurement), followed by a weakening of the clap with a soft blowing wind-like murmur as phase 2, followed by a strengthening of the clap and disappearance of the murmur as phase 3, followed by a dull tone as phase 4, and finally the disappearance of the sound as phase 5 (i.e., diastolic pressure, which is higher than the diastolic pressure measured by direct measurement).
Factors affecting measurement accuracy: The Koch tone method is susceptible to subjective factors that can lead to systematic error, terminal digit preference, which refers to rounding blood pressure readings to the nearest whole number, and observer prejudice, which refers to the fact that, based on a known specific blood pressure threshold, the recorded blood pressure value is closer to that value. (Observer error) based on a known specific blood pressure threshold and the recorded blood pressure value is close to that value.)
Oscillometric method (cuff oscillation wave)
In 1876, Marey discovered the phenomenon of pressure fluctuations, and the oscillometric method of electronic blood pressure measurement was established in the 1990s.
Principle: When the cuff is slowly deflated (2-3 mmHg/s) after the automatic inflation of the cuff over 30 mmHg systolic pressure, when the blood vessels under the cuff open and blood flow passes through, the change in the volume of the blood vessels causes a change in the volume of the cuff and generates an oscillation wave that is transmitted to the pressure sensor through the catheter. The amplitude of the oscillation wave becomes larger and larger as the pressure in the cuff decreases and blood flow increases. The cuff pressure corresponding to the point of maximum amplitude of the oscillation wave is equivalent to the average arterial pressure, the cuff pressure corresponding to the fluctuation point of half of the forward peak is the systolic pressure, and the cuff pressure corresponding to the fluctuation point of 75%-80% of the backward peak is the diastolic pressure.
Factors affecting measurement accuracy: The oscillometric method is not affected by subjective factors, but this technique suffers from characteristic coefficient errors, envelope fitting errors, etc. Mean arterial pressure can be significantly underestimated in elderly patients with stiff arteries and widened pulse pressures.
Ultrasound technique and others
The ultrasound technique is used to measure blood pressure by directly detecting the state of restored arterial blood flow and is of great value in the measurement of systolic blood pressure in infants, pediatric patients, and patients with weak Koch’s tone, in addition to the finger-prick method and tensiometry.
Section II sphygmomanometer
At present, the commonly used sphygmomanometers include mercury sphygmomanometer, aneroid sphygmomanometer, electronic sphygmomanometer and hybrid sphygmomanometer. sphygmomanometer) and so on.
Mercury sphygmomanometer: At present, the mercury sphygmomanometer is still the gold standard for the indirect method of blood pressure measurement, which consists of an adjustable pressurized balloon, a built-in balloon cuff and a mercury column manometer to measure blood pressure by the Koch’s tone method. It is not recommended for home blood pressure measurement due to its requirement for observer measurement skills.
Non-liquid sphygmomanometer: Also known as empty box pressure gauge sphygmomanometer or spring type sphygmomanometer, it is basically the same structure and pressure measuring principle as mercury sphygmomanometer, but its accuracy is not as good as mercury sphygmomanometer, and the error can exceed 4 mmHg.
Electronic sphygmomanometer: Most of them adopt the principle of oscillometric method, which can facilitate self-measurement of blood pressure and store data of systolic pressure, diastolic pressure, mean arterial pressure, heart rate, measurement time, etc. and draw blood pressure trend graph. However, it cannot be used in individuals with rapid atrial fibrillation, connected to an artificial heart-lung machine or Parkinson’s disease, or even in individuals.
Hybrid sphygmomanometer: Applying the principle of Koch’s tone method to measure blood pressure, it combines the advantages of mercury sphygmomanometer and electronic sphygmomanometer, avoids the problems associated with mercury and fluid-free manometers, eliminates the end digit preference and observer stigma, etc., and may completely replace mercury sphygmomanometers in the future.
Sphygmomanometer maintenance and validation: Mercury sphygmomanometers (at least every 6 months for medical use and at least every 12 months for general use), fluid-free sphygmomanometers (every 6 months), and electronic sphygmomanometers (every 1-2 years) must be professionally maintained and validated to ensure sphygmomanometer accuracy.
Section III Clinic measurement of blood pressure
There are three main methods of blood pressure measurement: office or clinic blood pressure measurement, self-measurement of blood pressure at home, and ambulatory blood pressure monitoring) three methods.
Upper extremity blood pressure measurement requirements and precautions
Table 1 AHA cuff size recommendations
Arm circumference (cm) Cuff size Cuff size (cm)
22-26 Small adults 12×22
27-34 Adult 16×30
35-44 Large adult 16×36
45-52 Adult thigh 16×42
1.Blood pressure monitor: up to standard mercury column blood pressure monitor or electronic blood pressure monitor.
2, suitable cuff: cuff airbag length and width of at least 80% and 40% of the upper arm arm circumference, respectively (46% when the error is minimal), large arm circumference should use large-size cuffs, children should use small-size cuffs. “Misuse of the cuff” (miscuffing), i.e., a disproportion between the size of the cuff and the arm, will lead to overestimation (too small a cuff leads to “cuff hypertension”) or underestimation of blood pressure (too large a cuff leads to missed hypertension). In clinical practice, a “standard” cuff with a 35-cm-long, 12-cm-wide balloon is often used. The “adjustable cuff” design concept has been proposed but not yet implemented, so it is currently recommended that a range of cuffs be available (see Table 1). Patients with a large arm circumference and a short upper arm can have their blood pressure measured by placing the cuff on the forearm and listening for sound on the radial artery (although this can overestimate systolic blood pressure), or by using a validated wrist blood pressure monitor placed at the heart level.
3, before the measurement requirements: the person being measured within 30 minutes before the measurement is prohibited exercise, smoking, drinking coffee, tobacco, alcohol, etc..
4, measurement requirements: the person being measured after emptying the bladder, in a suitable temperature, quiet environment in the house, sitting back chair (so that the back has to rely on, the back is not supported by the diastolic blood pressure can increase by 6mmHg), bare upper arm (with support, no support can lead to a 10% increase in blood pressure and heart rate increase) and the heart at the same level, legs touching the ground (not crossed, legs crossed can increase systolic blood pressure 2-8mmHg ) at least 5 minutes of quiet rest.
5, the cuff should be wrapped evenly against the skin on the exposed upper arm of the subject, the cuff balloon midline is located on the surface of the brachial artery, the lower edge of the cuff should be about 2-3 cm on the elbow bend, the stethoscope body parts are placed lightly on the brachial artery pulsation.
6, measurement of rapid inflation to 30 mmHg above the vanishing point of the radial artery pulsation, and then deflate at a rate of 2-3 mmHg/sec, measured diastolic pressure reading, rapid deflation to zero; slow heart rate should slow down the deflation rate.
7.During the deflation process, look at the surface of the mercury column with the descending mercury column with both eyes, observe the vertical height of the convex surface of the mercury column in the 1st and 5th phases of the Koch sound and record it. Some subjects may have an auscultation gap (the first Koch sound appears and disappears quickly, then reappears and eventually disappears as the fifth Koch sound, i.e., a silent period between systolic and diastolic blood pressure, which may be caused by venous filling of the upper extremity and less antegrade blood flow in the arteries, and is more likely to appear in the elderly and hypertensive patients with end-target organ damage, and may be a reflection of pressure fluctuations in the arteries). The systolic pressure should be read at the time of the first Koch sound, not when the sound reappears.