Femoral Nerve Block
- Indications: Anterior thigh and knee surgery
- Landmarks: Femoral (inguinal) crease, femoral artery pulse
- Nerve Stimulation: Twitch of the patella (quadriceps) at 0.2-0.5 mA current
- Local anesthetic: 20 mL
- Complexity level: Basic
Regional anesthesia anatomy
Distribution of anesthesia
- Anterior division:
- Middle cutaneous
- Medial cutaneous
- Muscular (sartorius)
- Posterior division:
- Saphenous nerve (most medial)
- Muscular (individual heads of the quadriceps muscle)
- Articular branches (hip and knee)
The patient is in the supine position with both legs extended. In obese patients, a pillow placed underneath patient's hips may facilitate palpation of the femoral artery and block performance.
Needle insertion site is labeled immediately lateral to the pulse of the femoral artery. All landmarks should be outlined with a marking pen.
- Note that this technique differs from common descriptions of the femoral nerve block, where the needle is inserted at the level of the inguinal ligament. Instead, in this technique the needle is inserted at the level of the femoral crease, a naturally occurring, oblique skin fold positioned a few centimeters below the inguinal ligament.
- The femoral crease can be accentuated in obese patients by asking an assistant to retract the lower abdomen laterally. The retraction of the abdomen should be maintained throughout the procedure to facilitate palpation of the femoral artery and block performance.
The femoral nerve innervates a number of muscle groups. A visible or palpable twitch of the quadriceps muscle (patella twitch) at 0.2-0.5 mA current is the optimal response.
Failure to localize the femoral nerve
- The most common response to nerve stimulation with this technique is twitch of the sartorius muscle. This results in a band-like contraction across the thigh without movement of the patella.
- It should be kept in mind that sartorius muscle twitch is not reliable because the branches to the sartorius muscle may be outside the femoral sheath. When the sartorius muscle twitch occurs, the needle is simply redirected laterally and advanced several mm deeper.
When stimulation of the quadriceps muscle is not obtained on the first needle pass, the palpating hand should not be moved from its position. First, visualize the needle plane in which the stimulation was not obtained and follow this algorithm:
- Ensure that the nerve stimulator is properly connected and functional.
- Withdraw the needle to the skin, redirect 10-15o laterally, and repeat the needle advancement.
Response Obtained Interpretation Problem Action No response The needle is inserted either too medially or too laterally Femoral artery not properly localized or the palpating hand moved during the procedure Follow the systematic lateral angulation and reinsertion of the needle as described in the technique Bone contact The needle contacts hip or superior ramus of the pubic bone The needle is inserted too deep Withdraw to the level of the skin and reinsert in another direction Local twitch Direct stimulation of the illiopsoas or pectineus muscle Too deep insertion Withdraw to the level of the skin and reinsert in another direction Twitch of the sartorius muscle Sartorius muscle twitch The needle tip is slightly anterior and medial to the main trunk of the femoral nerve Redirect the needle laterally and advance deeper 1-3 mm Vascular puncture Blood in the syringe invariably indicates placement into the femoral artery Too medial needle placement Withdraw and reinsert laterally 1 cm Patella twitch Stimulation of the main trunk of the femoral nerve None Accept and inject local anesthetic
Choice of local anesthetic
A femoral block can be accomplished with as little as 10 mL of local anesthetic. However, we often use larger volumes of local anesthetic (e.g., 20-25 mL), because the local anesthetic often disperses underneath fascia iliaca laterally and results also in block of the lateral femoral cutaneous nerve of thigh. The block of lateral cutaneous nerve of the thigh, in return, confers anesthesia to the lateral aspect of the thigh, which nicely complements the femoral nerve block.
The choice of the type and concentration of local anesthetic should be based on whether the block is planned for surgical anesthesia or pain management. Long-acting local anesthetic should be avoided in ambulatory patients undergoing relatively minor procedures as ambulation is affected by prolonged motor block of the quadriceps muscle.
The onset times and duration of anesthesia with different types and concentrations of local anesthetics and the addition of vasoconstrictor.
Anesthesia (hrs) Analgesia (hrs) 3% 2-chloroprocaine (+ HCO3) 10-15 1 2 3% 2-chloroprocaine (+ HCO3 + epi) 10-15 1.5-2 2-3 1.5% Mepivacaine (+ HCO3) 15-20 2-3 3-5 1.5% Mepivacaine (+ HCO3 + epi) 15-20 2-5 3-8 2% lidocaine (+ HCO3 + epi) 10-20 2-5 3-8 0.5% ropivacaine 15-30 4-8 5-12 0.75% ropivacaine 10-15 5-10 6-24 0.5 Bupivacaine (or l-bupivacaine) 15-30 5-15 6-30
Block Dynamics and Perioperative Management
This technique is associated with minimal patient discomfort, because the needle passes only through the skin and adipose of the femoral inguinal region. However, many patients feel uncomfortable being exposed during palpation of the femoral artery and appropriate sedation is necessary for the patient's comfort and acceptance. Midazolam 1-2 mg after patient is positioned and alfentanil 250-500µg just before the local infiltrations suffices for most patients. A typical onset time for this block is 10-15 minutes, depending on the type, concentration, and volume of local anesthetic used. The first sign of onset of blockade is the loss of sensation of the skin over the medial aspect of the leg below the knee (saphenous nerve). Weight bearing on the blocked side is impaired and this should be clearly explained the patient to prevent the risk of falls.
Complications and How to Avoid Them
Infection - Just like with any other procedure, use a strict aseptic technique
- Catheters at his location are difficult to keep sterile and should probably removed after 48 hours
Hematoma - Avoid advancement of the needle when the patient reports pain; this may indicate insertion of the needle through the illiopsoas or pectineus muscles
- When the femoral artery or vein are punctured, the procedure should be stopped and a firm and constant pressure applied over the femoral artery for 2-3 minutes before proceeding with the blockade.
- In a patient with difficult anatomy or severe peripheral vascular disease, use a single-shot smaller gauge needle to localize the femoral nerve before proceeding with a larger gauge needle for the continuous technique.
Vascular Puncture - Never redirect the needle medially!
- The needle is first inserted just lateral to the femoral artery and the consequent insertions and redirections should all be progressively more lateral
Nerve Injury - Use nerve stimulation and slow needle advancement
- Distinct paresthesia is almost never elicited with femoral nerve block and should not be sought
- Do not inject when the patient complains of pain or when high pressures on injection are met
Other Instruct the patient on the inability to bear weight on the blocked extremity
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