Thermal dilution assessment of proximal aortic and left ventricular mixing chambers in dogs

The adequacy of left ventricular mixing and the effect of mixing in the left ventricle and aorta on the measurement of the residual fraction by thermal dilution were assessed in open-chest dogs. Temperature was recorded at seven ventricular and two aortic sites during ventricular injections of iced saline at heart rates of 100, 150 and 200 bpm. The residual fraction, K, was calculated for each beat from the inscribed dilution curves and compared with the actual temperature at each measurement site and heart rate. Temperature was non-uniform throughout the ventricle after saline injections indicating incomplete tracer mixing. Thermal equilibration was slowest at the apex and decreased as heart rate increased. K, however, was similar for all sites in the ventricle and aorta at each heart rate by the third beat after saline injection. Thus, in spite of incomplete mixing, the beat-to-beat washout of indicator quickly stabilized which suggests that a reproducible measurement of functional volume can be obtained by this technique.