Mahutte C K, Jaffe M B
Department of Medicine, Veterans Affairs Medical Center, Long Beach, CA 90822, USA.
J Clin Monit. 1995 Mar;11(2):99-108. doi: 10.1007/BF01617731.
We have investigated the effect of measurement errors on cardiac output, calculated via three different Fick methods. In method 1, the classic O2 Fick equation is expressed in terms of oxygen uptake (VO2), arterial pulse (SaO2) and venous oximetry (SVO2) saturations. The second method, a modified CO2 Fick method, is obtained by replacing VO2 in method 1 with carbon dioxide production (VCO2) divided by the respiratory quotient. In method 3, cardiac output is expressed as VCO2 divided by the product of the SaO2-SVO2 difference and a constant. This constant is determined from initial measurements of VCO2, SaO2, SVO2, and thermodilution cardiac output (Qth). This determination of the constant results in equality of the initial cardiac output of method 3 with the simultaneously determined Qth and, therefore, is similar to performing an autocalibration. For each of the three preceding Fick methods, we derive general expressions that explicitly show how measurement errors (random and systematic) in the Fick variables (VO2, VCO2, SaO2, and SVO2) propagate into errors in calculated cardiac output. The errors in theoretically calculated cardiac output decrease as the SaO2-SVO2 difference increases, except for the systematic error in method 3. The systematic error of method 3 is constant and depends only upon the accuracy of the initial Qth. Analytic expressions for the sensitivity of calculated cardiac output to errors in individual Fick variables are also obtained. Using estimates from the literature for typical systematic and random measurement errors in the Fick variables, the resultant errors in cardiac output are numerically calculated. The effect of random measurement errors on errors in calculated cardiac output was comparable among the three methods. However, the systematic error was least with method 3. Total errors (random and systematic) were comparable among the three methods. Using these numerical measurement errors, we conclude that continuous cardiac output may be calculated with comparable accuracy with each of these methods.
我们研究了测量误差对通过三种不同菲克法计算的心输出量的影响。在方法1中，经典的氧菲克方程根据氧摄取量（VO₂）、动脉血氧饱和度（SaO₂）和静脉血氧饱和度（SVO₂）来表示。第二种方法是改良的二氧化碳菲克法，通过用二氧化碳产生量（VCO₂）除以呼吸商来替代方法1中的VO₂得到。在方法3中，心输出量表示为VCO₂除以SaO₂与SVO₂差值和一个常数的乘积。这个常数由VCO₂、SaO₂、SVO₂和热稀释法心输出量（Qth）的初始测量值确定。这个常数的确定使得方法3的初始心输出量与同时测定的Qth相等，因此类似于进行自动校准。对于上述三种菲克法中的每一种，我们推导了一般表达式，明确展示了菲克变量（VO₂、VCO₂、SaO₂和SVO₂）中的测量误差（随机误差和系统误差）如何传播到计算的心输出量误差中。理论计算的心输出量误差随着SaO₂ - SVO₂差值的增加而减小，但方法3中的系统误差除外。方法3的系统误差是恒定的，仅取决于初始Qth的准确性。还获得了计算的心输出量对各个菲克变量误差的灵敏度的解析表达式。利用文献中对菲克变量典型系统误差和随机测量误差的估计值，对心输出量的结果误差进行了数值计算。三种方法中随机测量误差对计算的心输出量误差的影响相当。然而，方法3的系统误差最小。三种方法的总误差（随机误差和系统误差）相当。利用这些数值测量误差，我们得出结论，使用这些方法中的任何一种计算连续心输出量的准确性相当。
