A system has been developed to monitor continuously the components of the oxygen Fick equation: oxygen consumption by a gas exchange analyzer and arterio-venous oxygen difference by pulse and fiberoptic oximetry. A computer-based system was developed which calculates cardiac output and other variables every 20 sec. Continuous Fick (CF) cardiac output was compared to thermodilution (TD) cardiac output in 21 ventilated post-cardiac surgery patients. A total of 237 simultaneous cardiac output measurements had a range between 2 and 11 L/min. The correlation between CF and TD cardiac outputs was r =.86, with an equation of TD cardiac output = 0.92 CF cardiac output plus; 1.16. There was a significant (p.001) difference between the two methods of cardiac output estimation. The CF method was consistently lower than TD; this difference was greater at lower flows. CF cardiac output measurement is practical; it offers distinct advantages in viewing cardiac output together with oxygen demand and oxygen extraction.
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