ACADEMY PUBLIC POLICIES

Measuring blood pressure - Accuracy of readings

What are the factors that may influence an accurate reading of BP?

There has been a great deal of research into the factors that influence and control accurate BP measurement. They can be divided into health professional-related factors, patient- related factors and instrument-related factors. Common mistakes made by health professionals when measuring BP include the use of an inappropriately short and/or narrow BP cuff. This increases both systolic and diastolic readings. It is recommended that at least 80% of the arm should be encircled by the bladder of the cuff.

Sufficient time should be allowed between BP measurements. It is recommended that a minimum of two readings at any one consultation should be taken. It is also important that these two readings should be taken at least 30 seconds apart. Often the doctor or nurse may deflate the cuff too fast and inaccurately read the mercury off the sphygmomanometer. The following guides for accurate BP measurement should be followed:

• You should be seated in a quiet environment with your arm resting on a support that places the mid-point of your upper arm at the level of your heart.

• A large cuff, with the bladder encircling at least 80% of the arm, should be used.

• The lower edge of the cuff should be at least 2 cm above your elbow joint.

• The cuff should be inflated at 10 mmHg increments and the doctor or nurse should be feeling the pulse in your wrist or arm whilst doing this. A note should be made of the level at which the pulse disappears during inflation and subsequently disappears during deflation.

• The flat part (‘diaphragm’) of the stethoscope should be placed over the elbow joint in your arm. The bladder should be deflated steadily to 20 mm above the level found by disappearance of the pulse. Then the bladder should be deflated by 2 mm per second.

• The doctor or nurse should be listening in with the stethoscope and making note of the first appearance of repetitive sounds (these are known as ‘Korotkoff sounds’). They should then take note of the mercury level at which the repetitive sounds disappear (this corresponds to your diastolic BP).

• Recording of BP should be rounded upwards to the nearest 2 mm of mercury.

The last time I visited the clinic, I had to rush in from work. Would this have altered my reading?

Yes. Many everyday activities can increase your BP. For instance, brisk walking can increase your systolic BP by up to 12 mmHg and your diastolic BP by 6 mmHg. For this reason, when you have your BP checked, it is recommended that you are resting for at least 5 minutes prior to the reading being taken. If you ran for the bus or hurried to the clinic, you should sit quietly for about half an hour before any measurement is taken. It would be worth telling the receptionist this and ask if you could delay your visit until you feel more rested.

Is the accuracy of BP measurements affected by whether I’m overweight or thin?

Measurements on people with very thin arms underestimate BP by 5–10 mmHg, however carefully they are performed. There is no way round this, and it is probably the reason why research studies generally show that high BP in very thin people appears to carry greater risks than the same levels of BP in fat people (because extremely thin people often have some other illness).

The cuff used to block flow through the artery during measurements contains an inflatable rubber bladder. This should reach round 80% of the arm circumference. In larger people, if it is too short, BP measurements may be overestimated by up to about 20 mmHg, a serious margin of error.

This is a common cause of incorrect diagnosis and unnecessary treatment. It can be avoided only by using a larger cuff. Well-equipped surgeries and hospitals have such cuffs, but doctors and nurses don’t always remember to use them. Many family doctors and hospitals don’t even possess outsize cuffs. If you have a larger width arm (more than about 30 cm circumference), I’m afraid you must learn (gently and courteously but firmly) to insist on the use of an outsize cuff. If you don’t do this, you may easily be over treated.

I know this is silly, but I hate going to the doctor’s. Does my state of mind affect my BP measurements?

Yes it does. If you know you are anxious or frightened when you see your doctor or nurse, it is important to tell them this. Your BP measurement could be affected by what is known as ‘white coat hypertension’. You can then either arrange to have more measurements at the clinic, so that you get used to the procedure or become less anxious, or you can arrange to measure your own BP at home.

Are there any other important causes of misleading BP readings?

Apart from recent exertion, pain, fear, anger, and embarrassment and so on, other important causes are:

• Alcohol

• Smoking

• Coffee

• Some kinds of medication

• A full bladder

• Stress.

Large quantities of alcohol, taken slowly and steadily over months, or quickly in a binge, can raise BP substantially in many people. This is a common cause of sustained high BP in young men.

Several commonly used drugs, both prescribed and across-the counter, tend to raise BP. Never forget to remind your doctor or nurse what drugs you are taking, whether bought or prescribed. If your bladder is full, your BP could rise; in people with a BP normally around 130/70 mmHg, BP may rise substantially. This can happen easily in a doctor’s waiting room – if you want to go to the toilet but are afraid of losing your place in the queue, for example. It can also happen to men with benign enlargement of the prostate admitted to hospital with acute retention of urine.

Will it matter whether my BP is measured in the morning or the evening?

Most people have higher BP for a couple of hours before and after waking. In theory, BP measured in your family doctor’s office early in the morning should therefore be a bit higher than if measured in the evening. In practice this seems not to happen probably because few people see their doctor within 2 hours of rising. Comparison of millions of BP measurements in a large survey in the USA showed no significant difference between

measurements made in the morning and in the afternoon.

You have mentioned ‘white coat hypertension’? Could you explain what this is?

This is when BP is elevated when measured during a surgery or outpatient clinic but is otherwise normal. This phenomenon usually occurs in response to the measurement of BP by a doctor or nurse. In people with normal BP, there is generally little or no difference between their BP reading at a clinic or in a surgery compared to their usual BP reading. However, in some people, substantial differences between clinic and usual BP are consistently found, with the higher readings occurring in situations where a doctor or nurse has made the BP reading. This phenomenon of white coat hypertension is more commonly seen in women and older people.

As many as 20% of people diagnosed with high BP at clinics or in surgery may have entirely normal BPs when measured during the rest of the day. In these individuals, other BP measuring techniques are recommended so that their usual pressure is accurately recorded.

So how will someone know whether my BP reading is a ‘white coat hypertension’ reading?

One of the reasons why BP should be taken at least twice at each consultation is to make sure that someone’s BP has been given the opportunity to return to the usual or normal level if it was raised at the first reading.

The white coat effect is often most pronounced when someone first enters the examination area, and declines rapidly over time. Your doctor will suspect white coat hypertension if there is a substantial difference between initial BP reading when the person enters the surgery or clinic and a subsequent reading towards the end of the consultation.

Another strategy for reducing the white coat effect is to have a less ‘threatening’ health professional take the BP recording. Often clinics use nurses or technicians trained in BP measuring to take people’s BP. The white coat effect occurs less when health professionals other than doctor take BP readings.