Jacques E. Chelly
Lower extremity blocks, alone or in combination with general anesthesia, represent an interesting alternative to neuroaxial blocks and general anesthesia alone. Contrary to common belief, lower extremity blocks are not difficult to perform. Most lower extremity blocks are often performed at some distance from the surgical site, which often produces a more extended motor and sensory block than the one required for the surgery. In contrast to the endless discussion about the advantages and disadvantages of using a nerve stimulator, paresthesia, or a transarterial approach for upper extremity blocks, there is a clear consensus favoring the use of nerve stimulators for lower extremity blocks.
Approaches to Lower Extremity Blocks
Lower extremity blocks can be performed with the patient in various positions (lateral, prone, or supine). Because lower extremity surgery usually requires at least two blocks (sciatic and lumbar plexus or a part of it), and because mobilization of the patient may be difficult or painful (e.g., morbid obesity, arthritis, trauma), the choice of technique takes into consideration the need to limit the mobilization of the patient. For surgery at the knee requiring sciatic and femoral blocks in a trauma patient who cannot be mobilized, anterior approaches to these two nerves are indicated. In contrast, when the patient can assume a prone or lateral position, a parasacral, posterior, or subgluteal approach to the sciatic nerve combined with a lumbar plexus approach is possible.
Some consideration should also be given to the choice of the block according to the associated surgical requirements. For example, placement of the tourniquet at the thigh or the calf requires a lumbar plexus block or a saphenous nerve block, respectively. Although the arthroscopic knee diagnostic procedure may be performed under a single femoral nerve block, any knee surgery involving the posterior aspect of the knee also requires a block of the sciatic nerve. This is accomplished using a parasacral, posterior, gluteal/subgluteal, anterior, and lateral or popliteal approach to the sciatic nerve. Although all these approaches are appropriate for surgery below the knee, it seems that a gluteal/subgluteal or a lateral or posterior popliteal approach is favored. Finally, it is possible to obtain a complete block of the foot with an ankle block. However, the use of a combined sciatic and lumbar plexus/femoral/saphenous provides better postoperative analgesia.
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Table 10-1. Peripheral Nerve Block Techniques for Common Lower Extremity Surgery |
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It is also important to take into consideration the duration of the expected postoperative pain. Thus, a continuous nerve block is indicated for postoperative analgesia for lower extremity surgery associated with over 24 hours of moderate to severe pain. These continuous nerve blocks are indicated for both hospitalized and ambulatory patients. Since in most cases a sciatic and lumbar plexus block is indicated, it is necessary to place either one perineural catheter (sciatic for major foot surgery, femoral/lumbar plexus for hip and knee surgery) or two perineural catheters to cover both territories (sciatic and lumbar plexus/femoral catheter for total knee replacement). In this latter instance, the lumbar plexus/femoral catheter is infusing continuously whereas the mode of infusion of the sciatic catheter is controlled by the patient (Table 10-1).
Surgical Indications for Lower Extremity Nerve Blocks
Trauma
Patients with trauma to the lower extremity may greatly benefit from blocks for pain control. In this type of situation, the block often needs to be performed with the patient in a supine position.
Vascular Surgery
For surgeries related to vascular diseases involving the lower extremities (below-the-knee amputation, debridement, and skin grafting), femoropopliteal and tibial arterial reconstruction, as well as for the removal of varices, the use of lower extremity blocks is of special interest because of their lack of effects on the cardiovascular system. However, their use remains limited. In this regard, it is important to recognize that a sciatic nerve block is unnecessary for removing the internal saphenous veins. A femoral block, or preferably, a lumbar plexus block that includes L2 is adequate. Finally, anticoagulant therapy is a contraindication to a sciatic nerve block using either a posterior or an anterior approach.
Orthopedic Surgery
There are a number of indications for nerve blocks in orthopedic surgery of the lower extremity (Table 10-1).
Hip Surgery
It is unclear whether a peripheral nerve block can adequately provide anesthesia for hip surgery and whether a single or a continuous lumbar plexus or a three-in-one block represents the best approach for postoperative pain control after total hip replacement. Irrespective of the approach, it is also important to recognize that the sciatic nerve also provides innervation to the hip. Consequently, consideration should be given for a combined lumbar plexus and high sciatic block for hip surgery.
Surgery of the Knee and Below the Knee
Sciatic and femoral or saphenous blocks are required to satisfy surgical requirements in this area. Sciatic nerve blocks using single injection or continuous infusions are also appropriate for pain control of tibial fractures (rods, placement of external or internal fixation).
Foot Surgery
Although ankle blocks represent the classic approach to surgical and postoperative analgesia for foot surgery, gluteal, subgluteal, and popliteal sciatic nerve blocks associated with saphenous nerve blocks when necessary are gaining acceptance. Selective blocks of the sciatic nerve branches also represent an alternative to limit the extent of motor block.
Characteristics of Lower Extremity Blocks
Doppler ultrasound can be used to facilitate identification of the femoral artery (morbidly obese patient) and fluoroscopy has been used to identify the dorsal surface of the ischium in patients in a prone position. The indications for both methods still remain limited.
Because the sciatic nerve is the largest nerve that we block, sciatic nerve blocks have the longest onset and duration of peripheral blocks commonly performed by anesthesiologists. A minimum of 15 to 30 minutes, and even 45 minutes, is required to allow a complete block. In contrast to upper extremity blocks, neither the addition of bicarbonates or epinephrine reduces the onset time of lower extremity blocks. Although a higher concentration of local anesthetic solution may reduce the onset time of a block, this long onset time needs to be taken into consideration when defining the anesthesia strategy. This contrasts with a femoral block that only requires 5 to 10 minutes for its onset. On the other hand, these blocks often last more than 12 to 16 hours (especially in diabetic patients) when performed with 0.5% to 0.75% of ropivacaine. This duration needs to be taken into consideration in the follow-up of the patient.
Various local anesthetics in various concentrations have been used to block the sciatic and femoral nerves. Except for ankle blocks, we favor the use of 1.5% lidocaine or mepivacaine and 0.5% ropivacaine. The total volume of local anesthetic solution required to block a lower extremity is often 40 to 50 mL (sciatic and lumbar plexus blocks). In this regard, the better safety profile of ropivacaine compared with bupivacaine or even levobupivacaine makes ropivacaine especially attractive when multiple lower extremity blocks are performed. The use of local anesthetics with shorter half-lives may also be indicated to favor early recovery of motor function.
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