The conductance catheter, which is placed in the right ventricle, establishes an electric field between an apical and basal electrode by the passage of a high-frequency current. Current is held constant while adjacent pairs of intervening electrodes measure the conductivity of the surrounding blood (which greatly exceeds that of the surrounding myocardium) along this right ventricular long axis ( DiCkS.te.!D.,e.LaL 1995). However, the technique has limitations.
1. The electrode pairs measure in cross-sectional dimensions. Thus shortening of the longitudinal axis is poorly assessed.
2. Blood resistivity varies with time, and increasing flow velocity may reduce blood conductivity and lead to underestimated values.
3. Structures surrounding the ventricle contribute to the conductance signal.
The last problem might be partly overcome by the proximal injection of 10 ml of cooled saline (2-4 °C). Since where G(t) is the variation of conductance with time, sb is the specific conductivity of blood, a is a dimensionless constant, L is the electrode distance, V(t) is the variation of volume with time, and Gp is the conductance of surrounding structures, such an injection would cause a transient decrease in left ventricular blood conductivity and allow Gp to be calculated. This technique has as yet only been applied to the left ventricle. There are also specific difficulties in applying this technique to the right ventricle. Because of the complex right ventricular geometry, the current density varies between electrode pairs. Right ventricular trabeculation distorts uniform distribution of the excitation current in the right ventricle and leaves some regions unscanned. Finally, the right ventricular wall is thinner than that of the left ventricle, and current leakage is greater. Impedance methods may significantly underestimate the stroke volume, and therefore correction factors are required. Nonetheless, stroke volumes measured by conductance and thermodilution correlate closely.
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