Suitable location and optimum bladder width for the occluding cuff used for indirect arterial blood pressure measurements at the wrist.

Using a finite element method (FEM) and a volume oscillometric method with a photo-electric plethysmograph, we tried to determine which location(s) and which bladder width(s) of the occluding cuff would allow the most accurate indirect blood pressure measurements at the human wrist. Nuclear magnetic resonance imaging of the wrist allowed us to determine the geometry of two-dimensional (2D) finite element models. FEM analysis showed that the pressure transmission ratio (calculated tissue pressure: externally applied pressure) from the cuff to the radial artery (RA) was 100% when the cuff was placed over the site (L1) at which the RA crosses the most protuberant spot on the volar aspect of the distal end of the radius. In fact, the mean blood pressure (BPM) measured at this site in human subjects coincided with that measured at the upper arm (BPMA). In addition, a three-dimensional (3D) FEM was constructed by extending the 2D FEM at site L1 in the longitudinal direction to investigate the relation between pressure transmission to the RA and cuff bladder width. FEM analysis using the 3D finite element model showed that for all cuff widths greater than 0.25 times the diameter of the wrist, the external pressure was transmitted almost completely to the RA beneath the cuff center. Moreover, when the cuff width was as large as 0.45 times the diameter of the wrist, the BPM measured at site L1 in human subjects was similar to the BPMA. The theoretical and experimental results in the present study both suggest that for accurate indirect blood pressure measurements at the wrist, the L1 site is a suitable location for the occluding cuff and a cuff whose bladder width is more than one-third but less than one-half the wrist diameter can be used.