Blood pressure is a moving number

Blood pressure is not a fixed property like your height. It rises and falls through the day in response to activity, posture, emotion, temperature, caffeine, pain and sleep. It is typically at its lowest during sleep and rises around the time you wake.

A reading taken in a clinic room captures one point on that curve. Sometimes that single point represents your usual blood pressure well. Sometimes it does not, and treating it as though it does can send things in the wrong direction.

Two ways a clinic reading can mislead

Blood pressure that is higher in the clinic than elsewhere. Often called white coat hypertension, this is a genuine and well-recognised effect. The reading is real, but it reflects the setting rather than the person's usual blood pressure. Acting on it alone can lead to medication that is not needed.

Blood pressure that is normal in the clinic but higher elsewhere. Sometimes called masked hypertension, this is the more concerning of the two, because raised blood pressure outside the clinic goes unrecognised. It can be more common in people who are working, under stress, or who smoke.

Neither pattern can be identified from a single clinic measurement. That is the practical reason your doctor may want readings collected away from the clinic before making a diagnosis or changing treatment.

What out-of-clinic measurement adds

There are two established approaches, and they answer slightly different questions.

Home monitoring uses your own validated upper-arm monitor, with readings taken in a consistent way over several days. It is convenient, repeatable, and gives your doctor a picture of your typical daytime blood pressure.

Ambulatory monitoring uses a cuff worn for 24 hours that inflates automatically at set intervals, including while you sleep. Because it continues overnight, it shows something home monitoring cannot: whether your blood pressure falls during sleep as it normally should. It also produces a large number of readings without you having to remember to take them.

Which one suits you is a decision for your doctor, based on what they are trying to establish.

How to take a good reading at home

Technique affects the number more than most people expect. If you are measuring at home, these steps help make the readings meaningful:

• Use a validated upper-arm monitor. Wrist and finger devices are generally less reliable.
• Check the cuff fits your arm. A cuff that is too small can read high.
• Avoid caffeine, smoking and exercise for 30 minutes beforehand.
• Empty your bladder first.
• Sit and rest quietly for five minutes before the first reading.
• Sit with your back supported, feet flat on the floor, legs uncrossed.
• Rest your arm on a table so the cuff sits at about heart height.
• Do not talk during the measurement.
• Take two or three readings a minute apart, and record them all.

Measure at roughly the same times each day, usually morning and evening, and bring the full record to your appointment rather than only the numbers that looked good.

A note on interpreting numbers

The thresholds used to define high blood pressure differ depending on how the reading was taken. Averages from ambulatory or home monitoring are generally assessed against lower thresholds than clinic readings, because the clinic setting itself tends to raise the number.

This is one reason it is worth resisting the urge to interpret your own readings against a figure found online. Your doctor considers the measurement method, the pattern across readings, your age, and your other risk factors together. A single high number is a prompt for a conversation, not a diagnosis.

What a 24 hour monitor involves

If your doctor refers you for ambulatory monitoring, a cuff is fitted to your non-dominant arm and connected to a small recorder. Fitting takes about 15 minutes. You then go about your normal day and sleep in it overnight, keeping it dry, and return it the following day along with a short diary of your activities and any symptoms.

The cuff inflates at intervals through the day and night. Most people find it more noticeable at night than during the day. Keeping your arm still and relaxed while it inflates gives a better reading.

Where Heartscope fits in

We provide 24 hour blood pressure monitoring on referral across our Melbourne clinics. A referral from your GP or specialist is normally required, and a refundable security deposit applies for the monitor. Your recording is analysed and reported back to your referring doctor, who will discuss what it means for you.

If you have questions about how to prepare, call us on 1800 202 111.

This article is general information only. It is not a substitute for individual medical advice. Please speak with your doctor about your own circumstances.