Download Update 11 - Update in Anaesthesia - WFSA

56Update in AnaesthesiaNERVE BLOCKS FOR ANAESTHESIA AND ANALGESIA OF THE LOWERLIMB - A PRACTICAL GUIDE: Femoral, Lumbar plexus, SciaticDr Simon Morphett, Specialist Registrar in Anaesthetics, Derriford Hospital, Plymouth, UK.IntroductionThe purpose of this guide is to provide a detailed, step bystep description of how to safely and reliably perform nerveblocks for surgery and pain relief on the lower limb, forthe non-specialist anaesthetic practitioner. It covers femoralnerve blockade, lumbar plexus blockade using the inguinalparavascular approach and sciatic nerve blockade. Nodescription is given of more distal blocks, since the majorityof the limb is readily anaesthetised with the techniquesdescribed and as a group they are relatively simple, reliableand commonly used.Conduct of nerve blocksIt is recommended that all blocks on major nerves becarried out on patients that are awake or only lightlysedated, as it is believed that this decreased the risk ofserious nerve damage, and will not hide the signs ofunexpected local anaesthetic toxicity. Sedated patients muststill be able to communicate with the operator during thenerve block procedure. It is also recommended that a nervestimulator be used if one is available as this increases thesuccess rate of the blocks especially in inexperiencedhands.Local anaesthetic drugs and dosagesFor fast onset and short duration blocks, lignocaine(maximum 4mg/kg) or lignocaine with adrenaline1:200,000 (6mg/kg) can be used. When injected into aplexus or near a large nerve such as the sciatic nerve theblock will come on at about 10-20 mins and last for 4-8hours. The adrenaline will prolong the block but maypossibly increase the risk of nerve damage throughischaemia. Bupivacaine in a maximum dose of 3mg/kg willgive a block of a major nerve that will start at 20-30 minutesand last as long as 18 hours. There is no value in addingadrenaline to bupivacaine except for local skin infiltration.AnatomyThe nerve supply of the lower limb is derived from thelumbar and sacral plexuses, a network of nervescomposed of the anterior primary rami of all the lumbarand the first three sacral nerve roots (and sometimes witha contribution from the twelfth thoracic nerve root). Arisingfrom these plexuses are the five main nerves that innervatethe lower limb.The lumbar plexus: This gives rise to the femoral nerve,obturator nerve and lateral cutaneous nerve of the thigh.The femoral nerve runs in the groove between the psoasmajor and iliacus muscles, with a covering of these muscles’fascia. It enters the thigh passing under the inguinalligament, where it is lateral to the femoral artery, whosepulsations are used to help locate the nerve. The femoralnerve block is performed at this point and there are twoimportant features of the anatomy. Firstly, below the inguinalligament, the femoral nerve divides into anterior andposterior branches, the anterior (superficial) branchsupplying sensation to the skin of the anterior and medialthigh and a posterior (deep) branch that supplies thequadriceps muscles, the medial knee joint, and the skinon the medial side of the calf and foot (via the saphenousnerve). Therefore, the block should not be performedlower than just distal to the inguinal ligament, in order notto miss one of the branches. Secondly, as it enters thethigh, the femoral nerve has two fascial layers covering it,the fascia lata and the fascia iliaca. This is in contrast tothe femoral artery, which is only covered by the fascia lataalone. This means that the nerve will lie in a different tissueplane than the artery and usually a little deeper. Thesecoverings can be used in blocking the nerve. (Seetechniques).The lateral cutaneous nerve of the thigh and the obturatornerve both have important sensory distributions (to thethigh and knee). This article does not cover blocks of thesenerves as single entities. However, they can readily beblocked in conjunction with the femoral nerve, using thesame technique, to produce a “3-in-1” block and this isdescribed later. (see Blockade of the lumbar plexus usingthe inguinal paravascular approach).The sacral plexus: This gives rise to the sciatic nerveand the posterior cutaneous nerve of the thigh. Althoughthese nerves are formed separately within the plexus, theypass through the pelvis and buttock together and with thetechniques described here are blocked with the sameinjection. Hence they are considered here as a single nervetrunk, and unless specifically stated, “sciatic nerve” willrefer to both the sciatic nerve and the posterior cutaneousnerve of the thigh.