Biomedical subjects

O C Deale

Publications and source records attributed to O C Deale.

9 recordsLinked to original sources

PubMed2001

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.

Animals↗

PubMed1995

Three-dimensional uniform grid modeling of electrical defibrillation on a data parallel computer.

Finite element modeling has played an increasingly important role in the study of defibrillation. In order to model well the complex anatomical details, a large number of elements are required in the finite element grid, leading to a large set of equations that often cannot be solved effectively with the computational power of conventional computers. In this paper, we describe the use of a data parallel computer to provide the memory and reduction in solution time for solving these large finite element problems. Using a uniform grid and a nodal assembly technique, the discretized problem domain can be mapped efficiently to the parallel computer, allowing the solution of problems with over two million unknowns. The finite element algorithm for a three-dimensional inhomogeneous anisotropic body is described together with its parallel implementation. Test results for a canine torso model constructed from CT images are also presented.

Algorithms↗

PubMed1995

Control of high common mode voltage during transthoracic defibrillation.

A high common mode voltage (Vcm) 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 Vcm, and therefore, a reduction in Vcm improves the accuracy of the potential gradient data. Experiments were conducted on 5 dogs to determine whether Vcm can be controlled using a bridge circuit. The bridge circuit consisted of a 5 k omega 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, Vcm 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 Vcm. It was determined that Vcm could be reduced to approximately zero at a given electrode on the heart. In addition, for the 5 dogs studied, the maximum measured Vcm on the heart was only 10% of the transthoracic voltage when the bridge circuit was balanced for an interior point in the heart.

Animals↗

PubMed1994

Linearity of transthoracic conductance with respect to electrode force and area during high-voltage defibrillation shocks.

Canine transthoracic conductance (GT) was measured during high-voltage defibrillation shocks to test the hypothesis that (GT) is a linear function of electrode force (F) and electrode area (A). Symmetric protocols were used to compensate for changes in (GT) 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, GT was linear (r = 0.996, 0.995, 0.971, 0.992, 0.995) over five dogs for 30 N < or = F < or = 70 N. For a constant F = 50 N, GT was linear (r = 0.992, 0.998, 0.994, 0.992) over four dogs for 20 cm2 < or = A < or = 60 cm2, and in one dog (r = 0.996) for 40 cm2 < or = A < or = 90 cm2. The quantitative relationship demonstrated for GT and F and A can be used in the design of experiments and interpretation of results used for validation of numerical defibrillation models.

Animals↗

PubMed1990

Nature of defibrillation: determinism versus probabilism.

The gradual transitions that are found between unsuccessful and successful shock strengths in percent success or dose-response curves suggest that defibrillation is a probabilistic phenomenon. This concept appears to be reinforced by the fact that a frequency distribution is observed in defibrillation threshold data and that a dose-response relationship is also obtained by integration of the frequency distribution. The purpose of this study was to investigate whether a deterministic threshold model (based on experimental results) could produce 1) gradual transitions in dose-response curves, and 2) a threshold frequency distribution for individual subjects. In the experimental phase of the study, a linear deterministic relationship was found between transthoracic threshold current and defibrillation episode number (other variables held constant) in pentobarbital-anesthetized dogs. The correlation coefficient for each dog was between 0.77 and 0.98 (P less than 0.01), and both positive and negative slopes were found. Based on these results, threshold current was modeled for computer simulation as a linear function of episode number. The model was thus purely deterministic with no random variability. For each simulated experiment, several parameters were varied: order of shocks (increment, decrement, random order), slope of threshold function, and percent error of the initial threshold. Several hundred computer simulations were performed to determine the effect of varying these parameters. In all cases, threshold-frequency distributions and sigmoidal dose-response curves with gradual transitions were produced. The results of this investigation demonstrate that the apparent probabilistic behavior of defibrillation can be produced by a deterministic relationship.

Animals↗

PubMed1990

Effect of epicardial patch electrodes on transthoracic defibrillation.

To improve survival rates in patients undergoing surgical ablative procedures for malignant ventricular tachycardia (VT), a frequent practice is to implant epicardial patch electrodes at the time of map-guided surgery. After operation, patients with inducible VT often receive an automatic internal cardioverter/defibrillator (AICD) implant, whereas patients with noninducible VT usually do not. In the event that spontaneous, hypotensive VT or ventricular fibrillation should subsequently occur in the patient with noninducible VT, however, transthoracic defibrillation and resuscitation may prove difficult, because the patch electrodes are insulated with silicone rubber that can reduce the amount of current traversing the myocardium and thus can increase transthoracic defibrillation threshold (DFT). In this study, DFT was determined in mongrel dogs to test the hypothesis that epicardial patch electrodes elevate threshold. This study was also designed to assess the effect of patch electrode orientation and size on DFT. In the first protocol (perpendicular orientation), small epicardial patch electrodes (surface area, 30 cm2) were sutured to the epicardial surfaces of the anterior right and posterior left ventricles in 15 dogs so that the center axes of the patch electrodes were coincident and perpendicular to the coincident center axes of the transthoracic electrodes. The effect of two large epicardial patch electrodes (surface area, 53 cm2) on transthoracic DFT was also examined in eight of these dogs. In the second protocol (parallel orientation), small patch electrodes were sutured to the lateral surfaces of the right and left ventricles in seven dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Intrathoracic current flow during transthoracic defibrillation in dogs. Transcardiac current fraction.

To achieve transcardiac threshold current during transthoracic defibrillation, a considerably larger current must be delivered to the thorax to compensate for the shunting effect of the lungs, the thoracic cage, and other elements of the torso. This shunting effect is thus an important determinant of transthoracic defibrillation threshold and can be quantified by the transcardiac current fraction (FC, the ratio of transcardiac to transthoracic threshold currents). Previous estimates of FC have ranged from as low as 3% to as high as 45%. The purpose of of this study was to quantify both FC and the major intrathoracic current pathways. Transthoracic and intrathoracic voltages and currents were simultaneously measured during high-voltage transthoracic shocks in 20 dogs. With correction factors determined from another set of 12 dogs, these raw data were corrected to compensate for field distortion caused by the presence of the intrathoracic electrodes, and the adjusted data were fit to a resistive network model. The results showed that 82% of the transthoracic current was shunted by the thoracic cage, while 14% was shunted by the lungs. The remaining 4% (FC) is the portion that passed through the heart. There was good agreement between the two independent methods used to calculate FC. Analysis based on the model indicated that FC was 3.7%, whereas FC determined by direct measurement with calibrated electrodes was 4.2%. Therefore, the results of this study, in contrast to earlier estimates of FC, show that defibrillation in dogs is achieved by only 4% of the total transthoracic current.

Animals↗

PubMed1990

Relation between transcardiac and transthoracic current during defibrillation in humans.

Conceptually, transthoracic defibrillation threshold current can be considered a function of at least two quantities. It is directly proportional to the transcardiac threshold current and inversely proportional to the transcardiac current fraction (FC) or the ratio of transcardiac and transthoracic current. Although experimental and theoretical estimates of FC have been as high as 45%, previous measurements in humans have not been made. This study was designed to quantify FC in humans. During intraoperative testing of the automatic implantable cardioverter defibrillator, transthoracic rescue shocks of 200-400 J were delivered when the device failed to defibrillate. Simultaneous transthoracic voltage (VT) and transcardiac voltage (VC) between two implanted epicardial patch electrodes were measured. The ratio, VC/VT, was 0.04 +/- 0.03 (mean +/- SD) in 10 patients. In 16 dogs, a comparison was made between direct measurement of FC and VC/VT. FC was determined with a specially designed electrode system, which was calibrated to account for field distortion introduced by the electrodes. There was no significant difference between FC and VC/VT, which were both approximately 0.05, suggesting that VC/VT was statistically equivalent to FC. The results of this study, therefore, indicate that during transthoracic defibrillation in humans, approximately 4% of transthoracic current traverses the heart. This relatively small percentage of current results from the existence of parallel pathways, such as the thoracic cage and lungs, which shunt current around the heart.

Adolescent↗

PubMed1987

Relationship between canine transthoracic impedance and defibrillation threshold. Evidence for current-based defibrillation.

The electrical parameter used to define defibrillation strength is energy. Peak current, however, may more accurately reflect the field quantities (i.e., electric field strength and current density) that mediate defibrillation and therefore should be a better clinical descriptor of threshold than energy. Though transthoracic impedance is a major determinant of energy-based threshold and is sensitive to operator-dependent changes in impedance (electrode-subject interface), an ideal threshold descriptor should be invariant with respect to these changes in impedance. We therefore compared the relative invariance of energy- and current-based thresholds when transthoracic impedance was altered by one of two methods: (a) change in electrode size (protocol A) or (b) change in electrode force (protocol B). In protocol A, impedance was altered in each dog by a mean of 95%. Energy thresholds determined at both low and high impedance were 44 +/- 21 J (mean +/- SD) and 105 +/- 35 J, respectively, P less than 0.0001. In contrast, peak current (A) thresholds were independent of transthoracic impedance, 22 +/- 5 A (low impedance) vs. 24 +/- 6 A (high impedance), P = NS. Energy and current thresholds showed a similar relationship for animals tested in protocol B. Therefore, current-based thresholds, in contrast to energy thresholds are independent of operator-dependent variables of transthoracic impedance and are invariant for a given animal. These results suggest that redefining defibrillation threshold in terms of peak current rather than energy provides a superior method of defibrillation.

Animals↗