STIMULATION OF EXCITABLE TISSUES - Design and Development of Medical Electronic Instrumentation - page 316

The primary factors determining whether su cient current fl flows to yield a desired clin-

ical e ect are impedance of the body tissues in the path of the current, electrode size and

position, stimulation parameters, and the electrical characteristics of the tissue to be
excited. These parameters are usually interrelated, as shown in Table 7.1 for the various
clinical areas in which electrical stimulation is used.
The most commonly used stimulation signal waveshapes are those shown in Figure 7.6.
The charge-balanced pulses of Figure 7.6 a ensure that no net charge is introduced to the

ff

Figure 7.6 These are the most common stimulation signal waveshapes and the generic circuits used to produce them. ( a ) Charge-balanced
pulses ensure that no net charge is introduced to the body. In the balanced bidirectional pulse pair, each stimulus pulse has two phases of
identical duration and identical current magnitude, but of opposite polarity. ( b ) True monophasic waveforms are seldom, if ever, used to stim-
ulate tissue because they introduce net charge through the tissue that can cause tissue damage. They are produced only when the energy
source is switched along the way to the tissue and there is no way for the electrode-tissue interface capacitance to discharge. ( c ) Monophasic
waveforms really tend to be asymmetric biphasic, as the net charge built up in the electrode-tissue interface or in a dedicated dc-blocking
capacitor discharges. Not all stimulators deliver a constant-current stimulus. Some generate the stimulus current by discharging a capacitor
into the tissue ( d ) or by using an impulse transformer to step-up the voltage ( e ).