J. Biomedical Science and Engineering, 2009, 2, 484-490 JBiSE
doi: 10.4236/jbise.2009.27070 Published Online November 2009 (http://www.SciRP.org/journal/jbise/).
Calibrating volume measurements made using the dual-field
Simon P. McGuirk1,2, David J. Barron1, Dan Ewert3, John H. Coote2
Department of Cardiac Surgery, Birmingham Children’s Hospital, Birmingham, UK; 2School of Clinical and Experimental Medicine,
University of Birmingham, Birmingham, UK; 3Department of Electrical and Computer Engineering, North Dakota State University,
Received 1 February 2009; revised 24 June 2009; accepted 16 July 2009.
ABSTRACT 1. INTRODUCTION
The conductance catheter technique allows real- The conductance catheter technique is an established
time measurements of ventricular volume based on method to measure the ventricular volume in real-time,
changes in the electrical conductance of blood based on the electrical conductance of blood within the
within the ventricular cavity. Conductance volume ventricular cavity [1,2,3]. Conductance volume meas-
measurements are corrected with a calibration coef- urements are based on the assumption that the electric
ficient, , in order to improve accuracy. However, field produced by the conductance catheter is homoge-
conductance volume measurements are also affected neously distributed within the ventricular cavity .
by parallel conductance, which may confound cali- However, theoretical and experimental studies have
bration coefficient estimation. This study was un- demonstrated that this assumption is not valid [4,5,6]. As
dertaken to examine the variation in using a a result, the conductance catheter tends to overestimate
physical model of the left ventricle without parallel the volume in small ventricles and underestimate the
conductance. Calibration coefficients were calculated volume in larger ventricles [2,3].
as the conductance-volume quotient ( V(t)) or the The dimensionless calibration coefficient, , was in-
stroke conductance-stroke volume quotient ( SV). troduced by Baan et al.  in order to account for the
Both calibration coefficients varied as a non-linear non-uniform conductance-absolute volume relationship
function of the ventricular volume. Conductance . This calibration coefficient represents the slope of
volume measurements calibrated with V(t) estimated the relationship between the conductance-derived vol-
ume and the true volume. The calibration coefficient,
ventricular volume to within 2.0 ± 6.9%. By contrast,
may also vary with ventricular volume. It is relatively
calibration with SV substantially over-estimated the
high in small animals , lower in humans  and in-
ventricular volume in a volume-dependent manner,
termediate values are found in dogs [3,8,9]. Experimen-
increasing from 26 ± 20% at 100ml to 106 ± 36% at
tal studies demonstrate that also varies during inferior
500ml. The accuracy of conductance volume meas-
vena caval occlusion  and may even fluctuate during
urements is affected by the choice of calibration coef-
the normal cardiac cycle [10,11].
ficient. Using a fixed or constant calibration coeffi- The tissues and fluid surrounding the ventricular cav-
cient will result in volume measurement errors. The ity also contribute to the measured conductance signal
conductance-stroke volume quotient is associated . This creates a volume offset called parallel conduc-
with particularly significant and volume-dependent tance. Parallel conductance also varies according to the
mea- surement errors. For this reason, conductance ventricular volume [4,7,8,9,12,13]. This volume- de-
volume measurements should ideally be calibrated pendent parallel conductance may confound the calibra-
with an alternative measurement of ventricular vol- tion coefficient estimation.
ume. This study was undertaken to examine the variation in
the calibration coefficient, , over a range of volumes
Keywords: Conductance Catheter; Calibration; Volume pertaining to clinical studies, in a physical model of the
Measurement left ventricle without parallel conductance.
Published Online Nove