When you have high blood pressure, or hypertension, your blood pressure is persistently higher than normal. Your blood pressure naturally varies throughout the day and night, going up and down in response to your emotions, levels of stress and the amount of exercise you take. Your blood pressure can even rise from the stress of having it measured by your doctor – a condition known as white coat hypertension. These fluctuations are usually temporary and your blood pressure soon reverts to your baseline level. When you have hypertension, however, your blood pressure remains high all the time, even at rest and when you are asleep. This constant, high blood pressure wears away at your artery linings and can lead to long-term damage if not controlled.
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How blood pressure varies during the day and night
The normal 24-hour variation in blood pressure is shown on the following graphs for someone with normal blood pressure, someone with high blood pressure, and a graph that superimposes the two.
Normal blood pressure
A normal blood pressure remains below 140/80 mmHg (apart from a normal excursion upwards due to exercise) during both the day and night. Blood pressure naturally falls to below 100/60 mmHg during sleep. Blood pressure then surges up just before you wake. During the day, blood pressure goes up and down a little depending on factors such as emotions, eating, sleeping and walking around.
Graph 1: Typical 24 hour blood pressure variation in someone with normal blood pressure
If this person had exercised briskly, their blood pressure would have risen temporarily up to 160/90mmHg or higher, due to they dynamic effects of exercise on the circulation, but would quickly have come back down again to normal levels.
High blood pressure
In someone with untreated high blood pressure, or hypertension, their blood pressure remains above 140/80 mmHg for most of the day and night.
Graph 2: Typical 24 hour blood pressure variation in someone with untreated high blood pressure
The diurnal or circadian variation of someone with untreated high blood pressure shows a similar pattern to that of someone with a normal blood pressure, in that it falls during sleep and surges before waking. This surge is particularly ‘dangerous’ for someone with high blood pressure, however, as it puts significant stress on the circulation.
Normal versus high blood pressure
When the above two graphs are superimposed, it becomes very obvious how much higher your blood pressure is through the day and night when you have hypertension – there is hardly any overlap with normal readings during the evening from 18.00 hours through until the waking hours.
Graph 3: Comparison of the 24 hour blood pressure variation in someone with normal versus someone with untreated high blood pressure
The aim of blood pressure treatment, through diet and lifestyle changes as well as through medication, is to bring your day-to-day blood pressure readings back down to the normal range.
How blood pressure is measured
The pressure within the circulation was originally assessed against the weight of mercury (Hg) it could support within a long, thin, glass tube. The length of the column was measured in millimetres (mm) and the pressures recorded in units known as millimetres of mercury (mmHg). These units, which date back to the eighteenth century, are still used today.
Traditional, mercury-filled sphygmomanometers remain available, along with modern aneroid devices in which mercury is replaced with a metal spring. Now we also have automated electronic devices that detect pressure pulsations transmitted to a cuff, and which are easy to use when monitoring your blood pressure at home.
When your blood pressure (BP) is measured, two readings are taken: the higher number, or systolic pressure, is created as your heart contracts, while the lower number, your diastolic pressure, is the background pressure in your circulation when your heart rests between beats.
Assessing Your Blood Pressure
Blood pressure is measured with the aid of a cuff placed around your arm or wrist. It’s important that the cuff is positioned level with your heart to get an accurate reading. The cuff is then rapidly inflated to a pressure (eg 180 mmHg or more) that should be high enough to temporarily stop the flow of blood in the underlying arteries.
The gold standard procedure is to use an upper arm cuff, and to place a stethoscope bell just above the crook of the elbow to listen to noises within the underlying brachial artery. The cuff must be the right size for your arm, as described here, or readings will be inaccurate.
Systolic blood pressure: As the pressure in the cuff is slowly released, there comes a point when the pressure within the underlying artery is able to overcome the pressure in the cuff so that blood squirts through. This produces a distinct tapping noise called a Korotkoff sound. The pressure at which this noise is first detected is taken as the highest pressure – your systolic blood pressure.
Your systolic blood pressure is the highest pressure reached within your circulation as your heart contracts to pump blood into your arteries.
Diastolic blood pressure: As pressure in the cuff continues to fall, the tapping sounds become louder, then change to a dull whooshing noise due to turbulence as the cuff continues to constrict blood flow within the underlying artery. The point at which this whooshing noise suddenly stops indicates when blood flow is no longer being constricted. This marks the point at which the pressure in your arteries and the cuff is identical, and is recorded as your lowest pressure – your diastolic blood pressure.
Your diastolic blood pressure is the lowest pressure reached within your circulation as your heart rests between beats.
Blood pressure recording
By convention, your systolic blood pressure is written over your diastolic blood pressure (eg 120/70 mmHg) and when spoken is described as, for example, ‘one twenty over seventy’.
Systolic blood pressure is therefore referred to as the ‘upper’ reading and diastolic blood pressure as the ‘lower’ reading.
Listen to blood pressure sounds
California State University have a great on-line simulator that lets you hear the sounds recorded through a stethoscope when a healthcare professional takes your blood pressure.
Select one of the three simulations provided at the link and click on the pump to inflate the dial (go on, it’s fun!) Then, when it tells you to, click on the valve to let the air out.
You will hear the first Korotkoff sound which pinpoints your systolic or upper blood pressure. Then you’ll hear the sound muffling before it disappears altogether at the pressure which represents your ‘lower’ diastolic BP.
If your blood pressure is raised, self-monitoring helps you maintain good control.
Click here for advice on choosing a blood pressure monitor to use at home.
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