Caudal anesthesia is one of the most popular regional blocks in children. This technique is usually performed after an inhaled or IV induction and is a useful adjunct during general anesthesia and for providing postoperative analgesia after genital, lower abdominal, and lower limb operations (1,2). The quality and level of the caudal blockade is dependent on the dose, volume, and concentration of the injected drug (3). To achieve a higher-level block, a larger volume of the local anesthetic solution is required. If the undiluted 0.25% solution of bupivacaine is used, the total dose of the drug is also increased and has the potential risk of increased toxicity. This double-blinded, prospective study compared the effect of two different volumes and concentrations of bupivacaine on the level and intensity of caudal blockade when a constant dose of 2 mg/kg of bupivacaine was used in children undergoing unilateral orchidopexy.
The IRB approved the study, and parental consent was obtained in each case. Fifty ASA status I un-premedicated children, 1 to 6 yr old, who were scheduled for unilateral orchidopexy as outpatients, were studied. To keep the study double-blinded, two separate anesthesiologists and a blinded observer were involved in each case. The same urologist performed all of the surgical procedures to maintain a uniform application of surgical stimulus. Each patient was randomly assigned to one of the two groups by following a computer-generated randomization table.
Anesthesiologist A performed the induction of general anesthesia by using oxygen, N2O, and halothane by face mask. Anesthesiologist B, who was not involved in the subsequent management of the child, performed a caudal block. Anesthesiologist A, who was unaware of the volume of local anesthetic injected into the caudal space, maintained the rest of the anesthetic by face mask. The volume of the caudal injection was adjusted so that each patient was given a constant dose of 2 mg/kg of bupivacaine. Group 1 patients received 0.8 mL/kg of 0.25% bupivacaine, or low volume/high concentration (LVHC), and Group 2 patients received 1 mL/kg of 0.2% bupivacaine (prepared by diluting 0.8 mL/kg of 0.25% bupivacaine into a 1 mL/kg solution, using preservative-free normal saline), or high volume/low concentration (HVLC). Both solutions also contained freshly added epinephrine in a concentration of 1:400,000 (2.5 μg/mL) and 0.1 mL of NaHCO3 per 10 mL of solution. The bupivacaine solutions were prepared immediately before injection. All patients were monitored with electrocardiogram, automated blood pressure device, temperature, capnography, and end-tidal gas spectrometry. Mask anesthesia was maintained in all patients by using 6 L of fresh gas flows (N2O/oxygen, 2:1) in a circle system. End-tidal gas measurement was achieved by placing the gas sampling tubing under the mask close to the lips in each patient. Recording of the end-tidal halothane concentration began 5 min after the performance of caudal blockade and every 5 min after that until the end of surgery. After the caudal blockade, there was a delay of at least 10 min before surgical incision and 15–20 min before the peritoneal traction occurred. During the first 10 min, all children were maintained at the same concentration of halothane (2%) along with N2O and oxygen (2:1). After the incision, Anesthesiologist A adjusted the inspired concentration of halothane according to the patient’s response to surgical stimulus. A uniform stimulus, consisting of digital traction on the spermatic cord during orchidopexy, was exerted by the same surgeon in each of these cases to evaluate the intensity and level of rostral spread of the caudal blockade. The surgeon, who was aware of the need to use an identical amount of traction force in each patient, used his thumb and index finger to bring the undescended testis down into the scrotal sac. In every case, the undescended testis was well palpable superficially in a low position, according to the surgeon’s evaluation. To compare the overall anesthetic depth between the two groups, minimum alveolar anesthetic concentration (MAC) minutes were calculated. The end-tidal halothane concentration was recorded every 5 min, and each value was multiplied by the number of minutes at that concentration. The expired halothane concentration was recorded at all times, including the time of the spermatic cord traction and the patient’s responses to that stimulus; i.e., tachycardia, hypertension, hyperventilation, and phonation were recorded. The inspired halothane concentration was increased to reverse these responses whenever they occurred, and this increase in halothane requirement was noted. If no response occurred during this stimulus, this was also noted. At the end of surgery, the patients were observed in the postanesthesia care unit. Recovery was assessed by the Steward recovery score (4) and pain by the Objective Pain Scale (5). Rescue fentanyl 1 μg/kg was administered for Objective Pain Scale scores of ≥6 (5). An acetaminophen suppository was given for pain in the short-stay recovery unit (SSRU).
The following data were prospectively collected by a blinded observer and compared: age (yr), weight (kg), surgical time (min), anesthesia time (min), recovery time (min), discharge time (min), halothane MAC minutes, the need to increase halothane concentration during peritoneal traction, and postoperative analgesic therapy with fentanyl and acetaminophen. Recovery time was defined as the time from when the patient entered the recovery room until the patient met Steward discharge criteria. Discharge time was defined as the time from when the patient entered the recovery room until the patient was discharged home from the SSRU. The continuous data were summarized by mean ± sd, median, and interquartile range. The continuous data were compared by using Student’s t-test and the Kruskal-Wallis test. The nominal data were compared by χ2 tests and Fisher’s exact test. A P value of <0.05 was considered statistically significant.
On the basis of the power analysis with the χ2 test and power = 0.80 (α = 0.05), a sample size of 51 per group was determined to detect a difference in proportions of increase in inspired halothane concentration, p1-p2 > 30%, when p2 = 20% in the HVLC group (Group 2). p1 is the percentage of patients requiring increase in halothane concentration in Group I and p2 is the percentage of patients requiring increase in halothane concentration in Group II. This was based on an anticipation of a percentage change in the incidence of response between the two groups. We terminated our study after studying 50 children because an unacceptably large number of children—15 (65.2%) of the 23 patients—who received the LVHC (Group 1) required an increase in inspired halothane concentration because of increased heart rate, blood pressure, and respiratory rate and phonation during spermatic cord traction, as opposed to the other group (HVLC; Group 2).
The two-sample Student’s t-test based on equal group size (n = 23;Table 1) will detect differences of size 0.85 sd at α = 0.05 and β = 0.20 (power = 0.80) if the Gaussian assumptions are met. The Kruskal-Wallis test will detect the same difference at the same significance level and a power more than 0.80, if the Gaussian assumptions needed for the Student’s t-test are unmet. Because the response variables are times to events and, hence, are non-Gaussian, we used the nonparametric Kruskal-Wallis test of significance. As per Fisher’s exact test (χ2 test) with equal group since (n = 23, n = 27, Table 1) it will detect the ratio p1/p2 = 2.6, (p2 = 25%) at α = 0.05 and β = 0.20 (power = 0.80) if the binomial distribution assumptions are met. Because Group 1 had a sample size of 23, we used Fisher’s exact test.
A total of 50 patients were studied; 23 in Group 1 (LVHC) and 27 in Group 2 (HVLC). There were no significant differences (P > 0.05) between the two groups with regard to their age (yr), anesthetic time (min), discharge time (min), halothane MAC minutes, and postoperative analgesic therapy with fentanyl, acetaminophen, or both (Table 1). In the patients who received the LVHC (Group 1), 15 (65.2%) of the 23 patients had a 15%–20% increase in heart rate, blood pressure, or respiratory rate from baseline and had an incidence of phonation requiring an acute increase in the concentration of halothane. In the patients who received the HVLC (Group 2), only 8 of the 27 patients had a 15%–20% increase in heart rate, blood pressure, or respiratory rate from baseline along with an incidence of phonation. These patients (29.6%) in Group 2, therefore, required less frequent increases in the inspired concentration of halothane during spermatic cord traction as compared with those who received the LVHC (Group 1) (P = 0.022). In the recovery room, there was no significant difference in rescue treatment between the two groups: four (17.4%) of the children in Group 1 required rescue treatment with fentanyl as compared with two (7.4%) of the children in Group 2 (P = 0.372). There was no difference in the need for acetaminophen treatment in the SSRU. There was no difference in the ability to ambulate between the two groups, and there was no incidence of prolonged stay secondary to motor weakness in any patient.
Caudal anesthesia with bupivacaine 0.25% in conjunction with light general anesthesia provides excellent intraoperative and postoperative analgesia in children undergoing genitourinary procedures (1). There are several formulae for determining the appropriate dose of caudal anesthesia in children. The variables determining the quality and level of caudal blockade are the volume, dose, and concentration of the injected drug. The rostral spread of caudal analgesia depends on the volume of local anesthetic injected according to Armitage (3). Bupivacaine 0.25% in volumes of 0.5, 1, and 1.25 mL/kg will provide analgesia to sacral, lower thoracic, and midthoracic dermatomes, respectively. When this formula results in a volume of local anesthetic more than 20 mL, Armitage further suggested decreasing the bupivacaine concentration to 0.2% to prevent the unpleasant sensation of lower limb weakness or heaviness.
We traditionally used 0.8 mL/kg of 0.25% bupivacaine for caudal analgesia and found that this volume was often inadequate to block the spermatic cord traction response during orchidopexy. Our goal was to use a larger volume of a smaller concentration to keep the total mass of drug within a safe limit. Bupivacaine 0.125 was considered. Wolf et al. (6) compared postoperatively administered caudal blocks with three different concentrations of bupivacaine: 0.0625%, 0.125%, and 0.25%. This study demonstrated that children who received caudal blocks performed with 0.125% bupivacaine at the completion of surgery had postoperative analgesia that was just as effective as that produced by 0.25% bupivacaine, but with less evidence of motor weakness. Rice et al. (personal communication, 2002) showed that the intraoperative analgesia provided by 0.125% bupivacaine was indistinguishable from that provided by 0.25%. The advantage of using 0.125% bupivacaine was that all patients were able to ambulate and meet discharge criteria earlier than the children whose caudal blocks were performed with the more concentrated solution. The study by Gunter et al. (7), however, showed that children receiving 0.2% bupivacaine had lower pain scores on arrival to the postanesthesia care unit than those receiving 0.125% bupivacaine. This is the reason we chose 0.2% instead of 0.125% to compare with the traditional concentration of 0.25% bupivacaine. This allowed us to keep the drug mass the same while producing a 20% increase in volume.
The pharmacokinetic variables of bupivacaine after caudal anesthesia in children are similar to those in adults, except for a large volume of distribution and a slightly longer terminal half-life in children (8). Plasma bupivacaine concentrations larger than 4 μg/mL have been postulated to carry a risk of producing convulsions (9). In a study of 45 children (4 months to 12 years old), Eyres et al. (10) demonstrated that caudally administered 0.25% bupivacaine in a dose of 3 mg/kg produces plasma levels well below the toxic limits (1.2–1.4 μg/mL). Despite this safety, the potential for inadvertent IV and/or intraosseous injection of the drug cautions one to use the smallest possible bupivacaine dose during caudal blockade.
Testicular innervation is derived from the aortic and renal plexuses and sympathetic fibers connecting to the T10 and T11 segments of the spinal cord through the thoracic splanchnic nerves. Postoperative analgesia, therefore, can be accomplished by a T10-level block. However, during orchidopexy surgery, a higher level of blockade (up to T4) may be necessary to block the peritoneal stimulation arising from spermatic cord traction. In our clinical experience, a caudal injection volume of 0.75 mL/kg of 0.25% bupivacaine does not provide adequate blockade of the peritoneal stimulation during spermatic cord traction. This is evidenced by the patient’s phonation, hyperventilation, and increase in blood pressure and heart rate, requiring the general anesthetic level to be increased. This response carries the risk of inducing laryngeal spasm. Increasing the volume of 0.25% bupivacaine to 1 mL/kg blocks the signs of the peritoneal stimulation but may result in prolonged motor blockade of the lower extremities. This is especially marked when epinephrine is added to the bupivacaine solution. Use of a more dilute solution avoids this problem.
The fact that there was no difference in postoperative pain between the two groups in our study is not surprising. The response to spermatic cord traction is a transient stimulus due to peritoneal traction (T4 to T6). Postoperative discomfort is due to skin incision and possible testicular irritation, which is blocked by a T10-level block.
The addition of epinephrine and sodium bicarbonate to the bupivacaine solution, as we did in our study, is controversial. Bromage (11) has stated that the addition of epinephrine to bupivacaine will improve the quality of blockade. Epinephrine increases the duration of caudal analgesia in older infants and children (12). Gunter et al. (personal communication, 2002) studied the effect of adding epinephrine 1:200,000 to bupivacaine in conscious infants and found that epinephrine prolonged the duration of caudal analgesia. Fisher et al. (13) looked at the effect of epinephrine in producing a delay in voiding and in enhancing caudal analgesia. In their study, although the caudal block with epinephrine-containing solution did prolong the time to postoperative voiding, the delay was not clinically significant.
Alkalinization of local anesthetics for regional anesthesia has been of interest to clinicians as a means of improving the quality and onset of neural blockade (14). We mixed 0.1 mL of NaHCO3 with 10 mL of bupivacaine to accelerate the onset of the local anesthetic block in all patients. Alkalinization of bupivacaine has been suggested as a method to produce a more rapid onset of caudal anesthesia, although this has not been studied in an objective way because of the difficulty in determining the onset of the sensory block in anesthetized children.
In conclusion, our results suggest that in children undergoing orchidopexy, a caudal block with a larger volume of dilute bupivacaine is more effective than the smaller volume of the more con-centrated solution in blocking the peritoneal stimulation during spermatic cord traction, without compromising the quality of postoperative analgesia.
The authors thank Deirdre Savoy, Graphics Presentation Specialist, Department of Anesthesiology, Children’s National Medical Center, for technical assistance.
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