AIMS OF STUDY
After spinal cord injury with loss of bladder function, reservoir function and voiding control can be restored by sacral anterior root stimulation (SARS) with deafferentation. Previous studies with modified, size-adapted Finetech Brindley electrodes revealed very good results in selective sphincter blockade and simultaneous bladder stimulation using modified quasitrapezoidal (QT) pulses in an anodal block technique. The aim of this study was to apply four QT-pulses, determined in previous trials, by means of a multichannel bladder stimulator.

METHODS
In acute animal trials, lumbal laminectomy (L4-L7) and sacral deafferentation (S1-S4) were performed in 6 male anaesthetized foxhounds. The sacral anterior root S2 was placed into a modified tripolar Brindley electrode. Two mono- and two biphasic quasitrapezoidal (QT) pulses were applied in uni- and bilateral trials. Sphincter pressure was urodynamically monitored. Current parameters that demonstrated the best sphincter pressure reduction in unilateral application were utilized for multichannel stimulation. Bilateral measurements were analyzed, evaluated and compared with the multichannel sphincter pressure results. A two-channel current source (Fraunhofer Institute, IMBT) with two direct arbitrary programmable signal channels initiated stimulation. Each anterior root was stimulated separately.

RESULTS
A selective urethral sphincter blockade was achieved in all QT series. In bilateral stimulation trials the average sphincter pressure could be reduced to 8.05 % of its maximum value. In multichanal stimulation sphincter pressure reduction to 4.78 % was seen. For maximal sphincter blockade the average current applied (1.1 to 1.2 mA) was the same with multichannel stimulation and with bilateral stimulation.

CONCLUSIONS
It is possible to achieve selective urethral sphincter relaxation with the application of quasitrapezoidal pulses in an anodal block stimulation technique. The application of individually specified current parameters to each sacral nerves enables better sphincter blockade than bilateral stimulation using the same parameters for each side with the application of the same amount of current. Multichannel stimulation is essential in the development of an neurostimulator that will induce coordinated and synergic bladder contractions. In the near future it will be possible to adjust the signal and current in this stimulator for simultaneous parameter application in accordance with bladder filling.