Effect of epidural administration of bupivacaine (Marcaine) on monosynaptic reflex-depression

The L5 and S1-segments of the spinal cord and relatively resistant to blockade. The extent and depth of the block are usually tested by pinprick. H-reflex depression (Hoffmann 1922, Magladery and McDougal 1950) was used to quantify differences between the 0.5%-solution and the 0.75%-solution of bupivacaine used for epidural anaesthesia. The H-reflex is a monosynaptic reflex representing the excitability of the motorneuron pool of the L5-S1 region of the spinal cord. Our results show a difference between the two concentrations favouring the 0.75% solution. The H-reflex is also a component in the complexity of control and servocircuits that play a role in the well-functioning of the spinal cord. Consequently we used its full recovery to rule out lasting toxic or irritating effects on the spinal cord. Bupivacaine 0.75% is not available in Europe whereas in the USA there is already wide experience in using this concentration. A comparison between 0.75% and 0.5% solutions reveals two important advantages in favour of the 0.75% solution: A higher intensity of motorblock [4, 15]. A lower incidence of incomplete blockade of the most resistant segments (L5-S1) [7, 8].