Effect of measurement errors on cardiac output calculated with O2 and modified CO2 Fick methods
Tóm tắt
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 (
$$\dot Vo_2 $$
), arterial pulse (Sao2) and venous oximetry (Svo2) saturations. The second method, a modified CO2 Fick method, is obtained by replacing
$$\dot Vo_2 $$
in method 1 with carbon dioxide production (
$$\dot Vco_2 $$
) divided by the respiratory quotient. In method 3, cardiac output is expressed as
$$\dot Vco_2 $$
divided by the product of the Sao2-Svo2 difference and a constant. This constant is determined from initial measurements of
$$\dot Vco_2 $$
, 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 (
$$\dot Vo_2 $$
,
$$\dot Vco_2 $$
, 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.
Tài liệu tham khảo
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