Indirect blood pressure measurements: a matter of interpretation.

The indirect measurement of arterial blood pressure is discussed from the standpoint of the intravascular phenomena that occur under the cuff during the production of Korotkoff sounds. The influence of the physical and elastic properties of the vessel wall and blood flow parameters on indirect blood pressure measurement were investigated by using the results of model studies carried out in a properly scaled, simulated mechanical system under controlled conditions. The results of those studies indicate that auscultatory readings may be subject to large errors, depending on the thickness and stiffness of the artery wall. These results are shown to be consistent with comparisons of direct and indirect blood pressures in human subjects in that errors of the order of 30 to 50 mm Hg are not uncommon in the elderly or in patients with advanced arteriosclerosis. The errors discussed here are inherent to the auscultatory technique and are not related to instrument calibration, auditory acuity, or office technique.