FACTORS CONFOUNDING IMPEDANCE CATHETER VOLUME MEASUREMENTS INVITRO

The impedance catheter allows continuous measurement of ventricular vo lume. External influences have been described as causing parallel shif ts in impedance-measured volumes; however, factors affecting impedance measurements in a nonparallel manner have not been fully characterize d. Accordingly, an impedance catheter was placed inside a latex balloo n into which known volumes of normal saline solution were injected. Co nductive and nonconductive materials were individually placed within t he balloon. Impedance was measured with materials touching (T) or not touching (NT) the catheter. Impedance-measured volumes were plotted ve rsus actual volumes. Compared with the line of identity (LID), a stati stical difference (p < 0.05) was found in the slopes in the presence o f metallic objects only. These included a pacing lead (T, NT) (m(T) = 1.32, m(NT) = 1.29 versus m(LID) = 1.00), titanium (T) (m(T) = 1.68 ve rsus m(LID) = 1.00), and aluminum (NT) (m(NT) = 0.72 versus m(LID) = 1 .00). These changes in slope indicate nonparallel effects on impedance that confound the ability of the impedance catheter to determine volu mes in vitro. These observations imply that serial calibration of both the slope constant (alpha) and the intercept (parallel conductance) o f impedance may be necessary for in vivo measurements of ventricular v olume based on impedance in the presence of metallic objects.