In Reply:—We appreciate the interest of de Jong et al. in the difficulties of calibration using oscillometry. Oscillometric blood pressure measurement provides, in descending order of accuracy, estimates of mean, systolic, and diastolic pressures. To determine a gain factor and offset to apply to the piezoresistive measurement using oscillometry, there are three degrees of freedom (mean, systolic, and diastolic) to determine two variables (gain and offset). With this additional degree of freedom, infinite calibration rules can be created. One rule is to consider the difference between systolic and diastolic pressure for calculating the gain. This method may be problematic because it combines the errors in the two measures most subject to inaccuracies. A linear calibration using only the mean and systolic measures might be proposed as possibly less subject to noise because the diastolic measurement may add more incremental noise than information. A quadratic calibration formula might be proposed because it would use the extra degree of freedom. Given these possibilities, we chose to simply correct for the offset in mean pressure alone, and in our study, we considered only mean pressure because it is the least error-prone. Oscillometric calibration is the major source of disagreement between measurements of blood pressure determined using arterial tonometry and intraarterial measurement. Further investigation may permit the improvement of calibration using oscillometry.
Lawrence C. Siegel, M.D., Assistant Professor of Anesthesia.
John G. Brock-Utne, F.F.A.(S.A.), Professor of Anesthesia.
Jay B. Brodsky, M.D., Professor of Anesthesia, Department of Anesthesia, Stanford University School of Medicine, Stanford, California 94305–5117.
(Accepted for publication January 23, 1995.)