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Title: Artificial control of the lower urinary tract by sacral root stimulation
Author: Costa Monteiro, Lucia Maria
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1995
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Micturition disorders are humiliating disabilities which cause major clinical, social and financial problem for a substantial part of the population. In many cases it lacks any adequate conventional solution. The clinical application of the anterior sacral root stimulation introduced by Brindley and his co-workers (1992) has been one of the most successful application of functional electrical stimulation to control the lower urinary tract. It has been used in several countries world-wide to control micturition. However, its more widespread acceptance is limited by the unsuitable recruitment characteristics of bipolar stimulating electrodes. Normal micturition requires co-ordinated bladder contraction and sphincter relaxation. The sacral roots contain large somatic motor fibres supplying the skeletal muscle of the external sphincter mixed in with small autonomic motor fibres supplying the bladder. During sacral root stimulation the larger fibres are activated more easily than the smaller ones and so, the external urethral sphincter contracts and close before the bladder is activated, making normal voiding difficult. The solution would be reverse that recruitment selection and allows bladder contraction with a relaxed sphincter. This research tests a tripolar stimulation strategy which is thought to be able to activate selectively small autonomic motors axons whilst blocking excitation in larger ones. This approach is based on initial mathematical modelling studies (Fitzpatrick, 1991). The construction of a tripolar electrode and the choice of stimulation current intensities and ratios were guided by the model predictions. The simulation results of the modelled tripolar electrode predict that the generation and propagation of action potentials in the larger axons can be anodally blocked at one end of the cuff electrode, called the blocking anode. Also, a second prediction is that is possible to regulate the intensity of the anodal current at the other side of the cuff electrode, called escape anode, in order to provide the selective blocking of the larger axons whilst allowing for the escape of action potentials in the smaller fibres. Two series of experiments were carried out to test the model predictions. The first series of experiments were developed to test the electrode and stimulation parameters, and if it is possible to achieve an anodal block at the blocking anode. Experiments performed in isolated rabbit sciatic nerve trunks, showed that the propagation of action potentials in all axons can be blocked at one end of the cuff, while unidirectional propagation of action potential still could be achieved at the escape end with tripolar stimulation. Generally, more current was needed to produce a block during the experiments than expected from the model, which can be explained because physiological circumstances during the experiments were not calculated. Block was achieved more frequently at an anodal ratio of 9:1 and trains of pulse of 0.3msec duration, confirming the model predictions. The second series of experiments were developed to test if it is possible to regulate the intensity of the anodal current at the escape anode, in order to provide the selective blocking of the larger axons whilst allowing for the escape of action potentials in the smaller fibres. Unilateral ventral sacral root stimulation was performed on adult males New Zealand white rabbits which were deeply anaesthetised with a mixture of 1-3% halothane in nitrous oxide and oxygen. Trains of pulse of 0.3msec duration lasting up to 2sec at 20Hz were delivered. Simultaneous recording of bladder and urethral pressures were made by conventional urodynamic methods. The root which produced the stronger bladder/urethral sphincter response was used throughout the experiment. The responses to stimulation of bladder, external urethral sphincter and coccygeal (skeletal) muscle, which is activated by stimulation of the same root, were compared during bipolar and tripolar stimulation. No spontaneous activity was recorded in the bladder or urethral sphincter when the animal was deeply anaesthetised. It was possible to produce activity in the lower urinary tract in 27 experiments. Average bladder pressures of 30cm H2O was developed during stimulation. Using bipolar stimulation, the lowest threshold currents were always associated with skeletomotor axons supplying muscles in the hip and in the base of the tail. Further increase in stimulus intensity recruited axons supplying the external urethral sphincter and the highest thresholds were associated with the parasympathetic axons supplying the bladder. These results confirmed the expected recruitment characteristics during conventional stimulation and were highly significant (p < 0.0001).
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available