RANDOM ZERO SPHYGMOMANOMETER VERSUS AUTOMATIC OSCILLOMETRIC BLOOD-PRESSURE MONITOR IS EITHER THE INSTRUMENT OF CHOICE

The Hawksley random zero sphygmomanometer was designed to eliminate ob server bias and two digit preference. We have measured blood pressure (BP) in a group of 62 young adults (median age 26.1 years, range 20.2- 31.3 years) with reflux nephropathy under standardised conditions (tha t is, in the morning, after a 2 h supine rest, before venepuncture, us ing a standard 12 x 23 cm adult size cuff appropriate for the machine used) utilising the random zero sphygmomanometer and the automatic osc illometric BP monitor (Dinamap(TM) 8100, Critikon). Seven consecutive recordings of right brachial BP at intervals of 2 min were taken using each instrument alternatively, and the first reading was discarded. T he first instrument used to measure BP was alternated between patients to eliminate bias on instrument preference. Random zero sphygmomanome ter was used as recommended by the manufacturers and Korotkoff phase V was used to measure the DBP. The observer and the equipment used were the same throughout the study period. The mean SEP and DBP were calcu lated to the nearest 1 mm Hg utilising the three recordings taken by e ach instrument. The limits of agreement and the repeatability coeffici ents for each method of measurement were assessed utilising the statis tical method described by Bland and Altman in 1986. The correlation co efficients among random zero and automatic oscillometric BP monitor fo r SEP and DBP measurements were 0.84 and 0.67, respectively. The avera ge BP (mean of random zero and automatic oscillometric BP monitor meas urement) plotted against the difference between the two methods of mea surement showed no relation between the difference and the average of measurement in the ranges of BP studied, that is, between 100 and 160 mm Hg systolic and 55 and 100 mm Hg diastolic. The mean of difference between random zero and automatic oscillometric BP monitor for SEP was -6.45 (s.d. 6.07) and for DBP +10.77 (s.d. 8.16) mm Hg. The limit of agreement for SEP measurement was +5.69 to -18.59 mmHg and for DBP was +27.09 to -5.55. The repeatability coefficients of random zero and au tomatic oscillometric BP monitor for systolic and diastolic measuremen ts were 8.64 and 7.04, and 9.72 and 6.62, respectively. Bland and Altm an analysis indicates major differences between the two methods of mea surement. The automatic oscillometric BP monitor could on average over -read the systolic by 6.45 mm Hg and under-read the diastolic by 10.77 mmHg compared with that of random zero. Furthermore, the limits of ag reement were wide enough for a normotensive to be inadvertently define d as a hypertensive on machine error alone. This clearly indicates tha t automatic oscillometric BP monitor and random zero BP measurements c annot be used interchangeably in clinical practice. Furthermore, the r epeatability coefficients, which should ideally be zero, are too large for either instrument to be considered as the gold standard for BP me asurement, although that of automatic oscillometric BP monitor was sup erior to that of random zero. This study highlights the importance of using nomograms generated by the same method of measurement for compar ison both in paediatric and adult practice for correct interpretation of BP.