THE EFFECT OF CHANGING EXCITATION-FREQUENCY ON PARALLEL CONDUCTANCE IN DIFFERENT-SIZED HEARTS

Objective: An important component of the ventricular volume measured u sing the conductance catheter technique is due to parallel conductance (Vc), which results from the extension of the electric field beyond t he ventricular blood pool. parallel conductance volume is normally est imated using the saline dilution method (Vc(saline dilution)), in whic h the conductivity of blood in the ventricle is transiently increased by injection of hypertonic saline. A simpler alternative has been repo rted by Gawne et al, [12], Vc(dual frequency) is estimated from the di fference in total conductance measured at two exciting frequencies and the method is based on the assumption that parallel conductance is ma inly capacitive and hence is negligible at low frequency. The objectiv e of this study was to determine whether the dual frequency technique could be used to substitute the saline dilution method to estimate Vc in different sized hearts. Methods: The accuracy and linearity of a cu stom-built conductance catheter (CC) system was initially assessed in vitro. Subsequently, a CC and micromanometer were inserted into the le ft ventricle of seven 5 kg pigs (group 1) and six 50 kg pigs (group 2) . Cardiac output was determined using thermodilution (group 1) and an ultrasonic flow probe (group 2) from which the slope coefficient (alph a) was determined. Steady state measurements and Vc estimated using sa line dilution were performed at frequencies in the range of 5-40 kHz. All measurements were made at end-expiration. Finally, Vc was estimate d from the change in end-systolic conductance between 5 kHz and 40 kHz using the dual frequency technique of Gawne et al. [12]. Results: The re was no change in measured volume of a simple insulated cylindrical model when the stimulating frequency was varied from 5-40 kHz, Vc(sali ne dilution) varied significantly with frequency in group 1 (8.63 +/- 2.74 ml at 5 kHz; 11.51 +/- 2.65 ml at 40 kHz) (p = 0.01). Similar res ults were obtained in group 2 (69.43 +/- 27.76 ml at 5 kHz; 101.24 +/- 15.21 ml at 40 kHz) (p < 0.001). However, the data indicate that the resistive component of the parallel conductance is substantial (Vc at 0 Hz estimated as 8.01 ml in group 1 and 62.3 ml in group 2). There wa s an increase in alpha with frequency in both groups but this did not reach significance. The correspondence between Vc(dual frequency) and Vc(saline dilution) methods was poor (group 1 R-2 = 0.69; group 2 R-2 = 0.22). Conclusion: At a lower excitation frequency of 5 kHz a smalle r percentage of the electric current extends beyond the blood pool so parallel conductance is reduced. While parallel conductance is frequen cy dependent, it has a substantial resistive component. The dual frequ ency method is based on the assumption that parallel conductance is ne gligible at low frequencies and this is clearly not the case. The resu lts of this study confirm that the dual frequency technique cannot be used to substitute the saline dilution technique. (C) 1998 Elsevier Sc ience B.V. All rights reserved.