Objectives To develop a valid statistical method of showing acceptable cardiac output (CO) trending ability when new CO monitors are compared to a reference standard, such as thermodilution, using polar coordinates. Design Developing a new statistical analytic method using historic data. Setting University Hospital Anesthesia and Intensive Care Department. Participants Data taken from previously published CO validation studies. Interventions Cartesian data were reanalyzed, being uplifted using Data Thief 3.0 software ( http://datathief.org/ ). Polar plots were constructed from this data. Central zone data (<0.5 L/min or <10% change) were excluded because they introduced statistical noise. Trial polar criteria were set using data from a study that compared 5 CO monitors against thermodilution. Then, these criteria were further validated using data extracted from 15 other studies. Mean (95% confidence intervals) polar angles were used. Measurements and Main Results Trial data suggest ±5° (angle) ±30° (95% confidence interval) as acceptance limits. Concordance rates (ie, >95%-90%) from 5 articles supported trending, and polar data from these studies concurred with the authors' pilot criteria. Favorable comments on trending also were found in 8 of 15 articles in which radial limits were less than ±32°. Good calibration was associated with a mean polar angle of less than ±5°. Conclusions Polar plots can be used to show the trending ability of CO monitors in comparative validation studies. They overcome the deficiencies of concordance analysis, which uses the direction of change as a statistic and ignores the magnitude of change in CO.