Korean J Anesthesiol Table 4. Subgroup Bland-Altman Analyses of the Cardiac Index LVEF CCI MAP SVRI NE ≥ 40 (n = 150) < 40 (n = 100) > 2.4 (n = 135) ≤ 2.4 (n = 115) 60-80 (n = 207) < 60 or > 80 (n = 43) 1,600-2,600 (n = 159) <1,600 or > 2,600 (n = 91) Not used (n = 137) Used (n = 113) FCI 2.6 ± 0.4 2.4 ± 0.4 2.7 ± 0.4 2.2 ± 0.3 2.4 ± 0.4 2.7 ± 0.5 2.5 ± 0.4 2.5 ± 0.5 2.5 ± 0.4 2.4 ± 0.5 CCI 2.7 ± 0.5 2.5 ± 0.4 2.9 ± 0.4 2.2 ± 0.2 2.6 ± 0.5 2.5 ± 0.5 2.5 ± 0.3 2.6 ± 0.7 2.6 ± 0.5 2.5 ± 0.5 Bias -0.05 -0.09 -0.19 0.08 -0.12 0.14 -0.06 -0.09 -0.11 -0.03 Limit of agreement % error 0.66 0.67 0.70 0.48 0.61 0.77 0.38 1.01 0.71 0.61 25.2 28.2 25.3 22.3 24.8 30.0 15.5 40.2 27.8 25.3 Jo, et al. The table presents the subgroup Bland-Altman analyses of the cardiac index as measured by the FloTrac/VigileoTM system (FCI, L/min/m2) and the pulmonary artery catheter (CCI, L/min/m2) according to the left ventricular ejection fraction (LVEF, %), the CCI, the mean arterial pressure (MAP, mmHg), the systemic vascular resistance index (SVRI, dyne·s/cm5/m2) and the use of norepinephrine (NE). n corresponds to the number of obtained data sets. FCI and CCI are presented as means ± SDs. The unit of bias and the limit of agreement are L/min/m2. None of the patients required a nicardipine or milrinone infusion during the study period. Discussion This prospective trial evaluated the accuracy of the FloTrac/ VigileoTM system-derived cardiac output in patients who were undergoing OPCAB, and we found clinically acceptable agreement between the FCI and CCI and even for the patients with a decreased LVEF or a low cardiac output status. The agreement was acceptable regardless of the MAP or the use of norepinephrine, except that the agreement was unacceptable (40.2%) when the SVRI was out of the normal range. Accurate determining the cardiac output constitutes an important axis of hemodynamic monitoring by providing the basis for guiding therapy to ensure adequate tissue perfusion, and especially in cardiac surgical patients. While the thermodilution method using the PAC with its attendant limitations is currently considered as a clinical standard, less invasive methods using arterial pressure waveform analysis are being extensively studied for their clinical feasibility [3- 10,14-17]. Among them, the FloTrac/VigileoTM system is the least invasive method because it does not require central venous access for external calibration to compensate for the interindividual differences in aortic compliance [3]. The SD of the pulse pressure is correlated to the stroke volume after adjustment for vascular compliance, which is estimated using the individual demographic data and the wave form characteristics such as skewness and kurtosis. Therefore, the operator dependency is minimized and the drift phenomena may be eliminated by automatic adjustment for the changes in the vascular tone. Yet the benefit of being less invasive provides a potential source to increased bias, and so this decreases the accuracy of the measured cardiac output. Indeed, earlier www.ekja.org validation studies have demonstrated conflicting results with acceptable accuracy observed mostly in patients with hemodynamically stable conditions [4-7]. With the improved algorithm of the FloTrac/VigileoTM system (software version 1.07), the rate of adjustment of the internal variables for estimating the vascular tone is reduced from 10 to 1 minutes combined with a reduction of the pulse wave detection noise. Validation studies of this second generation device have demonstrated more consistent results in cardiac surgeries with good agreement for the OPCAB and cardiac surgeries that are done under cardiopulmonary bypass as well [8-10]. However, none of the previous studies used a modified Bland-Altman analysis of agreement for repeated measures, and the cardiac output in the previous studies was measured and compared at multiple time points from each patient. Moreover, the previously reported evidence is limited regarding the accuracy of the cardiac output, as measured by the FloTrac/VigileoTM system, in patients with a decreased LVEF and in those patients with a lower than normal or low cardiac output status, which is when accurate estimation of the cardiac output is especially required to decide whether therapies targeted to improve oxygen delivery should be initiated. Moreover, considering that the peripheral arterial pulse wave is the summation of the reflected waves with the fundamental waves determined by the interaction between the left ventricular output and the capacitance of the vascular tree [18], abnormal ranges of the MAP and SVRI and the use of vasopressors could all be potential sources of error in the pulse contour analysis-derived cardiac output measurement. Yet no comprehensive data exist regarding the influence of these factors on the accuracy of the cardiac output measured by the FloTrac/VigileoTM system. The results of this current trial indicate that the cardiac output measured by the FloTrac/VigileoTM system consistently demonstrated good agreement with a percentage error ≤ 30% 241