Bioelectric sources heart

Bioimmittance is measured in vivo or in vitro. The tissue may be kept alive and perfused under ex vivo conditions. Bioimmittance can be measured with two-, three- or four-electrode systems. With four electrodes, one electrode pair is current carrying and the other pair picks up the corresponding potential difference somewhere else in the tissue. If the measured voltage is divided by the applied current, the transfer impedance is calculated. If no voltage is measured, the transfer impedance is zero. This is equivalent to the bioelectricity case in which a signal from the source, such as the heart, is transferred to the skin surface electrodes. Zero transfer impedance does not mean the tissue conducts well, only that no signal transfer occurs. With the bioimpedance two-electrode technique, the transfer factor is eliminated because current application and signal pickup occur at the same site, which means that measured impedance reflects tissue electrical properties more directly. 

The potentiometer, used for accurate determinations of voltage in standards laboratories, has various applications in biological research. One, of course, is the accurate determination of various bioelectric potentials. In addition, many instruments can be constructed about the potentiometer as a central core. Figure 5.10 illustrates the basic potentiometer circuit. The instrument consists of two batteries, a standard reference cell and a working battery. In some instrumentation systems, the working battery is replaced by a regulated power supply, and a zener diode reference source is used in place of the standard cell. The heart of the instrument is a very accurately calibrated resistance, which is called a slide wire. It is usually in the form of a helix wrapped about a solid core. In operation, switch 1 is connected to the standard cell. The key (switch 2) is tapped at intervals... 

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