Blood pressure measurement
I have been told that the old-fashioned mercury sphygmomanometer is the gold standard for measuring blood pressure. Is this true and, if so, why?
Of all the patient observations routinely undertaken by nurses, the recording of blood pressure (BP) is potentially the most unreliable and incorrectly performed (British Hypertension Society, 2009). Good practice can significantly reduce errors and help ensure that BP readings are correct.
Three BP recording devices are commonly used: the mercury sphygmomanometer; the aneroid sphygmomanometer; and the automated device (Dougherty and Lister, 2008). All should be used following national standards and guidelines to help ensure BP readings are accurate (British Hypertension Society, 2009).
The mercury sphygmomanometer is considered generally reliable and accurate and has traditionally been the device for recording BP (Dougherty and Lister, 2008). As nurses use their hearing to record the Korotkoff sound, it could be argued that it is the gold standard as nurses can always rely on their ears.
In addition, recording BP with a mercury sphygmomanometer allows nurses to use other senses to assess the cardiovascular system. For example, touch allows a nurse to check pulses and assess skin temperature (cool peripheries and hypotension suggests that shock may be present).
The correct procedure for recording BP using a mercury sphygmomanometer must be followed (Dougherty and Lister, 2008). The equipment should be in good working order and regularly maintained following the manufacturer’s instructions. Inaccurate and unreliable BP readings can result if the equipment is faulty or poorly maintained, or if the healthcare professional’s technique is poor (British Hypertension Society, 2009).
Errors in BP measurement using a mercury sphygmomanometer can occur for several reasons, including (British Hypertension Society, 2009):
Defective equipment, for example leaking tubing or a faulty valve;
Failure to ensure the mercury column reads 0mmHg at rest;
Too rapid deflation of the cuff;
Use of an incorrectly sized cuff – if it is too small the BP will be overestimated and if it is too large it will be underestimated;
The cuff is not at the same level as the heart;
Failure to observe the mercury level properly – the top of the mercury column should be at eye level;
Poor technique, for example failing to notice when the sounds disappear;
Digit preference, for example rounding readings up to the nearest 5mmHg or 10mmHg;
Observer bias, for example expecting a young patient’s BP to be normal.
However, concerns have been raised over mercury sphygmomanometers. Medical devices containing mercury are not banned in the UK, although the Medical Devices Agency recommends that mercury-free products should be used where possible (MDA, 2000). Some establishments do not use mercury sphygmomanometers.
It could be suggested that, if the mercury sphygmomanometer is considered the gold standard for recording BP, then the automated BP device and the aneroid sphygmomanometer should not be used.
However, both of these devices are considered reliable, and their use should adhere to national standards and guidelines (British Hypertension Society, 2009).
When the automated BP device was first used, its accuracy and reliability were questioned (Beevers et al, 2001). Improved technology has since led to more accurate and reliable devices (Beevers et al, 2001). Some have been tested and approved by the British Hypertension Society (2009).
Both the automated device and the aneroid sphygmomanometer have limitations. For example, if a patient has a weak thready pulse associated with atrial fibrillation, the automated electronic device can be inaccurate (Beevers et al, 2001). The aneroid sphygmomanometer is susceptible to inaccuracies or damage that would not be apparent to a nurse (O’Brien et al, 2001).
In summary, the old-fashioned mercury sphygmomanometer is the gold standard. It should be used following manufacturer’s instructions and national guidelines (British Hypertension Society, 2009). This will ensure a reliable BP reading. Automated devices and aneroid sphygmomanometers are reliable in most situations and should be used following manufacturers’ instructions and national guidelines (British Hypertension Society, 2009). Nurses should be familiar with each of the devices’ limitations.
Phil Jevon, PGCE, BSc, RN, is resuscitation training officer/clinical skills lead, Manor Hospital, Walsall.
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Beevers, G. et al (2001) Blood pressure measurement part I. Sphygmomanometry: factors common to all techniques. BMJ; 322, 981–985.
British Hypertension Society (2009) How to Measure Blood Pressure.
Dougherty, L., Lister, S. (2008) The Royal Marsden Hospital Manual of Clinical Nursing Procedures. Oxford: Blackwell Publishing.
Medical Devices Agency (2000) Blood Pressure Measurement Devices – Mercury and Non-Mercury. London: MDA.
O’Brien, E. et al (2001) Blood pressure measuring devices: recommendations of the European Society of Hypertension. BMJ; 322, 531–536.