Monopolar systems

The principal circuit diagram for a bipolar system is shown in Fig. 15-2a and the current distribution in the case of breakdown of a monopolar system in Fig. 15-2b. I and II are the respective ac systems. 

Fig. 15-2 Stray current interference from high-voltage dc transmission installations (a) bipolar system, (b) monopolar system.

RFTA The monopolar system which has been used up to now will probably be replaced in the near future by a bipolar system. Coagulation necrosis of up to 5 cm can be achieved. However, RFTA sometimes has a negative impact in areas near the large vessels, since the cooling effect may cause the tumour cells close to the vessel walls to survive, with a risk of recidivation. In special cases, RFTA can also be applied as an open (operative) technique. A combination of ablation therapy and resection is possible. RFTA has several advantages it is relatively low in cost, there is less parenchymal loss, the procedure is safe and can be repeated. (339) (s. p. 785)... 

In monopolar systems every electrode has to be connected to the power supply. The overall current is the sum of the currents between all anodes and cathodes, and the cell votlage is applied between every electrode pair. Monopolar connection requires high current and low voltage supply bipolar systems require the opposite (Fig. 3). 

In the monopolar system, each cell unit in one electrolyzer is connected in parallel and each electrolyzer is connected in series. Voltage across one electrolyzer is low and current passing through one electrolyzer is high. Each electrolyzer unit is connected to the next thru bolted bus bar connections which adds... 

FIGURE 10.10 Skin surface electrode systems. Monopolar systems are usually not ideally monopolar, see text below. The three- and four- electrode systems measure transfer immittance. Electrode functions M, measuring CC, current carrying R, reference PU, pick up. 

Small area contacts occur, for example, with pacemaker electrodes, catheter electrodes, and CC fluid-fiUed cardiac catheters. Small area contact implies a monopolar system with possible high local current densities but low current levels in the external circuit. This is called a microshock situation. The 50/60 Hz safe current hmit for a small area contact with the heart is 10 i,A in normal modes. The dilfer-ence between macro- and microshock dangerous current levels is, therefore, more than three orders of magnitude. 

Figure 6.3 Current constriction, (a) Monopolar system where the resistance is increased by the smaller electrode and current constricting geometry (b) bipolar system.

With no current through the electrolytic cell, it does not matter whether the electrodes are large or small the equilibrium potentials are the same. But with current flow, the current density and therefore the voltage drop and the polarization, will he much higher at the small electrode. An increased potential drop will occur in the constrictional current path near the small electrode, and in general the properties of the small electrode will dominate the results. The small electrode will be the electrode studied, often called the working electrode. It is a monopolar system, meaning that the effect is determined hy one electrode. The other electrode becomes the indifferent or neutral electrode. Note that this division is not true in potentiometry, electrode area is unimportant under no-current conditions. 

Polarography is voltammetry, preferably with the dripping mercury electrode and with a diffusion-controlled current in a monopolar system. 

PU electrode situated on an inactive skin site. The other electrode is then situated on an active skin site. This constitutes a monopolar system. If both electrodes are situated on an active skin site, no signal (potential difference) is measured. Function is independent of surface area (as are all PU electrodes), but small electrodes pick up more noise. Not CC, current density J = 0. Indifferent electrode shall be situated far from bioelectric sources. Examples ECG right leg, other extremities, Wilson terminal. EEG ear flip, nose. Electrodermal activity elbow. 

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