Tissue impedance

There are sources of nonlinearity both in electrode polarization and tissue impedance (Schwan 1992). Onaral and Schwan (1982) studied electrode polarization impedance and found that the limit voltage of linearity was about 100 mV and frequency independent. The corresponding hmit current is of course impedance dependent and therefore frequency dependent, and may be about 5 pA/cm at a frequency of 1 Hz. 

TABLE 20.2 Typical Tissue Impedance Ranges during ... 

A. Yiifera, A. Rueda, J.M. Munoz, et al., A Tissue Impedance Measurement Chip for Myocardial Ischemia Detection, IEEE Transactions on Circuits and Systems I Regular Papers, vol. 52, no. 12, pp. 2620-2628, 2005. 

Extracellular liquids usually dominate the in-phase conductivity. Therefore, the low-frequency temperature coefficient is that of electroljrtes, approximately +2%/°C. The quadrature e or Y" components have a smaller temperature coefficient (e.g., approximately —0.5%/°C) but are dependent on the measuring frequency in relation to the characteristic frequencies of the dispersions. The effect of hyperthermia and freezing on tissue impedance was studied by Yu et al. (2004). 

In low-frequency applications (<100 Hz), the skin impedance is very high compared with the polarization impedance of wet electrodes and deeper tissue impedance. The SC consists of dead and dry tissue, and its admittance is very dependent on the state of the superficial layers and the water content (humidity) of the surrounding air in contact with the skin before electrode onset. In addition, the sweat ducts shunt the SC with a very variable DC conductance. Sweat fills the ducts and moisturizes the surrounding SC. Therefore, the state of the skin and measured skin admittance at the time of electrode... 

The contact medium represents an electrolytic volume DC resistance in series with the polarization and tissue impedances. 

In most cases, electrode polarization is a nuisance because it is only the tissue impedance that is of interest. Electrode polarization impedance introduces errors in tissue impedance... 

Polarization comprises both dynamic and static properties. An AC voltage u connected to a skin electrode pair generates an AC current in the electrode wires. The impedance is Z = uH and it comprises both the tissue impedance and the double electrode polarization impedances. They are physically coupled in series. Can we avoid the electrode... 

Figure 7.37 Monopolar impedance of insulated needle with exposed tip in situ in living porcine tissue. Impedance data dominated by electrode polarization below =1 kHz. Notice, however, polarization impedance dependence on tissue type. Courte of Hava rd Kalvoy.

Geddes et al. (1975a) measured the current/voltage characteristics with a 5 ms duration heavily damped sinusoidal defibrillator pulse. Standard defibrillator electrodes 3.5" in diameter (60 cm ) were used with current pulses up to 80 A. The electrodes were face-to-face at 1 cm distance with the space filled with a 8.4 Q-cm electrode paste. It was found that the impedance of both electrodes (defined as the ratio of peak voltage to peak current) at such current levels was only a fraction of 1 Q. With the usual thorax tissue impedance of about 50 Q, little energy therefore is lost in electrode polarization processes. The 0.01 Hz impedance of the same electrode pair with small linear AC current levels was found to be about 2 kQ. This shows the extreme degree of nonlinearity. 

Immittance is the dependent variable in the Cole equations. For most biological systems, it is observed that the center of the impedance circular arc locus is situated below the real axis in the Wessel diagram. This was clear from the late 1920s, and Cole and Fricke published diagrams and equations based upon a frequency-independent phase angle. But in 1940, Kenneth S. Cole proposed the following empirical equation z = Zoo + (ro — r[Pg.348]

The neutral plate is often split in two, and a small eurrent is passed between the two plates via the skin and tissue. Impedance is measured, and if this impedance is outside preset or memory set hmits, the unit will warn to inform about poor and dangerous plate contact (cf. Figure 10.21). 

Rudolf Hoeber discovered the frequency dependence of conductivity of blood and postulated the existence of cell membranes (1911). Philippson in 1921 measured tissue impedance as a function of frequency and found that the capacitance varied approximately as the inverse square root of the frequency. He called this a polarization capacitance similar to that found for the metal/electrolyte interphase. In the late 1920s, Gildemeister found the constant phase character of tissue, and Herman Rein found electro-osmotic effects. 

McAdams, E.T. and J. Jossinet (1995). Tissue impedance a historical overview. Physiological measurement 76, A1. 

Whenever electricity flows across a circuit, there is a resistance to flow encountered by the electrons. For pacing systems, the resistance is determined by the complex interaction of multiple components. Because some of these components are also characterized by the ability to retain charge or capacitance, the term impedance is preferred. At the time of lead implantation, it is this complicated series of resistance and capacitance factors that are measured and are referred to as system impedance. For a pacing circuit, the system impedance has five basic components a low, purely resistive conductor impedance, a high cathode electrode impedance, complex polarization effects at the electrode-tissue interface, a low tissue impedance, and the anode electrode impedance (Fig. 1.3). 

Fig. U Schematic of a bipolar lead illustrating the factors involved in determining system impedance. The arrows denote current flow. Resistance to current flow occurs at the lead conductor (conductor resistance), at the cathode-tissue interface (cathode impedance and polarization), in the myocardium (tissue impedance) and at the anode (anode impedance). The largest contributors to system impedance are the cathode impedance and polarization effects.

Johnson MD, Otto KJ, Kipke DR (2005) Repeated voltage biasing improves unit recordings by reducing resistive tissue impedances. TREE Trans Neural Syst Rehabil Eng 13 160-165... 

In larger lesions, the array is withdrawn and redeployed anteriorly at 1.5- to 2.0-cm intervals into the tumor. After the hooks are fully deployed, the electrode is connected to the RF generator. The heating protocol for all the available needles is provided by the manufacturer. In general, the generator is started at a lower power setting and is subsequently increased in 10-W increments at 1-min intervals. The endpoint of RF application is the appearance of a rapid increase in tissue impedance (Roll-off) around the electrode. When this occurs, the RF application is automatically terminated. This process is repeated starting at 70% of the roll-off power until a second increase of the impedance occurs. Since the time point of the rapid increase in tissue impedance cannot be predicted, the ablation time needed varies dramatically between different tumor sizes and nodules. At the end of the procedure the needle track is ablated to prevent any tumor dissemination. Therefore, the tip has to be unisolated prior to the insertion of the needle. 

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