Thursday, March 22, 2012

Measuring blood pressure

(To our junior colleagues and students) 

1. Patient should be comfortable, lying or sitting with the arm supported. Actively holding the arm up by the patient raises blood pressure. The patient should avoid smoking or drinking coffee or tea shortly before measurement as this may raise blood pressure.
2. Mercury sphygmomanometer is reliable. Aneroid sphygmomanometer is reliable if calibrated against a mercury one.
3.   Apply the cuff neatly around the upper arm well above the cubital fossa (to leave a place for the stethoscope). Any of the two arms may be used. However if the pressure is measured for the first time, especially in an elderly patient, it is advisable to measure it in both arms and consider the higher one as the true representation of the patient’s blood pressure. It is not uncommon, especially in elderly people, to get a lower reading in one arm as a result of atherosclerosis in the arteries supplying that arm. Subsequent measurements in such a patient should use the arm with the higher pressure.
4. The arm should be at the same level as the heart. The pressure inside the brachial artery decreases when the arm is raised and increases when it is lowered. The position of the sphygmomanometer is not important because the cuff and the tubing are filled with air and the pressure in a container filled with gas (unlike liquid) is the same at any point regardless of its position.
5. If you find (more often in women) that rolling up the arm clothing will constrict the upper arm and it is not practical to ask the patient to undress, then it is better to apply the cuff over the clothing provided it is not thick. Thin clothing does not significantly impair transmission of pressure from the cuff to the arm or transmission of sound from the arm to the stethoscope.(1)
6.  It is advisable to develop the habit of measuring blood pressure by palpation first. It has the following advantages:
a)      It gives an idea about the systolic pressure so that when you take it by auscultation you only raise the pressure to a little above the systolic before starting to deflate. In this way you avoid raising it too high which is painful and may result in a reflex rise of blood pressure.
b)      It avoids the possibility of raising the pressure in the cuff to a point in the silent gap (in patients who have a silent gap) and starting deflation. You will then wrongly consider the reappearance of sounds (phase three Korotkoff sound) as the systolic pressure.
c)       It makes you check the pressure twice which is advisable. The second measurement has been found to be generally lower than the first and more representative of the real.
7.     Apply the stethoscope over the brachial artery in the cubital fossa and avoid inserting it between the cuff and the arm. The hard structure of the stethoscope may interfere with the even distribution of pressure on various points of the arm circumference.
8.  Deflation of the cuff should be slow to give time for the mercury column or the dial pointer to change position as the pressure drops. Too rapid deflation gives a higher reading as a result of inertia of the mercury or the pointer causing it to lag behind the decreasing pressure inside the cuff.
9. Diastolic blood pressure (as measured intra arterially) falls between phase four (sudden muffling) and phase five (complete disappearance) of Korotkoff sounds but nearer the latter. So it is better to take disappearance of sounds as the diastolic pressure except in the occasional case when the sounds persist down to a very low level or zero.

(1) Hovsrpian R.,  Al-Haddad M.,  Abdulla K., Comparison of blood pressure measurements in bare arm, clothed arm and forearm,  J. Fac. Med. Baghdad, 1996, 38, 221-224.

Saturday, March 3, 2012

Confidence Interval

(To our junior colleagues and students)

Confidence interval is a statistical term frequently encountered in papers describing various kinds of medical research. It is one of the terms that postgraduate medical students need to know.
The following examples serve to explain it.
If you want to find the mean weight of a group of 30 men, you measure the weight of each one, add the figures and divide by 30. The result is the mean weight of the group.
If you want to find the mean body weight of men residents of a big city, it is not practical to do the same because of the large number involved. Statisticians get around this by taking a random sample of the men in question. They calculate the mean weight of the men in the sample and consider it a satisfactory representation of the required mean of the total. They may take e.g. a thousand men, chosen randomly from various districts of the city, measure the weight of each and then add and divide to find the mean. They consider this representative of the mean body weight of the men of that city. Now imagine yourself to be the person who requested that mean because you wanted to assess the nutrition status of the people in the city and you asked a statistician to do it for you. Imagine also that the mean weight of the thousand men was 60 kg. You may then have the following dialogue with the statistician:
·         Are you sure the figure you gave me is exactly the same as the figure you would have obtained had you taken all the men in the city?
·         No, most probably it is not, but it is very near that figure and is sufficient for your purpose.
·         How near is it? How much is the difference?
·       I cannot tell you the exact difference because I do not know the true figure of all the men in the city. We statisticians usually deal with probabilities. I can tell you the probability of the difference being of a certain magnitude. I can work out from the data of the thousand men a figure we call the Standard Error (SE). In fact I have already done that and found the standard error of the mean of the sample to be 2. We know from statistics laws and rules that the probability of the difference being not more than one standard error (1SE) is approximately 67% and being not more than 2SE approximately 95%. In other words I can tell you that I am practically 95% confident that the true figure of the mean weight of all men in the city is within 2SE above and below the figure of 60 i.e. between 56 and 64 kg. That is what I mean when I say the mean body weight of the sample of men is 60 kg. and its 95% Confidence Interval (CI) is 56 - 64.
The 95% probability (or confidence) becomes approximately 67% if you choose 1SE above and below the mean as the limits of your confidence interval and approximately 99% if you choose 2.5 SE. The 95% (i.e. mean ± 2SE) is commonly used and if the percentage is not written it usually means 95%.
I used the mean as an example to explain the confidence interval. The same applies to other parameters like proportions when samples are used instead of the total.