Anesth Analg 2002;94:1465-1468
© 2002 International Anesthesia Research Society
PEDIATRIC ANESTHESIA
The Minimum Local Anesthetic Concentration of Ropivacaine for Caudal Analgesia in Children
Xiao-Ming Deng, MD,
Wen-Jing Xiao, MD,
Geng-Zhi Tang, MD,
Mao-Ping Luo, MD, and
Kun-Ling Xu, MD
Department of Anesthesiology, Plastic Surgery Hospital, Chinese Academy of Medical Sciences, Peking Union Medical College, Beijing, China
Address correspondence and reprint requests to Wen-Jing Xiao, Department of Anesthesiology, Plastic Surgery Hospital, Chinese Academy of Medical Sciences, Peking Union Medical College, Beijing, China, 100041. Address e-mail to wenjingxiao{at}hotmail.com
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Abstract
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Ropivacaine is a new long-acting amino-amide local anesthetic. The concentrations of ropivacaine used in caudal analgesia range from 0.1% to 0.5%. The purpose of this study was to determine the minimum local analgesic concentration of ropivacaine to provide caudal analgesia in children. In a prospective, randomized, double-blinded clinical study, we studied 26 ASA grade I patients aged 1 to 5 yr who were scheduled for hypospadias operation under general anesthesia with caudal ropivacaine analgesia. General anesthesia was maintained with an end-tidal enflurane concentration of 0.8% in 100% oxygen (0.5 minimum alveolar anesthetic concentration [MAC]). Each child received 1 mL/kg of ropivacaine solution through a caudal catheter. The first child received ropivacaine 0.2%, and subsequent concentrations were determined by the analgesic response of the previous patient to the initial skin incision by use of Dixon’s up-and-down sequential allocation. The testing interval was set at 0.025%. The minimum local analgesic concentration of ropivacaine for caudal analgesia under general anesthesia with 0.5 MAC enflurane was 0.11% (95% confidence interval [CI], 0.09%–0.12%), and the 95% effective concentration was 0.13% (95% CI, 0.12%–0.21%). In conclusion, the minimum local analgesic concentration of ropivacaine to provide effective caudal analgesia in children under general anesthesia with 0.5 MAC enflurane was found to be 0.11% (95% CI, 0.09%–0.12%).
IMPLICATIONS: Ropivacaine is a new long-acting amino-amide local anesthetic. The concentrations of ropivacaine used in caudal analgesia ranged from 0.1% to 0.5%. This study determined the minimum local analgesic concentration of ropivacaine to establish effective caudal analgesia under general anesthesia with 0.5 minimum alveolar anesthetic concentration enflurane in children.
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Introduction
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Caudal block is one of the most commonly performed regional anesthetic techniques in small children (1,2). Caudal block is usually placed after the induction of general anesthesia and is used to provide adjunct intraoperative anesthesia as well as postoperative analgesia in children undergoing surgical procedures below the level of the umbilicus. Caudal analgesia can reduce the amount of inhaled and IV anesthetic administration, attenuate the stress response to surgery, facilitate a rapid, smooth recovery, and provide good immediate postoperative analgesia (3,4).
Ropivacaine, a long-acting amide local anesthetic related structurally to bupivacaine, has been used for pediatric caudal anesthesia (5). The concentrations of ropivacaine used in caudal analgesia range from 0.1% to 0.5% (5–8), and the optimal dosage regimen of ropivacaine has not been determined. The minimum local analgesic concentration (MLAC) of local anesthetics has been developed to serve as a benchmark for epidural dosing in labor (9). We performed a prospective, randomized, double-blinded study to determine the MLAC of ropivacaine to establish caudal analgesia with light general anesthesia in children undergoing hypospadias repair.
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Methods
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After obtaining approval from the Hospital Ethics Committee and written parental informed consent, we enrolled 26 children, aged 1 to 5 yr, ASA physical status I, who were scheduled to undergo elective hypospadias repair. All were scheduled to receive general anesthesia combined with caudal ropivacaine analgesia. Children with coagulopathy, neuromuscular disease, local skin infections of the caudal area, or spinal diseases were excluded. All children were premedicated with scopolamine 0.01 mg/kg and midazolam 0.15 mg/kg IM, approximately 30 min before the operation. Anesthesia was induced via face mask with enflurane and 60% nitrous oxide in oxygen. After placement of an IV cannula, propofol 2 mg/kg and succinylcholine 1 mg/kg were given to facilitate orotracheal intubation. All children were allowed to breathe spontaneously with manual assistance after the induction. The anesthesia was maintained with an end-tidal enflurane concentration of 0.8% in 100% oxygen (0.5 minimum alveolar anesthetic concentration [MAC] from our previous trial). Intraoperative fluid management comprised lactated Ringer’s solution at a rate of 5–10 mL · kg-1 · h-1. After the anesthetic induction, the patients were placed in the left lateral position, and a caudal block was performed. Under sterile conditions, a 22-gauge IV catheter with an inner stylet was inserted through the sacrococcygeal ligament into the caudal space. The stylet was then removed, and the catheter was advanced 0.5–1 cm and kept in place for ropivacaine administration and possible rescue analgesia. After negative aspiration of blood or cerebrospinal fluid, each child received ropivacaine 1 mL/kg (Naropin; Astra Pharmaceuticals, NSW, Sweden) diluted with 0.9% wt/vol saline to achieve the desired dose at room temperature. The first child received ropivacaine 0.2%, which is a common concentration used during caudal analgesia. Thereafter, the concentration of ropivacaine received by a particular subject was determined by the analgesic response of the previous patient to the initial skin incision, according to Dixon’s up-and-down sequential method (10). The testing interval was set at 0.025%. Skin incision began at 20 min after the caudal block. An independent, blinded observer recorded noninvasive mean arterial blood pressure (MAP) and heart rate (HR) just before and after surgical incision and every 5 min thereafter until anesthesia was discontinued. After the skin incision, the children were observed for at least 1 min for signs of gross purposeful muscular movement and hemodynamic stability. The assessment was performed by an observer who was blinded to the concentration tested. Three outcomes were possible:
- 1. Effective: This required the absence of any gross purposeful muscular movement and the absence of an increase in MAP or HR of more than 20% compared with baseline values obtained just before the surgical incision, with end-tidal enflurane concentration maintained at approximately 0.8% (0.5 MAC). Effective analgesia directed a decrement of 0.025% ropivacaine concentration for the next child.
- 2. Ineffective: This followed the signs of gross purposeful muscular movement and an increase in MAP or HR of more than 20% compared with the baseline values obtained just before the surgical incision. Rescue analgesia was offered with 0.5 mL/kg of lidocaine 1% through the caudal catheter. Ten minutes after the caudal rescue, skin incision restarted, and an effective analgesia indicated the end of the study and directed a 0.025% ropivacaine increment for the next child.
- 3. Failure: No caudal analgesia because of technical failure. This directed that the same concentration be repeated for the next child.
After the assessment, anesthesia was maintained with 0.5–1 MAC enflurane in 100% oxygen. If the patient responded to the operation with an increase in MAP or HR, IV fentanyl 1 µg/kg was administered. Postoperative pain was assessed, and IV morphine or oral tramadol was offered at the discretion of the anesthesiologist. Because of the variation in the concentration of ropivacaine used in this study, no attempt was made to assess the postoperative analgesia effects of caudal ropivacaine.
Demographic data were collected and are presented as mean (range) and median (interquartile range) as appropriate. The up-and-down sequences were analyzed by probit test, which enabled MLAC with 95% confidence limits of the mean to be derived. We also analyzed our data by a logistic regression test to obtain the probability of no movement versus the concentration of ropivacaine. Analyses were performed with the following software: Microsoft Excel 97 (Microsoft Inc., Redmond, WA) and SPSS for Windows 10.0 (SPSS Inc., Chicago, IL).
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Results
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Of the 26 children entered into this study, one was rejected because of subcutaneous injection, leaving 25 for analysis. Their mean age was 3.7 yr (1–5 yr), mean weight was 15.5 kg (10–25 kg), and mean height was 102 cm (80–125 cm). The mean surgery duration was 91.6 min (45–155 min). The sequences of effective and ineffective caudal analgesia are shown in Figure 1. Curves constructed on the basis of logit analysis of responses from these patients (Fig. 2) revealed that the MLAC of ropivacaine for caudal analgesia under general anesthesia with 0.5 MAC enflurane was 0.11% (95% confidence interval [CI], 0.09%–0.12%), and the 95% effective concentration (EC95) was 0.13% (95% CI, 0.12%–0.21%). The children with effective caudal analgesia did not need fentanyl during the operation.
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Figure 1. The sequential responses of 25 children to incision with the up-and-down method. The testing interval of ropivacaine concentration was 0.025%.
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Figure 2. Dose-response curve for ropivacaine plotted from probit analysis. The minimum local analgesic concentration of ropivacaine for caudal analgesia was 0.11% (95% confidence interval, 0.09%–0.12%), and the 95% effective concentration was 0.13% (95% confidence interval, 0.12%–0.21%).
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Discussion
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The MAC is the most widely used measure of anesthetic potency for volatile anesthetics. The use of the MLAC for epidural local anesthetics also has been established as a research tool in the investigation of analgesia requirements in labor (9). The aim of this study was to devise a clinical model to determine the EC50 of local anesthetics of children during caudal analgesia. In this study of patients aged one to five years, the ropivacaine MLAC for caudal analgesia under general anesthesia with 0.5 MAC enflurane during the operation was 0.11% (95% CI, 0.09%–0.12%), and the EC95 of ropivacaine was 0.13% (95% CI, 0.12%–0.21%). Although EC95 is usually more relevant clinically, EC50 would provide a more sensitive research tool because of the respective positions of each on the cumulative concentration-response curve. EC50 corresponds to the inflection point where the slope is largest.
Ropivacaine use has been increasing in caudal analgesia in pediatric practice with its potential to produce differential neural blockade with less motor block and reduced cardiovascular and neurological toxicity (11). Since the introduction of ropivacaine for use in adult patients, a number of clinical studies have been published regarding the use of this new local anesthetic in children (3,5–8,12).
The most important variables that determine the effectiveness of caudal analgesia for a specific local anesthetic are the volume of solution and the concentration of local anesthetic solution (13). From a practical point of view, 1 mL/kg is the most common volume of local anesthetic solution used for pediatric operation below T10-level analgesia. However, the concentration of ropivacaine used in pediatric caudal blockade ranged from 0.1% to 0.5% (5–8). If the concentration of local anesthetic is too diluted, caudal block may not provide effective analgesia. Increasing the concentration of the local anesthetic would probably block fibers more satisfactorily, but this would also increase the total dose administered and, thus, the potential for systemic toxicity. The optimal concentration of a local anesthetic should provide effective analgesia and few side effects. The estimate of the MLAC of local anesthetics would be helpful in determining the optimal concentration.
Toxic drug complications from local anesthetics have been reported after caudal blockade. Although the result of the MLAC may reduce the local anesthetic dosage administered in caudal analgesia, efforts to minimize the risks of complications during caudal anesthesia must also be directed toward those measures that reduce accidental IV, intraosseous, or intrathecal injections (14).
Determination of the MLAC of local anesthetics may be affected by IV or inhaled anesthetics. In this study, the premedication with midazolam and scopolamine and the anesthesia induction with propofol and inhaled anesthetic enflurane all would reduce the MLAC result.
This is the first report of an estimate of MLAC for caudal ropivacaine in children. The MLAC of ropivacaine for caudal analgesia under general anesthesia with 0.5 MAC enflurane during an operation was 0.11% (95% CI, 0.09%–0.12%), and the EC95 was 0.13% (95% CI, 0.12%–0.21%).
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Accepted for publication January 16, 2002.
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Abstract
Introduction
