Antharvedi Anne

Accuracy of the conductance catheter for measurement of ventricular volumes seen clinically: effects of electric field homogeneity and parallel conductance

The conductance-volume method is an important clinical tool which allows the assessment of left ventricular function in vivo. However, the accuracy of this method is limited by the homogeneity of electric field the conductance catheter produces and the parallel conductance of surrounding structures. This paper examines these sources of error in volumes seen clinically, The characteristics of electric field within a chamber were examined using computer simulation. Nonconductive and conductive models were constructed and experimental measurements obtained using both single-field (SF) and dual-field (DF) excitation. Results from computer simulations and in vitro measurements were compared to validate the proposed theoretical model of conductance-volume method. The effects of field homogeneity and significance of parallel conductance in volume measurement were then determined. The results of this study show that DF provide a more accurate measure of intraventricular volume than SF, especially at larger volumes. However, both significantly underestimate true volume at larger volumes. In addition, the parallel conductance due to the chamber wall is significant at small volumes, but diminishes at larger volumes. Furthermore, the effect of parallel conductance beyond the chamber wall may be negligible. This study demonstrates the limitations in applying current conductance technology to patients with dilated hearts.

Identification of the high-risk population for serious burn injuries

Abstract The incidence of serious burn injuries has been determined for the Commonwealth of Virginia during a 21-month period. The data set included all patients with burn injuries who entered the hospital and were considered serious enough to require inpatient treatment. The risk for burn injury was calculated by age, race and sex. Examination of the data identified a high-risk population that was prone to serious burn injuries. The demographic characteristics of the burn population has been correlated with their socio-economic status.

Epidemiology of minor burn injuries

The incidence of minor burn injuries has been determined for the Commonwealth of Virginia during a 17-month period. The data set included all patients with burn injuries receiving treatment in the emergency department that did not require hospitalization. The risk of burn injury was calculated by age, race and sex. Examination of the data revealed large differences in the magnitude of age-specific incidence rates between men and women and the white and non-white population. Differences in the economic status of the population may explain a large proportion of the variation in the crude burn rates.

Use of Signal Representation to Identify Abnormal Motor Unit Potentials in Macro EMG

Macro electromyography (EMG) is a recently described recording technique allowing a nonselective recording of motor unit activity. The pick-up area of the electrode, the cannula of a modified single-fiber electrode, covers the entire motor unit territory. The motor unit potential (MUP) is obtained by averaging the cannula signals that are time locked to a single-fiber action potential. The MUP waveform is relatively insensitive to the position of the electrode within the motor unit. The amplitude of the MUP has been used as a measure of the size of the motor unit and normal values for the individual and median MUP amplitude have been defined.

Effect of Maximal versus Submaximal Expiratory Effort on Spirometric Values

Data from flow volume curves suggest that flow measured at the mouth may be lower with maximal effort than with submaximal effort. We consequently studied the effect of maximal vs. submaximal expiratory effort on spirometric values. Significant increases in FVC, FEV1 or FEF25-75% with submaximal effort occurred in 12 of 38 subjects. All subjects who improved spirometric values had peak flows with submaximal efforts that varied from 91 to 65% of peak flow with maximal efforts. We conclude that spirometric values in some subjects are larger with submaximal rather than maximal effort.

Optimal Ventilation of Critically Ill Patients

The intent of this research was to provide a theoretical approach for the optimal adjustment of controlled ventilators used in treating respiratory insufficiency in critically ill patients who have undergone major thoracic or abdominal surgery. In order to solve the optimization problem, it was necessary to develop a mathematical description of the process. The number of interactions involved, as well as the changing nature of the actual system, has been represented by an extremely complex model which may defy solution (Guyton 1973, Grodins 1967). It was therefore decided to describe the respiratory plant by a simple mathematical model that includes the pertinent information regarding the performance of the actual gas transport process. Variations in plant parameters were taken into account by providing an adaptive control philosophy that involves resolving the optimization problem (Sage 1966) to determine the following optimal ventilator settings: 1) tidal volume, 2) respiratory frequency and 3) fraction of inspired oxygen.

Evaluation of an emergency medical service referral system for burn patients

It is the purpose of this report to examine the process of burn care of hospitalized burn patients in the Commonwealth of Virginia over a 21 month period. Eighty-nine per cent of the hospitals within the state participated in the study, the results of which provide a positive indication of the performance of our statewide emergency medical service referral system. This system was successful in redistributing patients, with the more severely burned patients being treated in the specialty burn treatment facilities. The process of burn care within the hospital setting was also consistent with the patients severity of injury.

Use of the conductance cathieter to determine left ventricular volume: current leakage beyond the left ventricular cavity

The conductance catheter offers the only method for the determination of left ventricular volume on a continuous basis in patients. Two potential limitations of this technique include inhomogeneity of the electric field and current leakage into adjacent structures. The objectives of this study were to determine the extent of current leakage into adjacent structures and the source of additional volume detected when the electric field is made more homogeneous. Both nonconductive and conductive cylinders over the range of left ventricular volumes seen clinically were constructed. The conductance catheter was radially centered in the models and a signal conditioner-processor was used to generate the electric field and calculate the corresponding volume. The conductive models were surrounded by saline to simulate surrounding structures. We demonstrate that electric current does not extend beyond the model wall, implying that adjacent structures such as the right ventricle are unlikely to affect the conductive volume measurements in vivo. Second, rhe increased homogeneity of the electric field detects more chamber but not more wall volume. INTRODUCTION The conductance volume measurement method employs a multi-electrode catheter to generate an electric field in the left ventricle and measure instantaneous segmental resistance from which cardiac volumes are determined. It is the only instrument for the determination of left ventricular volume on a continuous basis in patients during studies of left ventricular function [l]. There are two potential limitations in equating conductance volume with chamber volume. The first involves the inhomogeneity of the electric field. Baan et af introduced a new method called dual field excitation. Analytical analysis has shown that a more homogeneous electric field is generated using this new excitation technique [ 2 ] . As a result, more volume is detected using dual field excitation. A second limitation involves the possibility of current leakage into adjacent structures, and has been termed parallel conductance. The objectives of this study were to determine: 1) the extent of current leakage beyond the left ventricular chamber and 2) the source of additional volume detected by dual field excitation. METHODS Nine nonconductive Plexiglas cylinders with volumes ranging from 6 to 340 ml were constructed. Ten conductive carbon black-polypropylene cylinders were cast with volumes ranging from 19 to 340 ml. The resistivity, rho, of the carbon black polypropylene was determined to be 24*5 ohm-cm (n=7). The inner cavity of the cylinders were filled with a sodium chloride solution of resistivity 11 ohmcm in order to approximately simulate the 1 :2 resistivity ratio of blood to myocardium [3]. A signal conditioner processor was used to generate the electric field and calculate the corresponding volume. Experiments were conducted using three different excitation modes of the conductance catheter: single field (SF), dual field 0.25 (DF-.25), and dual field 0.30 (DF-.30). The 0.25 and 0.30 modes of dual field excitation represent the current ratios of the correcting field to the original field [ 2 ] . Wall volume detected by the conductance catheter was determined by subtracting the volume signal of the nonconductive models from the volume signal of the conductive models. Volumes from the conductive models were measured with the cylinders surrounded first by air and then by sodium chloride solutions of two different resistivities (rho = 11, rho = 22 ohm-cm) in order to simulate surrounding blood and cardiac tissues, respectively . RESULTS The conductance catheter estimation of the Plexiglas cylinder volumes was found to be accurate in volumes less than 125 ml (Figure 1). However, in volumes seen clinically ( > 125 ml), the conductance catheter underestimated true volume in a nonlinear fashion. Dual field excitation increased the total volume detected, but did not enable the conductance catheter to detect the true chamber volume of the larger models. In the smaller conductive cylinders (<200 ml), the conductance catheter overestimated the true chamber volume, but did not exceed the chamber plus conductive wail volume (Figure 2) . However, in the larger conductive cylinders (> 200 ml), the conductance catheter 0-780313771/93

MICROPROCESSOR-BASED JITTER ANALYSIS IN SINGLE FIBER ELECTROMYOGRAPHY

3.00 631993 IEEE 903 underestimated true chamber volume. In both the nonconductive and the conductive models, the addition of dual field excitation increased the total volume detecled at larger total volumes (> 125 ml), but not at smaller total volumes (<50 ml). In addition, the use o f dual field excitation did not significantly increase the detection of conductive wall volume (Figure 3). The submergence of the conductive cylinders in sodium chloride solutions (rho = 1 I , rho = 22 ohm-cm) to simulate adjacent blood volumes and cardiac tissues, respectively, did not change the volume detected by the conductance catheter (Figure 4). = E 3 5 0

Book Review / Erratum

In single fiber electromyographic recordings of action potentials from two muscle fibers belonging to the same motor unit, a variability in the interval between action potentials in successive discharges is observed. This is largely due to variations in transmission time from nerve to muscle. This variability, called “jitter,” is typically expressed as the mean consecutive difference (MCD). Increased jitter has been clinically used to diagnose the defect of neuromuscular transmission that characterizes myasthenia gravis. We have developed a microprocessor-based system that automatically measures the jitter in single fiber EMG. A pulse lasting for the duration of the interpotential interval (IPI) is generated using analog and digital hardware. A microprocessor system based on INTEL 8080 is employed to measure IPI in groups of 20 and up to 10 groups in a single run. Elementary statistical analysis is performed and the results are displayed on a CRT and a monitor scope. The hardware preprocessing of the signal makes it feasible to measure jitter with precision up to one microsecond. Complete control of data acquisition and processing is available through software commands.