Regional Anaesthesia

There are a number of regional anaesthetic techniques that enable abdominal surgery with the patient standing. These procedures are almost exclusively reserved for rumi­nants and have the advantage that the patient remains standing so that there is much less danger of regurgitation.

11.6.1 Paravertebral anaesthesia

This technique involves the injection of a local anaesthetic alongside the dorsal and ventral rami of the spinal nerves at T13, L1 and L2 as they emerge from the interverte­bral foramina. The technique produces analgesia of the body wall including the parietal peritoneum. The patient remains standing and fully conscious. The GSA neurons that have touch and pain receptors in the body wall are anaesthetised in the localised area. The anaesthetised ventral rami (T13, L1, L2) contain GSE neurons that innervate the striated muscle in the localised region of the body wall. The parasympathetic system does not have any neurons in the thoracolumbar spinal nerves. The sympathetic system does have preganglionic motor neurons within the ventral roots of the thoracolumbar spinal cord. However, the ratio of pre- to postganglionic sympathetic neurons is about 1:20 so that the system is capable of a widespread response or a compensation for the anaesthetised neurons. The result is that, whereas the GSA and GSE neurons are effec­tively anaesthetised, there are plenty of sympathetic GVA neurons to maintain function.

11.6.2 Pudendal nerve block

This procedure is used to produce anaesthesia of the penis and prepuce of the bull. The main trunk of the pudendal nerve divides into two terminal branches:

1) The dorsal nerve of the penis supplying GSE fibres to the retractor penis muscle and GSA fibres providing sensory innervation to the free end of the penis.

2) The superficial perineal nerve, which divides into a preputial branch providing sensation (GSA) to the preputial membrane and a scrotal branch supplying cutaneous sensa­tion (GSA) to the scrotum.

In addition, there is a pudendal branch of the ischiatic nerve that originates from spinal segments L6, S1 and S2 and that can also be anaesthetised. The procedure involves rectal palpation and anaesthesia of the pudendal nerve in the ischiatic foramen.

Epidural anaesthesia is now preferred to pudendal nerve block.