Biomedical subjects

Jonathan W Valvano

Publications and source records attributed to Jonathan W Valvano.

9 recordsLinked to original sources

PubMed2006

Impact of physiological variables and genetic background on myocardial frequency-resistivity relations in the intact beating murine heart.

Conductance measurements for generation of an instantaneous left ventricular (LV) volume signal in the mouse are limited, because the volume signal is a combination of blood and LV muscle, and only the blood signal is desired. We have developed a conductance system that operates at two simultaneous frequencies to identify and remove the myocardial contribution to the instantaneous volume signal. This system is based on the observation that myocardial resistivity varies with frequency, whereas blood resistivity does not. For calculation of LV blood volume with the dual-frequency conductance system in mice, in vivo murine myocardial resistivity was measured and combined with an analytic approach. The goals of the present study were to identify and minimize the sources of error in the measurement of myocardial resistivity to enhance the accuracy of the dual-frequency conductance system. We extended these findings to a gene-altered mouse model to determine the impact of measured myocardial resistivity on the calculation of LV pressure-volume relations. We examined the impact of temperature, timing of the measurement during the cardiac cycle, breeding strain, anisotropy, and intrameasurement and interanimal variability on the measurement of intact murine myocardial resistivity. Applying this knowledge to diabetic and nondiabetic 11- and 20- to 24-wk-old mice, we demonstrated differences in myocardial resistivity at low frequencies, enhancement of LV systolic function at 11 wk and LV dilation at 20-24 wk, and histological and electron-microscopic studies demonstrating greater glycogen deposition in the diabetic mice. This study demonstrated the accurate technique of measuring myocardial resistivity and its impact on the determination of LV pressure-volume relations in gene-altered mice.

Animals↗

PubMed2005

Effects of the time response of the temperature sensor on thermodilution measurements.

Thermodilution is widely used to measure cardiac output, ejection fraction and end diastolic volume. Even though the method is based on dynamic temperature measurements, little attention has been paid to the characterization of the dynamic behavior of the temperature sensor and to its influence on the accuracy of the method. This paper presents several theoretical and empirical results related to the thermodilution method. The results show that, at flow velocities above 0.2 m s(-1), the response of temperature sensors embedded in Swan-Ganz catheters can be accurately described by a convolution operation between the true temperature of the blood and the impulse response of the sensor. The model developed is used to assess the influence of the probe response on the measurement of cardiac output, and this study leads us to the conclusion that the probe response can cause errors in the cardiac output measurement, but this error is usually small (2% in cases with a high degree of arrhythmia). The results show that these small errors appear during arrhythmias that affect the R-R interval and when the real temperature distribution at the pulmonary artery does not possess a shape with perfect temperature plateaux.

Animals↗

PubMed2005

Nonlinear conductance-volume relationship for murine conductance catheter measurement system.

The conductance catheter system is a tool to determine instantaneous left ventricular volume in vivo by converting measured conductance to volume. The currently adopted conductance-to-volume conversion equation was proposed by Baan, and the accuracy of this equation is limited by the assumption of a linear conductance-volume relationship. The electric field generated by a conductance catheter is nonuniform, which results in a nonlinear relationship between conductance and volume. This paper investigates this nonlinear relationship and proposes a new nonlinear conductance-to-volume conversion equation. The proposed nonlinear equation uses a single empirically determined calibration coefficient, derived from independently measured stroke volume. In vitro experiments and numerical model simulations were performed to verify and validate the proposed equation.

Animals↗

PubMed2004

Analysis of a thermal method for assessing endothelial dysfunction.

The presence of atherosclerosis not only affects the normal functioning of the coronary blood vessels but also of the peripheral vasculature. Property measurements made in the peripheral vasculature hence do reflect the condition of the coronary blood vessels. The endothelial cells form the inner lining of the blood vessels, and are responsible for the release of nitric oxide (NO) in order to control the vascular tone. Under normal conditions the artery will dilate in response to increased blood flow, mediated by the release of NO. The hampering of this normal response, caused by certain cardiovascular diseases, is referred to as endothelial dysfunction (EDF). Occlusion of the arm using a standard blood pressure cuff for five minutes followed by sudden release of the occlusion is known to create a reactive hyperemia in a normally functioning vasculature. We propose to measure the EDF by attempting to create this reactive hyperemia in the arm and measuring the temperature response in the hand and forearm, using a computer-based data acquisition system. The rate of temperature fall during occlusion and the temperature rate of rise after release are combined to assess EDF. An engineering analysis of the instrument was performed. Initial studies on normal subjects have indicated that the rate of rise is significantly higher than the rate of fall of temperature.

Arm↗

PubMed2004

Design of instrumentation and data-acquisition system for complex admittance measurement.

Instantaneous left ventricular volume measurements have been made for many years using a tetrapolar conductance catheter. The main objective is to determine the efficiency of the beating heart, using a tetrapolar catheter inserted in the left ventricle of transgenic mice. The effect of the parallel myocardium contribution must be removed from the total measurement. A dual-frequency technique involving 1 kHz and 100 kHz was chosen because it has been established that the imaginary part (the capacitive reactance) of the complex admittance of the cardiac muscle is much smaller in the lower frequency than at the higher frequency. The design involves generation of an accurate frequency source for both the frequencies careful selection of operational amplifiers for the current conversion stage so that the current is not too large to kill the mouse and that it is capable of performing at high frequencies. The band pass filter stage involved careful design with minimal overlap of the pass bands of both the channels. The overall circuit was designed so that there is minimal shift in the phase due to the circuit elements alone. Work also involved design of GPIB--based data acquisition system using LabVIEW and a digital oscilloscope for effective data acquisition even at high frequencies, which are normally limited by the sampling frequency. This data acquisition system is currently being used in laboratory studies in vivo.

Animals↗

PubMed2003

An instrument to measure the heat convection coefficient on the endocardial surface.

This work describes the fundamentals and calibration procedure of an instrument for in vivo evaluation of the heat convection coefficient between the endocardium and the circulating blood flow. The instrument is to be used immediately before radio-frequency cardiac ablation is performed. Thus, this instrument provides researchers with a valuable parameter to predict lesion size to be achieved by the procedure. The probe is a thermistor mounted in a Swan-Ganz catheter, and it is driven by a constant-temperature anemometer circuit. A 1D model of the sensor behaviour in a convective medium, the calibration procedure and the apparatus are explained in detail. Finally, a performance analysis of the instrument in the range of 200-3500 W m(-2) K(-1) shows that the average absolute error of full scale is 7.4%.

Animals↗

PubMed2003

In vivo measurements of heat transfer on the endocardial surface.

A catheter-based instrument was used to measure the heat transfer on the right atrial and ventricular endocardial surfaces of two pigs in vivo. The heat transfer parameters will assist in calculating the proper dose for radio-frequency ablation. The time constant of the device was 0.05 s. It was found that the average heat convection coefficient varies significantly both spatially and temporally on the endocardium. The average heat convection coefficients found were between 510 and 4800 W m(-2) K(-1).

Animals↗

PubMed2002

Cyclic capillary electrophoresis.

A strategy is described here for increasing both the resolution and the flexibility of capillary electrophoresis performed in a sieving medium of ungelled polymer. This strategy is based on analysis and, sometimes, re-analysis that is done in several stages of constant-field electrophoresis. Enhancement-stages are between the analysis-stages. An enhancement-stage (i) increases the separation between peaks, while (ii) moving DNA molecules in the reverse direction. An enhancement-stage is based on an electrophoretic ratchet generated by a pulsed electrical field that can be zero-integrated. The ratchet-generating pulses are longer than the field pulses that have previously been used to improve the resolution of DNA molecules. No limit has been found to the resolution enhancement achievable. Apparently, diffusion-induced peak broadening is inhibited and, in some cases, may be reversed by the ratchet. The enhancement-stages are critically dependent on the electrical field-dependence of a plot of electrophoretic mobility as a function of DNA length. To generate the pulsed electrical field, a computer-controlled system with a time resolution of 30 microseconds has been developed. Programming is flexible enough to embed other pulses within ratchet-generating pulses. These other pulses can be either the previously used, shorter field-inversion pulses or high-frequency periodic oscillations previously found to sharpen peaks.

Computers↗

PubMed2002

Measurement of ejection fraction with standard thermodilution catheters.

Right ventricle ejection fraction (RVEF) is clinically used to evaluate right ventricular function. The thermodilution method can be modified to estimate the RVEF. However, this method requires a thermistor with a fast time response in order to yield correct estimates. Digital signal processing techniques that were developed in previous works, allow the use of industry-standard slow time response thermistors for the measurement EF. However, these algorithms were not automated, and the works did not present a complete evaluation of the method's performance. This article presents a modified automated version of these algorithms, and uses numerical and in vitro simulations to test their performance. In the simulations, the measured ejection fraction was compared to the true ejection fraction. RVEFs ranging from 0.20 to 0.80 were tested for heart rates ranging from 30 to 120 heart beats per min. Statistical analysis of data showed that the new method presents an improved performance.

Algorithms↗