Equipment

Mercury sphygmomanometers

The mercury sphygmomanometer was introduced to clinical medicine by Scipione Riva-Rocci in 1896. A decade later Nikolai Korotkoff discovered that sounds are audible as an occluding cuff is deflated and the stethoscope became as indispensable to the measurement of blood pressure as the mercury manometer. It is likely that this traditional technique, which has contributed so much to our knowledge of hypertension over the past century, will soon disappear from clinical practice. There are three reasons for this: mercury is likely to be banned from clinical use because of the danger of toxicity; accurate, automated devices are available to replace the mercury sphygmomanometer; and with the use of 24 h ambulatory blood pressure measurement in clinical practice, more reliance is being placed on blood pressure behaviour than on casual measurement of blood pressure levels.

Banning mercury from hospital wards raises another issue of even greater importance for clinical medicine than that of rendering the mercury sphygmomanometer obsolete. If the millimetre of mercury is no longer the unit of measurement for blood pressure, there can be little scientific argument against its replacement with the Système International (SI) unit, the kilopascal.

The British Hypertension Society has instructed its Working Party on Blood Pressure Measurement to consider the implications of these issues for clinical practice and draw up recommendations for the smooth implementation of the necessary changes. Towards this end kilopascal equivalents to millimetres of mercury are shown in the table.

The mercury sphygmomanometer consists of a manometer, an inflatable bladder in a cuff, and an inflation-deflation device. Before any measurement is attempted the equipment must be checked to make sure that it is appropriate and in good order. If any part of the apparatus is defective or unsuitable, alternative equipment must be used.

Points to check when assessing equipment

Manometer - visibility of meniscus; calibration
Cuff - condition; length and width of inflatable bladder
Inflation-deflation device - possible malfunction; control valve
Stethoscope - condition
Maintenance - regularity; responsibility

Features affecting accuracy of the mercury sphygmomanometer

• When the sphygmomanometer is not in use, the top of the mercury meniscus should rest at exactly zero without pressure applied; if it is above or below this level the device needs to be serviced.
• The scale should be clearly calibrated in 2 mm divisions from 0 to 300 mm Hg and should indicate accurately the differences between the levels of mercury in the tube and in the reservoir.
• The diameter of the reservoir must be at least 10 times that of the vertical tube, or the vertical scale must correct for the drop in the mercury level in the reservoir as the column rises.
• Substantial errors may occur if the manometer is not kept vertical during measurement. Calibrations on floor models are especially adjusted to compensate for the tilt in the face of the gauge. Stand mounted manometers are recommended for hospital use. This allows the observer to adjust the level of the sphygmomanometer and to perform measurement without having to balance the sphygmomanometer precariously on the side of the bed.
• The air vent at the top of the manometer must be kept patent as clogging will cause the mercury column to respond sluggishly to pressure changes and overestimate pressure.
• The control valve is one of the commonest sources of error in sphygmomanometers and when it becomes defective it should be replaced. Spare control valves should be available in hospitals and a spare control valve should be supplied with sphygmomanometers.

Advice to be included in the instructions accompanying a sphygmomanometer using mercury

Guidelines and precautions

A mercury-type sphygmomanometer should be handled with care. In particular, the instrument should not be dropped or treated in any way that could result in damage to the manometer. Regular checks should be made to ensure that there are no leaks from the inflation system and that the manometer has not been damaged so as to cause a loss of mercury.

Health and safety when handling mercury

Exposure to mercury can have serious toxic effects; absorption of mercury results in neuropsychiatric disorders and, in extreme cases, nephrosis. Therefore precautions should be taken when carrying out any maintenance to a mercury sphygmomanometer.
All maintenance necessitating handling of mercury should be conducted by the manufacturer or specialised service units.

Mercury spillage

When dealing with a mercury spillage, wear latex gloves. Avoid prolonged inhalation of mercury vapour. Do not use an open vacuum system to aid collection.
Collect all the small droplets of spilt mercury into one globule and immediately transfer all the mercury into a container, which should then be sealed.
After removal of as much of the mercury as practicable, treat the contaminated surfaces with a wash composed of equal parts of calcium hydroxide and powdered sulphur mixed with water to form a thin paste. Apply this paste to all the contaminated surfaces and allow to dry. After 24 h, remove the paste and wash the surfaces with clean water. Allow to dry and ventilate the area.

Manometer

Mercury column manometer The meniscus should be clearly visible and not obscured by oxidised mercury on the inside of the glass. Before inflation it must be at zero.

Cuff

The cuff consists of an inflatable bladder within a restrictive cloth sheath. The bladder, tubing, connections, bulb and valves should all be sound. The sheath containing the bladder should also be in good condition and have a secure fastening. Provided it is long enough to wrap round the arm and be easily secured, the length of the sheath is not important.  It is the dimensions of the bladder within the sheath that affect the accuracy of blood pressure measurement.

Bladder dimensions

Miscuffing

Too narrow or too short a bladder will cause overestimation of blood pressure. Too wide or too long a bladder may cause underestimation of blood pressure. The former has the effect in clinical practice of overdiagnosing and the latter of underdiagnosing hypertension. Either error has serious implications.

Provided bladder length is such as to encircle 80% of arm circumference bladder width is not so critical, provided it is not less than 12 cm. Most arms do not readily accommodate bladders with widths greater than 13 cm as the bladder is likely to encroach on the antecubital fossa.

• Bladder too small = undercuffing = overestimation of BP
• Bladder too large = overcuffing = underestimation of BP     
• Undercuffing is more common than overcuffing

Arm circumference

The mean arm circumference in many European countries is about 30 cm. Knowing that measurement will be most accurate with a cuff containing a bladder that will encircle 80% of arm circumference, it can be calculated that a bladder measuring 12 x 26 cm would correctly cuff 79% of European arms; it would incorrectly cuff 21% of arms - 10% from undercuffing and 11% from overcuffing. Further, a bladder that is suitable for Northern Europeans may be too large for the leaner arms of, for example, Brazilians.

These findings suggest that the optimum bladder dimensions should be recommended according to the arm circumference of the population for which the recommendation applies.

The BHS is advising cuff manufacturers on the design for a cuff containing a bladder that will completely encircle all adult arms without overcuffing (leading to underestimation of blood pressure), or undercuffing (leading to overestimation of blood pressure).  Until such a cuff becomes available, the most accurate measurement of blood pressure will be obtained using cuffs containing bladders with the dimensions shown below.  However, if these cuffs are not available, using a cuff containing a bladder with the dimensions 35 x 12 cm will avoid undercuffing (the greatest source of error) in adults, at the risk of some overcuffing in subject with slim arms.

It would seem appropriate, therefore, to have available three cuffs containing bladders with the following dimensions:

• A standard bladder measuring12 x 26 cm for most adult arms
• An "obese" bladder measuring 12 x 40 cm for obese arms
• A "small" bladder measuring 10 x 18 cm for lean adult arms and children

Footnote: the British Hypertension Society Working Party has previously recommended a cuff containing a bladder 12 x 35 cm, on the basis that such a cuff would give accurate blood pressure measurements in most adults.

The dimensions of the bladder should be clearly shown on each cuff, with a prominent marker indicating the centre of the bladder.

Inflation-deflation devices

Failure to achieve a pressure of 40 mm Hg above the estimated systolic blood pressure or 200 mm Hg after 3-5 seconds of rapid inflation is a sign of possible equipment malfunction. So too is inability of the equipment to deflate smoothly when the controlling release valve is operated at 2-3 mm/s or at each pulse beat. When such problems occur the unit should be set aside and clearly marked with instructions for defective parts to be repaired or replaced. Faulty control valves, leaks, dirty vents and perished tubing are simple to repair. The most common source of error in the inflation-deflation system is the control release valve, which can easily be replaced.

Deflation that is either jerky or too rapid may result in the systolic pressure being underestimated and the diastolic pressure overestimated. If, on the other hand, deflation is too slow the patient may suffer pain, even bruising, and blood pressure may be overestimated.

Stethoscope

The stethoscope should be of good quality and in good condition with clean, well fitting earpieces.

Maintenance

The due date of the next service should be clearly marked on the sphygmomanometer. Sphygmomanometers should be serviced every 6 months. Replacement parts are cheap and should be readily available in the clinical area, together with a maintenance instruction booklet.

The responsibility for reporting faulty equipment or the lack of appropriate cuffs lies with the observer, who should always refuse to use defective or inappropriate equipment. The responsibility for arranging regular maintenance should be clearly defined for each clinical area.

Sphygmomanometer maintenance

• Responsibility for maintenance should be clearly defined.
• The date for routine 6-monthly service should be marked on the unit.
• Any defective equipment should be reported to the member of staff responsible.
• Replacement parts and instruction booklet should be available in the clinical area.

Aneroid sphygmomanometers

Aneroid sphygmomanometers were once popular because they are more compact than mercury sphygmomanometers, but their use is now discouraged because their accuracy deteriorates with use, leading usually to falsely low readings and a consequent underestimation of blood pressure. If an aneroid sphygmomanometer is used, its accuracy can be checked at different pressure levels by connecting it with a Y piece to the tubing of a standardised mercury column manometer. If recalibration is necessary this must be done by the manufacturer.

Automated devices

Most automated devices work on one of three principles:  the detection of Korotkoff sounds by a microphone or the detection of arterial blood flow by ultrasound or oscillometry. Until recently automated devices depended on Korotkoff sound detection using an electronic microphone shielded from extraneous noise in the pressure cuff; blood pressure was recorded on a print-out or indicated on a digital display. The microphones are sensitive to movement and friction, however, and may be difficult to place accurately. Manufacturers are turning therefore to oscillometric detection of blood pressure, in which cuff placement is not critical. Until recently the accuracy of automated devices was questionable but accurate automated sphygmomanometers for clinic or home measurement are now available, although tests of reliability after a period in use have not been performed. As with all equipment, the user is advised to seek independent evidence of validation from the manufacturer.

Automated device for blood pressure measurement.

Automated devices for measuring blood pressure in the home; some of these may be suitable for measuring blood pressure in the clinical setting.

An automated device for measuring ambulatory blood pressure.

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© BHS 1999