why blood pressure goes up and down

This is a puzzling subject for many patients, not least because blood pressure readings do vary in any one person from hour to hour and day to day.

However, as I’m sure you realise, correct measurement is essential, as high blood pressure must never go undiagnosed.

Officially known as hypertension, it causes readings consistently above 140/90, and has a number of bad consequences for future health. Not only is it harmful for the heart, but it raises the risk of stroke, and damages the kidneys and eyes, too.

When your blood pressure is tested, the reading produces two numbers.

The first represents the pressure in your blood vessels when your heart contracts. Each contraction sends out around a teacupful of blood; the pressure forces it throughout the body to move along the blood vessels.

At this point your blood pressure is at its highest, known as systolic pressure.

When the heart relaxes and is refilling, preparing for the next contraction, the pressure drops back. This lower reading is your diastolic pressure

A healthy blood pressure is around 120/80, though it’s not a fixed figure — as with weight, there is a spectrum that is considered healthy. But although measuring blood pressure may seem simple, there are a number of important factors that can influence your reading.

Smoking briefly pushes up blood pressure (this may not apply to you), as does coffee. Both must be avoided for at least an hour before a reading is taken. Measurement in cold surroundings, or while you are talking, will also result in a higher reading, as both increase heart rate.

The type of machine used is also important: the old-fashioned mercury ones are the most accurate — these are the ones where you squeeze on a rubber bulb to pump up the cuff. But these are slowly being abandoned because of health and safety obsessions — mercury, if spilled, is toxic to the environment.

However, the advantage of the automatic types is that they are easier to read and less training is needed to use them correctly. Both work by inflating the cuff until it cuts off blood flow, then slowly deflating it and measuring the point at which blood starts to flow again through the arm.

The use of a proper cuff size is also essential: too small and it will give inaccurately high readings. The cuff must be carefully placed: never over clothing, and if a rolled-up sleeve is at all tight around the arm then clothing must be removed.

Furthermore, you should be seated comfortably for five minutes before the reading is taken, and during measurement your arm should be at the level of your heart.

Another factor is angst — one in five patients found to have high readings in a doctor’s office is eventually found to have normal blood pressure, a condition known as ‘white coat hypertension’.

Many surgeries avoid this by asking nurses to take the readings (as they seem to provoke less anxiety than doctors) and asking patients to take their own measurements on an automatic machine at home.