Blood pressure measurement

doi:10.1136/bmj.322.7293.1043

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Part II—Conventional sphygmomanometry: technique of auscultatory blood pressure measurement

Gareth Beevers,
Gregory Y H Lip,
Eoin O'Brien

The measurement of blood pressure in clinical practice by the century-old technique of Riva-Rocci/Korotkoff is dependent on the accurate transmission and interpretation of a signal (Korotkoff sound or pulse wave) from a subject via a device (the sphygmomanometer) to an observer. Errors in measurement can occur at each of these interactionary points of the technique, but by far the most fallible component is the observer.

Rose classification of observer error

Systematic error
Terminal digit preference
Observer prejudice

Observer error

In 1964, Geoffrey Rose and his colleagues classified observer error into three categories.1

Observer training techniques

Direct instruction by an experienced observer
Instruction manuals and booklets
Audiotapes
Video films
CD Rom presentations

Systematic error

This leads to both intraobserver and interobserver error. It may be caused by lack of concentration, poor hearing, confusion of auditory and visual cues, etc. The most important factor is failure to interpret the Korotkoff sounds accurately, especially for diastolic pressure.

Terminal digit preference

This refers to the phenomenon whereby the observer rounds off the pressure reading to a digit of his or her choosing, most often to zero. Doctors may have a 12-fold bias in favour of the terminal digit zero; this has grave implications for decisions on diagnosis and treatment, although its greatest effect is in epidemiological and research studies in which it can distort the frequency distribution curve and reduce the power of statistical tests.2

Observer prejudice or bias

This is the practice whereby the observer simply adjusts the pressure to meet his or her preconceived notion of what the pressure should be. It usually occurs when there has been recording of an excess of pressures below the cut-off point for hypertension and it reflects the observer's reluctance to diagnose hypertension. This is most likely to occur when an arbitrary division is applied between normal and high blood pressure, for example 140/90 mm Hg. An observer might tend to record a …