The answer to the question – how is blood pressure measured with or by a blood pressure machine – is not a discourse on how to perform a blood pressure measurement. It is more. It is insight into how a device allows you to measure BP or how it measures the reading itself, depending on the type of the device. Knowing how the device works makes the BP reading it yields more meaningful.
Blood pressure (BP) is the force exerted against the walls of arteries, expressed in terms of mm/Hg, as blood flows through them. Direct measurement of it is possible by means of a catheter inserted into an artery. But this method is impractical and in rare use this day and age, except to monitor the BP of very ill patients in hospitals settings.
Blood pressure measurement basic concepts
The method of BP measurement in most common use today is indirect. Instead of measuring the pressure directly inside of an artery, determining the amount of pressure around the artery that cuts off blood flow through it is the accepted equivalent. Arteries don’t open and close naturally between heartbeats to produce sounds. Neither are heart sounds audible in vessels. Therefore, an indirect measurement of BP requires artificially causing blood flow through an artery to produce sound as the heart pumps blood out. This is what a blood pressure machine does.
BP measurement most commonly involves the use of a device consisting of a cuff that wraps around the arm. Inflation of the cuff with air causes it to apply pressure to the tissues surrounding the brachial artery – the main artery in the arm. In general, the tissue pressure is equal to the cuff pressure displayed by a manometer during inflation of the cuff. The pressure in the tissues surrounding the artery compresses the artery. When the compression reaches a certain level the artery collapses and blood flow through it ceases.
If the surrounding tissue pressure drops to a level equal to or just below the pressure within the artery blood forces the artery open and blood flows through as the heart pumps. This produces Korotkoff sounds – vibrations of the walls of the artery from the forced opening of the vessel. The use of a stethoscope placed over the artery is required to hear these sounds. They correlate with the first heart sound of each heartbeat, but are not the heart sounds per se.
When the external pressure around the brachial artery drops below the pressure within it while the heart is filling, Korotkoff sounds cease to be audible. The reason is the artery no longer collapses when the heart fills with blood; thus it does not open due to force within it when the heart pumps blood out.
Blood pressure measurement devices
There are basically two types of devices for measuring BP indirectly. They are manual and digital. A manual device allows a person to measure the BP. A digital or electronic device makes the measurement itself. There are human actions required with the use of either type of device. But the digital device is simpler to use.
Sphygmomanometer is the medical name for the tool that allows the manual measuring of BP. Another common name for it is manual blood pressure monitor. Some in the medical community refer to it as an aneroid (clock face) device.
The unit consists of a flat rubber bag enclosed in a cuff, a manometer -pressure measuring unit – and a rubber bulb with a valve which serves as an air pump. Rubber tubing connects the bulb to the bag and the bag to the measuring unit.
Squeezing the bulb inflates the bag with air and applies pressure to the surrounding cuff. Closing the valve on the bulb by turning the valve knob clockwise allows air to move into the bag but not out during squeezing of the bulb. Therefore with each squeeze the amount of air in the bag increases.
The cuff is made of plastic, fabric, or other material that does not stretch. Thus, it creates pressure in the tissues around the brachial artery as air enters the enclosed bag.
Opening the valve by turning the knob counterclockwise allows air to escape. This causes the pressure in the cuff and in the tissues around the artery to fall. The rate at which the pressure drops depends on how open the valve is.
The procedure for measuring BP with this device involves:
- Properly placing the cuff around the arm
- Inflating the cuff to a reading which is 20 mmHg or higher than the anticipated systolic BP (or starting at 180 mmHg – 200 mmHg if you cannot anticipate the BP)
- Slowly releasing the air from the bag by opening the valve while listening with a stethoscope placed over the brachial artery for the appearance of Korotkoff sounds
- Continuing the cuff deflation and listening with a stethoscope until Korotkoff sounds cease to be heard
Manual units are also available for-self-measurement of BP. They differ from the regular ones in that they have an attached stethoscope which overlies the brachial artery when the cuff is in its correct position.
The digital blood pressure machine
The digital blood pressure machine is similar to the manual unit with respect to the use of cuff pressure to produce arterial vibrations. But instead of listening for Korotkoff sounds the machine senses the vibrations from the forced opening of the artery and converts them into electronic signals.
It matches the initial appearance of those signals with the cuff pressure at the time and the disappearance of them with the corresponding cuff pressure. It then displays both readings on a screen at the end of the process. Those readings are the systolic BP and diastolic BP.
From a user standpoint, the main features of a digital blood pressure machine that differ from a manual device are the following:
- Cuff inflation and deflation occur automatically with the push of a button.
- It does not require watching a monometer and listening to Korotkoff sounds during the process.
- The machine itself performs the BP measurement.
- Some devices can make wrist BP measurements.
- Most devices can store readings in their memory.