Your Body Type and Blood Pressure

!±8± Your Body Type and Blood Pressure

Is there scope for human error in making blood pressure measurements? Yes, of course, as with everything else humans do. Blood pressure measurements need to be made carefully and by people who have been trained in how to make them.

For example, there are some hospitals where all blood pressure measurements recorded end in zeros for both systolic and diastolic pressure. If you read to the nearest 2 mmHg (the marks on the scale) then on average only 20 percent of accurate measurements should end in zero. This means that the doctors and nurses in those hospitals were measuring only to the nearest 10 mmHg. Hospitals now are tending to use electronic machines, which are incapable of preferring zeros, but this apparently greater accuracy can conceal readings which may be just as inaccurate if the machines are carelessly used, for example by lowering the cuff pressure too fast.

Apart from carelessness, the main source of human error comes from using a cuff that is too small or deflating the cuff too fast. In order to get an accurate reading, the pressure in the cuff must be deflated at roughly 2 mmHg per second. If it is deflated faster than this, then blood pressure will be systematically underestimated, by an amount proportional to the speed of deflation - in other words, the faster the deflation, the less accurate the reading.

So if this is the case, why would someone deflate the cuff too fast? One simple reason is that the person may be in a hurry. Another is that having an inflated cuff around your arm squeezing it tight is uncomfortable, sometimes even painful and it can be tempting to deflate it quickly to remove the discomfort. Some of the better electronic sphygmomanometers can solve this problem, as they have mechanisms of deflating the cuff automatically - but then someone has to set the mechanism at the correct slow rate. Obviously, these problems are not insurmountable - they can all be easily avoided by being careful and paying attention to detail.

The accuracy of your blood pressure measurements can also be affected by whether you are fat or thin. The cuff used to block the flow of blood to your arm through your brachial artery when measurements are being taken contains an inflatable rubber bladder. This should reach right around the full girth of the arm. If it is too short, blood pressure measurements may be overestimated by about 20 mmHg, a critical margin of error. This is a common cause of erroneous diagnosis and unnecessary treatment. It can be avoided only by using a larger cuff. Well-equipped general practitioners and hospitals have such cuffs, but don't always remember to use them and, unfortunately, some of them don't posses any outsized cuffs. If you have large upper arms (more than 30 cm or 12 inches in circumference), you must learn to insist gently and courteously but firmly on the use of an outsized cuff. If you don't do this, you may easily be over treated.

Electronic sphygmomanometers present a similar problem. Because they depend on a sensor sewn into the cuff, serious measurement errors can result if the cuff used is too short. If you have large upper arms, you will again need to insist on an outsize cuff being used. Some models do have long cuffs as standard: cuffs containing a 35 cm (14 inch) rubber bladder inside will do for arms up to 42 cm (16 inch to 17 inch) circumference and few arms are bigger than this. Measurements on people with exceptionally thin arms underestimate blood pressure 5-10 mmHg however carefully they are performed. There is no way around this and it is probably the reason why research studies generally show that high blood pressure in very thin people appears to carry greater risks than the same levels of blood pressure in fatter people.