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A Comparison of High Volume/Low Concentration and Low Volume/High Concentration Ropivacaine in Caudal Analgesia for Pediatric Orchiopexy

Hong, Jeong-Yeon, MD*; Han, Sang W., MD; Kim, Won O., MD*; Cho, Jin S., MD*; Kil, Hae K., MD*

doi: 10.1213/ane.0b013e3181b20c52
Pediatric Anesthesiology: Case Report
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Chinese Language Editions

BACKGROUND: It is unclear whether the volume or concentration of local anesthetic influences its spread and quality of caudal analgesia when the total drug dose is fixed.

METHODS: We performed this study in a prospective, randomized, observer-blind manner. Children aged 1–5 yr received a constant dose of 2.25 mg/kg of ropivacaine prepared as either 1.0 mL/kg of 0.225% (low volume/high concentration [LVHC], n = 37) or 1.5 mL/kg of 0.15% solution (high volume/low concentration [HVLC], n = 36). Both solutions contained radiopaque dye.

RESULTS: The median spread levels with ranges in the HVLC group (confirmed by fluoroscopic examination) were significantly higher (T6, T3-11) than in the LVHC group (T11, T8-L2). There were no significant differences in recovery times, postoperative pain scores, or side effects between the two groups. After discharge, fewer children in the HVLC group required rescue oral acetaminophen compared with the LVHC group (50.0% vs 75.7%). First oral acetaminophen time was found to be significantly longer with HVLC patients than LVHC patients (363.0 min vs 554.5 min).

CONCLUSIONS: We confirmed (with fluoroscopy) that a caudal block with 1 mL/kg ropivacaine spreads to T11 and to T6 with 1.5 mL/kg. If the total dose is fixed, caudal analgesia with a larger volume of diluted ropivacaine (0.15%) provides better quality and longer duration after discharge than a smaller volume of more concentrated ropivacaine (0.225%) in children undergoing day-case orchiopexy. The spread level of ropivacaine correlated significantly with the first oral acetaminophen time after discharge.

From the *Department of Anesthesiology and Pain Medicine, Anesthesia and Pain Research Institute; and †Department of Urology, Urological Science Institute, Yonsei University College of Medicine, Seoul, Korea.

Accepted for publication May 26, 2009.

Address correspondence and reprint requests to Hae Keum Kil, MD, Department of Anesthesiology and Pain Medicine, Severance Hospital, 134 Sinchon-dong, Seodaemun-gu, Seoul 120-752, Korea. Address e-mail to hkkil@yuhs.ac or jenyhongg@yuhs.ac.

Caudal anesthesia is one of the most commonly used regional blocks for postoperative analgesia in pediatric surgery.1 Although it is a versatile block, one major limitation of the single-injection technique is the relatively short duration of analgesia.1 The quality and level of the block are dependent upon dose, volume, and concentration of the local anesthetic solution. By increasing the volume of local anesthetics, a higher initial level of blockade might extend the analgesic duration.

Until now, it has been unclear whether the volume or concentration of local anesthetic influences the spread and quality of caudal analgesia when the total drug dose in fixed.2,3 In children, limited data regarding the relative effects of volume and concentration of local anesthetic solutions used for caudal analgesia are available, and the conclusions are still under debate.1,4,5 In addition, no study has assessed the spread of local anesthetics and its related analgesic duration.

We conducted a randomized, prospective, observer- blinded study to demonstrate the spread of epidurally administered local anesthetic using fluoroscopic examination and to compare analgesic durations between high volume/low concentration (HVLC) and low volume/ high concentration (LVHC) ropivacaine when a constant dose was administered to children undergoing day-case unilateral orchiopexy.

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METHODS

The IRB of our institution approved this study, and parental consent was obtained for each patient. Seventy-three ASA physical status I unpremedicated children, aged 1–5 yr, who were scheduled for day-case unilateral orchiopexy were enrolled in this prospective, randomized, observer-blinded study. Patients were excluded from the study if they had a history of allergic reactions to any local anesthetics, bleeding diathesis, infections at the puncture sites, or preexisting neurological disease. Two anesthesiologists and a blinded observer were involved with each patient. The same urologist performed all surgical procedures. Each patient was randomly assigned to one of the two groups with the use of a computer-generated randomization table. On the day of the preanesthetic visit, parents were taught to use the visual analog pain score system (VAS, 0 = “no pain” and 10 = “the worst imaginable pain”). Standard monitoring was applied, and anesthesia was induced with 8 vol% of sevoflurane in 100% oxygen by an anesthesiologist who was unaware of the patient's group assignment. Tracheal intubation was performed without muscle relaxants, and mechanically controlled ventilation was used to maintain end-tidal carbon dioxide at 35 ± 5 mm Hg. After induction of anesthesia, a caudal block was performed by a second anesthesiologist who was not involved in the subsequent management of the patient. With the patient in the lateral position, a 5 cm short beveled 22-gauge caudal needle was inserted after measuring the optimal angle using ultrasonography (LOGIQe™, GE Healthcare, Wauwatosa, MI).6 After identifying the caudal space (using the loss-of-resistance technique with saline), children received a fixed dose of freshly prepared ropivacaine, either 1.0 mL/kg of 0.225% (LVHC, n = 37) or 1.5 mL/kg of 0.15% (HVLC, n = 36) containing diluted radiopaque dye of 75 mg/mL (Omnipaque™, GE Healthcare, County Cork, Ireland). Drugs were injected slowly at a rate of 1 mL/3 s. The patient was then placed in the supine position for fluoroscopic examination. Fluoroscopic images of the lumbo-sacral or thoracic-sacral area were saved, and the spread levels were determined by an anesthesiologist blinded to the protocol and their determinations were confirmed by an independent radiologist.

Surgery was allowed to begin 10 min after performing the block. Peripheral oxygen saturation, heart rate, and noninvasive arterial blood pressure were monitored and recorded throughout the surgery. All measurements were recorded at 5-min intervals. End-tidal sevoflurane was adjusted according to clinical variables (arterial blood pressure or heart rate within 20% of baseline or absence of responses such as movement, tearing, or sweating). The minimum alveolar anesthetic concentration-hour was calculated.

After emerging from anesthesia, patients were managed by an observer blinded to group allocation in the recovery room (RR). Postoperative pain was assessed using the Children's Hospital of Eastern Ontario Pain Scale (CHEOPS).7 Rescue fentanyl (0.5μg/kg) was administered if two coupled observations separated by a 5-min waiting period yielded CHEOPS ≥4. Motor blocks were assessed with a modified Bromage score (0: no motor block, 1: able to move legs, and 2: unable to move legs).8 Postoperative sedation was evaluated using 8-point modified Ramsay Sedation Scale.9

Recovery times were defined as the time from when the patient entered the RR to when they could drink water, void, and were discharged. The discharge criteria for children included clear consciousness, stability of vital signs, ability to tolerate oral fluids and void, age-appropriate level of ambulation, and absence of side effects. A telephone interview with the patient's parents 24 hr later assessed the demands for additional analgesics. The interviewer, who was blinded to the treatment group, documented these data in the patient's medical record. The time to first supplemental oral acetaminophen (first acetaminophen time) was defined as the time from the end of surgery to the first registration of a VAS (0–10) ≥4 by parent's observation.10

Sample size calculation was based on the data published by Verghese et al.,4 who found the incidence of postoperative rescue analgesics to be 90.3% and 66.7% in children who received caudal analgesia in LVHC (0.25% bupivacaine, 0.8 mL/kg) and HVLC (0.2% bupivacaine, 1.0 mL/kg), respectively. Thirty-one patients in each group allowed an α of 0.05 and β of 0.1. Power analysis for the incidence of rescue analgesics was also performed based on a similar study by Silvani et al.1 They demonstrated that first rescue analgesics were needed after 520 min in the LVHC group (0.375% ropivacaine, 0.5 mL/kg) and 952 min in the HVLC group (0.1% ropivacaine, 1.8 mL/kg) after hypospadia repair. The calculated sample size was at least 27 in each group using an α value of 0.05 and power of 0.9. Seventy-three patients were enrolled for protocol omissions. Differences between the groups were analyzed using Student's t-test, Mann–Whitney rank sum test, χ2 test, and Fisher's exact test when appropriate. A linear regression analysis was used to determine whether epidural spread level of ropivacaine is associated with first oral acetaminophen time after discharge. A P value <0.05 was considered significant.

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RESULTS

Seventy-three children were studied, of which 37 were administered LVHC and 36 were administered HVLC. There were no significant differences between the two groups with regard to age, weight, height, duration of anesthesia and surgery, and fluid administration during surgery (Table 1). There were no caudal block failures in any patient.

Table 1

Table 1

The spread levels (median with ranges) of ropivacaine confirmed by fluoroscopic examination (Fig. 1) were T11 (T8–L2) in the LVHC group and T6 (T3–11) in the HVLC group (Fig. 2, P = 0.010).

Figure 1.

Figure 1.

Figure 2.

Figure 2.

Differences in intraoperative hemodynamic changes between the two groups were not statistically significant (Fig. 3). The minimum alveolar anesthetic concentration-hours of the two groups were similar. No patient received additional analgesics intraoperatively.

Figure 3.

Figure 3.

In the RR, there were no significant differences in recovery profiles between the two groups (Table 2). There were no patients who needed rescue analgesics in the RR. There were no significant differences in the incidences of vomiting, flushing, deep sedation, or motor block. However, three patients in the LVHC group showed modified Bromage scale scores of 3 and had a prolonged stay after emergence. No children experienced motor block in the HVLC group. CHEOPS assessed at 30-min intervals in the RR showed similarities within the two groups (Fig. 4).

Table 2

Table 2

Figure 4.

Figure 4.

After discharge, more children who received LVHC required rescue oral acetaminophen compared with those receiving HVLC (Table 2). The first oral acetaminophen time was significantly longer with HVLC patients than LVHC patients. The spread level of ropivacaine correlated significantly with the first oral acetaminophen time after discharge in both the groups (Fig. 5).

Figure 5.

Figure 5.

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DISCUSSION

The results of this study demonstrated that caudal administration of HVLC of ropivacaine provided longer postoperative analgesia compared with LVHC. We also noted that LVHC increased the spread of ropivacaine from T11 to T6 and that there was a significant correlation between the spread of ropivacaine and the analgesic duration.

The two principal dosing variables for local anesthetics are the concentration and the volume of solution administered. In children, few studies have evaluated the quality and duration of caudal blocks against concentration and volume of the local anesthetic. However, these previous reports had conflicting results. Silvani et al.1 found that caudal blocks with a HVLC regimen produced prolonged analgesia and fewer side effects compared with a LVHC regimen in children undergoing hypospadia repair. Their results were similar to those of our study, but 0.375% of ropivacaine was not the usual concentration for children and the incidence of motor weakness was more than 50%. In addition, the total dose of ropivacaine was not taken into consideration. Verghese et al.4 demonstrated that a caudal block with HVLC is more effective than LVHC in blocking the peritoneal response during spermatic cord traction during orchiopexy. They performed a caudal block with a fixed dose of 0.2% and 0.25% bupivacaine, but only produced a 20% difference in volume. Furthermore, they could not differentiate postoperative analgesic duration because of the addition of epinephrine and sodium bicarbonate to their solutions. On the other hand, Schrock and Jones5 observed that even increasing the total dose by increasing the volume of a fixed concentration (0.175%) of bupivacaine did not extend the duration of caudal analgesia in children undergoing inguinal herniorraphy.

Caudal analgesia regresses from the site of lowest anesthetic concentration distal from the injection and caudally toward the highest concentration.8 This model of regression could be explained by diffusion and uptake into the surrounding vasculature, with the effect at each dermatome diminishing as the minimal concentration for action is reached. The pharmacokinetic variables of ropivacaine after caudal administration in children are well established. Karmakar et al.11 demonstrated that caudally administered 0.2% ropivacaine at a dose of 2 mL/kg produced a peak plasma concentration of 1.12 μg/mL, which is well below the toxic limits (2.2 μg/mL) in children aged 1–7 yr. There was no clinical evidence of toxicity in any of our children, but one must bear in mind the effects of general anesthesia, which can mask signs of central nervous system toxicity. No data regarding plasma concentrations of ropivacaine are available for comparing different volumes and concentrations with a fixed dose, and further investigation is needed.

The required volume of caudal analgesic has been evaluated by several authors. Volume per kilogram dosage produces a tight linear correlation with the number of dermatomes anesthetized.12 According to the Armitage formula,13 we usually used 0.5–1.25 mL/kg of 0.2% ropivacaine for caudal analgesia for pediatric urologic surgery, as seen in several previous reports.14–16 Our goal was to use a larger volume with a smaller concentration to keep the total dose of the drug within a safe limit, thus 0.225% (3 mL of 0.75% ropivacaine added to 7 mL of saline) and 0.15% (2 mL of 0.75% ropivacaine added to 8 mL of saline) ropivacaine were considered safe. This dosage allowed us to keep the drug mass the same while producing a 50% increase in volume. The level of block was determined by fluoroscopic examination in individual patients of this study, and we confirmed that a 50% increase in the injectate volume from 1.0 to 1.5 mL/kg produced five segments higher level of block from T11 to T6 and that the level of caudal block correlated significantly with analgesic duration. Of significance, we also observed a wide range of spread levels for both the groups. Larger injected volumes into the epidural space spread out not only along the longitudinal axis of the epidural space but also into the lateral intervertebral foramina.17,18

Our results also demonstrated that there were no differences in arterial blood pressures and heart rates between the two groups despite the significantly higher spread level of the HVLC compared with the LVHC. The mechanism for this lack of hemodynamic sympathectomy was postulated to be the immaturity of the sympathetic nerve system. In addition, it is also possible that the smaller blood volume that is present in the lower extremities of a young child compared with that of an adult may account for less venous pooling and therefore less hemodynamic change.19

One of the major end points of this study, the time to first oral acetaminophen, represents the parent's subjective impression of the child's pain. As oral acetaminophen was first administered after discharge, parents were frequently the sole assessor of their child's analgesic requirements. Although parental assessment of pain has not been well studied, we used observer VAS measures of pain to determine the need for rescue analgesic after discharge. A number of studies have provided varying levels of support for the validity of CHEOPS for the assessment of pain in postoperative children. However, as a consequence of the tight observational and recording intervals, and the numerous types of behavior, evaluating pain is burdensome for the parent. Furthermore, Beyer et al.20 found that CHEOPS scores were generally low after discharge from the RR and that over time, self-reports of pain worsened. Thus, CHEOPS may be valid only during the immediate postoperative period. Tarbell et al.10 also noted that the strong correlation between CHEOPS and observer VAS measures of pain might mean it is more practical to use VAS.

In conclusion, we confirmed that the spread level was T11 (T8-L2) with 1 mL/kg of 0.225% ropivacaine and T6 (T3-11) with 1.5 mL/kg of 0.15% ropivacaine. If the total dose is fixed, a caudal block with HVLC ropivacaine provides a longer analgesic duration after discharge than LVHC ropivacaine in children undergoing day-case orchiopexy. The analgesic duration does depend on the spread level of ropivacaine in pediatric caudal blocking.

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