Robert E. Dodde

Thermal-Electric Finite Element Analysis and Experimental Validation of Bipolar Electrosurgical Cautery

Cautery is a process to coagulate tissues and seal blood vessels using heat. In this study, finite element modeling (FEM) was performed to analyze temperature distribution in biological tissue subject to a bipolar electrosurgical technique. FEM can provide detailed insight into the tissue heat transfer to reduce the collateral thermal damage and improve the safety of cautery surgical procedures. A coupled thermal-electric FEM module was applied with temperature-dependent electrical and thermal properties for the tissue. Tissue temperature was measured using microthermistors at different locations during the electrosurgical experiments and compared to FEM results with good agreement. The temperature- and compression-dependent electrical conductivity has a significant effect on temperature profiles. In comparison, the temperature-dependent thermal conductivity does not impact heat transfer as much as the temperature-dependent electrical conductivity. Detailed results of temperature distribution were obtained from the model. The FEM results show that the temperature distribution can be changed with different electrode geometries. A flat electrode was modeled that focuses the current density at the midline of the instrument profile resulting in higher peak temperature than that of the grooved electrode (105 versus 96°C). DOI: 10.1115/1.2902858

Electrosurgical Vessel Sealing Tissue Temperature: Experimental Measurement and Finite Element Modeling

The temporal and spatial tissue temperature profile in electrosurgical vessel sealing was experimentally measured and modeled using finite element modeling (FEM). Vessel sealing procedures are often performed near the neurovascular bundle and may cause collateral neural thermal damage. Therefore, the heat generated during electrosurgical vessel sealing is of concern among surgeons. Tissue temperature in an in vivo porcine femoral artery sealed using a bipolar electrosurgical device was studied. Three FEM techniques were incorporated to model the tissue evaporation, water loss, and fusion by manipulating the specific heat, electrical conductivity, and electrical contact resistance, respectively. These three techniques enable the FEM to accurately predict the vessel sealing tissue temperature profile. The averaged discrepancy between the experimentally measured temperature and the FEM predicted temperature at three thermistor locations is less than 7%. The maximum error is 23.9%. Effects of the three FEM techniques are also quantified.

Bioimpedance of soft tissue under compression

In this paper compression-dependent bioimpedance measurements of porcine spleen tissue are presented. Using a Cole-Cole model, nonlinear compositional changes in extracellular and intracellular makeup; related to a loss of fluid from the tissue, are identified during compression. Bioimpedance measurements were made using a custom tetrapolar probe and bioimpedance circuitry. As the tissue is increasingly compressed up to 50%, both intracellular and extracellular resistances increase while bulk membrane capacitance decreases. Increasing compression to 80% results in an increase in intracellular resistance and bulk membrane capacitance while extracellular resistance decreases. Tissues compressed incrementally to 80% show a decreased extracellular resistance of 32%, an increased intracellular resistance of 107%, and an increased bulk membrane capacitance of 64% compared to their uncompressed values. Intracellular resistance exhibits double asymptotic curves when plotted against the peak tissue pressure during compression, possibly indicating two distinct phases of mechanical change in the tissue during compression. Based on these findings, differing theories as to what is happening at a cellular level during high tissue compression are discussed, including the possibility of cell rupture and mass exudation of cellular material.

Monopolar Electrosurgical Thermal Management for Minimizing Tissue Damage

In this study, a novel thermal management system (TMS) is developed for the minimization of thermal spread created by a monopolar electrosurgical device, the most commonly used surgical instrument. The phenomenon of resistive heating of tissue is modeled using the finite-element method (FEM) to analyze the electrical potential and temperature distributions in biological tissue subjected to heat generation during monopolar electrosurgery. Ex vivo experiments are used to validate the FEM by comparing the model predicted and experimentally measured temperatures. The predicted FEM maximum temperature 1.0 mm adjacent to the electrode is within 1% of the experimentally measured maximum temperature using a standard monopolar pencil electrode. A TMS consisting of adjacent cooling channels produces coagulation volumes 80% that of standard monopolar procedures while maintaining comparable temperatures in the targeted tissue below the electrode. In vivo temperatures using a device incorporating a TMS at distances of 2 and 3 mm adjacent to the electrode edge are maintained below temperatures known to damage tissue.

A pulsatile blood vessel system for a femoral arterial access clinical simulation model

The model-based, rapid-prototyping-enabled design and manufacture of a pulsatile blood vessel (PBV) for high-fidelity mannequin-based clinical simulations is presented. The PBV presented here is a pressurized, flexible tube with alternating fluid pressure created by a pump to mimic the behavior of a human vessel in response to pulsatile pressure. The use of PBVs is important for the fidelity of a clinical simulator that requires residents to palpate and/or access the vessel. In this study, a PBV is presented which features the integration of 3D modeling using patient-specific computed tomography (CT) data, mold fabrication using rapid-prototyping, and finite element method for estimating the required pumping pressure to generate the same level of force (about 1.5 N) experienced by the user through palpation. The relationship between this palpation force and the vessel pressure is studied using two strategies: finite element analysis (FEA) and experiments in a femoral arterial access simulator with a pump, artificial vessel, and surrounding phantom tissue. The experimental results show a discrepancy of 8.7% from the FEA-predicted value. Qualitative validation is done by exposing and surveying 19 interventional cardiology residents at four major educational institutions to the simulator for accuracy of its feel. The overall survey results are positive.

A Novel Technique for Demonstrating the Real-Time Subsurface Tissue Thermal Profile of Two Energized Surgical Instruments

Currently the primary methods for obtaining hemostasis during minimally invasive surgery include systems that incorporate either ultrasonic or radiofrequency energy. Although the use of improved energy systems has dramatically facilitated laparoscopic dissection and hemostasis, the threat of thermal collateral damage remains and detracts from the usefulness of these devices. Traditionally the thermal spread from these instruments has been determined with a combination of in situ dynamic thermography and histopathologic studies that result in varying degrees of cross-correlation. The ability to obtain a real-time subsurface tissue thermal profile without the use of thermography in either an electrosurgical or ultrasonic device is feasible with a microthermistor placed at a predetermined distance and depth from the instrument tool edge. A comparison with traditional thermographic measurements and their correlation with histologic study are warranted with future studies.

Impedance of tissue-mimicking phantom material under compression

Abstract The bioimpedance of tissues under compression is a field in need of study. While biological tissues can become compressed in a myriad of ways, very few experiments have been conducted to describe the relationship between the passive electrical properties of a material (impedance/admittance) and its underlying mechanical properties (stress and strain) during deformation. Of the investigations that have been conducted, the exodus of fluid from samples under compression has been thought to be the cause of changes in impedance, though until now was not measured directly. Using a soft tissue-mimicking phantom material (tofu) whose passive electrical properties are a function of the conducting fluid held within its porous structure, we have shown that the mechanical behavior of a sample under compression can be measured through bioimpedance techniques.

Microbubble transport through a bifurcating vessel network with pulsatile flow

Motivated by two-phase microfluidics and by the clinical applications of air embolism and a developmental gas embolotherapy technique, experimental and theoretical models of microbubble transport in pulsatile flow are presented. The one-dimensional time-dependent theoretical model is developed from an unsteady Bernoulli equation that has been modified to include viscous and unsteady effects. Results of both experiments and theory show that roll angle (the angle the plane of the bifurcating network makes with the horizontal) is an important contributor to bubble splitting ratio at each bifurcation within the bifurcating network. When compared to corresponding constant flow, pulsatile flow was shown to produce insignificant changes to the overall splitting ratio of the bubble despite the order one Womersley numbers, suggesting that bubble splitting through the vasculature could be modeled adequately with a more modest constant flow model. However, bubble lodging was affected by the flow pulsatility, and the effects of pulsatile flow were evident in the dependence of splitting ratio of bubble length. The ability of bubbles to remain lodged after reaching a steady state in the bifurcations is promising for the effectiveness of gas embolotherapy to occlude blood flow to tumors, and indicates the importance of understanding where lodging will occur in air embolism. The ability to accurately predict the bubble dynamics in unsteady flow within a bifurcating network is demonstrated and suggests the potential for bubbles in microfluidics devices to encode information in both steady and unsteady aspects of their dynamics.

Quantification of Ultrasound Correlation-Based Flow Velocity Mapping and Edge Velocity Gradient Measurement

This study investigated the use of ultrasound speckle decorrelation‐ and correlation‐based lateral speckle‐tracking methods for transverse and longitudinal blood velocity profile measurement, respectively. By studying the blood velocity gradient at the vessel wall, vascular wall shear stress, which is important in vascular physiology as well as the pathophysiologic mechanisms of vascular diseases, can be obtained. Decorrelation‐based blood velocity profile measurement transverse to the flow direction is a novel approach, which provides advantages for vascular wall shear stress measurement over longitudinal blood velocity measurement methods. Blood flow velocity profiles are obtained from measurements of frame‐to‐frame decorrelation. In this research, both decorrelation and lateral speckle‐tracking flow estimation methods were compared with Poiseuille theory over physiologic flows ranging from 50 to 1000 mm/s. The decorrelation flow velocity measurement method demonstrated more accurate prediction of the flow velocity gradient at the wall edge than the correlation‐based lateral speckle‐tracking method. The novelty of this study is that speckle decorrelation‐based flow velocity measurements determine the blood velocity across a vessel. In addition, speckle decor‐relation‐based flow velocity measurements have higher axial spatial resolution than Doppler ultrasound measurements to enable more accurate measurement of blood velocity near a vessel wall and determine the physiologically important wall shear.

In Vivo Vascular Wall Shear Rate and Circumferential Strain of Renal Disease Patients

This study measures the vascular wall shear rate at the vessel edge using decorrelation based ultrasound speckle tracking. Results for nine healthy and eight renal disease subjects are presented. Additionally, the vascular wall shear rate and circumferential strain during physiologic pressure, pressure equalization and hyperemia are compared for five healthy and three renal disease subjects. The mean and maximum wall shear rates were measured during the cardiac cycle at the top and bottom wall edges. The healthy subjects had significantly higher mean and maximum vascular wall shear rate than the renal disease subjects. The key findings of this research were that the mean vascular wall shear rates and circumferential strain changes between physiologic pressure and hyperemia that was significantly different between healthy and renal disease subjects.