Cardiac output monitoring Introduction Estimating the cardiac output constitutes an essential part of the hemodynamic monitoring during cardiac surgery as it provides the basis for therapeutic interventions to ensure adequate tissue perfusion. For that purpose, a pulmonary artery catheter (PAC) using the thermodilution method has been considered a ‘clinical standard’ along with its attendant limitations [1]. The invasive nature of the PAC carries the risk of various complications, including damage to the cardiac valves and pulmonary artery rupture, and the clinical value of the data obtained from the PAC remains a matter of heated debate [2]. The FloTrac/VigileoTM system (Edwards Lifesciences, Irvine, CA, USA) is a less invasive method of acquiring continuous data on the cardiac output and this system uses pulse contour analysis. It requires only standard radial artery catheterization and it empirically correlates the standard deviation (SD) of the pulse pressure to the stroke volume on the basis of the patient’s characteristics after automatic adjustment for actual vascular compliance and thus, it does not require external calibration [3]. The earlier validation studies of the FloTrac/VigileoTM system have demonstrated conflicting results [4-7]. With the updated software (version 1.07) that provides a reduced time window (1 min) for vascular adjustment, the recent clinical studies that focused on cardiac surgeries have demonstrated promising results [8-10]. Still, the evidence regarding its accuracy is limited for surgeries accompanied with changes in the vascular compliance, and especially for patients with a decreased left ventricular ejection fraction (LVEF) and for patients with a low cardiac output status, and accurately estimating the cardiac output is of particular importance for clinical decision making. Furthermore, no comprehensive data exist regarding the influence of factors that affect the pulse contour. We evaluated the accuracy of the cardiac output measured by the FloTrac/VigileoTM system (software version 1.07), as compared to the cardiac output measured by a PAC, in patients who are undergoing off-pump coronary bypass surgery (OPCAB), and we included patients with a decreased LVEF. We also evaluated the influence of the mean arterial pressure (MAP), the systemic vascular resistance index (SVRI) and the use of vasopressor on the performance of the FloTrac/VigileoTM system. Materials and Methods This study was approved by the Institutional Review Board of our hospital and written informed consent was obtained from all subjects. The study subjects were 50 patients (30 patients with LVEF ≥ 40%, 20 patients with LVEF < 40%) who were scheduled for elective, isolated, multivessel OPCAB between June 2007 and February 2008. The LVEF was measured with 238 Vol. 60, No. 4, April 2011 transthoracic echocardiography and using the biplane modified Simpson’s method 1 day prior to surgery by cardiologists who were not aware of this study. The patients with preexisting pulmonary disease, concomitant valvular heart disease, cardiac arrhythmias and peripheral vascular disease were excluded from the study. The patients’ cardiac medications were continued until the morning of the surgery, except for diuretics and antiplatelet agents. All the patients received 0.05-0.1 mg/kg of morphine intramuscularly as premedication 1 hr before their operation. Upon arrival at the operating room, standard monitoring devices were applied including a PAC (Swan-Ganz CCOmbo CCO/SvO2TM, Edwards Lifesciences LLC, Irvine, CA, USA), which was inserted via the right internal jugular vein and connected to an analysis system (VigilanceTM, Edwards Life- sciences LLC, Irvine, CA, USA) for continuous monitoring of the cardiac index (CCI) and the mixed venous oxygen saturation (SvO2). The radial artery was also cannulated in all the patients with a 20-G cannula (BD Angiocath PlusTM, Becton Dickinson Korea Ltd. Korea) and this was connected to a FloTrac/VigileoTM sensor for estimating the continuous arterial pressure waveform analysis cardiac index (FCI). All the transducers were zeroed at the mid-axillary level and care was taken to ensure that the pressure waveform was not dampened during the study period. Anesthesia was induced with intravenous midazolam (0.03- 0.07 mg/kg) and sufentanil (1.5-2.0 μg/kg), and anesthesia was maintained with sevoflurane (0.8-1.5%) and a continuous infusion of sufentanil (0.5-1.5 μg/kg/hr). Neuromuscular blockade was achieved by administering rocuronium (0.9 mg/ kg) and this was maintained with a continuous infusion of vecuronium (1-2 μg/kg/min). Isosorbide dinitrate 0.5 μg/kg/ min was infused in all the patients throughout the study period. The patients’ lungs were ventilated with a tidal volume of 8-10 ml/kg, with an I : E ratio of 1 : 2, at a rate of 8-12 breaths/min of 40% oxygen with air and a positive end-expiratory pressure of 5 cmH2O during the surgery. Intravascular volume replacement was managed with crystalloid and colloid solutions to maintain the pulmonary capillary wedge pressure between 8-16 mmHg according to the baseline values prior to manipulation of the heart and after completion of grafting. During the period of heart displacement, the crystalloid solution was infused at a fixed rate of 6-8 ml/kg/hr, whereas the colloid solution was infused to compensate for the amount of blood loss collected by a cell salvage device. The blood salvaged by the cell salvage device was reinfused to the patient before the end of the surgery. The hemodynamic management was as follows; 1) maintenance of the MAP between 60-80 mmHg with either a norepinephrine or nicardipine infusion with a 10-20o Trendelenburg position and/or a norepinephrine infusion during heart displacement), 2) infusion of milrinone in the patients with a SvO2 < 60% for longer www.ekja.org