Cardiac output monitoring at various time points of measurements during OPCAB. Good agreement could also be demonstrated in the patients with a decreased LVEF (< 40%) and at time points of a decreased CCI (≤ 2.4 L/min/m2), including the period of heart displacement for obtuse marginalis artery grafting when the hemodynamic derangement is most severe [19]. This seems to be attributable to the improved algorithm of the FloTrac/VigileoTM system that attenuates the bias induced by the hemodynamic changes that accompany various surgical situations such as sternotomy, opened pericardium, heart displacement and cumulative ischemia-reperfusion injury of various degrees. It is interesting to note was that the FloTrac/VigileoTM system performed well regardless of the range of the MAP or the use of vasopressor. However, in contrast to the best agreement (percentage error: 15.5%) demonstrated when the SVRI was within the normal range, the accuracy of the cardiac output measured by the FloTrac/VigileoTM system was poor with unacceptable agreement (percentage error: 40.2%) when the SVRI was outside the normal range. Unacceptable agreement had previously been demonstrated in a study of patients with liver cirrhosis and who were undergoing liver transplantation with a low SVRI [20]. FCI has also been demonstrated to be unreliable when phenylephrine was administered [21,22]. However, in those previous studies, phenylephrine administration increased the MAP to 82-90 mmHg, while the mean MAP could be maintained between 73-76 mmHg in the current study. Although the SVRI was not calculated in both of the previous studies, an increase in the MAP in our study was accompanied by a decrease in the CCI, suggesting a significant increase in afterload and thus the SVRI over the normal range. It seems that the extent of vasodilation or vasoconstriction, as manifested by extreme ranges of the SVRI, has the most influential impact on the arterial pressure waveform [23], regardless of the MAP or the use of vasopressor. Yet it is difficult to compare and state that this poor performance for a SVRI outside the normal range is a consistent finding since the data regarding this matter is limited at best. In order to improve the device’s performance, this issue should also be addressed and incorporated when developing a more refined algorithm for the FloTrac/VigileoTM system. In this study, the FCI was compared to the CCI and not with the cardiac index measured by intermittent PAC thermodilution, which could be a limitation. However, numerous studies have validated the accuracy of the CCI as compared to intermittent PAC thermodilution and even to electromagnetic measurement of the aortic blood flow, which is considered the ‘gold standard’ in cardiac laboratories [24,25]. Moreover, one of the benefits of the FloTrac/VigileoTM system is the minimized operator dependency, which is not the case for the intermittent PAC thermodilution method. Indeed, the variation between two 242 Vol. 60, No. 4, April 2011 series of three intermittent thermodilution cardiac output measurements is still as high as 15%, and in that regard the CCI has better reproducibility with negligible bias and so it is the less operator-dependent method [26]. The CCI value is an average made over 3-8 minutes of time and when the trend of the FloTrac/VigileoTM system is set to 5 minutes, it would be more ideal to compare the FCI to the CCI rather than comparing the FCI to the cardiac output as measured by the intermittent PAC thermodilution method. Likewise, since the response time of the STAT mode of the PAC to changes in the cardiac output has been demonstrated to be approximately 270 seconds [27], we used the STAT mode at T3 to minimize a potential source of error. In conclusion, the cardiac output measured by the FloTrac/ VigileoTM system was reliable even in patients with a decreased LVEF and who were in low cardiac output status during OPCAB. The modified algorithm appears to be robust in various situations of hemodynamic changes such as during the period of heart displacement and also regardless of the use of vasopressor. However, additional refinement of the algorithm should be done to improve the reliability when the SVRI is outside the normal range. References 1. Jhanji S, Dawson J, Pearse RM. Cardiac output monitoring: basic science and clinical application. Anaesthesia 2008; 63: 172-81. 2. Sandham JD, Hull RD, Brant RF, Knox L, Pineo GF, Doig CJ, et al. Canadian Critical Care Clinical Trials Group. A randomized, controlled trial of the use of pulmonary-artery catheters in high-risk surgical patients. N Engl J Med 2003; 348: 5-14. 3. Manecke GR. 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