The Blood Pressure Meter

Source: Dr. Emmanuel K. Coomson, Regional Hospital, Koforidua, [email protected] - Ghanaian Chronicle

The sphygmomanometer, or 'sphyg' for short, is a device used to measure blood pressure. It can also be called a blood pressure meter. Blood pressure is the force exerted by the blood on blood vessel walls as the heart pumps it around the body. When the blood pressure is higher than normal it is called hypertension or high blood pressure. Hypertension is a major killer in our society through strokes, heart attacks and kidney diseases.

Hypertension usually does not present with any symptoms so most people who have it don't know they do, until they have their blood pressure checked during a screening programme or on a visit to the hospital. For some unfortunate individuals, they only get to know they had hypertension only after the disease has given them a stroke or a heart attack.

Hypertension has therefore been called the silent killer. Thus, it was a blessing that the sphyg was invented to enable us determine our blood pressure at all times.

Human blood pressure was first measured in 1847 by Carl Ludwig. He did this by inserting a cannula (narrow tube) directly into the artery of the patient. This cannula was then connected to a U-shaped manometer (a device that measures pressure) from which the pressure was measured. As you can imagine this method did not catch up with medical practitioners because there are a lot dangers involved in inserting a cannula into a patient's blood stream.

It was not until 1881, when Samuel Siegfried Karl Ritter von Basch invented the sphygmomanometer. His device consisted of a water-filled bag connected to a manometer. His device measured blood pressure fairly accurately without sticking anything into the body of the patient.

Scipione Riva-Rocci developed a more accurate sphygmomanometer which used mercury instead of water in 1896. This design was the prototype of the modern mercury sphygmomanometer.

Riva-Rocci's sphygmomanometer was spotted by the American neurosurgeon Harvey Cushing who seeing the potential benefit of the device sent the design to the United States in 1901. The design was modified for more clinical use and the sphygmomanometer became commonplace.

The word sphygmomanometer comes from the Greek word sphygmós meaning 'pulse', and the scientific term manometer meaning 'pressure meter'.

The manual or classical sphygmomanometer consists of an inflatable cuff, a measuring unit (the mercury manometer), a tube to connect the two, and an inflation bulb also connected by a tube to the cuff.

The cuff is normally placed around the left arm, at roughly the same vertical height as the heart while the subject seated with the arm supported.

The human heart pumps out blood about 72 times every minute into the blood vessels. Between any two pumping actions, the heart relaxes and refills with blood. The pressure that the heart generates when it pumps out blood is called the systolic pressure and the pressure generated when the heart relaxes is called diastolic pressure. These two readings make up one's blood pressure. Thus blood pressure is recorded as two figures; systolic over diastolic readings. Eg. 120/80 mmHg.

The classical sphygmomanometer is used with a stethoscope. The stethoscope is used to listen to the flow of blood in the brachial artery. The stethoscope enables the one measuring the blood pressure to accurately tell the exact systolic and diastolic readings by listening to the various changes in the sounds the blood makes as it flows through the blood vessel narrowed to various degrees by the cuff applied to the arm.

By observing the mercury in the column while releasing the air pressure with a control valve, one can read on the scale the values of the blood pressure in mm Hg.

The classical sphyg must be operated by a trained person. Mercury manometers are considered to be the “gold standard” of measurement because they do not go out of calibration. For this reason they are often required in clinical trials of pharmaceuticals and for clinical evaluations of determining blood pressure for high risk patients including pregnant women.

There are digital blood pressure machines with manual or automatic inflation. These are electronic, easy to operate and practical in noisy environments because they do not require the use of a stethoscope. These have sensors that enable them derive the blood pressure. They are useful when they are regularly compared with the classical type and adjusted. They are fond of giving inaccurate readings.

The digital portable wrist blood pressure meters and Digital portable finger blood pressure meters have automatic inflation. They are powered by batteries and are more convenient for travel purposes. They are applied to the wrist or finger. Again these too are more portable and easy to operate, but less accurate.

Every one above the age of 30 years must have their blood pressure checked at least twice every year. While having your blood pressure checked it is very important that you should have rested for at least 15 minutes if you walked to the hospital. You should also have a peaceful thought in your mind and not be angry with the nurse or be thinking about how long a time you will have to wait for the doctor. All these states of mind tend to increase the blood pressure.

If you have hypertension and are on antihypertensive medication, it is important to take your drugs daily as prescribed and report to the hospital when you have about five day's stock of drugs left and not completely run out. This is because the doctor would want to know if the medication is working well for you by checking you pressure while you are taking the medication. If you don't take the medications for a few days, the blood pressure will go up and that can lead to a stroke or heart attack.