Key result
The optimal characteristic ratio for estimating systolic blood pressure using the oscillometric method was 0.565, whereas tested commercial devices used different ratios leading to average errors of 4.8 to 6.2 mmHg.
Commercially available oscillometric blood pressure monitors use varying characteristic ratios that deviate from the optimal value of 0.565, leading to systematic over- or underestimation of systolic blood pressure.
May cause systolic BP errors in practice; leaves open standardization of characteristic ratios in device validation.
We present a robust method for testing and calibrating the performance of oscillometric non-invasive blood pressure (NIBP) monitors, using an industry standard NIBP simulator to determine the characteristic ratios used, and to explore differences between different devices. Assuming that classical auscultatory sphygmomanometry provides the best approximation to intra-arterial pressure, the results obtained from oscillometric measurements for a range of characteristic ratios are compared against a modified auscultatory method to determine an optimum characteristic ratio, Rs for systolic blood pressure (SBP), which was found to be 0.565. We demonstrate that whilst three Chinese manufactured NIBP monitors we tested used the conventional maximum amplitude algorithm (MAA) with characteristic ratios Rs = 0.4624±0.0303 (Mean±SD) and Rd = 0.6275±0.0222, another three devices manufactured in Germany and Japan either do not implement this standard protocol or used different characteristic ratios. Using a reference database of 304 records from 102 patients, containing both the Korotkoff sounds and the oscillometric waveforms, we showed that none of the devices tested used the optimal value of 0.565 for the characteristic ratio Rs, and as a result, three of the devices tested would underestimate systolic pressure by an average of 4.8mmHg, and three would overestimate the systolic pressure by an average of 6.2 mmHg.
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Celler et al. (2018) studied Blood pressure measurement (n=102). Oscillometric blood pressure measurement vs. Modified auscultatory method was evaluated on Optimal characteristic ratio (Rs) for systolic blood pressure. The optimal characteristic ratio for estimating systolic blood pressure using the oscillometric method was 0.565, whereas tested commercial devices used different ratios leading to average errors of 4.8 to 6.2 mmHg.
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