Tools such as Simpleware ScanIP+FE and COMSOL Multiphysics allow us to gain a better understanding of bioimpedance measurements without actually doing the measurements. This tutorial will cover the steps needed to go from a 3D voxel data set to a model that can be used to simulate a transfer impedance measurement. Geometrical input data used in this tutorial are from MRI scan of a human thigh, which are converted to a mesh using Simpleware ScanIP+FE. The mesh is merged with electrical properties for the relevant tissues, and a simulation is done in COMSOL Multiphysics. Available numerical output data are transfer impedance, contribution from different tissues to final transfer impedance, and voltages at electrodes. Available volume output data are normal and reciprocal current densities, potential, sensitivity, and volume impedance sensitivity. The output data are presented as both numbers and graphs. The tutorial will be useful even if data from other sources such as VOXEL-MAN or CT scans are used.

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Pump thrombosis and stroke are serious complications of left ventricular assist device (LVAD) support. The aim of this study was to test the ability of an accelerometer to detect pump thrombosis and thromboembolic events (TEs) using real-time analysis of pump vibrations. An accelerometer sensor was attached to a HeartWare HVAD and tested in three in vitro experiments using different pumps for each experiment. Each experiment included thrombi injections sized 0.2-1.0 mL and control interventions: pump speed change, afterload increase, preload decrease, and saline bolus injections. A spectrogram was calculated from the accelerometer signal, and the third harmonic amplitude was used to test the sensitivity and specificity of the method. The third harmonic amplitude was compared with the pump energy consumption. The acceleration signals were of high quality. A significant change was identified in the accelerometer third harmonic during the thromboembolic interventions. The third harmonic detected thromboembolic events with higher sensitivity/specificity than LVAD energy consumption: 92%/94% vs. 72%/58%, respectively. A total of 60% of thromboembolic events led to a prolonged third harmonic amplitude change, which is indicative of thrombus mass residue on the impeller. We concluded that there is strong evidence to support the feasibility of real-time continuous LVAD monitoring for thromboembolic events and pump thrombosis using an accelerometer. Further in vivo studies are needed to confirm these promising findings.

We have recently demonstrated that accelerometer-based pump thrombosis and thromboembolic events detection is feasible in vitro. This article focuses on detection of these conditions in vivo. In an open-chest porcine model (n = 7), an accelerometer was attached to the pump casing of an implanted HeartWare HVAD. Pump vibration was analyzed by Fast Fourier Transform of the accelerometer signals, and the spectrogram third harmonic amplitude quantified and compared with pump power. Interventions included injection of thrombi into the left atrium (sized 0.3–0.4 ml, total n = 35) and control interventions; pump speed change, graft obstruction, and saline bolus injections (total n = 47). Graft flow to cardiac output ratio was used to estimate the expected number of thrombi passing through the pump. Sensitivity/specificity was assessed by receiver operating characteristic curve. Graft flow to cardiac output ratio averaged 66%. Twenty-six of 35 (74%) thrombi caused notable accelerometer signal change. Accelerometer third harmonic amplitude was significantly increased in thromboembolic interventions compared with control interventions, 64.5 (interquartile range [IQR]: 18.8–107.1) and 5.45 (IQR: 4.2–6.6), respectively (p < 0.01). The corresponding difference in pump power was 3 W (IQR: 2.9–3.3) and 2.8 W (IQR: 2.4–2.9), respectively (p < 0.01). Sensitivity/specificity of the accelerometer and pump power to detect thromboembolic events was 0.74/1.00 (area under the curve [AUC]: 0.956) and 0.40/1.00 (AUC: 0.759), respectively. Persistent high third harmonic amplitude was evident at end of all experiments, and pump thrombosis was confirmed by visual inspection. The findings demonstrate that accelerometer-based detection of thromboembolic events and pump thrombosis is feasible in vivo and that the method is superior to detection based on pump power.

In this project, we have studied the use of electrical impedance cardiography as a possible method for measuring blood pulse wave velocity, and hence be an aid in the assessment of the degree of arteriosclerosis. Using two different four-electrode setups, we measured the timing of the systolic pulse at two locations, the upper arm and the thorax, and found that the pulse wave velocity was in general higher in older volunteers and furthermore that it was also more heart rate dependent for older subjects. We attribute this to the fact that the degree of arteriosclerosis typically increases with age and that stiffening of the arterial wall will make the arteries less able to comply with increased heart rate (and corresponding blood pressure), without leading to increased pulse wave velocity. In view of these findings, we conclude that impedance cardiography seems to be well suited and practical for pulse wave velocity measurements and possibly for the assessment of the degree of arteriosclerosis. However, further studies are needed for comparison between this approach and reference methods for pulse wave velocity and assessment of arteriosclerosis before any firm conclusions can be drawn.

We have previously demonstrated that accelerometer-based vibration analysis detects thromboembolism and pump thrombosis in HeartWare Left Ventricular Assist Device (HVAD) using the third harmonic frequency (pump_speedx3). Thromboembolism also affected the amplitude of the nonharmonic frequencies. The aim of this study was to determine whether nonharmonic-amplitude (NHA) analysis can improve the diagnosis of thromboembolic complications. An accelerometer was attached to HVAD in three in vitro and seven in vivo experiments. Control interventions, including load and pump speed alternations (n = 107), were followed by thromboembolic events (n = 60). A sliding fast-Fourier-transform was analyzed, and changes in NHAs were quantified in the acute phase and in a steady state. Receiver operating characteristic curves were constructed with cutoff values of NHA to detect thromboembolic events. Positive predictive values were calculated on the basis of a specificity of 1. In the acute phase, NHA change was 6.5 times higher under thromboembolism than under control interventions (p < 0.001). Most thromboembolic events lead to concomitant changes in both NHA and third-harmonic amplitude. Combining the two methods improved the PPV by 8.3%. At steady state, signal changes predominantly demonstrated either NHA or third-harmonic changes. Combined signal analysis improved the PPV by 36%. This method enhanced the detection of thromboembolism and pump thrombosis in the HVAD.

Electrosurgery is a commonly used device in the operating room, but it has some adverse effects, which are only partly described in literature. Interference issues are well described, but unintended heating in and around implants is not well studied. We simulated different scenarios using a Finite Element Model to investigate unintended heating caused by electrosurgery. We looked at different shapes, sizes, and active electrode placements. We found that all these factors play a role in the amount of heating.

Focused impedance measurements (FIM) are used in several fields, and address the problem of measuring the volume impedance of an object within a volume conductor. Several electrode configurations are possible, and these have different properties. Sensitivity fields of four configurations have been investigated. We present one new development of an existing FIM configuration, and we made finite element models of the configurations to analyse and compare them both graphically and numerically. The models developed have a variable-sized mesh that allows us to build complex models that fit easily in computer memory. We found that one configuration in particular, FIM4, was superior to the others in most aspects. We also analysed the effects of very high sensitivities in and under the electrodes. We found that even if the sensitivity is very high under the electrodes, the effects of inhomogeneities were not as high as one might expect.