VALIDITY OF THERMAL DILUTION TECHNIQUE FOR MEASUREMENT OF CARDIAC-OUTPUT IN RATS

The validity of the thermal dilution technique for the measurement of cardiac output was verified in experiments on a circulation model and on anesthetized rats under open- and closed-chest conditions. In the c irculation model thermal dilution was compared with direct (F(dir)) an d electromagnetic (F(elm)) flowmetry. Flow values measured in the circ ulation model with the thermal dilution (F(th)) technique correspond w ell with direct flowmetry (F(th) = 0.92 F(dir) + 7.0; r = 0.888) and w ith electromagnetic flowmetry (F(th) = 0.95 F(elm) + 1.2; r = 0.990). In the anesthetized rat cardiac output was determined with thermal dil ution and simultaneously with Fick's method and/or with electromagneti c flowmetry. Fick's method and electromagnetic flowmetry resulted in i dentical cardiac output values (CO(Fick) = 0.95 CO(elm); r = 0.865), w hereas the thermal dilution technique yielded unequivocally higher val ues. The extent of overestimation is much more pronounced at low cardi ac output than at a high output. The study clearly demonstrates that t his overestimation is due to heat diffusion, which is obviously of gre ater significance in small animals than in large animals or humans. Th erefore, the thermal dilution technique is not appropriate for the mea surement of cardiac output in the rat.