Analysis of uncertainty in theoretical methods of cardiac output measurement using the "Monte Carlo" technique.

We have developed three models which describe the relationship between cardiac output and the uptake of volatile anaesthetic agents, based on the Fick equation, and determined if these models could provide useful methods of cardiac output measurement. Because many variables are involved in the calculation of cardiac output using these methods, a "Monte Carlo" simulation was performed to investigate the combined effect of uncertainties in several variables on the resultant cardiac output estimate. We found that the single-breath model was most accurate when the inspired concentration was large, while the rebreathing model was better with smaller inspired concentrations. The three-breath model was the least accurate under all conditions studied. Volatile anaesthetics were generally more accurate than nitrous oxide, with both enflurane and halothane more accurate than isoflurane. The "Monte Carlo" technique provides a valuable tool for analysis of errors in measurement methods.