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Principles and techniques of blood pressure measurement

Although the mercury sphygmomanometer is widely regarded as the “gold standard” for office blood pressure measurement, the ban on use of mercury devices continues to diminish their role in office and hospital settings. To date, mercury devices have largely been phased out in US hospitals. This has led to the proliferation of non-mercury devices and has changed (probably for ever) the preferable modality of blood pressure measurement in clinic and hospital settings. In this article, the basic techniques of blood pressure measurement and the technical issues associated with measurements in clinical practice are discussed. The devices currently available for hospital and clinic measurements and their important sources of error are presented. Practical advice is given on how the different devices and measurement techniques should be used. Blood pressure measurements in different circumstances and in special populations such as infants, children, pregnant women, elderly persons, and obese subjects are discussed.

The standard location for blood pressure measurement is the brachial artery. Arm blood pressure monitor that measure pressure at the wrist and fingers have become popular, but it is important to realize that systolic and diastolic pressures vary substantially in different parts of the arterial tree with systolic pressure increasing in more distal arteries, and diastolic pressure decreasing.

The auscultatory method

Although the auscultatory method using mercury sphygmomanometer is regarded as the ‘gold standard’ for office blood pressure measurement, widespread implementation of the ban in use of mercury sphygmomanometers continues to diminish the role of this technique.72 The situation is made worse by the fact that existing aneroid manometers, which use this technique, are less accurate and often need frequent calibration.72 New devices known, as “hybrid” sphygmomanometers, have been developed as replacement for mercury devices. Basically, these devices combine the features of both electronic and auscultatory devices such that the mercury column is replaced by an electronic pressure gauge, similar to oscillometric devices, but the blood pressure is taken in the same manner as a mercury or aneroid device, by an observer using a stethoscope and listening for the Korotkoff sounds.72

The oscillometric technique

This was first demonstrated by Marey in 1876,38 and it was subsequently shown that when the oscillations of pressure in a sphygmomanometer cuff are recorded during gradual deflation, the point of maximal oscillation corresponds to the mean intra-arterial pressure.32,39,97 The oscillations begin at approximately systolic pressure and continue below diastolic (Fig. 1), so that systolic and diastolic pressure can only be estimated indirectly according to some empirically derived algorithm. This method is advantageous in that no transducer need be placed over the brachial artery, and it is less susceptible to external noise (but not to low frequency mechanical vibration), and that the cuff can be removed and replaced by the patient during ambulatory monitoring, for example, to take a shower. The main disadvantage is that such recorders do not work well during physical activity when there may be considerable movement artifact. The oscillometric technique has been used successfully in ambulatory blood pressure monitors and home monitors. It should be pointed out that different brands of oscillometric recorders use different algorithms, and there is no generic oscillometric technique. Comparisons of several different commercial models with intra-arterial and Korotkoff sound measurements, however, have shown generally good agreement.

Devices incorporating this technique use an ultrasound transmitter and receiver placed over the brachial artery under a sphygmomanometer cuff. As the cuff is deflated, the movement of the arterial wall at systolic pressure causes a Doppler phase shift in the reflected ultrasound, and diastolic pressure is recorded as the point at which diminution of arterial motion occurs. Another variation of this method detects the onset of blood flow at systolic pressure, which has been found to be of particular value for measuring pressure in infants and children.18 In patients with very faint Korotkoff sounds (for example those with muscular atrophy) placing a Doppler probe over the brachial artery may help to detect the systolic pressure, and the same technique can be used for measuring the ankle-brachial index, in which the systolic pressures in the brachial artery and the posterior tibial artery are compared, to obtain an index of peripheral arterial disease.

The finger cuff method of Penaz

This interesting method was first developed by Penaz63 and works on the principle of the “unloaded arterial wall.” Arterial pulsation in a finger is detected by a photo-plethysmograph under a pressure cuff. The output of the plethysmograph is used to drive a servo-loop, which rapidly changes the cuff pressure to keep the output constant, so that the artery is held in a partially opened state. The oscillations of pressure in the cuff are measured and have been found to resemble the intra-arterial pressure wave in most subjects (Fig. 2). This method gives an accurate estimate of the changes of systolic and diastolic pressure when compared to brachial artery pressures;63 the cuff can be kept inflated for up to 2 hours. It is now commercially available as the Finometer and Portapres recorders and has been validated in several studies against intra-arterial pressures.61,84 The Portapres enables readings to be taken over 24 hours while the subjects are ambulatory, although it is somewhat cumbersome.

The increasing use of wrist blood pressure monitor for both self-and ambulatory monitoring has necessitated the development of standard protocols for testing them. The two most widely used have been developed by the BHS52 and Association for the Advancement of Medical Instrumentation (AAMI) in the United States.2 Both require the taking of three blood pressure readings in 85 subjects (chosen to have a variety of ages and blood pressures) by trained observers and the device being tested. The BHS protocol requires that a device must give at least 50% of readings within 5 mm Hg and 75% within 10 mm Hg with the two methods (grade B), and the AAMI requires that the average difference between the two methods not exceed 5 mm Hg with a standard deviation of less than 8 mm Hg. One of the limitations of the validation procedures is that they analyze the data on a population basis and pay no attention to individual factors. Thus, it is possible that a monitor will pass the validation criteria and still be consistently in error in a substantial number of individuals.23

Devices for clinic and hospital measurement

Mercury sphygmomanometers

The design of mercury sphygmomanometers has changed little over the past 50 years, except that modern versions are less likely to spill mercury if dropped. As indicated earlier, although the use of mercury sphygmomanometer is widely regarded as the ‘gold standard’ for office blood pressure measurement, widespread implementation of the ban in use of mercury devices continues to diminish their role in office and hospital settings. To date, mercury devices have largely being phased out in US hospitals.43 The reason is not because any more accurate device has been developed but because of concerns about the safety of mercury. Currently the two alternatives for replacement of mercury are aneroid sphygmomanometer and electronic (oscillometric) devices.

Aneroid devices

The ban on mercury sphygmomanometer has placed new interest in alternative methods, of which aneroid devices are the leading contenders. The error rates reported with regards to accuracy of aneroid devices in older hospital surveys range from 1% in one survey,8 to 44% in another.44 Validation studies conducted a decade ago indicated that they could be accurate.4,96 A most recent study, which compared the use of mercury versus aneroid device in the setting of a large clinical trial across over 20 clinical sites, also found it to be accurate.36 This is the best evidence yet attesting to the accuracy of aneroid devices.

Sources of error with the auscultatory method

Some of the major causes of a discrepancy between the conventional clinical measurement of blood pressure and the true blood pressure are listed in Table 2. The measurement of blood pressure typically involves an interaction between the patient and the physician (or whoever is taking the reading), and factors related to both may lead to a tendency to either overestimate or underestimate the true blood pressure or to act as a source of bi-directional error. As shown in Table 2, there may be activities that precede or accompany the measurement that make it unrepresentative of the patient’s “true” pressure. These include exercise and smoking before the measurement as well as talking during it.

The white coat effect and white coat hypertension

One of the main reasons for the growing emphasis on blood pressure readings taken outside the physician’s office or clinic is the white coat effect, which is conceived as the increase of blood pressure that occurs at the time of a clinic visit and dissipates soon thereafter. Recent studies indicate that the mechanisms underlying the white coat effect may include anxiety, a hyperactive alerting response, or a conditioned response29,55 In one of these studies, we assessed office blood pressure, ambulatory blood pressure, and anxiety scores on three separate occasions one month apart in 238 patients. We found the largest white coat effect occurred in the physician’s presence, and the noted white coat effect was a conditioned response to the medical environment and the physician’s presence rather than a function of the patients’ trait anxiety level (See Figure 4). The white coat effect is seen to a greater or lesser extent in most if not all hypertensive patients but is much smaller or absent in normotensive individuals. It usually has been defined as the difference between the clinic and daytime ambulatory pressure.91 A closely linked but discrete entity is white coat hypertension, which refers to a subset of patients who are hypertensive according to their clinic blood pressures but normotensive at other times. Thus, white coat hypertension is a measure of blood pressure levels, whereas the white coat effect is a measure of blood pressure monitor with extra large cuff.

What distinguishes patients with white coat hypertension from those with true or sustained hypertension is not that they have an exaggerated white coat effect but that their blood pressure is within the normal range when they are outside the clinic setting. White coat hypertension is important clinically because it appears to be a relatively low-risk condition compared to sustained hypertension (defined by an elevated blood pressure in both the clinic and ambulatory settings).19 It can only be diagnosed reliably by accurate automatic home digital blood pressure monitor and home self-monitoring as described later. Observer error and observer bias are important sources of error when sphygmomanometers are used. Differences of auditory acuity between observers may lead to consistent errors, and digit preference is very common, with most observers recording a disproportionate number of readings ending in 5 or 0.60 An example is shown in Fig. 5 of readings taken by hypertension specialists, who are clearly not immune to this error. The average values of blood pressure recorded by trained individual observers have been found to vary by as much as 5 to l0 mm Hg.17 The level of pressure that is recorded may also be profoundly influenced by behavioral factors related to the effects of the observer on the subject, the best known of which is the presence of a physician. It has been known for more than 40 years that blood pressures recorded by a physician can be as much as 30 mm Hg higher than pressures taken by the patient at home, using the same technique and in the same posture.3 Physicians also record higher pressures than nurses or technicians.37,73 Other factors that influence the pressure that is recorded may include both the race and sex of the observer.

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A study on the flow characteristics of butterfly valve with baffles

The butterfly valve was originally used where a tight closure was not absolutely necessary. However, over the years, these valves have been manufactured with fairly tight seals made of rubber or elastomeric materials that provide good shut off similar to other types of valves. Butterfly valves are used where space is limited. Unlike gate valves, butterfly valves can be used for throttling or regulating flow as well as in the full open and fully closed position. The pressure loss through a butterfly valve is small in comparison with the gate valve. The L/D ratio for this type of valve is approximately one-third of that of a gate valve. Butterfly valves are used in large and small sizes. They may be hand wheel–operated or operated using a wrench or gearing mechanism.

Concentric butterfly valves are bidirectional. Double offset and triple offset bi-offset butterfly are also bidirectional but with preferred flow (pressure) direction, such as flow from the stem side. Fig. 2.96 shows the preferred flow direction of a flanged end double offset butterfly valve.

Butterfly valves tend to be cheaper than gate valves because they require less material and less civil works. They are also easier to operate against unbalanced water pressures as the disc pivots about an axis on or near the pipe axis. Consequently butterfly valves are now commonly used in water distribution systems. Butterfly valves can be metal seated or resilient seated; in the latter case the seat is usually made of natural or synthetic rubber and is commonly fixed to the body of valves of smaller sizes or to the disc. Plate 28(b) shows a resilient seated butterfly valve.

Resilient seated valves can remain virtually watertight, even after prolonged use in silty water. Therefore, resilient seats are usually specified for isolating valves in distribution systems. Resilient seated valves may also be used for control purposes but, if operated at small openings, the seal may be damaged. Solid rubber is the material usually used for resilient seatings: inflatable seals have been used on very large valves but not always with success. Metal seated butterfly valves do not have tight shut-off characteristics and are mainly intended for flow control purposes where they need to be held in the partially open position.

Distribution network pipe systems are now designed to produce self-cleaning velocities at least once every 24 hours and should not need swabbing as part of normal operation. A transfer pipeline may need to be swabbed periodically. Butterfly valves on the line prevent the passage of foam swabs (except for very soft ones) but this does not usually pose a problem if the valves are spaced sufficiently far apart to allow the pipe to be cleaned in sections. Short lengths of pipe either side of the valve are made removable so that the cleaning apparatus can be inserted and removed.

Butterfly valves should normally be mounted with the spindle horizontal since this allows debris in the pipe invert to be swept clear as the valve is closed. Where the spindle is vertical solids can lodge under the disc at the spindle and cause damage to the seal. Disc position indicators are useful and strong disc stops integral with the body should be specified, so that the operator can feel with certainty when the disc is fully closed or fully open.

Butterfly valves have been made to very large diameters (10 m or more) operating under very high heads and at high water velocities (20 m/s or more) and have proved successful in use. However, when a centre-pivoted butterfly valve is to be used for flow control purposes the maximum velocity of approach to the valve should be limited to 5 m/s. Resilient seated valves can be specified to have no visible leakage on seat test but the range of acceptable seat leakage rates for metal seated valves varies from about 0.004 to 0.04 l/h per 100 mm of nominal diameter (DN), at the specifier’s choice. However, a low rate for a high pressure differential would be expensive to achieve and difficult to maintain with metal seats. For some control applications, an acceptable seat leakage rate of about 0.4 l/h per 100 mm DN may be appropriate.

If a valve may be required to remain in place closed on removal of the pipe on one side for a temporary operation, it must be flanged for bolting to a pipe flange on the other side. ‘Wafer’ butterfly valves whose bodies are sandwiched between pipe flanges do not achieve this. Use of such valves for isolation of air valves allows maintenance to be carried out on the air valve in situ with the pipeline in service but does not allow removal and replacement of the air valve under pressure. Since replacement of air valves is likely to be cheaper than in situ refurbishment, flanged isolating valves are preferred in such situations.

The butterfly valve is a rotary valve in which a disk-shaped seating element is rotated 90° to open or close the flow passage. They are used in throttling service, particularly where large-size valves with automatic actuators are required. Butterfly valves cannot be used where a nonobstructed, full opening is needed. They offer a size and weight advantage over plug and ball valves.

Conventional butterfly valves are used mainly in low-pressure water service and throttling applications. The seats, disk, and shaft are in the same plane. The seat is obtained by an interference fit between the disk and resilient (flexible) liner. This type of fit is shown in Figure 4.64. The tightness of the seat is limited by the operating torque of valve and the seal between the shaft and the liner. The sealing characteristics of this valve are poor and leakage usually occurs.

The high-performance butterfly valve provides good sealing characteristics and a tight shutoff. The disk is essentially an off-center slice of a ball, and the seating mechanism of this valve is similar to that of a ball valve. The disk and seats of this valve are offset from the shaft and shaft sealing in this valve is not critical. Many valves offer a primary seat made of a resilient material and a secondary metal-to-metal seal making them “fire-safe.” High-performance butterfly valves are available in pressure classes as high as ANSI 900 and can be used in applications requiring tight shutoff.

The gate valve has a unique body style unlike the other valves we have discussed. The butterfly uses a circular plate or wafer operated by a wrench to control flow. A 90° turn of the wrench moves the wafer from a fully open position to a fully closed position. The wafer remains in the stream of flow and rotates around a shaft connected to the wrench. As the valve is being closed, the wafer rotates to become perpendicular to the direction of flow and acts as a dam to reduce or stop the flow.

Traditional butterfly valves now work at high pressure drops across the disc which can be both metallic and “soft”. Upper and lower temperature limits are the same, by and large, as those for globe valves, depending on duty and material of construction. The butterfly construction is especially suitable for high temperatures. Bodies can be fabricated from bar and plate and the seals can be mounted on cooling extensions away from the main flow.

Butterfly valves can be used as a control valve and also as a shut-off valve, as discussed in Chapter 3, Section 3.3.3, against high pressure drops of regularly up to 415 barg. Depending upon the materials of construction and the seat design a butterfly control valve may have very limited shut-off pressure drops. Some 100 barg valves are only rated for 4 barg shut-off differential.

globe valve should have a range of possible shaft diameters for each nominal valve size in order to handle the variation in torque due to various operating pressure conditions and packing box friction. Shafts should not be made of material prone to creep, such as some austenitic stainless steels. In these situations a precipitation hardening stainless steel such as 17-4PH is preferred. The corrosion resistance of such materials, equivalent to AISI 304, must be borne in mind. The disc must withstand high differential pressures. Some valves do have restrictions on the maximum throttling differential pressure, 35% of pressure rating in some cases.

Generally, butterfly valves are used for the inlet control and bypass valves. They are inexpensive to manufacture, and their actuators are able to operate in accordance with the requisite response times. Butterfly valves do, however, have the disadvantage of a tightly curved characteristic.

Additional non-linearities arise from the fact that valves of different nominal sizes are operated in sequence. An initial improvement in the control response was achieved in that the steady-state duty point characteristics for operation with and without the expander were stored in function generators in the controller. Depending on the operational state, the output of the process controller (regenerator pressure, or differential pressure, between the regenerator and the reactor) is applied to one or the other of these characteristics. In the event of an expander trip, the system immediately switches from one characteristic to the other. This results in linearization of the characteristic profile, so that the process controller is able to operate independent of the duty point concerned and independent of the operating mode (i.e., with or without the expander). This switching between characteristics in the event of an emergency trip also ensures that the bypass valves are always driven at maximum actuating speed to their new steady-state position in accordance with the prevailing operating conditions. All this is performed independent of the prevailing duty point (i.e., whether the system is operating at partial load or at overload).

In the industry, globe valves, which are commonly used to precise control the flow rate along with opening and closing flow in pipes, are technically and economically limited in valve size due to structural instability related to complex internal flow passage. Butterfly valves, on the other hand, have advantages such as low weight and low manufacturing costs, but it is difficult to control the flow rate at an opening angle of 60°or higher and flow is unstable in the case of butterfly valve. Therefore, the purpose of this study is to have characteristics of flow rate of the globe valve that is used in the industry at the same diameter of pipeline and flow stability due to uniformity of flow through the baffle hole by adding 1/2 baffle to forged valve. Hole size of baffles were set at 5, 7 and 9 mm and baffles were set at the rear of the butterfly valve. To verify the method of numerical analysis, the results of experimental study were compared with the results of numerical study. As a results, it is confirmed that characteristics of flow rate of butterfly valve with baffle is similar to globe valve in the case of hole size 5 mm. In addition, flow pattern is to be stable by analyzing turbulence energy. Consequently, when applying baffle to butterfly valve, it is possible to reduce the flow unstability and change the flow rate of butterfly valve.

 

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