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.

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