Anil T. G. Kottam

Electrical Conductivity and Permittivity of Murine Myocardium

A classic problem in traditional conductance measurement of left ventricular (LV) volume is the separation of the contributions of myocardium from blood. Measurement of both the magnitude and the phase of admittance allow estimation of the time-varying myocardial contribution, which provides a substantial improvement by eliminating the need for hypertonic saline injection. We present in vivo epicardial surface probe measurements of electrical properties in murine myocardium using two different techniques (a digital and an analog approach). These methods exploit the capacitive properties of the myocardium, and both methods yield similar results. The relative permittivity varies from approximately 100 000 at 2 kHz to approximately 5000 at 50 kHz. The electrical conductivity is approximately constant at 0.16 S/m over the same frequency range. These values can be used to estimate and eliminate the time-varying myocardial contribution from the combined signal obtained in LV conductance catheter measurements, thus yielding the blood contribution alone. To study the effects of albumin on the blood conductivity, we also present electrical conductivity estimates of murine blood with and without typical administrations of albumin during the experiment. The blood conductivity is significantly altered (p < 0.0001) by administering albumin (0.941 S/m with albumin, 0.478 S/m without albumin).

Real time Pressure-Volume loops in mice using complex admittance: measurement and implications

Real time left ventricular (LV) pressure-volume (P-V) loops have provided a framework for understanding cardiac mechanics in experimental animals and humans. Conductance measurements have been used for the past 25 years to generate an instantaneous left ventricular (LV) volume signal. The standard conductance method yields a combination of blood and ventricular muscle conductance; however, only the blood signal is used to estimate LV volume. The state of the art techniques like hypertonic saline injection and IVC occlusion, determine only a single steady-state value of the parallel conductance of the cardiac muscle. This is inaccurate, since the cardiac muscle component should vary instantaneously throughout the cardiac cycle as the LV contracts and fills, because the distance from the catheter to the muscle changes. The capacitive nature of cardiac muscle can be used to identify its contribution to the combined conductance signal. This method, in contrast to existing techniques, yields an instantaneous estimate of the parallel admittance of cardiac muscle that can be used to correct the measurement in real time. The corrected signal consists of blood conductance alone. We present the results of real time in vivo measurements of pressure-admittance and pressure-phase loops inside the murine left ventricle. We then use the magnitude and phase angle of the measured admittance to determine pressure volume loops inside the LV on a beat by beat basis. These results may be used to achieve a substantial improvement in the state of the art in this measurement method by eliminating the need for hypertonic saline injection

Evidence of Time-Varying Myocardial Contribution by In Vivo Magnitude and Phase Measurement in Mice

Cardiac volume can be estimated by a conductance catheter system. Both blood and myocardium are conductive, but only the blood conductance is desired. Therefore, the parallel myocardium contribution should be removed from the total measured conductance. Several methods have been developed to estimate the contribution from myocardium, and they only determine a single steady state value for the parallel contribution. Besides, myocardium was treated as purely resistive or mainly capacitive when estimating the myocardial contribution. We question these assumptions and propose that the myocardium is both resistive and capacitive, and its contribution changes during a single cardiac cycle. In vivo magnitude and phase experiments were performed in mice to confirm this hypothesis.

Novel approach to admittance to volume conversion for ventricular volume measurement

The conductance catheter is a widely used tool to determine ventricular volumes in animal models. A tetra-polar catheter is inserted into the ventricle to measure instantaneous conductance, which is a combination of ventricular blood and surrounding myocardium. Various techniques have been used to separate the blood conductance signal from the combined measured signal [1], [2]. The blood conductance is then converted to volume using a linear relationship proposed by Baan [1] or an improved non linear relationship proposed by Wei [3]. We propose a novel approach that uses the combined blood-muscle signal to calculate volume, thereby eliminating the need to subtract out the muscle. In vivo experiments were performed in mice to validate this new approach and the results were compared with volumes obtained using ultrasound imaging.

Design of a wireless telemetric backpack device for real-time in vivo measurement of pressure-volume loops in conscious ambulatory rats

Pressure - Volume (PV) analysis is the de facto standard for assessing myocardial function. Conductance based methods have been used for the past 27 years to generate instantaneous left ventricular (LV) volume signal. Our research group has developed the instrumentation and the algorithm for obtaining PV loops based on the measurement of real - time admittance magnitude and phase from the LV of anaesthetized mice and rats. In this study, the instrumentation will be integrated into an ASIC (Application Specific Integrated Circuit) and a backpack device will be designed along with this ASIC. This will enable measurement of real-time in vivo P-V loops from conscious and ambulatory rats, useful for both acute and chronic studies.

Murine Heart Volume: Numerical Comparison and Calibration of Conductance Catheter Models

A full set of finite-element method (FEM) studies of the catheter within a cylindrical cuvette and within an elliptical cuvette are presented along with novel insight on the fundamental electromagnetic properties of the catheter. An in vitro experiment with modified small mouse pressure-volume catheters was conducted and the results are presented as a validation of the FEM models. In addition, sensitivity analysis on the electrode size and position is conducted and the results allow for a novel calibration factor based on catheter geometry to be presented. This calibration factor is used in conjunction with Weis conductance volume equations to reduce the average measured error in cuvette volume measurements from 26.5% to 5%.

Validation of a defibrillation lead ventricular volume measurement compared to three-dimensional echocardiography

BACKGROUND There is increasing evidence that using frequent invasive measures of pressure in patients with heart failure results in improved outcomes compared to traditional measures. Admittance, a measure of volume derived from preexisting defibrillation leads, is proposed as a new technique to monitor cardiac hemodynamics in patients with an implantable defibrillator. OBJECTIVE The purpose of this study was to evaluate the accuracy of a new ventricular volume sensor (VVS, CardioVol) compared with 3-dimenssional echocardiography (echo) in patients with an implantable defibrillator. METHODS Twenty-two patients referred for generator replacement had their defibrillation lead attached to VVS to determine the level of agreement to a volume measurement standard (echo). Two opposite hemodynamic challenges were sequentially applied to the heart (overdrive pacing and dobutamine administration) to determine whether real changes in hemodynamics could be reliably and repeatedly assessed with VVS. Equivalence of end-diastolic volume (EDV) and stroke volume (SV) determined by both methods was also assessed. RESULTS EDV and SV were compared using VVS and echo. VVS tracked expected physiologic trends. EDV was modulated -10% by overdrive pacing (14 mL). SV was modulated -13.7% during overdrive pacing (-6 mL) and increased over baseline +14.6% (+8 mL) with dobutamine. VVS and echo mean EDVs were found statistically equivalent, with margin of equivalence 13.8 mL (P <.05). Likewise, mean SVs were found statistically equivalent with margin of equivalence 15.8 mL (P <.05). CONCLUSION VVS provides an accurate method for ventricular volume assessment using chronically implanted defibrillator leads and is statistically equivalent to echo determination of mean EDV and SV.