We have developed equipment suitable for electrical bioimpedance measurement in human hearts, and we have developed a method of detecting capture for CRT devices. We have used bioimpedance to improve treatment of cardiac disease, but not to detect cardiac disease. This means that the aims we set are partially reached. The equipment is in use today in other CRT studies, so we expect to reach the last aim as well. The objectives are considered individually:
• Evaluate feasibility of getting useful results from measurements in a human heart using pacemaker leads during the acute phase of the implantation by analysing finite element models.: The work on finite element models have shown that it is probable to get useful results form four electrode measurements, and to some degree that it is feasible to get results from three electrode measurements. If we, connect the work on three electrode models with knowledge of variable tissue properties in myocardium, we can conclude that also three electrode measurement will give useful results. To be able to quantify the feasibility, we developednew measures of quality that are useful for evaluating measurement set-ups in in
silico experiments. We therefore conclude that the objective is fulfilled.
• Design, build, and test a measurement system suitable for use on human hearts:
We did build a measurement system that successfully has been used for measurements on human subjects. We have demonstrated that the system is capturing 190 spectrums per second with five frequencies form 20 kHz to 750 kHz. The results are presented in real-time, and are saved to file simultaneously for post-processing. We therefore conclude that the objective is fulfilled.
• Start building a knowledge base: We have done a series of measurements withvarying set-ups, and we have collected valuable knowledge of possibilities, limitations, and expected results for a number of set-ups. We are still learning and adjusting the information we have, but we have started. We therefore conclude that the objective is fulfilled.
• Demonstrate practical use of electrical bioimpedance in cardiology: We developed a novel method to determine loss of capture in pace electrodes used for delivering CRT therapy based on electrical bioimpedance. The method is based on bioimpedance measurements within the heart, and has been tried in a clinical study. We therefore conclude that the objective is fulfilled.
The work presented here has enabled our group to continue exploiting electrical bioimpedance as a tool in cardiology.
Reported studies pertaining to needle guidance suggest that tissue impedance available from neuromonitoring systems can be used to discriminate nerve tissue proximity. In this pilot study, the existence of a relationship between intraoperative electrical impedance and tissue density, estimated from computer tomography (CT) images, is evaluated in the mastoid bone of in vivo sheep. In five subjects, nine trajectories were drilled using an image-guided surgical robot. Per trajectory, five measurement points near the facial nerve were accessed and electrical impedance was measured (≤1 KHz) using a multipolar electrode probe. Micro-CT was used postoperatively to measure the distances from the drilled trajectories to the facial nerve. Tissue density was determined from coregistered preoperative CT images and, following sensitivity field modeling of the measuring tip, tissue resistivity was calculated. The relationship between impedance and density was determined for 29 trajectories passing or intersecting the facial nerve. A monotonic decrease in impedance magnitude was observed in all trajectories with a drill axis intersecting the facial nerve. Mean tissue densities intersecting with the facial nerve (971-1161 HU) were different (p <;0.01) from those along safe trajectories passing the nerve (1194-1449 HU). However, mean resistivity values of trajectories intersecting the facial nerve (14-24 Ωm) were similar to those of safe passing trajectories (17-23 Ωm). The determined relationship between tissue density and electrical impedance during neuromonitoring of the facial nerve suggests that impedance spectroscopy may be used to increase the accuracy of tissue discrimination, and ultimately improve nerve safety distance assessment in the future.