A comparison of the random-zero and standard mercury sphygmomanometers

Both the standard mercury sphygmomanometer and the random-zero sphygmomanometer have been used in epidemiological studies and clinical trials. Problems arise in comparing studies since, in addition to other methodological differences, the readings obtained with the random-zero sphygmomanometer have been found to be lower than those obtained with the standard mercury sphygmomanometer. In the present study, blood pressures were measured in 66 subjects to examine the comparability of findings with the two instruments. Trained observers measured blood pressures simultaneously using a double-headed stethoscope and one cuff connected to the two sphygmomanometers. Use of instrument was randomly assigned for each blood pressure measurement; each observer was unaware of the other's blood pressure reading. Readings were lower with the random-zero sphygmomanometer; mean difference ranged from 2.5 to 3.3 mm Hg for systolic pressure and 1.9 to 2.7 mm Hg for diastolic pressure. Digit distributions recorded by the two observers for the standard mercury sphygmomanometer and the random-zero sphygmomanometer were not significantly different for either systolic or diastolic blood pressure. Intraindividual variation was greater with the random-zero sphygmomanometer than with the standard mercury sphygmomanometer. These data do not indicate that one instrument is clearly superior to the other, although in studies where the observer seeks to reduce the bias of multiple readings per person, the random-zero sphygmomanometer may be the more appropriate instrument. Critical to the use of either instrument are careful training, standardization, certification, and periodic recertification of observers.