A Study on Sphygmomanometers

Introduction

A Sphygmomanometer or BP (Blood Pressure) Monitor is a device that measures blood pressure, particularly in arteries. It is composes of an inflatable rubber cuff, which is wrapped around the arm. A measuring device indicates the cuff's pressure. A bulb inflates the cuff and a valve releases pressure. A stethoscope is used to listen to arterial blood flow sounds which are called “Korotkoff” sounds. The name Sphygmomanometer is a combination of Greek word "Sphygmos" which means pulse and the scientific term "Manometer" which means pressure meter.

Definition of Blood Pressure:

The human heart is a muscular pump. When it is contracting (heartbeat) it pushes blood through blood vessels called arteries.

The pushing force exerted on artery walls by the blood flow is called blood pressure. The pressure is dependent on the rate and force at which the heart is contracting (heartbeat) and also on the diameter and elasticity of the walls of the arteries.

High blood pressure puts a lot of stress on the blood vessels and the heart. It is important to monitor and if necessary reduce the blood pressure to prevent damage to the blood vessels and the heart.

Readings of Blood Pressure:

A blood pressure reading consists of two numbers:

1.     Systolic

2.     Diastolic.

Systolic refers to systole, the phase when the heart pumps blood out into the aorta. Diastolic refers to diastole, the resting period when the heart refills with blood. At each heartbeat, blood pressure is raised to the systolic level, and, between beats, it drops to the diastolic level.

Systolic and diastolic pressures are commonly stated as systolic 'over' diastolic and measured in Millimeters of Mercury. For example, 120 over 80 (120 / 80 mmHg).

The average normal blood pressure for a resting adult is 120/80 mmHg.

History of Sphygmomanometers:

English clergyman Stephen Hales was the first person to suggest that blood pressure can be measured and he demonstrated this using a horse as his subject in the year 1733.

During the 19th century scientists like Jean Leonard Marie Poiseuille and Carl Ludwig designed primitive mercury based devices that could measure the systolic blood pressure. These early devices were not very practical for regular clinical use due to the fact that they were invasive in nature. As strange as it sounds, tubes had to be inserted into the blood vessels (arteries) for the device to register blood pressure.

During the second half of the 19th century scientists like Etienne Jules Marey and Pierre Potain developed noninvasive devices to measure blood pressure.

Austrian physician Samuel Siegfried Karl von Basch invented the sphygmomanometer in 1881

In 1896 Scipione Riva-Rocci further developed the sphygmomanometer by give it the all familiar modern day look and feel by adding an inflatable cuff for the arm and an attached bulb for pumping air into the cuff.

By the end of the 19th century it was only possible to record systolic pressure and diastolic pressure was not detectable. In 1905, Russian surgeon Nikolai Korotkoff first described the technique to measure diastolic pressure. Using a stethoscope, he identified blood flow sounds now known as “Korotkoff sounds” observed while the cuff is inflated and deflated.Since then the basic working principles of measuring systolic and diastolic blood pressure has remained unchanged.

Recent developments in electronics and sensor technology has led the way to replace mercury filled manual sphygmomanometer devices with automatic electronic sensor based sphygmomanometers. These devices are slowly gaining wider acceptance in clinical settings and is the preferred choice for personal monitoring due to its relative ease of use.

Components of Mercury BP Monitor

The sphygmomanometer has several key components to help determine the blood pressure of the patient.

The Cuff: Is the part that is placed around the patients arm. It fills with air to apply pressure to the arm and limits the amount of blood flow to the area being monitored.

Cuff is connecting to Tube and Bulb: The bulb is squeezed repeatedly to allow air pressure to build up in the cuff. The tubing is used to send the air to and from the cuff.

At the base of the tubing, near the bulb, is a Valve: This valve is used to slowly release the air to get the proper reading.

Gauge: The gauge shows the amount of pressure being placed on the arm for the reading.

Working Mechanism:

The sphygmomanometer cuff is inflated to well above expected systolic pressure. As the valve is opened, cuff pressure (slowly) decreases. When the cuff's pressure equals the arterial systolic pressure, blood begins to flow past the cuff, creating blood flow turbulence and audible sounds. Using a stethoscope, these sounds are heard and the cuff's pressure is recorded. The blood flow sounds will continue until the cuff's pressure falls below the arterial diastolic pressure. The pressure when the blood flow sounds stop indicates the diastolic pressure.

Measuring Blood Pressure:

A conventional or “old school” blood pressure monitoring device is the Mercury Sphygmomanometers. An inflatable cuff attached to the blood pressure monitor is placed around the patient's upper arm while the patient is seated beside a table by resting the arm on the table. A stethoscope is placed below the cuff, around the inside of the elbow.

Air is pumped into the cuff and this increases pressure around the arm and the examiner observers the pulse sounds till the blood flow stops to the arm being examined. Then air pressure is slowly released by deflating the cuff and the point at which the blood flow is restored is recorded as the systolic pressure. The reading indicates the maximum output pressure generated by the heart while its pumping.

The examiner continues to deflate the cuff and usually around the maximum deflation the pulse sounds disappears completely and this is recorded as the diastolic pressure. The reading indicates the pressure in the circularly system when the heart is relaxing.

With children, the clicking sound does not disappear but changes to a soft muffled sound. Because sounds continue to be heard as the cuff deflates to zero, the reading of the gauge at the point where the sounds change is recorded as the diastolic pressure.

Note:

• It’s very important to choose the correct size of the Cuff. Depending on the circumference of the patient’s upper arm, cuff size of appropriate length should be used. Inappropriate cuff size can result in incorrect readings.
• Using a small cuff may result in erroneously high blood pressure readings, while a larger cuff may give out erroneously low blood pressure readings. The inflatable part of the cuff should cover at least 80% of the circumference of the patient's upper arm.
• Cuff size (length) comes in different variants like Child's cuff (<24cm), Regular adult cuff (24-30.5cm), Large adult cuff (33-42cm)
• The cuff should be firmly applied with the centre of the inflatable part over the brachial artery.
• The cuff must be free of leaks.
• The mercury should be clean and at the zero mark before use.
• During cuff inflation the mercury should rise smoothly, and stop immediately inflation stops.

Purpose of BP Monitoring:

A sphygmomanometer is used to establish a baseline at a healthcare encounter and on admission to a hospital. Checking blood pressure is also performed to monitor the effectiveness of medication and other methods to control hypertension, and as a diagnostic aid to detect various diseases and abnormalities.

A sphygmomanometer can be used or encountered in a variety of settings:

·        Home

·        Hospital

·        Primary care clinic or professional office

·        Ambulance

·        Dental office

·        Pharmacy and other retail establishment

Types:

There are three types of sphygmomanometers:-

1. Mercury Sphygmomanometer:

Blood pressure readings produced by a conventional sphygmomanometers based on mercury is considered as the golden standard in the health industry.

Mercury based produces have been banned in many countries due to the toxicity associated with the metal. However, some exceptions have been made for medical devices like conventional sphygmomanometers that are used in clinical settings due to the lack of alternative medical devices that can produce similar accurate readings.Mercury sphygmomanometers have a manually inflatable cuff that is attached to the measuring unit by tubes. The measuring unit has a transparent tube containing mercury that is calibrated and marked in millimeters of mercury (mmHg).

During the recording procedure, it’s important to place the unit on a flat surface on its upright position. The healthcare worker operating the unit can read the reading at eye level to the unit.

These devices are very delicate and special care should be taken while operating, storing or transporting the unit. Accidental dropping of the unit can result in the rupture of the mercury containing tube and the spilling of the toxic mercury metal.

Advantages of Mercury Sphygmomanometers:

      I.           It’s a simple medical device and the device is very durable. If properly used, this device can be used for a lifetime.

   II.           This device can produce accurate readings and do not require any readjustments or recalibrations.

 Disadvantages of Mercury Sphygmomanometers:

      I.           It’s a bulky medical device that should be handled carefully to prevent damage to the mercury containing tube.

   II.           Operating this device requires practice and is not suitable for household uses.

 III.           People with hearing or visual disabilities cannot use this device. The reading should be read on a flat surface at eye level.

2. Aneroid Sphygmomanometer:

Aneroid means “without fluid”. These devices do not use mercury and is considered as a safer alternative when compared to mercury based sphygmomanometers.

The recording procedure using an aneroid sphygmomanometer is very similar to a conventional mercury based Sphygmomanometers requiring inflating and deflating the cuff with the exception that most Aneroid devices come with an attached stethoscope to the cuff.The device consists of cuff that is attached by tubes to a dial gauge marked in millimeters of mercury (mmHg). Inside the gauge head the device uses mechanical parts to convert the cuff pressure into a gauge based reading.

Aneroid Sphygmomanometers comes in various form factors and some of the commonly found variants are:

·        Pocket Aneroid Sphygmomanometer: It is the most popular variant due to its compact design, portability and low cost. It very popular among medical students and nurses.

·        Palm Aneroid Sphygmomanometer: It is popular in clinical and Emergency medical service (EMS) environments where cuffs of various sizes are required. The bulb and dial gauge is designed in a compact form factor to be operated by one hand. It is very easy to switch different cuff sizes as per requirement in emergency situations.

·        Clock-style Aneroid Sphygmomanometer: These variants typically have larger dials for viewing from a distance. They are usually found in the doctor's office, clinics or nursing homes. The dial gauge can be wall mounted, desktop or attached to a portable stand for mobility.

Advantages of Aneroid Sphygmomanometers:

      I.           They are cheaper, more portable and less expensive compared to mercury sphygmomanometers

   II.           The aneroid gauge can be placed in any position for easy reading. Dials comes in various sizes for comfortable and quick interpretation.

 III.           This also come with build in stethoscope.

 IV.           The aneroid gauge can be attached to the cuff for single hand operation of the device.

 Disadvantages of Aneroid Sphygmomanometers:

      I.           The aneroid gauge is a delicate mechanism. Special care should be taken to prevent accidental bumping or dropping of the gauge.

   II.           Aneroid gauges require periodic cross checking with mercury sphygmomanometer to make sure that the internal mechanisms are working perfectly. It may require recalibration by experts if the device is giving faulty reading.

 III.           Like mercury sphygmomanometer, people with hearing or visual disabilities cannot use the device.

 IV.           Operating aneroid sphygmomanometers requires practice.

3.  Automatic Digital Sphygmomanometer:

Oscillometric devices are commonly referred as Automatic Digital Sphygmomanometers or Digital Sphygmomanometers. These devices use an electronic pressure sensor for measuring the blood pressure and the readings are given out digitally on a display.

These devices have inflatable cuffs like Mercury or Aneroid Sphygmomanometers and the cuff is attached to the electronic unit. However, the main difference is in the technique used for measuring the blood pressure.

Mercury or Aneroid Sphygmomanometers reports are based on the sounds produced by the blood flowing inside the arteries. Digital Sphygmomanometers evaluates and measures the oscillations of the arteries using pressure sensors.

As the cuff is inflated and then deflated later, oscillations occurs. These oscillations are processed using an algorithm to produce systolic and diastolic values that are digitally displayed on the device display.

It is interesting to note that the working principles behind the oscillometric devices were discovered even before Korotkoff’s technique came into existence. However, due to various factors oscillometric approach remained unpopular till recently.

Automatic Digital Sphygmomanometers are usually battery operated. Some models designed for field use have dual power sources like battery and solar cells. They come in two variants:

·        Full-automatic blood pressure monitors: These devices have an electric pump for inflating the cuff. The operation of the device is very easy and requires minimum inputs from the user. Once the cuff is placed on the upper arm, the device can be switched on and the reports are produced automatically.

·        Semi-automatic blood pressure monitors: The user has to inflate the cuff manually by hand using the bulb like a conventional device. Once inflated, the device can then start deflating the cuff automatically and beyond this point the reading is produced in a similar way as an automatic device. These devices consume less power and can be more suitable for field operations where resources might be limited.

·        Wrist blood pressure monitors: These are digital blood pressure monitors that work similar to upper arm blood pressure monitors. It can be used by individual who find arm based devices uncomfortable or painful. However, medical experts do not recommend these devices for everyone, due to the possibility of receiving false reading due to improper use. Blood pressure monitors are very sensitive to body position and special care should be taken during their use in order to get accurate readings.

Advantages of Automatic Digital Sphygmomanometers:

      I.           The device is very compact and portable. Operating the device is extremely easy and this is the preferred device of choice for personal monitoring at home.

   II.           Since most of the critical operations are done automatically during the recording process, chances of human error are minimum.

Disadvantages of Automatic Digital Sphygmomanometers:

      I.           The device is delicate and proper care should be taken while handling the device. Repairing the device can be complicated and in most cases the device has to be serviced by the manufacturer.

   II.           Even the most advanced devices can produce incorrect reading with some individuals. Therefore, it necessary to periodically counter check with conventional Mercury sphygmomanometers for accuracy.

Problems in Mercury Sphygmomanometer:

·        Mercury vapour is poisonous: Any maintenance should be performed in an area of good ventilation. Store mercury in a plastic bottle with a little water placed on top of the mercury. Be careful not to inhale black mercuric oxide powder during cleaning procedures.

·        Black discoloration of the mercury: The mercury should be a clean silver colour. With time, a black powder (mercuric oxide), forms on the surface. A little black powder in the column does not matter. If there is a large amount, the mercury should be removed from the sphygmomanometer and the column and reservoir cleaned. Do this by laying the machine on its side with the reservoir downwards. Remove the column, ensure that you do not lose the leather disc at the top. Undo the reservoir top and pour the mercury out into a plastic bottle.

Blow the reservoir and the leather disc in the reservoir top clean with compressed air and wipe with a cloth. (If you don't have compressed air, take an old blood pressure inflation bulb, find a large bore needle, file off the tip and fit it to the inflation bulb. By squeezing the inflation bulb you will have a source of compressed air). Clean the inside of the column and replace it. Remember to replace the leather disc at the top and the washer at the bottom.

·        Replace the mercury in the reservoir to the zero mark: Use a syringe and needle to draw up the mercury from the plastic bottle. Keep the needle under the surface of the mercury to avoid returning any black powder. Replace the reservoir top with its sealing ring, connect the cuff and check the system is airtight by inflating the cuff until the mercury is at the top of the column. Check that it does not spill out-if this happens, the top leather disc is faulty or missing. (This disc should allow the air to pass in and out as the mercury rises and falls, but not allow mercury to escape.)

·        Mercury continues to rise slowly after stopping inflation: This is caused by the air at the top of the column failing to escape through the top leather disc quickly enough as the mercury rises up the column. When the sphygmomanometer is used, this fault may result in abnormally high readings as the mercury falls more slowly than the cuff pressure, due to the faulty leather disc restricting the air entering the top of the column.

The cause of these faults is in the top leather disc. It is either too thick or dirty. It should be removed as described before and cleaned. Replace it and test the sphygmomanometer. If the fault persists, remove the disc. Holding it between your finger and thumb on a flat surface, gently scrape it with a scalpel blade. Turn it continually to ensure it keeps its round shape and take care not to put a hole in it. Refit.

·        The mercury does not rise but the cuff inflates: This indicates blockage at D in the figure or a kinked or obstructed tube.

·        The cuff will not inflate or mercury rise: This indicates that there is a leak. Check valves A, B and C, the rubber bladder, tubing and connections. Rubber bladders may be repaired with an ordinary bicycle tyre puncture kit. Valve B can usually be removed and cleaned. Valve A may be a small ball bearing which can be removed from its cage and, with care, cleaned and replaced. Valve C cannot normally be removed.

After full assessment reassemble the sphygmomanometer and test.

For Further Readings…..

Procedures of using a Sphygmomanometer:

? To begin blood pressure measurement, use a properly sized blood pressure cuff. The length of the cuff's bladder should be at least equal to 80% of the circumference of the upper arm.

? Wrap the cuff around the upper arm with the cuff's lower edge one inch above the antecubital fossa.

? Lightly press the stethoscope's bell over the brachial artery just below the cuff's edge. Some health care workers have difficulty using the bell in the antecubital fossa, so its suggested that using the bell or the diaphragm to measure the blood pressure.

? Rapidly inflate the cuff to 180mmHg. Release air from the cuff at a moderate rate (3mm/sec).

? Listen with the stethoscope and simultaneously observe the dial or mercury gauge. The first knocking sound (Korotkoff) is the subject's systolic pressure. When the knocking sound disappears, that is the diastolic pressure (such as 120/80).

? Record the pressure in both arms and note the difference; also record the subject's position (supine), which arm was used, and the cuff size (small, standard or large adult cuff).

? If the subject's pressure is elevated, measure blood pressure two additional times, waiting a few minutes between measurements.

? A Blood Pressure Of 180/120mmhg Or More Requires Immediate Attention!

Subject:

Position: supine, seated, standing.

In seated position, the subject's arm should be flexed.

The flexed elbow should be at the level of the heart.

If the subject is anxious, wait a few minutes before taking the pressure.

Precautions:

? Patient position for using sphygmomanometer

? Aneroid and digital manometers may require periodic calibration.

? Use a larger cuff on obese or heavily muscled subjects.

? Use a smaller cuff for pediatric patients.

? For pediatric patients a lower blood pressure may indicate the presence of hypertension.

? Don't place the cuff over clothing.

? Flex and support the subject's arm.

Note:

In some patients the Korotkoff sounds disappear as the systolic pressure is bled down. After an interval, the Korotkoff sounds reappear. This interval is referred to as the "auscultatory gap." This pathophysiologic occurrence can lead to a marked under-estimation of systolic pressure if the cuff pressure is not elevated enough. It is for this reason that the rapid inflation of the blood pressure cuff to 180mmHg was recommended above. The "auscultatory gap" is felt to be associated with carotid atherosclerosis and a decrease in arterial compliance in patients with increased blood pressure.

Interpretation:

Blood pressure readings must be interpreted in relation to a person's age, physical condition, medical history, and medications being used.

Maintenance:

Devices should be checked and calibrated annually by a qualified technician to ensure accurate readings. This is especially important for automatic sphygmomanometers.

Normal results:

One elevated reading does not mean that hypertension is present. Repeated measurements may be required if hypertension is suspected. The blood pressure measurement is recorded and compared with normal ranges for an individual's age and medical condition, and a decision is made on whether any further medical intervention is required.

Conditions Diagnosed by Measuring BP:

? Hypertension

? Hypotension

? Pulsus paradoxus

? Pulsus alternates

? Coarctation of aorta (Hypertension in upper limb, hypotension in lower limb)

? Aortic incompetence (Hill’s sign)

Reason for Unequal Blood Pressure in Two Arms of Same Individual:

? In normal individuals, BP may vary due to recording at different times in two arms.

? Supravalvular aortic stenosis (right sided higher BP)

? Preductal coarctation of aorta (right sided higher BP)

? Unilateral occlusive disease of the arteries-Atherosclerosis, embolism, aortoarteritis, thoracic outlet syndrome, etc. (BP will be low on the affected side)

References:

·        3 Types of Blood Pressure Monitoring Devices - Sphygmomanometers - Medicwiz. 2017. 3 Types of Blood Pressure Monitoring Devices - Sphygmomanometers - Medicwiz. [ONLINE] Available at: https://www.medicwiz.com/medtech/diagnostics/3-types-of-blood-pressure-monitoring-devices-sphygmomanometers. [Accessed 02 December 2017].

·        American Diagnostic Corporation - Core Medical Device Manufacturer. Stethoscopes, Blood Pressure, Thermometry, and EENT. 2017. American Diagnostic Corporation - Core Medical Device Manufacturer. Stethoscopes, Blood Pressure, Thermometry, and EENT. [ONLINE] Available at: https://www.adctoday.com/learning-center/about-sphygmomanometers/history-sphygmomanometer. [Accessed 01 December 2017].

·        EBME. 2017. Maintenance of a Mercury Sphygmomanometer.. [ONLINE] Available at: https://www.ebme.co.uk/articles/maintenance/344-maintenance-of-a-mercury-sphygmomanometer. [Accessed 03 December 2017].

·        HowStuffWorks. 2017. How does a blood pressure gauge (sphygmomanometer) work? What exactly is blood pressure? | HowStuffWorks. [ONLINE] Available at: https://science.howstuffworks.com/innovation/everyday-innovations/question146.htm. [Accessed 02 December 2017].

·        MedicineNet. 2017. Medical Definition of Sphygmomanometer. [ONLINE] Available at: https://www.medicinenet.com/script/main/art.asp?articlekey=15771. [Accessed 01 December 2017].

·        Practical Clinical Skills. 2017. Sphygmomanometer . [ONLINE] Available at: https://www.practicalclinicalskills.com/sphygmomanometer. [Accessed 02 December 2017].

·        Understanding the Parts of a Blood Pressure Cuff   . 2017. Understanding the Parts of a Blood Pressure Cuff . [ONLINE] Available at: https://bloodpressuremonitorguide.com/understanding-the-parts-of-a-blood-pressure-cuff/. [Accessed 07 December 2017].