Agreement of systolic, mean, and diastolic arterial BP (SAP, MAP, and DAP, respectively) was assessed traditionally with Bland-Altman and trend analysis and clinically safety was quantified with error grid analyses. Invasive radial and non-invasive finger BP were measured, of the latter B ̂P Rad and B ̂P Bra were transformed. This observational study was conducted on thirty-three non-cardiac surgery patients. In this study we determined whether this new algorithm shows better agreement with invasive radial BP than the original one and whether in the operating room this algorithm can be used safely. A modified concordanceĪ new algorithm was developed that transforms the non-invasive finger blood pressure (BP) into a radial artery BP ( B ̂P Rad), whereas the original algorithm estimated brachial BP ( B ̂P Bra). The radial limits of agreement replace the horizontal limit lines in the Bland and Altman plot. The bias becomes the mean of the polar angles formed by these data points and reflects the difference in calibration between the reference and test methods. Full- and half-circle formats can be drawn. Polar plots present the data from a 4-quadrant plot in a similar format to a Bland and Altman plot but with a radial distribution of data points about a polar origin 3 (Fig 2).
3 The following variables based on the polar plot and polar angle (θ) were chosen: (1) the mean polar angle (or angular bias) and (2) 95% CIs (limits of radial agreement) of ResultsĪ total of 18 articles with 4-quadrant plots were identified, 2 of which were excluded because the range of values of the data was too small for polar analysis.21, 22 Discussion In Bland and Altman analysis, the bias and limits of agreement are used to assess agreement. Statistical variables that assessed trending ability and described the polar plot needed to be identified. A number of steps were required to transform the X-Y ΔCO data into polar data, and these steps are presented later (Fig 1).
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