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Machines and Instrumentation
Published in Pradeep Venkatesh, Handbook of Vitreoretinal Surgery, 2023
Ohm’s law [electric conduction]. This law is applicable during the use of wet-field cautery [cautery is a misnomer because no passive heat is being used] and diathermy. It states that the current flow across two points of a conductor is directly proportional to the voltage difference across the points. Current flowing through the tissue causes a raise in temperature resulting in coagulation [e.g., stop intraocular bleed] or focal tissue necrosis [e.g., create retinotomy].
Electrophysiology
Published in A. Bakiya, K. Kamalanand, R. L. J. De Britto, Mechano-Electric Correlations in the Human Physiological System, 2021
A. Bakiya, K. Kamalanand, R. L. J. De Britto
where Iext is the external stimulus current from outside to inside. Ic, INa, IK and IL are the ionic currents from inside to outside. According to Ohms law:
Brain stimulation and epilepsy: electrical stimulus characteristics
Published in Hans O Lüders, Deep Brain Stimulation and Epilepsy, 2020
Reduced to its simplest form, a neuro-stimulator consist of a power supply (i.e. a battery), a pair of electrodes in contact with the tissue, extension wires to connect the electrodes to the battery and a ‘switch’ that enables the power to be intermittently connected to the electrodes (Figure 4.1). Ohm’s law governs the relationship between the voltage and current. Much of the basic understanding of nerve cell electro-physiology was discovered as a result of studies carried out with intracellular electrodes referenced to electrodes in the extracellular space. Neurostimulators used for neuromodulation, however, make use of extracellular electrodes to generate the voltage/current fields.
Effect of anisotropy in myocardial electrical conductivity on lesion characteristics during radiofrequency cardiac ablation: a numerical study
Published in International Journal of Hyperthermia, 2022
Kaihao Gu, Shengjie Yan, Xiaomei Wu
As established in simulation studies that performed validation of RFCA computer models with isotropic MEC, matching the initial impedance of the overall system is a common way to unify the model with the experimental setup by adjusting the MEC in simulation [37,41]. A proper MEC under IC that can produce results close to the use of AC (i.e., the actual MEC in a real situation) can therefore be simultaneously determined. However, there was an inevitable deviation of the lesion dimension between the two scenarios even if the initial impedances were matched. The lesion depth and width of the computer model tended to be larger and smaller than the experimental results, respectively. The average differences in lesion depth and width between simulation and experiment from these two studies were both approximately 10%. These findings suggest that the computational lesion is relatively more spherical because of the isotropic setting of cardiac tissue. If the isotropic MEC is determined by averaging the anisotropic MEC, we found that the initial impedance of the ablation system under IC was always smaller than the one under AC regardless of the ablation sites. This was believed to produce higher tissue temperature and larger lesion volume under IC since the electrical field inside the myocardium was strengthened based on Ohm’s law
Does early activation within hours after cochlear implant surgery influence electrode impedances?
Published in International Journal of Audiology, 2022
Aniket A. Saoji, Weston J. Adkins, Madison K. Graham, Matthew L. Carlson
In cochlear implants, reverse telemetry is used to measure electrode impedances and assess integrity of the cochlear implant electrode array. Electrode impedance measurements are typically used to interrogate open or short electrodes. Electrode impedances also play an important role in optimising electrical stimulation parameters and ultimately determine the size of the external battery and sound processor. Using the equation V = I * R (Ohms law), the electrode impedance (R) and the compliance voltage (V) of the device are used to determine the maximum current (I) that can be delivered on a cochlear implant electrode. Lower impedances allow for stimulation using a lower compliance voltage, which consumes less battery power. This opens the possibility of a smaller external battery and sound processor, improving retention and cosmetics (e.g. Langner et al. 2017). Additionally, for electrodes with lower electrical impedances, comfortable loudness levels are likely to be achieved by simply increasing the current amplitude. For electrodes with higher impedances, wider pulse duration is sometimes needed beyond an increased current level to achieve comfortable loudness levels (e.g. Saoji et al. 2021). In some patients, wider pulse duration can lead to reduced rate of stimulation (e.g. Büchner et al., 2005) or limited spectral maxima and adversely affect cochlear implant outcome (e.g. Berg et al. 2019). As such, maintaining low, stable impedances is of great interest.
Deep brain stimulation programming strategies: segmented leads, independent current sources, and future technology
Published in Expert Review of Medical Devices, 2021
Bhavana Patel, Shannon Chiu, Joshua K. Wong, Addie Patterson, Wissam Deeb, Matthew Burns, Pamela Zeilman, Aparna Wagle-Shukla, Leonardo Almeida, Michael S. Okun, Adolfo Ramirez-Zamora
A DBS system involves 1) an electrode that is placed into a brain target (gray matter, white matter, or an interface), 2) an implantable pulse generator (IPG) which includes a battery and electronic circuit used to generate the electrical field, and, 3) an extension cable connecting the electrode to the IPG [56]. Depending on the DBS system, the electrical source can be voltage- or current-driven. Most recently approved devices have been built for constant current stimulation since the traditional constant voltage DBS systems are vulnerable to impedance changes and uneven electrical distribution across tissues. By Ohm’s law (V ≈ I.Z), voltage (V) is directly proportional to impedance (Z) and current (I). Hence, in a constant-voltage system, an increase in impedance will result in a decrease in current and a reduction in stimulation delivered.