ACADEMY PUBLIC POLICIES

Measuring blood pressure - Ambulatory monitoring

A friend of mine had a 24-hour BP recording done. What is this and why is it performed?

What you are describing is ‘ambulatory’ BP monitoring. Ambulatory BP monitoring is a more rigorous and intensive way of measuring somebody’s BP in whom one of the following factors may be suspected:

• When the measurement shows unusual variability in the clinic;

• Where someone has ‘uncontrolled hypertension’ – this is high BP that has not been reduced to a target BP level after intensive drug treatment has been given;

• When people are suffering from very low BP. Low BP may affect your ‘activities of daily living’. ‘Postural hypotension’ is a condition that can occur in such situations.

This is when BP fails to adjust when a person stands up. In normal circumstances our BP increases when we stand up, but in people with postural hypotension, their BP does not increase on standing with the effect that they may feel dizzy or light-headed. In more severe cases it can cause fainting or a fall.

The most common reason for using ambulatory BP monitoring is to diagnose white coat hypertension.

Ambulatory BP readings help distinguish between people with white coat hypertension and people who have sustained hypertension.

Ambulatory BP monitors became widely available for the first time in the late 1980s. They still depend on periodic inflation of a cuff compressing the upper arm, and thus disturb sleep seriously in some people, and may be distracting while driving a car. They don’t interfere much with ordinary activities while you are sitting, but heavy work is impossible. They are usually set to measure BP at 2-hourly intervals (the best for statistical analysis). Though they are designed to be used over 48 hours, few people can tolerate them for more than 24 hours continuously.

They are a great help in sorting out ‘white coat hypertension’ from ‘real’ high BP, and thus help to avoid starting people on a lifetime of unnecessary treatment. The evidence that they provide is not always easy to interpret because all the important evidence that we have on the value and limitations of long-term treatment for high BP at various levels is based on surgery readings by means of mercury sphygmomanometers.

What are the advantages and disadvantages of ambulatory BP monitoring?

Ambulatory BP monitoring permits the non-invasive measurement of BP over a prolonged period of time (usually 24 hours). It was first developed as a research tool in the 1960s and 1970s – it has now become a popular way of assessing the average BP reading. Its advantages are that it provides a more representative estimate of someone’s BP reading compared to isolated clinical readings. In addition, BP values derived from ambulatory readings are better markers for the risk of possible organ damage in the future caused by hypertension (what is called ‘prognostic information’) than usual BP readings at the surgery or clinic.

The main disadvantage is that ambulatory BP monitoring is an expensive technology. It also requires specialist involvement, as interpretation of ambulatory readings can be complex and requires specialist training.

Monitoring at Home

A friend of mine has an electronic BP monitor at home. What are the advantages and disadvantages of self measuring BP?

Automated electronic devices provide timed printouts of BP and remove many of the sources of error associated with conventional BP measurement. The critical point is to make sure that any electronic device that you use when measuring your own BP has been validated against a conventional mercury sphygmomanometer.

There are various points to remember when taking your own BP reading:

• Sit quietly and comfortably without distractions in a warm room throughout the procedure.

• Your measurement arm (normally the left arm in right-handed people) should be supported (usually by a table) at about the same level as your heart (the level of the nipple in men). You’ll need a small cushion or a book under your arm to ensure this. If your arm is far above or below your heart, you’ll get a false reading.

• Make sure you have no tight clothing above the cuff. If you just roll up your sleeve, it may be so tight that it puts pressure on the brachial artery itself, and thus gives a false low BP measurement. At least while you’re learning how to use the device, it’s wise to remove all clothing from the arm before you begin the measurement.

Electronic monitoring devices come with an instruction book, which nowadays is usually adequate. As they vary in how fully they are automated, and in the various symbols and icons that they use on their displays, you will need to read this instruction book to fully understand what you are doing. Most of them have some sort of automatic inflation of the cuff, followed by automatic deflation, triggered by recognition of systolic pressure.

The rate of deflation may or may not be variable. If it is, set it to the slowest rate possible and don’t speed this up later, even if it makes the procedure more comfortable. If it is not variable, check the time it takes to go from, say, 140 mmHg to 80 mmHg against the seconds hand on your watch. This should take 1 minute and 20 seconds. If it takes less than a minute, don’t buy or use the machine.

Electronic monitoring devices recognize systolic and diastolic pressures either through a microphone, which recognizes the appearance and disappearance of regular tapping sounds, or through a transducer, which recognizes a pulse wave.

Transducers are more efficient (but more expensive), and less prone to pick up extraneous signals. In both cases, the part of the cuff containing the sensor must be placed accurately over the brachial pulse. Failure to do this is the commonest cause of inaccurate or failed BP reading. The brachial pulse can be felt along the crease of your inner arm. It lies on the inside part of your biceps tendon.

Virtually all electronic monitoring devices measure your pulse as well as BP. These are useful indicators of your state of mind while you are measuring your BP. Rates over about 80 beats a minute suggest some anxiety.

The common causes of false measurements at home are anxiety, fear, anger, pain, embarrassment and cold. If you feel relaxed and are not distracted by other things going on, you can be reasonably confident that the pressures you record are correct. If you really cannot relax, and always feel anxious when measuring your own BP, say so, because this factor needs to be taken into account by your doctor. Cold is a powerful raiser of BP, and if you have come in from a frosty winter’s day, take half an hour or so to warm up before measuring your BP.

Are there any situations where electronic devices are not recommended?

In some situations the oscillometric technique that electronic monitors use is not reliable and accurate in people with specific problems. The commonest cause is in people who have an irregular pulse, most notably atrial fibrillation. This is a condition where the upper chambers of the heart, the atria, ‘fibrillate’ rather than beat regularly.

Atrial fibrillation is not life-threatening as the main chambers of the heart, the ventricles, continue to beat regularly. Atrial fibrillation is associated with a greater risk of suffering a stroke and you will probably be given a blood-thinning drug either in the form of aspirin or warfarin. Atrial fibrillation can be detected by the doctor taking an electrocardiogram (ECG) reading.

A friend of mine has been told that BP readings need to be adjusted when electronic devices as opposed to mercury sphygmomanometers are used. What does this mean?

It is recognized that repeated daytime measurements of BP produces systematically lower BP readings compared with isolated surgery or clinic readings. Because of this phenomenon, treatment thresholds (the BP level for initiating treatment) and target threshold (the target BP which should be aimed at when taking BP-lowering drugs) need to be adjusted downwards when the doctor makes a decision based on self-measuring electronic devices or ambulatory BP readings. The general recommended difference between clinic and daytime mean BP readings is thought to be about 12/7 mmHg.

What are the advantages and disadvantages of taking my own BP readings by means of an electronic device?

The advantages of taking your own readings are that multiple readings can be obtained over a prolonged period of time allowing better definition of your true BP. As no medical personnel are involved, distortion due to the white coat effect is far less likely.

The disadvantages are that many of these devices are inaccurate and may produce false readings. This leads to incorrect interpretation of the BP measurements from the device.

Another reason to take your BP concerns monitoring of your BP once you have started BP treatment. Some people find this form of self monitoring reassuring.

However, there are two caveats that should be remembered: firstly, self-readings need to be adjusted downwards by 12/7 mmHg so as to be consistent with surgery/clinic readings; secondly, there is very little evidence that home self-measurement of BP is an effective way to help people reach target BP and may cause unnecessary anxiety when spuriously ‘high’ or ‘low’ readings are read.