FEASIBILITY AND DURATION OF UNMYELINATED VAGAL NERVE BLOCKADE IN NEONATAL LAMB. 1991:

Recent studies have shown that vagal innervation plays a significant role in the ventilatory control during the neonatal life. We have also shown that vagal innervation is critical for the maintenance of effective neonatal breathing shortly after birth. However, it is not clear which of the two receptor-afferent-fibre (myelinated or unmyelinated) systems play the major role in the maintenance of neonatal breathing. Furthermore, it is not known if and for how long the perineural application of capsaicin will block the unmyelinated fibres in the sheep species. Therefore, we studied 4 neonatal lambs to test the hypotheses that 1) capsaicin will block the unmyelinated vagal fibres and 2) such blockade will be maintained for at least 10 days. Observing aseptic conditions and under halothane general anesthesia, one vagus nerve was instrumented with 2 tripolar cuff style electrodes (90-100 mm apart). Compound action potentials (CAPs) were evoked by monophasic electrical stimulation at the distal electrode and were recorded at the proximal electrode. Threshold for the CAP was defined as the minimum amount of current which would evoke a potential twice the size of the background noise. Typically, a maximal A-wave, representing large myelinated vagal fibres, could be evoked at 5 times threshold and a stimulus duration of 0.1 ms. A maximal C-wave, representing unmyelinated vagal fibres, could be evoked at 10 times threshold at a stimulus duration between 0.5 and 5 msec. Conduction block of unmyelinated fibres was achieved by administering 0.1-0.2 ml of 1% capsaicin to a small area between the proximal and distal electrodes. After 10 minutes of application, A-wave remained unchanged whereas the C-wave was diminished by 90%. No further decrease was observed after one hr of application. Furthermore, the blockade was still present when retested after 10 days. Therefore, we conclude that perineural capsaicin application preferentially blocked the conduction of unmyelinated vagal fibres and such blockade lasted at least for 10 days. These results demonstrate the feasibility of a technique which may elucidate the relative role (s) of myelinated and unmyelinated receptor-afferent-fibres systems in the perinatal control of breathing.