Combined femoral-sciatic nerve block is well suited for below-knee surgery (1,2). A pneumatic tourniquet is often used to provide a bloodless operating field during such surgery (3,4). When the tourniquet is located below the knee, it could interfere with the operative field and potentially lead to common peroneal nerve trauma (5). That is why a tourniquet at the proximal thigh is usually used. In this circumstance, many authors advocate performing a proximal sciatic nerve approach because it provides anesthesia of the posterior femoral cutaneous nerve (PFCN) at the same time as the sciatic nerve (6,7). However, this is not supported by data.
The aim of the present, prospective, randomized, blinded study was to compare thigh tourniquet tolerance when a proximal (classical posterior) or a popliteal sciatic nerve approach was performed in patients scheduled for elective foot surgery.
After informed consent and with IRB approval, 120 ASA physical status I–II patients scheduled for elective foot surgery under combined femoral/sciatic nerve blockade were included in this prospective, randomized study. Exclusion criteria included the presence of coagulation disorders, infection at the site of block placement, preoperative sciatic or femoral neuropathy, diabetic patients, and inability to comprehend a visual analog pain (VAS) scale.
No premedication was given. In all patients, an IV line and standard ASA monitors were placed. A femoral (F) nerve block was performed following Winnie et al.’s approach (8). A 50-mm short beveled, conducting needle (Stimuplex, Braun, Melsungen, Germany) connected to a peripheral nerve stimulator (PNS) (Stimuplex HNS11, Braun) was introduced 1 cm lateral to the F arterial pulse and 2 cm below the inguinal ligament. It was advanced until stimulation of the F nerve (i.e., patellar ascension) was obtained. Its position was judged adequate when output <0.5 mA (pulse duration, 100 μs) still elicited muscular twitches. After negative aspiration test for blood, 15 mL of 1% mepivacaine with epinephrine 1:200,000 was injected. Patients were then randomly divided into two groups of 60 by a random number generator. In Group 1, a posterior popliteal sciatic nerve block was performed following Singelyn et al.’s approach (1). In Group 2, a classical posterior sciatic nerve block was performed following Labat’s approach (9). In both groups, a 100-mm, 21-gauge, short-beveled insulated needle (Stimuplex, Braun) connected to a PNS was used. Its position was judged adequate when output < 0.5 mA (pulse duration: 100 μs) still elicited foot twitches (i.e., dorsiflexion or plantar flexion). After a negative aspiration test for blood, 25 mL 1% mepivacaine with epinephrine 1:200,000 was injected. Comfort during block performance was evaluated using a VAS (0 = fully comfortable to 100 = highly uncomfortable). Sensory block (loss of cold sensation by using an ether-soaked swab) was assessed in the distribution of the F (anterior aspect of the thigh and medial aspect of the calf, saphenous nerve), lateral cutaneous (LCT) nerve of the thigh (lateral aspect of the thigh at the level of the greater trochanter), tibial (T) (plantar aspect of the foot), common peroneal (CP) (dorsal aspect of the foot), and posterior femoral cutaneous nerve (PFCN) (posterior aspect of the thigh) at 10, 20, and 30 min after injection. Sciatic nerve block was defined as complete (anesthesia of T and CP at 30 min after injection), partial (anesthesia of T or CP at 30 min after injection), or failed (no anesthesia of T and CP at 30 min after injection). In all patients, a 14-cm wide pneumatic tourniquet was placed at the root of the thigh. After limb exsanguinations by gravity, it was inflated to 300 mm Hg. Tourniquet tolerance (0 = no pain, 1 = minimal pain, 2 = moderate pain, 3 = severe pain) was assessed at inflation and thereafter every 10 min until the end of surgery by a resident not involved in block performance. When a score was ≥2, patients received 5 μg sufentanil IV. If insufficient, general anesthesia was performed. Surgical success rate was defined as total (no complementary anesthesia), partial (supplemental local anesthesia or intraoperative sedation with IV opioids), or failure (general anesthesia).
Duration of tourniquet inflation and surgery and patient satisfaction score at the end of the surgery (VAS: 0 = never again to 100 = totally satisfied) were also recorded.
Based on a previous study (10), we hypothesized that we would observe at least a 50% reduction in thigh tourniquet tolerance between groups. A power analysis (percentage of patients with a good tourniquet tolerance at 40 min of 80% in Group 2 and 40% in Group 1) estimated that 45 patients would be needed in each group to provide a 95% chance of detecting such reduction at the 0.05 level of significance. To improve the clinical significance of our results, we decided to include 60 patients per group. Statistical analysis was performed using SAS® software (SAS Institute Inc, Cary, NC.). Results were expressed as mean ± sd. Parametric data (age, weight, height, comfort during block performance, and satisfaction score) were compared using Student’s t-test. Discrete variables (sex ratio, sensory block, success rate, and side effects) were compared using χ2 test or Fisher’s exact test when appropriate. To compare tourniquet tolerance, a survival analysis was performed using the Kaplan-Meier method. Log-rank and Wilcoxon’s tests were used to compare groups. A P value < 0.05 was considered significant.
One-hundred-eighteen patients were finally included. Two patients (complex foot arthrodesis) in Group 2 were excluded because of an unexpected prolonged duration of surgery requiring general anesthesia.
Patient demographics, time to perform the block, and type of surgery were comparable in the groups (Tables 1 and 2). Block performance was significantly more comfortable in Group 1 than in Group 2 (P < 0.01). At t30, completeness of sciatic nerve (94% versus 98% in groups 1 and 2, respectively), F nerve, and LCT nerve of the thigh was comparable in both groups. When compared with Group 1, a significant difference in the incidence of PFCN anesthesia was noted at t30 in Group 2 (P < 0.01). Surgical success rate was comparable in both groups (total 93% versus 97% and partial 7% versus 3%, in Groups 1 and 2, respectively).
Tolerance of thigh tourniquet in each group is presented in Figure 1. No difference was found between groups. In both groups, tourniquet pain increased with time. Because of a tourniquet pain, 3 patients in each group required IV opioids and 0 versus 2 patients required general anesthesia in Groups 1 and 2, respectively. In all those patients, sciatic and F nerve blocks were completed at 30 min and the duration of surgery was more than 60 min. The satisfaction score was comparable in both groups (92 ± 10 versus 87 ± 11, in Groups 1 and 2 respectively).
The present study demonstrates that, with an efficient F nerve block, both proximal and popliteal sciatic nerve approaches provide comparable thigh tourniquet tolerance. Tourniquet pain increases with time, and it is not affected by the presence of a PFCN block.
The mechanism of tourniquet pain remains controversial and many factors are thought to be implicated. Different authors have stated that local pain from skin compression plays a predominant role (11–13). During IV regional anesthesia, EMLA cream significantly prolonged the duration of tourniquet analgesia when compared with a control group (12). Nevertheless, as stated by Tsai et al. (13), such result is of little clinical significance because the technique is time consuming, expensive, and variably effective. During infraclavicular brachial plexus blockade, anesthesia of the medial cutaneous nerves by a skin infiltration of local anesthetic at the root of the arm improved arm tourniquet tolerance (14). However, tourniquet pain can complicate a spinal or epidural anesthesia despite adequate sensory anesthesia of the dermatome underlying the tourniquet (15,16). As demonstrated in the present study, complete skin anesthesia of the thigh did not improve tourniquet tolerance. The lack of difference between the two groups could be partially attributable to inadequate anesthesia of the obturator nerve, which innervates the medial thigh muscle compartment. This should be assessed by a specific prospective study. Thus, local pain from skin compression appears to be one component of tourniquet pain but probably not a predominant one. This may explain the lack of effect of complete block of the PFCN we observed.
Other factors such as tourniquet size, inflation pressure, and duration (17,18) and type of anesthesia (19), or adjuvant to the local anesthetic solution (20,21) have been implicated.
A wide tourniquet cuff is more efficient at a lower occlusive pressure than a narrow one (22). It has been recommended to inflate the cuff to a pressure 150 mm Hg more than the patient’s systolic blood pressure (3). In the present study, a 14-cm wide tourniquet inflated at a 300 mm Hg pressure was used in all patients. Such inflation pressure is our standard institutional practice. A retrospective review of more than 650 patients who underwent orthopedic surgery demonstrated that the incidence of tourniquet pain and the associated hypertension correlated with tourniquet time (19). This is confirmed by our data; <15% of patients at 30 min but more than 50% of patients at 70 min complained of tourniquet pain and required supplemental analgesia. In the present study, hemodynamic variables were not specifically recorded. In addition to pain, the clinician must be vigilant for increasing arterial blood pressure as a significant side effect of prolonged tourniquet time (19).
Tourniquet pain is thought to be mediated by unmyelinated, slow-conducting C fibers that are normally inhibited by fast pain impulses conducted by myelinated A-δ fibers (4,23). Mechanical compression causes loss of conduction in nerve fibers with large ones being blocked before small ones. It is suggested that, after approximately 30 minutes of tourniquet inflation, the large A-δ fibers will be blocked, leaving still functioning C fibers uninhibited. Clonidine has been reported to depress nerve action potentials in C fibers (24). Intrathecal clonidine combined with local anesthetic has been shown to decrease the incidence of tourniquet pain in the lower limb (15). In patients undergoing upper limb IV regional anesthesia, clonidine added to the local anesthetic solution improved tourniquet tolerance (20,21). This beneficial effect has never been demonstrated during lower limb peripheral nerve blockade. This should be evaluated in a prospective, randomized, blinded study.
During the Labat’s approach, the needle has to be inserted through the gluteal muscles. Performed on the midline between muscles, the posterior popliteal approach avoids muscle trauma and would thus be more comfortable for the patient. This is demonstrated in the present study. Indeed more patients assessed block performance as uncomfortable (VAS > 30 of 100) in Group 2 than in Group 1 (8% versus 22% of patients respectively in groups 1 and 2). This is in accordance with previous results (25). It must be stressed that in the present study, patients received no sedation. Sedation before block performance routinely used in many institutions could mask early signs of systemic local anesthetic toxicity or of intraneural injection. The present study indicates that, in most patients, such sedation is not required to perform sciatic nerve block, particularly when the posterior popliteal approach is used.
In conclusion, this randomized, prospective, blinded study demonstrates that the Labat’s approach of the sciatic nerve provides no better tolerance of a thigh tourniquet than the popliteal approach. As it is as efficient and more comfortable for the patient, popliteal sciatic nerve block would be the preferred technique for below-knee surgery.
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© 2005 International Anesthesia Research Society
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