What is a Sphygmomanometer?
A sphygmomanometer also known as blood pressure monitor is a medical device used to measure the blood pressure. It is one of the most iconic medical device found virtually on every doctor’s table. The name Sphygmomanometer is a combination of Greek word "Sphygmos" which means pulse and the scientific term "Manometer" which means pressure meter.
What is 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.
What is normal blood pressure?
Normal blood pressure for adults is below 120/80 mmHg (millimeters of mercury).
A blood pressure monitoring device measures the blood pressure when the heart is pumping (Systolic Pressure) and when the heart is between beats or at rest (Diastolic Pressure) and gives reading as systolic pressure over diastolic pressure expressed in millimeters of mercury (e.g. 120/80 mmHg)
History of Sphygmomanometer
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.
How does a sphygmomanometer work?
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.
Does sphygmomanometer cuff size matter?
Yes, it’s important to choose the correct cuff size. 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)
3 Types of blood pressure monitors (Sphygmomanometers)
Techniques to accurately measure blood pressure has been around for more than 100 years. The meaning of the numbers obtained were not clear till the later half of the 20th century when links to heart disease and stroke were linked to high blood pressure. It was previously assumed that high blood pressure was a part of the natural process of ageing.
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
- It’s a simple medical device and the device is very durable. If properly used, this device can be used for a lifetime.
- This device can produce accurate readings and do not require any readjustments or recalibrations.
Disadvantages of Mercury Sphygmomanometers
- It’s a bulky medical device that should be handled carefully to prevent damage to the mercury containing tube.
- Operating this device requires practice and is not suitable for household uses.
- People with hearing or visual disabilities cannot use this device. The reading should be read on a flat surface at eye level.
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
- They are cheaper, more portable and less expensive compared to mercury sphygmomanometers
- The aneroid gauge can be placed in any position for easy reading. Dials comes in various sizes for comfortable and quick interpretation.
- The also come with build in stethoscope.
- The aneroid gauge can be attached to the cuff for single hand operation of the device.
Disadvantages of Aneroid Sphygmomanometers
- The aneroid gauge is a delicate mechanism. Special care should be taken to prevent accidental bumping or dropping of the gauge.
- 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.
- Like mercury sphygmomanometer, people with hearing or visual disabilities cannot use the device.
- Operating aneroid sphygmomanometers requires practice.
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
- 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.
- Since most of the critical operations are done automatically during the recording process, chances of human error are minimum.
Disadvantages of Automatic Digital Sphygmomanometers
- 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.
- 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.