The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [469]
Preprocedural considerations
Equipment
Sphygmomanometers
Sphygmomanometers which are uncalibrated or not working accurately are a cause of potential blood pressure measurement error (Curran 2009). If using a manual sphygmomanometer, check that the dial is set at zero or the mercury level is at zero prior to commencing (O’Brien et al. 2003). In addition, follow the manufacturer’s recommendations and local policies regarding servicing and care of the device.
Manual mercury sphygmomanometers
The Medicines and Healthcare products Regulatory Agency suggests that manual mercury sphygmomanometers should be gradually phased out and replaced with a dial or electronic manometer due to potential mercury leaks which are hazardous to both the environment and humans (MHRA 2005). Where mercury sphygmomanometers are used, there should be appropriate health and safety guidelines in place to guide staff, they should have access to mercury spillage kits, be trained in their use and be aware of how to dispose of the sphygmomanometers safely (MHRA 2005).
Manual aneroid sphygmomanometers
Aneroid sphygmomanometers measure blood pressure through a lever and bellows system which is more complex than the mercury sphygmomanometer. However, if it is damaged it may become inaccurate (O’Brien et al. 2003). If aneroid sphygmomanometers have a dial gauge then there is a need to be aware of the risk of damage with significant errors occurring with the calibration and setting of zero (MHRA 2005). As a result, O’Brien et al. (2003) recommend that these devices are serviced every 6 months to ensure accuracy.
Automated oscillometric sphygmomanometers
These devices show blood pressure on an electronic display (MHRA 2005). Some studies have found that the results differ between automated and manual blood pressure devices (Bern et al. 2007). Indeed, there are concerns that there is a greater need for validation of the devices so it is recommended that only devices which have passed recognized validation criteria should be used (BHS 2006, MHRA 2005, Stergiou et al. 2010, Williams et al. 2004). If levels of accuracy and reliability can be achieved then they do have certain advantages:
many devices combine blood pressure with other observations (O’Brien et al. 2003)
a printout can be obtained, reducing the risk of bias (O’Brien et al. 2003)
the preference of terminal digits (0 or 5) should be eradicated (O’Brien et al. 2003)
data can be stored on the device (O’Brien et al. 2003)
they enable the setting of alarms (MHRA 2005).
Practitioners must refer to the manufacturer’s instructions and be aware of the limitations of the device. If there is any doubt about a measurement then it should be verified by an accurate manual blood pressure reading. Patients who have arrhythmias, hypotension or hypertension or are acutely unwell should have a manual blood pressure taken rather than an automated one.
The cuff
The cuff is made of an inelastic material which encloses an inflatable bladder and encircles the arm. Cuffs that are too small yield a reading that is falsely high and large cuffs give a falsely low reading (Williams et al. 2004). In the correct size cuff the bladder should encircle 80% of the patient’s arm (Williams et al. 2004). Please see Table 12.2 for guidance to blood pressure cuff sizes.
Alternative adult cuffs (width × length, 12 × 35 cm) have been recommended for all adult patients, but can result in problems with over and undercuffing. The British Hypertension Society recommends that cuff size be selected based on arm circumference (Williams et al. 2004).
The inflatable bladder, valve, pump and tubing
In a manual sphygmomanometer, the system used to inflate and deflate the bladder consists of a bulb attached to the bladder with rubber tubing. When the bulb is compressed,