SINGLE INJECTION THERMODILUTION - A FLOW-CORRECTED METHOD

Background: Application of the Stewart-Hamilton equation in the thermo dilution technique requires now to be constant. In patients in whom ve ntilation of the lungs is controlled, flow modulations may occur leadi ng to large errors in the estimation of mean cardiac output. Methods: To eliminate these errors, a modified equation was developed. The resu lting how-corrected equation needs an additional measure of the relati ve changes of blood now during the period of the dilution curve. Relat ive flow was computed from the pulmonary artery pressure with use of t he pulse contour method. Measurements were obtained in 16 patients und ergoing elective coronary artery bypass surgery. In 11 patients (group A), pulmonary artery pressure was measured with a catheter tip transd ucer, in a partially overlapping group of 11 patients (group B), it wa s measured with a fluid-filled system. For reference cardiac output we used the proven method of four uncorrected thermodilution estimates e qually spread over the ventilatory cycle. Results: A total of 208 card iac output estimates was obtained in group A, and 228 in group B. In g roup B, 48 estimates could not be corrected because of insufficient pu lmonary artery pressure waveform quality from the fluid-filled system. Individual uncorrected Stewart-Hamilton estimates showed a large vari ability with respect to their mean. In group A, mean cardiac output wa s 5.01 1/min with a standard deviation of 0.53 1/min, or 10.6%. After flow correction, this scatter decreased to 5.0% (P< 0.0001). With no b ias, the corresponding limits of agreement decreased from +/- 1.06 to +/- 0.5 1/min after now correction. In group B, the scatter decreased similarly and the limits of agreement also became +/- 0.5 1/min after how correction. Conclusion: It was concluded that a single thermodilut ion cardiac output estimate using the now-corrected equation is clinic ally feasible. This is obtained at the cost of a more complex computat ion and an extra pressure measurement, which often is already availabl e. With this technique it is possible to reduce the fluid load to the patient considerably.