DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

EXTRACELLULAR STIMULATION 309membrane’s threshold voltage. Note that this is the reverse of what happens with intracellularstimulation, where excitation occurs at the anode.Depending on the arrangement of the electrodes, three stimulation modes can be distinguished:1. Bipolar. Both electrodes are close to the target tissue.2. Monopolar (also called unipolar). One electrode, normally the cathode, is close tothe target tissue, and the other (anode) is remote from the target tissue, making itssize and exact placement irrelevant.3. Field stimulation. Both electrodes are remote from the target tissue.The efficiency of bipolar and monopolar stimulation is similar. However, the currentdelivered in the monopolar mode often crosses through nontarget tissue on its way to theanode (yes, the conventional direction for current is in the opposite direction, but you knowwhat we mean) and is sometimes capable of stimulating these nontarget excitable cellsundesirably. Field stimulation is the most inefficient method but is very commonly the preferredmode of current delivery in nonchronic applications since it allows tissues to bestimulated using noninvasive skin-surface electrodes.A stimulus must be of adequate intensity and duration to evoke a response. If it is tooshort, even a strong pulse will not be effective. The stimulation threshold is defined as theminimum strength of stimulus (expressed either in volts or in milliamperes) required foractivation of a target tissue for a given stimulus duration. When thresholds for several durationsare put together on the same graph, a strength–duration curve is formed. The nicething about the strength–duration curve is that with one quick look one can determinewhether or not a stimulus will be effective. Any stimulus that falls above the curve willexcite the target tissue.As shown in the stylized strength–duration curve of Figure 7.4, stimulus current andduration can be mutually traded off over a certain range. For a short pulse, the effectivenessof a stimulus is characterized by the product of current I and duration t, where deliveredcharge Q It. Hence if the amount of charge required to activate the target tissue isQ threshold and the stimulus duration is t, the current I threshold required to achieve activationwill be I threshold Q threshold /t.It would seem from this relationship that the strength–duration curve should show adecline to near zero as stimulus duration is increased. However, the strength–durationcurve of real excitable tissue flattens out with long stimulus durations, reaching an asymptotecalled the rheobase. The root rheo means current and base means foundation; thus,the rheobase is the foundation, or minimum, current (stimulus strength) that will producea response. When the stimulus strength is below the rheobase, stimulation is ineffectiveeven when stimulus duration is very long.The reason for the difference between the actual behavior and that predicted byI threshold Q threshold /t is that the latter assumes that the membrane is an ideal capacitor. Thisis not the case, and the leakage resistance shows its effect during prolonged stimulation(large values of t). The equation fails to predict the charge transfer across the cell membranebecause under these conditions, more membrane current is carried by the leakageresistance and less is used to charge the membrane capacitance. Membrane potential thusrises exponentially to a plateau during prolonged stimulation instead of increasing linearlywith time.The strength–duration curve was characterized by Lapicque [1909] by the value of therheobase (in volts or milliamperes) and a second number called the chronaxie. The rootchron means time and axie means axis. The chronaxie is measured along the time axis andis defined as the stimulus duration (in milliseconds) that yields excitation of the tissue whenstimulated at twice the rheobase strength. In the strength–duration curve of Figure 7.4, the