Calibrating volume measurements made using the dual-field conductance catheter

The conductance catheter technique allows realtime measurements of ventricular volume based on changes in the electrical conductance of blood within the ventricular cavity. Conductance volume measurements are corrected with a calibration coefficient, α, in order to improve accuracy. However, conductance volume measurements are also affected by parallel conductance, which may confound calibration coefficient estimation. This study was undertaken to examine the variation in α using a physical model of the left ventricle without parallel conductance. Calibration coefficients were calculated as the conductance-volume quotient (αV(t)) or the stroke conductance-stroke volume quotient (αSV). Both calibration coefficients varied as a non-linear function of the ventricular volume. Conductance volume measurements calibrated with αV(t) estimated ventricular volume to within 2.0 ± 6.9%. By contrast, calibration with αSV substantially over-estimated the ventricular volume in a volume-dependent manner, increasing from 26 ± 20% at 100ml to 106 ± 36% at 500ml. The accuracy of conductance volume measurements is affected by the choice of calibration coefficient. Using a fixed or constant calibration coefficient will result in volume measurement errors. The conductance-stroke volume quotient is associated with particularly significant and volume-dependent measurement errors. For this reason, conductance volume measurements should ideally be calibrated with an alternative measurement of ventricular vol-