Sphygmomanometers

5. How accurate are the alternatives?

Non-mercury using blood pressure devices which rely on listening for blood sounds (auscultation ) share some sources of error found in use of mercury sphygmomanometers, notably the inaccuracies and observer bias which can arise from the auscultation itself.

They may have other, additional sources of error. Aneroid sphygmomanometers, for example, are inherently less accurate than mercury-using devices. The mechanical linkages are quite delicate and may be unstable over time, especially if handled roughly. Reliability of aneroid instruments is affected more by the design and build quality than is the case for mercury-using devices. They need to be calibrated at regular intervals. A recent study in primary care in the UK founds that more than half of aneroid devices had a cuff pressure measurement error greater than 3mmHg, compared with only 8 per cent of mercury and automated devices taken together. Automated auscultatory devices still call on the operator to respond to a visual display and record the pressure indicated when the system detects the critical blood sounds. They are also highly dependent on correct placement of the microphone which picks up the Korotkoff sounds . Automated oscillometric devices have a number of possible errors. The signals from pressure waves inside the artery are processed by a special algorithm, which is specific to the device and calculates the systolic and diastolic pressures for display. This avoids certain kinds of operator error. However, users of automated devices may be more likely to be untrained, and prone to errors such as using the wrong size of cuff, leaving the arm in the wrong position, or allowing the patient to move. They also give unreliable results in groups with conditions which can alter condition of the arteries, including those with diabetes, irregular heartbeat, pregnant women with pre-eclampsia , and the elderly.
Automated devices designed for use on the upper arm are more likely to give satisfactory results. Few finger or wrist devices have passed independent accuracy assessments, though a small minority of wrist devices achieved an acceptable standard. Wrist devices are prone to errors related to positioning of the wrist at heart level. Some are fitted with position sensors, but even those appear to give significantly different readings than arm devices in a large proportion of patients with raised blood pressure.