ABC of hypertension: Blood pressure measurement

Abstract

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 favourable measurement in a young healthy man with a borderline increase in pressure, but categorise as hypertensive an obese, middle aged man with a similar reading. Likewise, there might be observer bias in overreading blood pressure to facilitate recruitment for a research project, such as a drug trial. Observer prejudice is a serious source of inaccuracy, as the error cannot usually be demonstrated.3 View larger version: In this window In a new window Binaural stethoscope used for instruction in ausculatory blood pressure measurement Overcoming error by observer training The technique of auscultatory blood pressure measurement is a complicated one that is often taken for granted. Instruction to medical students and nurses has not always been as comprehensive as it might be, and assessment for competence in measuring blood pressure has been a relatively recent development.4 Ironically, these methods of achieving much needed improvement in performing the auscultatory technique have arrived as the mercury sphygmomanometer is under threat and as automated devices move in to replace the observer; these have included: direct instruction using a binaural stethoscope; the use of manuals, booklets, and published recommendations; audiotape training methods; videofilm methods, and, most recently, CD Rom methods. The CD Rom produced by the Working Party on Blood Pressure Measurement of the British Hypertension Society in 19985 incorporates instruction, with examples of blood pressure measurement using a falling mercury column with Korotkoff sounds and a means for the student to assess competence in the technique using a series of examples. The CD is accompanied by the British Hypertension Society booklet Blood pressure measurement: recommendations of the British Hypertension Society.6 Recommendations for observer training Training observers in clinical practice: nursing and medical students, doctors, paramedical personnel Instruction in the theory of hypertension and blood pressure measurement Booklet for reading, eg BHS Recommendations on blood pressure measurement Tutorial sessions with demonstrations using a binaural or multiaural stethoscope CD Rom demonstration using, eg, the BHS CD Rom CD Rom assessment Repeat CD Rom assessment until level of accuracy achieved Reassessment using BHS CD Rom every two years Training observers in research Measurement of blood pressure—highest possible standard Level of accuracy —90% of SBP and DBP within 5 mm Hg—100% within 10 mm Hg of an expert observer Instruction in the theory of hypertension and blood pressure measurement Audiogram to check auditory acuity Booklet for reading, eg BHS Recommendations on blood pressure measurement Tutorial sessions with demonstrations using a binaural or multiaural stethoscope CD Rom demonstration using, eg the BHS CD Rom CD Rom assessment Repeat CD Rom assessment until level of accuracy achieved Training and assessment repeated at least every three months Overcoming error with instrumentation As mentioned earlier, blood pressure measurement is subject to observer prejudice and terminal digit preference, introducing an error that is unacceptable for research work. Careful training of observers can reduce but not abolish these sources of error, some of which cannot be easily demonstrated. Because accuracy of measurement is particularly desirable in research, efforts have been made to devise devices that would minimise or abolish observer error.