Control of high common mode voltage during transthoracicdefibrillation

A high common mode voltage (V_cm) relative to earth ground is produced on the myocardium during the delivery of a defibrillator pulse and can generate a differential error signal when potential gradients are recorded with bipolar electrodes and isolation amplifiers. The error signal is proportional to V_cm, and therefore, a reduction in V_cm improves the accuracy of the potential gradient data. Experiments were conducted on 5 dogs to determine whether V_cm can be controlled using a bridge circuit. The bridge circuit consisted of a 5 kΩ power rheostat in parallel with the transthoracic resistance of the dog. The variable contact of the rheostat was connected to earth ground, and by adjusting the rheostat, V_cm on the myocardium could be varied. In each dog, 20 A shocks were delivered through stainless steel transthoracic electrodes. Point contact electrodes sutured to the epicardium were used to measure V_cm. It was determined that V_cm could be reduced to approximately zero at a given electrode on the heart. In addition, for the 5 dogs studied, the maximum measured V_cm on the heart was only 10% of the transthoracic voltage when the bridge circuit was balanced for an interior point in the heart     

Calibrated single-plunge bipolar electrode array for mapping myocardial vector fields in three dimensions during high-voltage transthoracic defibrillation

Mapping of the myocardial scalar electric potential during defibrillation is normally performed with unipolar electrodes connected to voltage dividers and a global potential reference. Unfortunately, vector potential gradients that are calculated from these data tend to exhibit a high sensitivity to measurement errors. This paper presents a calibrated single-plunge bipolar electrode array (EA) that avoids the error sensitivity of unipolar electrodes. The EA is triaxial, uses a local potential reference, and simultaneously measures all three components of the myocardial electric field vector. An electrode spacing of approximately 500 microm allows the EA to be direct-coupled to high-input-impedance, isolated, differential amplifiers and eliminates the need for voltage dividers. Calibration is performed with an electrolytic tank in which an accurately measured, uniform electric field is produced. For each EA, unique calibration matrices are determined which transform potential difference readings from the EA to orthogonal components of the electric field vector. Elements of the matrices are evaluated by least squares multiple regression analysis of data recorded during rotation of the electric field. The design of the electrolytic tank and electrode holder allows the electric field vector to be rotated globally with respect to the electrode axes. The calibration technique corrects for both field perturbation by the plunge electrode body and deviations from orthogonality of the electrode axes. A unique feature of this technique is that it eliminates the need for mechanical measurement of the electrode spacing. During calibration, only angular settings and voltages are recorded. For this study, ten EAs were calibrated and their root-mean-square (rms) errors evaluated. The mean of the vector magnitude rms errors over the set of ten EAs was 0.40% and the standard deviation 0.07%. Calibrated EAs were also tested for multisite mapping in four dogs during high-voltage transthoracic shocks.     

Simplified calibration of single-plunge bipolar electrode array for field measurement during defibrillation

In an earlier study, the authors presented a calibration technique for a triaxial bipolar electrode array (EA) that used 72 data points collected during a global sweep of the electric field vector relative to the EA axes. Although necessary for the initial characterization of the EAs, this data requirement has to be significantly reduced for the technique to become a practical tool. Therefore, in the present study, an analysis is performed to determine the relation between the number of data points used in the calibration and the mean root-mean-square error. The analysis shows that 18 data points can produce results nearly identical to those obtained with the 72-point calibration, thus reducing the required amount of data fourfold.     

Calibrated current divider network for precision current delivery during high-voltage transthoracic defibrillation

The design of a calibrated resistive-network current divider for precision current delivery during transthoracic defibrillation shocks is presented together with test results. The current divider presents a constant 50-ohm load to the defibrillator and thus maintains a constant pulse shape. Current is selected before the shock by setting three rheostats using a computer-generated calibration table. Following each shock, the data acquisition and display software updates the calibration table based on the measured value of transthoracic resistance. Over a range of 15-27 A, the root-mean-square (rms) error for delivered versus selected current was 0.48% for a 45-ohm resistive load, and 0.71% for a 100-ohm load. These test results were confirmed by animal experiments. Over 3 dogs, the rms error was 0.49% from 15-27 A and not greater than 1.5% over the entire 8-44 A range.     

Linearity of transthoracic conductance with respect to electrodeforce and area during high-voltage defibrillation shocks

Canine transthoracic conductance (G_T) was measured during high-voltage defibrillation shocks to test the hypothesis that (G _T) is a linear function of electrode force (F) and electrode area (A). Symmetric protocols were used to compensate for changes in (G _T) with respect to shock number (n). Stainless steel electrodes were employed with a force-control system for precise selection and control of both F and A at each shock. For a constant A=60 cm², G_T was linear (r=0.996, 0.995, 0.971, 0.992, 0.995) over five dogs for 30 N⩽F⩽70 N. For a constant F=50 N, G _T was linear (r=0.992, 0.998, 0.994, 0.992) over four dogs for 20 cm²⩽A⩽60 cm², and in one dog (r=0.996) for 40 cm²⩽A⩽90 cm². The quantitative relationship demonstrated for G_T and F and A can be used in the design of experiments and interpretation of results used for validation of numerical defibrillation models