EFFECT OF MEASUREMENT ERRORS ON CARDIAC-OUTPUT CALCULATED WITH O-2 AND MODIFIED CO2 FICK METHODS

We have investigated the effect of measurement errors on cardiac outpu t, calculated via three different Fick methods. In method 1, the class ic O2 Fick equation is expressed in terms of oxygen uptake (Vo(2)), ar terial pulse (Sao(2)) and venous oximetry (Svo(2)) saturations. The se cond method, a modified CO2 Fick method, is obtained by replacing Vo, in method 1 with carbon dioxide production (Vco(2)) divided by the res piratory quotient. In method 3, cardiac output is expressed as Vco(2) divided by the product of the Sao(2)-Svo(2) difference and a constant. This constant is determined from initial measurements of Vco(2), Sao( 2), Svo(2), and thermodilution cardiac output (Q(th)) This determinati on of the constant results in equality of the initial cardiac output o f method 3 with the simultaneously determined Q(th) and, therefore, is similar to performing an autocalibration. For each of the three prece ding Fick methods, we derive general expressions that explicitly show how measurement errors (random and systematic) in the Fick variables ( Vo(2), Vco(2), Sao(2), and Svo(2)) Propagate into errors in calculated cardiac output. The errors in theoretically calculated cardiac output decrease as the Sao,-Svo(2), difference increases, except for the sys tematic error in method 3. The systematic error of method 3 is constan t and depends only upon the accuracy of the initial Q(th) Analytic exp ressions 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 th e Fick variables, the resultant errors in cardiac output are numerical ly calculated. The effect of random measurement errors on errors in ca lculated cardiac output was comparable among the three methods. Howeve r, the systematic error was least with method 3. Total errors (random and systematic) were comparable among the three methods. Using these n umerical measurement errors, we conclude that continuous cardiac outpu t may be calculated with comparable accuracy with each of these method s.