Stroke volume in the dog: measurements by the impedance technique and thermodilution

The main objective of this paper was to assess the efficacy of a laboratory prototype cardio impedance meter, to determine stroke volume in experimental dogs, using Kubicek's classic model. A second objective, complementary to the first, was to evaluate the method itself, under the carefully controlled condition in which cardiac frequency was increased from the initial to the final value. Measurements were performed on ten mongrel dogs (average body weight = 15.2 kg, SD = 4.7), employing the standard thermodilution procedure as reference. Based on a total of 179 determinations (about 18 per animal), the following regression equation was obtained: SVd = (0.36 SVz + 3.94) ml where SVd = stroke volume by thermodilution, SVz = stroke volume by impedance, with a standard error of the estimate SEE = 4.26 ml, a coefficient of variation CV = 32.8%, and 0.84 as correlation r. Thus, the impedance technique overestimated consistently by an average factor in the order of 2. These conclusions were made: In the experimental dog, the impedancimetric method can measure non-invasively directional and relative changes in stroke volume; this permits the effect of several types of maneuvres to be followed on a beat-to-beat basis. The lack of accuracy would require the development of a more sophisticated mathematical model; however, a practical statistical expression for laboratory use is easily derivable, making the determination as if the measurement had been made with the thermodilution procedure.(ABSTRACT TRUNCATED AT 250 WORDS)