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 circulation model thermal dilution was compared with direct (Fdir) and electromagnetic (F(elm)) flowmetry. Flow values measured in the circulation model with the thermal dilution (Fth) technique correspond well with direct flowmetry (Fth = 0.92 Fdir + 7.0; r = 0.888) and with electromagnetic flowmetry (Fth = 0.95 F(elm) + 1.2; r = 0.990). In the anesthetized rat cardiac output was determined with thermal dilution and simultaneously with Fick's method and/or with electromagnetic flowmetry. Fick's method and electromagnetic flowmetry resulted in identical cardiac output values (COFick = 0.95 COelm; r = 0.865), whereas the thermal dilution technique yielded unequivocally higher values. The extent of overestimation is much more pronounced at low cardiac output than at a high output. The study clearly demonstrates that this overestimation is due to heat diffusion, which is obviously of greater significance in small animals than in large animals or humans. Therefore, the thermal dilution technique is not appropriate for the measurement of cardiac output in the rat.