Abstract
The conductance catheter technique allows real- time 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 cali-bration coefficient estimation. This study was un-dertaken 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 coeffi-cient 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-ume.
Highlights
The conductance catheter technique is an established method to measure the ventricular volume in real-time, based on the electrical conductance of blood within the ventricular cavity [1,2,3]
Compared with the line of identity (i.e. x = y), conductance volume measurements predicted using this model were equal to ventricular volume at approximately 150 ml; slightly overestimated ventricular volume when the absolute volume was less than 150 ml; but underestimated ventricular volume at volumes over 150 ml
This study investigated the accuracy of conductance volume measurements, and the effect of calibration, in a series of in vitro experiments that spanned the volume range observed in clinical studies [17]
Summary
The conductance catheter technique is an established method to measure the ventricular volume in real-time, based on the electrical conductance of blood within the ventricular cavity [1,2,3]. Conductance volume measurements are based on the assumption that the electric field produced by the conductance catheter is homogeneously distributed within the ventricular cavity [1]. The dimensionless calibration coefficient, α, was introduced by Baan et al [3] in order to account for the non-uniform conductance-absolute volume relationship [3]. This calibration coefficient represents the slope of the relationship between the conductance-derived volume and the true volume. The calibration coefficient, α may vary with ventricular volume It is relatively high in small animals [7], lower in humans [3] and intermediate values are found in dogs [3,8,9]. Experimental studies demonstrate that α varies during inferior vena caval occlusion [9] and may even fluctuate during the normal cardiac cycle [10,11]
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