Many of us have had our blood pressure measured at the doctor but don’t really understand why it’s important. Find out why (and how) your doctor measures your blood pressure, what’s normal and what’s not, and what to do if you have high blood pressure.
How is blood pressure measured?
Blood pressure is measured using an instrument called a sphygmomanometer. It consists of an inflatable cuff, an inflating bulb, and a gauge to show the blood pressure.
The cuff is wrapped around the upper arm, and inflated to a pressure which temporarily stops the blood from flowing through the brachial artery – the main blood vessel in your arm. At this point, the doctor will not be able to feel a pulse in the arm. The cuff pressure is then raised slightly above this point, and then slowly lowered.
As the pressure is lowered, your doctor will listen through a stethoscope for the sound of blood starting to pulse through the brachial artery again. The pressure reading on the sphygmomanometer when the sound of blood is first heard pulsing through the artery is known as the systolic pressure. As the doctor continues to lower the pressure in the cuff, the sounds will disappear. Usually, the point at which the sounds disappear is known as the diastolic pressure.
The systolic reading (the first number of the 2) indicates the pressure of blood within your arteries during a contraction of the left ventricle of the heart. The diastolic reading (the second number) indicates the pressure within the arteries when the heart is at rest. Blood pressure is measured in millimetres of mercury (mmHg), for example 120/80 mmHg (known as 120 over 80).
What is normal blood pressure?
According to the Heart Foundation of Australia, as a general guide:
- blood pressure below 120/80 mmHg can be classified as ‘optimal‘;
- blood pressure between 120/80 and 129/84 mmHg is ‘normal‘; and
- blood pressure between 130/85 and 139/89 mmHg is classified as ‘high-normal‘.
A person is defined by the Heart Foundation as having high blood pressure (hypertension) if they:
- have a systolic pressure greater than or equal to 140 mmHg; and/or
- a diastolic pressure greater than or equal to 90 mmHg.
High blood pressure is further classified as mild, moderate or severe as the pressure increases above this level.
|Blood pressure categories – Australia
|Blood pressure category
|Less than 120
|Less than 80
|More than or equal to 180
|More than or equal to 110
|Isolated systolic hypertension
|More than 140
|Less than 90
Low blood pressure, or hypotension, is not as easy to define as it is usually relative to a person’s normal blood pressure reading, and varies between different people. It generally refers to a blood pressure below an average of about 90/60 mmHg.
Getting an accurate reading
According to the Heart Foundation, the diagnosis of high blood pressure should be based on multiple blood pressure measurements taken on separate occasions.
It is recommended that you do not smoke or drink caffeine-containing drinks for 2 hours before having your blood pressure monitored, as this can cause an increase in your readings.
Self-monitoring of blood pressure in your own environment (home blood pressure monitoring) or ambulatory monitoring of blood pressure is also used to help diagnose high blood pressure.
For ambulatory blood pressure monitoring, you wear a portable automatic blood pressure machine for 24 hours while going about your usual daily routine. The portable monitor will measure your blood pressure multiple times during the 24 hours. Your doctor will ask you to record your activities over the 24 hours, plus any symptoms you experience (and when they occur). Variations in blood pressure are normal and may occur depending on where and when the blood pressure is taken.
Some people who have raised blood pressure readings taken at the doctor’s surgery actually have acceptable levels outside the surgery, when under normal stress levels. This is known as ‘white-coat’ hypertension.
There are also people with ‘reverse white-coat’ hypertension (also known as masked hypertension), who have normal blood pressure when measured in the clinic but high ambulatory blood pressure readings (those recorded during normal daily activities).
Keeping on target
Your target blood pressure may vary according to whether you have other conditions that can increase your risk of cardiovascular (heart and blood vessel) disease or conditions that have been caused by high blood pressure.
Raised blood pressure is a major risk factor for cardiovascular disease, and the higher your blood pressure, the greater your chance of having heart disease or stroke. For this reason it is important that you have your blood pressure monitored regularly, and that you always take any high blood pressure treatments that have been recommended, including medicines that have been prescribed.
Hypertension can also be controlled to a large extent by lifestyle measures such as:
- losing excess weight;
- undertaking regular physical activity;
- quitting smoking; and
- changing your diet (reducing your alcohol and salt intake, and following a healthy eating plan).
These steps can help to lower your blood pressure and reduce your risk of cardiovascular disease.