Systematic error of cardiac output measured by bolus thermodilution with a pulmonary artery catheter compared with that measured by an aortic flow probe in a pig model.

OBJECTIVE: The precision of thermodilution cardiac output measurement using a pulmonary artery catheter can be divided into random and systematic errors. This study determined the systematic component. DESIGN: Comparative validation against a transonic flow probe on the aortic root. SETTING: Animal research laboratory. PARTICIPANTS: Eight anesthetized pigs, weight 27 to 32 kg. INTERVENTIONS: Thermodilution measurements were compared to those from an aortic flow probe. One (or two) catheters were tested in each pig, with multiple paired readings recorded as cardiac output was varied pharmacologically by esmolol, epinephrine, or dopamine. Linear regression lines were drawn for each pig as well as the slope used to quantify systematic error. Regression analysis of data from each pig was used to assess trending. Bland-Altman analysis also was performed. MEASUREMENTS AND MAIN RESULTS: Systematic error derived from slope data was ± 26%. Trending was reliable in 8 out of 10 catheter placements (p value>0.95). Bland- Altman analysis (n = 77 basal anesthesia and 165 pharmacologic intervention data pairs) provided percentage errors of ± 23% and ± 34-39%, respectively. This percentage error increase was due to variations in calibration and the slopes of regression lines for each catheter tested as the lines diverged as cardiac output increased, widening the spread of data. CONCLUSIONS: Thermodilution does trend cardiac output reliably in most cases. However, the systematic error is large and has a significant effect on the percentage error as cardiac output increases. The precision error of ± 20% currently used for thermodilution measurement may be set too low and is dependent on the clinical setting.

Assessment of trending ability of cardiac output monitors by polar plot methodology.

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.