Strength-duration curve

FIGURE 30.6 Properties of central nervous system stimulation, (a) The strength-duration relationship describes the amplitude required for stimulation as a function of the stimulation pulse duration. Strength-duration curves for intracellular stimulation of different neural elements were constructed from data summarized in Ranck, 1976. (b) The current-distance relationship describes the threshold intensity required for stimulation as a function of the distance between the electrode and the neuron. Current-distance curves for axons and cells were constructed from data summarized in Ranck, 1976. 

This electric field produces action potential in excitable neuronal cells, which might result in activation of neuronal circuits when applied above certain threshold. The neuronal response depends not only on the electric field strength, but also on the pulse duration through a strength-duration curve of the form ... 

FIGURE 37.3 Neural strength-duration curve depicting stimulation threshold vs. duration. 

Bourland J.D., Nyenhuis I.A., Noe W.A., Schaefer J.D., Foster K.S., and Geddes L.A. Motor and sensory strength-duration curves for MRI gradient fields, in Proceedings of International Society Magnetic Resonance Medicine 4th Scientific Meeting and Exhibit, New York, 1996 p. 1724. 

FIGURE 14.1 Strength-duration curves for current, energy, and charge. Adequate current shocks are above and to the right of the current curve. (Modified from Tacker WA, Geddes LA. 1980. Electrical Defibrillation, Boca Raton, FL, CRC Press. With permission.)... 

Bourland, J. D., J. A. Nyenhuis, W. A. Noe, D. J. Schaefer, K. S. Foster, and L. A. Geddes, 1996, Motor and Sensory Strength-Duration Curves for MRI Gradient Fields, Abstracts of the Society of Magnetic Resonance, Fourth Meeting, New York p. 1724. 

Like voltage and current, a threshold energy strength-duration curve can be created. Referring to the energy formula, it is not surprising that threshold energy... 

Pulse duration 0.4-0.6 ms Pnlse durations of 0.4—0.6ms correspond with the nadir of the threshold energy strength-duration curve (Fig. 1.7). 

Safety margin 2 1 voltage 3 1 pulse duration The strength-duration curve must he taken into account when determining the optimal type of safety margin programming. 

Fig. 2.11 Strength-duration curve shows relationship between stimulation voltage and pulse width. Points on and above the curve capture, whereas those below do not. At short pulse widths, a small change in pulse width is associated with significant change in threshold amplitude however, this is not the case at longer pulse durations, where only a small change is seen. (Reprinted from Ellenbogen KA, ed. Cardiac Pacing, 2nd ed. Oxford Blackwell Science, 1992, with permission.)...

Factors such as method of measurement, type of electrode, drugs, and duration of lead implantation affect the threshold and strength-duration curve (29-31). In addition, a factor known as the Wedensky effect should sometimes be taken into consideration (32). The Wedensky effect posits that, when stimulation thresholds are measured by decrementing the stimulus voltage until loss of capture, the threshold is usually lower by 0.1-0.2 V than when gradually increased from subthreshold until capture is achieved. 

Fig. 19.4 Strength-duration curve created based on a pulse duration threshold at 2.5 V amplitude and a pulse amplitude threshold at a 1.0 ms pulse duration. This curve is automatically constructed by the Medtronic programmer based on testing with the Medtronic Kappa 400 pacemaker. If the output were to fall within the shaded area, it would be subthreshold. The two times and three times safety margin curves are also provided along with the location of the current programmed parameters (X). The curve, based on two data points, is generated from a library of templates which resides within the 9790 programmer.

Strength-Duration Curve Shorter Duration Pulses Require Larger Amplitudes... 

Figure 3. Strength-duration curve for a just visible contraction. Results pooled from three fibres.

Fig. 3 shows a strength-duration curve (data from three experiments) on a semilog plot. For long pulses our contraction threshold voltage seems to be more negative than the one reported by Kovacs Schneider (1978) but quite similar to the one reported by Constantin (1974) in intact fibres. 

FIGURE 28.11 Effect of pulse width on excitation threshold, (a) Strength-duration curve. The amplitude of the current required to reach threshold decreases with the width of the pulse. This effect can be derived theoretically by assuming that the charge on the cable at threshold is constant, (b) The threshold charge (amplitude x pulsewidth) injected into the tissue increases with the pulse width. Narrow pulse are recommended to minimize charge injection. 

The relationship between the strength (current) of an applied constant current pulse required to initiate an action potential and the duration of the pulse, known as the strength-duration curve, is shown in Fig. 13(a). The threshold current 7th decreases with increasing pulse width. At very long pulse widths, the current is... 

---