michigan hypertension core curriculum - State of Michigan

Measurement Methods
Early detection, treatment and control of hypertension require accurate blood pressure
(BP) measurement. 1 This task, which too often is left to unlicensed assistive personnel, should
be carefully done by the health care professional. Accuracy of measurement begins with
understanding the three methods used to obtain a BP reading, and ensuring that the equipment
to be used is accurate. The first method is auscultation with an approved and accurate BP
device. The mercury sphygmomanometer is considered to provide the gold standard of BP
measurement. However, due to concerns about environmental hazards these devices are being
phased out, and in Michigan as of January 2009 mercury sphygmomanometers can only be
used to check accuracy of other devices or used in a patient’s home. 2
As a result the mercury sphygmomanometers are being replaced with aneroid and/or oscillometric
devices. Aneroid devices also use auscultation to detect blood flow through the artery. BP readings
based on auscultation are subject to measurement error due to environmental factors (e.g., extraneous
room noise), personnel factors (e.g., education, hearing ability, terminal digit preference), and device
factors. Aneroid devices do not maintain stability over time and require frequent re-calibration (e.g.,
every 6 -12 months). The level of inaccuracy of BP measurements obtained with aneroid devices has
been found to range from 1% to 44%. 3 To overcome the errors of auscultation, an ocillometric method
may be used. The ocillometric method detects vibrations in the arterial wall that occur due to blood
flow, and transforms the vibrations into an electrical signal which is displayed as a digital readout of
BP. However, factors other than blood flow may affect the vibrations. Thus the oscillometric techniques
will underestimate the true BP in patients with arterial stiffness or dysrrhythmias. 4 The ocillometric
method has been used with a variety of measurement devices (e.g., upper arm, wrist, finger, and
ambulatory devices). Automated upper arm devices that measure BP at the brachial artery have
been shown to be reliable in clinical practice, and therefore their use is recommended over wrist
or finger devices. Finger devices are not recommended due to inaccuracies related to peripheral
vasoconstriction, alteration in BP at distal sites, and the error of limb position in relation to the heart
during measurement. 4,3 Wrist devices are increasingly being used especially with obese people since
the diameter of the wrist is usually not affected by obesity. However, wrist devices are subject to the
same errors as finger devices, with the addition of altered readings due to the flexion/ hyperextension
NKFM & MDCH 11