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REGIONAL ANESTHESIA

Skin Temperature During Regional Anesthesia of the Lower Extremity

Department of Anesthesiology, University of Düsseldorf, Düsseldorf; Germany

Address correspondence and reprint requests to Markus F. Stevens, MD, DEAA, Klinik für Anästhesiologie, Universitätsklinikum Düsseldorf, Postfach 101007; 40001 Düsseldorf; Germany. Address e-mail to markus.stevens{at}med.uni-duesseldorf.de.

	Abstract

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Increase in skin temperature (Ts) occurs early during neuraxial blocks. However, the reliability of Ts to predict successful peripheral block is unknown. Therefore, we investigated whether an increase in Ts more than 1°C precedes or follows an impairment of sensation after combined femoral and sciatic nerve block as well as after epidural anesthesia. In this prospective, nonrandomized study we determined Ts changes in 33 patients undergoing knee or foot surgery under femoral and sciatic nerve block and 10 patients undergoing epidural anesthesia. Perception and motor function were assessed every 5 min. An increase in Ts (1°C) at the foot occurred later after sciatic nerve block than after epidural anesthesia (10.3 ± 2.8 versus 5.0 min; P < 0.01). Alterations of Ts at skin innervated by the femoral nerve were <1°C. Ts increase preceded sensory block after sciatic nerve block in 6.6% of patients but indicated a successful block (sensitivity, specificity, and accuracy = 100%). We conclude that an increase of Ts is a reliable, but late, sign of successful sciatic nerve block. Therefore it is of limited clinical value. Ts changes after femoral nerve block are negligible and late.

	Introduction

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Increase in skin temperature (Ts) occurs early during neuraxial block. An increase in Ts takes 90 s to 2 min (1) during spinal anesthesia and 10 to 15 min (2–4) during lumbar epidural block (EPI). This increase in Ts is a sign of sympathetic block during EPI anesthesia as shown by microneurography (5,6). Sympathetic block has been reported to occur 10 min before the loss of pinprick sensation during EPI (7).

It was our clinical impression that, during peripheral nerve block, warming of the skin was a comparatively late sign of a successful block, but there are conflicting opinions on this matter (8).

It would be helpful in clinical practice to know the time sequence of Ts increase and sensory block during nerve block. Because onset of sciatic nerve block is highly variable and the success rate varies between 71% and 97% (9–13), an early sign of successful block would be advantageous. An increase in Ts after peripheral nerve block of a distal extremity should theoretically be substantial because in distal extremities low resistance arteriovenous anastomoses are numerous and richly innervated by sympathetic vasoconstrictor nerves (14). Furthermore, Ts increases faster after EPI anesthesia than after lumbar sympathetic block, suggesting that the velocity of Ts increase depends on the site of sympathetic block (2).

We investigated onset of sensory, sympathetic, and motor blocks in patients undergoing surgery during combined femoral and sciatic nerve block (FSB). We hypothesized that an increase in Ts follows sensory block during FSB and compared the results with those acquired during EPI.

	Methods

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After approval by the human investigation committee at our institution and informed patient consent, 43 patients were enrolled in the study. Thirty-three patients underwent orthopedic surgery of the knee or below with FSB, and 10 patients underwent hip surgery with EPI. Patients were excluded from the study if they were ASA physical status >III, had a contraindication to the use of local anesthetics or to nerve or epidural block (e.g., coagulopathy), or had clinical signs of peripheral neuropathy. Lactated Ringer’s solution 10 mL/kg was given IV before administering the designated block. All patients received alfentanil before performance of the blocks. All blocks were performed by one anesthesiologist with experience in both techniques.

Femoral block (3-in-1) was performed using Winnie et al.’s landmarks (15) and the sciatic nerve was approached anteriorly (16). In each case the femoral nerve was approached first. Nerves were located using a peripheral nerve stimulator (Stimuplex® HNS 11; B Braun, Melsungen, Germany) and an insulated needle (110 mm Contiplex® D; B Braun). The correct position of the needle was confirmed by appropriate motor response elicited with a square impulse of 100 µs width and a current of 0.4 mA. Local anesthetics were injected through the needle. For femoral nerve blocks we injected 30 mL of 1% prilocaine followed by 10 mL of 0.75% ropivacaine. Then we injected 20 mL of 1% prilocaine followed by 10 mL of 0.75% ropivacaine for the sciatic nerve block. A catheter (ø 0.45 mm, Perifix®; B Braun) was advanced if needed for postoperative pain control. Local anesthetics were injected over 3 min.

The epidural space was identified at the L4-5 or L3-4 interspace using a loss of resistance technique. A catheter was advanced 5 cm into the epidural space. The patients were positioned supine and a test dose of 3 mL of 1% lidocaine was given through the catheter, followed by 10 mL of 0.75% ropivacaine over a period of 5 min.

Starting at the end of the local anesthetic injection, Ts and sensory changes were measured every 5 min for 45 min. Ts measurements were performed using a non-contact infrared temperature probe (C600-M Biotherm Infrared Thermometer; Linear Laboratories, Fremont, Canada; sensitivity 0.1°C). Care was taken to measure Ts distant to joints and subcutaneous veins. Ts was measured at four points: the skin innervated by the sciatic nerve (plantar foot and dorsal foot) and the skin innervated by the femoral nerve (medial ankle and medial mid-tibia). Core temperature was monitored by infrared tympanic thermometry (First Temp Genius®; Sherwood Medical Industries Ltd, Sussex, UK). Room temperature was maintained at 23°C ± 0.9°C, and relative humidity was approximately 28%.

Pinprick sensation was assessed by a needle wheel, cold perception by a swab soaked with alcohol, touch perception by fine brush, and vibration sense by tuning fork. Sensory and motor functions were assessed using a 3-point scale, with 2 corresponding to normal sensation or muscle strength, 1 corresponding to a blunted sensation (compared with an unblocked extremity) or impaired motor function, and 0 corresponding to absence of sensation or motor function.

All data are presented as means and confidence intervals (95%). Calculations were made with SPSS version 12.0 (SPSS Inc., Chicago, IL). Comparisons of temperature changes from baseline were made by paired-samples Student’s t-test corrected for multiple comparisons (Bonferroni). An increase in Ts of at least 1°C was defined as a test criterion and related to the event of a successful block. Sensitivity was the ratio of the number of patients with both a Ts increase and sensory impairment over the total number of patients with sensory impairment. Specificity is the ratio of the number of patients without a Ts increase or sensory impairment over the total number of patients without a sensory impairment. Accuracy is the sum of the number of patients with a Ts increase and sensory impairment and the number of patients without a Ts increase and sensory impairment, divided by the total number of patients.

Based on clinical experience, we assumed that in 25% of peripheral blocks and 75% of EPI blocks the Ts increase preceded the impairment of sensation. To demonstrate a difference using the proportions described above and defining an = 0.05 and a power of 0.9, we enrolled 40 patients (power analysis). Fewer patients were included in the EPI group because data on Ts during EPI have been reported. Fisher’s exact test was used to evaluate the association of Ts increase and successful block and the sequence of Ts increase and sensory block between EPI and FSB. Comparisons of data between FSB and EPI were made by unpaired Student’s t-test not assuming equal variances of data and corrected for multiple comparisons (Bonferroni).

	Results

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Thirty-three patients scheduled for lower extremity surgery anesthetized by FSB were enrolled. In all cases femoral nerve block was successful, whereas sciatic nerve block was insufficient in three patients. Data from patients with unsuccessful blocks were not included in the time sequence evaluation but were used for calculation of specificity, sensitivity, accuracy, and the positive and negative predictive values.

Patient demographics are shown in Table 1. Core temperature declined 0.2°C during the FSB and 0.4°C during EPI (P < 0.05).

There was a significant difference of 1.0°C ± 0.9°C (P < 0.05) in baseline temperature between legs. After 5 min Ts of patients with a successful sciatic nerve block increased by 1.8°C ± 0.9°C (P < 0.001) at the plantar foot and by 0.7°C ± 0.5°C (P < 0.05) at the dorsal foot compared with baseline. Maximal Ts increase was 5.9°C ± 0.4°C (P < 0.001) at plantar foot and 3.8°C ± 0.5°C (P < 0.001) at the dorsal foot (Fig. 1). Ts increase preceded sensory block in 6.6% of all patients and was reached at the same time in 56.6% and followed sensory block in 36.6%. All measurements at the contralateral extremity showed a decrease of Ts over time. No sensory or motor changes were experienced by any patient in the contralateral leg.

View larger version (34K): [in this window] [in a new window]   Figure 1. Skin temperature (Ts), pinprick sensation, and motor strength examined for 45 min after injection of local anesthetic for sciatic nerve block (n = 30). Ts increased significantly after 5 min at the plantar foot (1.8°C ± 0.9°C; P < 0.001) and at the dorsal foot (0.7°C ± 0.5°C; P < 0.05). Ts at the contralateral extremity declined after 5 min (P < 0.01). Pinprick sensation declined and was impaired (intensity 1) after at least 10 min. Note that motor strength also declined to impaired status within 15 min but did not vanish completely.

In all patients with successful block, mean perceived intensity of pinprick, cold, touch, and vibration sensations declined over time (Fig. 1).

The three patients with an insufficient sciatic nerve block did not show any increase in Ts and no loss of sensation for 45 min. They underwent surgery under general anesthesia. The sensitivity, specificity, and accuracy of a Ts increase for a successful sciatic nerve block were 100% (88%–100%).

Ts at the medial ankle increased 0.5°C ± 0.3°C after 45 min (P < 0.05), whereas Ts at the medial mid-tibia did not increase (Fig. 2). No sensory or motor changes were experienced by any patient in the contralateral leg.

View larger version (27K): [in this window] [in a new window]   Figure 2. Skin temperature (Ts), pinprick sensation, and motor strength examined for 45 min after injection of local anesthetic for femoral nerve block (n = 30). Ts at medial ankle increased not until 45 min and only 0.5°C ± 0.3°C (P < 0.05). Ts at medial mid-tibia did not increase significantly after 45 min (P = 0.055). Pinprick sensation declined earlier (intensity 1 within 5 min) than after sciatic nerve block. Note that mean motor strength was impaired after 5 min and was completely blocked after 20 min.

In all patients with successful block, mean perceived intensity of pinprick, cold, touch, and vibration sensations declined over time (Fig. 2). The onset of sensory was shorter after femoral nerve block (12.3 ± 2.3 min) than after sciatic nerve block (20.3 ± 4.2 min; P < 0.01). Similarly, onset of motor block was shorter during femoral block (9.8 ± 0.9 min) than after sciatic nerve block (36.8 ± 4.5 min; P < 0.001).

Figure 3 summarizes the results during the first 25 min after 10 epidural anesthetics. Ts increased significantly within 5 min by 2.6°C ± 1.3°C (P < 0.01) at the plantar foot, whereas Ts increased 1.7°C ± 1.0°C (P < 0.05) after 10 min at the dorsal foot. Maximal Ts increase was 5.7°C ± 2.3°C (P < 0.001) at the plantar foot and 3.7°C ± 1.5°C (P < 0.01) at the dorsal foot (Fig. 3) compared to baseline. Ts increase preceded sensory block in 50% of patients. In all other patients a Ts increase and sensory block appeared at the same time. A complete sensory block and a Ts plateau were reached in each patient within 25 min.

View larger version (26K): [in this window] [in a new window]   Figure 3. Skin temperature (Ts), pinprick sensation, and motor strength assessed within 25 min after complete application of epidural anesthesia (n = 10). Ts increased within 5 min at the plantar foot (P < 0.01) and within 10 min at the dorsal foot (P < 0.05). Mean pinprick sensation declined and was impaired (intensity 1) after at least 10 min. Note that motor strength also declined to impaired status within about 15 min, but did not vanish completely.

It took 10.3 ± 2.8 min during sciatic nerve block until a 1°C increase in Ts of the plantar foot was achieved, whereas the same increase was reached within 5 min after all EPI anesthetics (P < 0.01). Loss of pinprick sensation (20.3 ± 4.2 versus 15.0 ± 4.4; P = 0.10) and loss of cold sensation (18.3 ± 4.4 versus 15.0 ± 4.4; P = 0.30) were not significantly different between sciatic nerve block and EPI, respectively.

The rate of Ts increase preceding a sufficient block was significantly more rapid after EPI (50%) than after sciatic nerve block (6.6%; P < 0.05).

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
This is the first study investigating the value of Ts increase during onset of peripheral nerve block. We demonstrated that a Ts increase is a reliable, but late, sign of successful sciatic nerve block. The Ts increase during femoral nerve block is negligible.

In contrast to Ts during EPI, as shown in Figure 3, time until Ts increase >1°C during peripheral nerve blocks is delayed. Thus, sensitivity, specificity, and accuracy of Ts increase are high, but Ts increase is not an early indicator of successful sciatic nerve block.

Another interesting finding is that the plantar foot seems to be the best place to observe Ts changes during regional anesthesia of the leg, perhaps because the glabrous skin contains the most subcutaneous blood vessels (14).

One may argue that initial block of the femoral nerve influences Ts in the area innervated by the sciatic nerve. This assumption is highly unlikely for two reasons. First, in the three cases with unsuccessful sciatic nerve block Ts did not change as in all other cases. Second, Ts in the area innervated by the femoral nerve increased only negligibly and late; thus an influence of the initial femoral nerve block on the early and large temperature changes after sciatic nerve block is hard to imagine.

An unexpected finding was the significant difference in baseline Ts at the plantar foot between legs. This might have been related to an aseptic inflammation of the diseased leg, which has been shown thermographically in orthopedic patients (17). Therefore, a Ts difference between both extremities to identify a sufficient nerve block may be misleading because the diseased leg may have an increased Ts even before the block. Thus, the relevant Ts for comparison is the baseline Ts of the same blocked leg. The increased baseline Ts of the diseased leg diminished the effect of the sciatic nerve block on Ts changes, and therefore the pure effect of the nerve block on Ts may be even higher.

The nonrandomized comparison of two different anesthetic techniques (combined nerve block versus EPI anesthesia) is a methodological problem. We favor the use of peripheral nerve blocks in patients undergoing surgery of the knee or below because of our perioperative thromboprophylaxis regimen and presumed increased risk of spinal hematoma in orthopedic patients (18,19).

The use of different local anesthetics between EPI and FSB was necessary because equal doses of the same local anesthetic used for both regional anesthesia procedures would result in a local anesthetic overdose in the EPI group or produce an incomplete FSB. Therefore, we applied a common combination of two local anesthetics for the FSB (20); these anesthetics have equal onset times during peripheral nerve block (21). Thus, it is unlikely that the divergent pattern of block development during EPI (Fig. 3) was related to the different local anesthetics.

The slower increase of Ts during the sciatic nerve block compared with EPI cannot be explained by a smaller extent of sympathetic block because the extent of the Ts increase of the leg was equal between groups. The mean Ts at the plantar foot increased from 28°C to 34°C in both groups (Fig. 1 and 3). The comparison of EPI with FSB must be interpreted carefully for methodological reasons.

Another unexpected finding of our investigation is that the pattern of onset is distinct between sciatic nerve block, femoral nerve block, and EPI. During femoral nerve block, the onset for all sensory and motor modalities was rapid, whereas during sciatic nerve block and EPI the motor block was delayed. During peripheral nerve blocks, the time sequence is presumably more dependent on the anatomy of the blocked structures than on the preference of local anesthetics for different nerve fibers. Typically (15), the femoral nerve is blocked near the site where it branches out (20). The sciatic nerve at the upper thigh is near 1 cm in the anterior posterior axis and up to 3.5 cm in the medial-lateral axis (22,23). This might explain the fast and simultaneous loss of sensory and motor function during femoral nerve block.

In conclusion, Ts is a reliable, but late, sign of successful sciatic nerve block. Therefore it is of limited clinical value. Ts changes after femoral nerve block are negligible and late.

	Footnotes

 
Accepted for publication October 28, 2005.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (6) Citing Articles via Google Scholar Google Scholar Articles by Stevens, M. F. Articles by Lipfert, P. Search for Related Content PubMed PubMed Citation Articles by Stevens, M. F. Articles by Lipfert, P. Related Collections Monitoring (Non-cardiac) Regional Anesthesia