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60Update in Anaesthesia Allow time for the local anaesthetic to takeeffect - at least 15 - 20 minutes will be required forsurgical anaesthesia.With the weaker concentrationsof bupivacaine, 30 - 45 minutes may be required.TechniquesThe femoral nerve and lumbar plexusAnatomy The femoral nerve has contributions from thesecond, third and fourth lumbar nerves. It is derived fromthe lumbar plexus and in fact lies within the same fascialenvelope as the lumbar plexus. This important fact maybe utilised to block most of the nerves originating in thelumbar plexus with a single injection distally, as localanaesthetic can be made to spread proximally within thisplane. (See anatomy)Technique (figure 2) The patient lies supine with the legextended, lying flat on the bed. The operator stands onthe side of the patient that is to be blocked. Firstly, identifythe point of injection, using the surface landmarks. For thefemoral nerve, this is just below (distal to) the inguinalligament. Palpate both the anterior superior iliac spine andthe pubic tubercle. The line between these two overliesthe inguinal ligament. It is often helpful to draw the linesthat are described on the skin. The femoral artery shouldlie at the midpoint of the inguinal ligament and it is necessaryto locate this by feeling for the pulse at this point. The sitefor injection is 1cm lateral to (outside of) the pulsations ofthe femoral artery and 1 - 2cm below (distal to) the line ofthe inguinal ligament. Having identified the site, it will bemore comfortable for the patient if a small amount of localanaesthetic is used to create a skin wheal (“bleb”) at theinjection point.An ordinary needle of length 3 - 4 cm and 21 - 23g inwidth is suitable for performing this block. It should beinserted perpendicular to the skin, but aiming slightlytowards the head of the patient.The following are two ways of carrying out the block. Inthe first technique, the operator attempts to locate the nerveby eliciting paraesthesiae, or failing this by depositing thelocal anaesthetic over a range of areas (the classicaltechnique of Labat 5 ). In the second technique, use is madeof the fascial layers overlying the nerve and a single injectiononly is employed (Khoo and Brown 2 ). In both cases, it isvery important to remember the anatomy. The femoralnerve lies adjacent to but slightly deeper than the structurescontained within the femoral sheath (the artery, vein andfemoral canal). This is because the nerve lies deep to thefascia iliaca, while the contents of the femoral sheath lie ontop of it.Figure 2: Left - Site of injection for femoral nerve block. Right - Transverse section.
Update in Anaesthesia 61The classical approach The needle is advancedthrough the skin, as described above, until the patientfeels paraesthesiae in the distribution of the femoralnerve. If a depth of 4 - 5cm is reached and noparaesthesiae are found, then it should be withdrawnto just below the skin and advanced again in a slightlymedial or lateral direction, repeating this until thepatient feels paraesthesiae. Once this occurs, the needleshould be fixed in position with one hand, resting thishand on the patient to try and minimise movement.With the other hand, the syringe containing localanaesthetic is then connected to the needle and gentleaspiration performed. If no blood is seen then 15 - 20mls of local anaesthetic is injected (aspirating againregularly to check for the presence of blood).The presence of paraesthesiae is the best indicator forcorrect positioning of the tip of the needle, but it is oftennot easy or even possible to locate the femoral nerve inthis manner.Alternatively as the needle is advanced alongside theartery, its pulsations may cause lateral (side to side)movement of the hub of the needle. If this is the case,the needle is slowly advanced and frequentobservations made until the point is reached when thelateral movements are at their greatest. This generallyrepresents a depth where the tip of the needle is justdeep to the artery and should be in the correct plane.The needle is then fixed, the syringe connected andaspiration performed as before. However, only 10 mlof local anaesthetic is injected at this site. This injectionis then supplemented by withdrawing the needleslightly redirecting the tip outwards (laterally),inserting to the same depth as before and injecting 3 -4 mls of local anaesthetic (after aspiration). Thisprocess is repeated 2 or 3 times, moving progressivelyfurther laterally, such that a total of 20 - 25mls of localanaesthetic is deposited in a “fan-shaped” area lateraland deep to the femoral artery.The single injection technique 2 This method has thevirtue of simplicity and generally involves less probing withthe needle. For this reason it is popular but requires somepractice for a high success rate.The site for injection is the same as already described.However, the needle is inserted directly perpendicular tothe skin. If the needle is held gently between thumb andforefinger, then a slight resistance is encountered at thefascia lata, followed by a definite loss of resistance, or“pop” as the needle penetrates this layer. The same thingis felt as the needle penetrates the fascia iliaca and comesinto the proximity of the femoral nerve. Therefore,immediately on feeling this second loss of resistance, or“pop”, the tip of the needle should be in the correct position.The needle is then fixed in position with one hand, theother hand again being used to connect a syringe, aspirateto check for blood and inject 20 ml of local anaesthetic.This technique is entirely dependent on being able to detectthe two points of loss of resistance as each of the fasciallayers is penetrated. This is much easier if a short-bevelneedle is available to use, as it does not pierce the fasciaquite as easily as an ordinary needle, making the feel ofthe layers more obvious. If a short-bevel needle is notavailable, then the same effect can be achieved using anordinary needle but blunting it prior to insertion. The“blunting” may be cleanly achieved by piercing the side ofthe protective plastic sheath (that comes with the needle)several times with the needle tip before performing theblock. The detection of paraesthesiae is not to berecommended when using this technique as the “blunted”needle is more likely to cause nerve damage.Use of a nerve stimulator If an insulated stimulatingneedle is available for use, then it is necessary to obtaincontraction of the quadriceps muscle group. This is mostreliably seen by movement of the patella and extension ofthe knee joint. (The movement of the knee is not normallyobvious, as the patient’s leg is usually flat on the bed andfully extended anyway.) The contractions should still bevisible at a stimulating current of 0.3 - 0.5 mA indicatingadequate proximity to the nerve. Exactly the sametechnique will be used if one wishes to perform a lumbarplexus block using this approach.Blockade of the lumbar plexus using the inguinalparavascular approachThis technique is also referred to as the “Winnie 3-in-1block” after the author who first described it 3 . It is so called,because it aims to block three nerves with the one injection:the femoral nerve, the lateral cutaneous nerve of the thighand the obturator nerve.Anatomy For most operations on the thigh and knee it isnot sufficient to block the femoral nerve alone. The lateralcutaneous nerve of the thigh supplies all the outside of thethigh and the obturator nerve supplies a variable amountof skin on the inner thigh just above the knee andcontributes to the innervation of the knee joint.All three nerves are derived from the lumbar plexus. Theplexus lies on the quadratus lumborum muscle and behind
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