Electrocardiographic interference and conductance volume measurements

The conductance catheter technique enables continuous ventricular volume measurements based on the electrical conductance of blood within the ventricular cavity. However, ventricular excitation also produces a measurable electrical signal within the ventricular cavity. This study was undertaken to investigate the relationship between the ventricular electrogram and conductance volume measurements in a physical model of the left ventricle without parallel conductance. The ventricular electrogram was simulated with an ECG signal, ECGinput connected to two ring electrodes within the model ventricle. Conductance volume measurements were made with and without ECGinput. The difference between these measurements, GECG(t), represented the conductance volume due to ECGinput. GECG(t) varied as a function of the first-derivative of ECGinput with respect to time (r2=0.92, P<0.001). GECG(t), This primarily affected volume measurements during ventricular depolarisation; during this phase the volume measurement error varied widely between –12% and +9%. As a re-sult, end-diastole could not be reliably identified on the pressure-volume loop. The accuracy of conduc-tance volume measurements during late diastole and early isovolumic contraction are substantially affected by the ventricular electrogram. This may result in a significant error in end-diastolic volume estimates, which has important implications for the quantitative assessment of ventricular function including, in particular, the assessment of chamber compliance.