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Anesthesia & Analgesia:
doi: 10.1097/00000539-200205000-00019
PEDIATRIC ANESTHESIA: Special Article

Testing Anal Sphincter Tone Predicts the Effectiveness of Caudal Analgesia in Children

Verghese, Susan T. MD; Mostello, Lucille A. MD; Patel, Ramesh I. MD; Kaplan, Richard F. MD; Patel, Kantilal M. PhD

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Author Information

Departments of Anesthesiology and Pediatrics, Children’s National Medical Center and George Washington University, Washington, DC

Presented in part at the annual meetings of the International Anesthesia Research Society, Honolulu, HI, March 11, 2000.

December 28, 2001.

Address correspondence and reprint requests to Susan T. Verghese, MD, Department of Anesthesiology, Children’s National Medical Center, 111 Michigan Ave., NW, Washington, DC 20010. Address e-mail to sverghes@cnmc.org.

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Abstract

In this study, we examined the effectiveness of caudal blocks and correlated it with the laxity of the patients’ anal sphincter before emergence from anesthesia in 178 children undergoing inguinal and/or penile surgery. Bupivacaine 0.25% in a volume of 0.6–1.25 mL/kg was used in all patients. The presence of a lax anal sphincter at the end of surgery correlated significantly with the reduced administration of narcotics intraoperatively and in the postanesthesia care unit (P < 0.001). The sensitivity of the sphincter tone test was 98.1% with a 95% confidence interval (CI) ranging from 94.3% to 99.6%. The specificity of the test was 94.4% with a 95% CI of 72.0%–100%. The positive predictive value of this test in predicting adequate caudal block was excellent (99.4%) with a 95% CI of 96.1%–100%. The negative predictive value was better than average (85%) with a 95% CI of 62.9%–95.4%. We conclude that a lax anal sphincter can predict the effectiveness of analgesia after pediatric caudal blockade. A tight sphincter may suggest the need to repeat the block before the child awakens, or consider alternate methods of postoperative analgesia.

Several studies have shown the superiority of caudal blockade compared with IV opioids in allowing patients early discharge home after ambulatory surgical procedures (1–4). Although the block is easily performed, the success is <100%, especially in inexperienced hands.

Because most caudal blocks in children are performed while the child is anesthetized, it is not possible to assess the effectiveness of the block by testing for sensory levels. Intraoperative effectiveness of the block is often assessed by the ability to decrease the concentration of inhaled anesthetics without the use of narcotics. Our postanesthesia care unit (PACU) nurses have observed that patients who manifest a lax anal sphincter (LAS) when acetaminophen suppositories are inserted appear to be awake, calm, and pain-free, suggesting a correlation between the success of caudal blockade and anal sphincter laxity. In this study, we prospectively correlated the intraoperative assessment of caudal blockade, perioperative narcotic requirements, and postoperative pain, with the laxity of the anal sphincter in children undergoing inguinal and/or urologic (penile) surgery. Our hypothesis was that a LAS can predict the effectiveness of analgesia after pediatric caudal blockade.

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Methods

This was a prospective study of caudal blockade effectiveness in patients who received the block as a part of their routine anesthetic regimen. The IRB’s permission to present the data was obtained. One hundred seventy-eight ASA physical status I and II children, undergoing inguinal (hernias, hydroceles, and orchiopexies) or penile (circumcision, hypospadias) procedures, were studied. All caudal blocks were placed after the induction of anesthesia (nitrous oxide and halothane) but before the surgical incision. Anesthesia residents in training and attending anesthesiologists performed all the caudal blocks using a standard single-shot technique. Bupivacaine 0.25% in a volume of 0.6 to 1.25 mL/kg, depending on the site of surgery, was used in all patients. For patients undergoing orchidopexies, hernia repair, and hydrocelectomies, a larger volume of 1–1.25 mL/kg was administered. For patients who had brief urethral surgeries, smaller volumes of 0.6–0.8 mL/kg bupivacaine were given. The hemodynamic and ventilatory response to skin incision and other surgical maneuvers were noted, and the anesthesiologist’s ability to decrease the inspired concentration of the inhaled anesthetic (halothane) to ≤1 minimum alveolar anesthetic concentration after 15 min of incision was recorded. The need for any supplemental opioids to control hemodynamic or ventilatory response to surgery was also recorded. Based on the ability to decrease the concentration of the inhaled anesthetic to <1 minimum alveolar concentration during the surgery, without any significant increase in hemodynamic (heart rate, blood pressure) or respiratory variables, the caudal block was determined to be clinically effective or successful. If there was a need to increase the concentration of the inhaled anesthetic and/or administer narcotics intraoperatively, the block was considered to be unsuccessful. At the end of the surgery and before emergence from anesthesia, a nurse who was blinded to the anesthesiologist’s assessment of the intraoperative effectiveness of the caudal blockade inserted a rectal suppository of acetaminophen and gauged the tone of the anal sphincter. This assessment was related to the experience of the normal tone observed when the same procedure was performed in hundreds of children undergoing otolaryngological surgery in which no caudal block is ever used.

In the PACU, recovery from anesthesia was evaluated by using the Aldrete score (5), and the efficacy of analgesia was assessed by a nurse who was unaware of the effectiveness of the caudal blockade by using the objective pain scale (OPS) (6). The OPS scoring system assigns a score of 0, 1, or 2 to 5 variables, including blood pressure, crying, movement, posture, and agitation (maximal score of 10). Any child who was judged to be in pain (score of ≥4) was given IV fentanyl (0.5 to 1 μg/kg).

The data collected included time from block to acetaminophen insertion, volume of bupivacaine used, the perioperative effectiveness of caudal blockade, quantity and the time of perioperative narcotics, time of insertion of the acetaminophen suppository, the laxity of the anal sphincter as gauged by digital palpation, and the OPS scores in the PACU. Fisher’s exact test was used to analyze the pain scores and the use of perioperative narcotics. Ninety-five percent confidence interval (CI) was used to estimate the proportion of successful caudal blockades; sensitivity, the probability of the sphincter tone test positive (lax sphincter tone) given that the caudal blockade was adequate; specificity, the probability of the sphincter tone test negative (tight sphincter tone) given that the caudal blockade was inadequate; positive predictive value (PPV), the probability of the caudal blockade adequate being given that the sphincter tone test was positive (LAS); and negative predictive value (NPV), the probability of the caudal blockade being inadequate given that the sphincter tone test was negative (tight sphincter tone [TAS]) (7,8). Fisher’s exact test was used to compare the sensitivity, specificity, PPV, and NPV of the new test and the standard method (i.e., “sphincter tone laxity” alone and intraoperative assessment of the caudal blockade by the anesthesiologist, and OPS, and PACU narcotics outcomes). Differences among predictive values or proportions were considered statistically significant when Fisher’s exact test yielded P < 0.05.

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Results

One hundred seventy-eight children, 1 mo to 7 yr of age (mean age 31.3 ± 31.5 mo), were studied. There were 11 girls (6.2%) and 167 boys (93.8%). Eighty-three percent were <5 yr of age and 92% <25 kg in weight. Among the patients studied, 95% were ambulatory surgical patients. Only two children required premedication with oral midazolam. The caudal space was identified in the first or second attempt in 97% of the cases and by the third and fourth attempt in 3% of the cases. There were three cases of partial skin infiltration with initial attempts, which were followed by successful caudal injection. There was no occurrence of bloody tap or cerebrospinal fluid aspiration. In 97% of cases, the duration of surgery was <120 min.

One hundred thirty of the surgeries were urologic: hypospadias repair, hydrocelectomies, circumcision, and orchidopexies. Forty-eight of the surgeries were inguinal hernia operations. Eighty percent of the patients received bupivacaine 0.25% in a volume of 1–1.25 mL/kg, whereas 20% received 0.6–0.8 mL/kg. The patients undergoing orchidopexies, hernia repair, and hydrocelectomies received the larger dose, and the patients who had penile surgeries had the smaller volumes of 0.6–0.8 mL/kg bupivacaine.

One hundred sixty children (89.9%) were assessed to have adequate intraoperative caudal blockades and 18 children were assessed to have inadequate caudal blockades. One hundred fifty-seven (98.1%) of these 160 patients had LAS tone at the end of surgery. Only 1 child who was judged to have an inadequate block had LAS (P < 0.001, Fisher’s exact test). The sensitivity of the sphincter tone test (LAS) given an adequate blockade was 98.1% (157/160) with a 95% CI of 94.3%–99.6%. The specificity of the test (TAS) given an inadequate caudal blockade was 94.4% (17/18) with a 95% CI of 72.0%–100%. The PPV of this test in predicting intraoperative adequate caudal block given a positive test (LAS) was 99.4% (157/158) with a 95% CI of 96.1%–100%. The NPV of this test to predict intraoperative inadequate caudal blockade given a negative test (TAS) was 85% (17/20) with a 95% CI of 62.9%–95.4%. The presence of a LAS at the end of surgery correlated significantly with the reduced administration of narcotics in the PACU (P < 0.001, Table 1). The PPV in predicting low OPS scores in PACU was excellent (98.1%) with a 95% CI of 94.3%–99.6% (Table 1). The NPV of inadequate postoperative analgesia, however, was only 45% and false negative error rate was 55%. The PPV of the test predicting need for PACU narcotics was 82.3% with a 95% CI of 75.5%–87.5%. However, the NPV for predicting lack of need for PACU narcotic was only 12 of 20 (60% with 95% CI of 38.6%–78.1%). The other comparisons were not significant (P > 0.05). The number of patients with a TAS was too small to test the hypothesis that combining the anesthesiologist’s assessment and anal sphincter tone testing would further increase the sensitivity in identifying patients with inadequate caudal blockade.

Table 1
Table 1
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Discussion

Caudal block has several advantages in children undergoing inguinal and/or penile surgery. A caudal block is simple to perform after induction of anesthesia, offers excellent intraoperative and postoperative analgesia, and enables the patient to return to normal activity quickly. Caudal block is superior to local nerve block in preventing pain after ambulatory pediatric urologic surgery (9). Caudal injection performed before surgery may also provide preemptive analgesia to children when compared with caudal injection or with local nerve blocks performed at the end of surgery (10).

When a successful caudal block is placed after the induction of anesthesia and before the incision, the requirements for inhaled anesthetics are usually decreased, and the need for intraoperative opioids is obviated; this allows for faster emergence and recovery (11). This technique is particularly advantageous in the ambulatory surgery patient.

Successful performance of a caudal block requires correct placement of the needle in the caudal epidural space, and injection of an appropriate volume of a local anesthetic solution. During the performance of the caudal block, the needle is assumed to be in the correct position if there is the distinct feel of “pop” as the needle pierces the sacrococcygeal membrane. Despite its simplicity, the presence of a “give” or “pop” and the sudden loss of resistance are subjective end points. Meticulous attention to visualizing and palpating surface landmarks can prevent misplacement of the needle. The injection of local anesthetics into a decoy hiatus can result in failure to achieve analgesia and increase the risk of adverse effects (12).

The effectiveness of a caudal block is often assessed in the intraoperative period by the ability to decrease the concentration of anesthetic gases and/or by the reduced need for supplemental narcotics both during and after surgery. Additionally, because caudal anesthesia causes a quick onset of full muscle relaxation of the anal area, changes in anal canal pressure have been used to measure the effectiveness of caudal and lumbar epidural anesthesia in adults (13). Anal canal pressure under resting conditions decreased after the injection of local anesthetics into the lumbar or caudal epidural space.

Tsui et al. (14) examined the effect of caudal electric needle stimulation to determine the accurate placement of the caudal needle. The needle placement was judged to be correct or incorrect depending on the presence or absence of anal sphincter contraction (S2-S4) to low current electrical stimulation (1–10 mA). In that study, the caudal block was determined to be successful based on whether or not opioids were necessary in the postoperative period. Although peripheral nerve stimulators are useful and effective in confirming needle position in the caudal space, this technique requires additional equipment and is not practical.

Experienced clinicians have observed that LAS and flaccidity of the penile corporal bodies were associated with successful caudal blockade. Our results indicate that this is a reliable simple clinical marker to predict successful caudal blockade.

In our study, all the children were given 0.25% bupivacaine in a volume of at least 0.6 mL/kg as a single shot injection before the surgical incision. This should produce a block at the sacral area or higher (15). The sensitivity of the sphincter tone test (LAS) given a successful blockade was 98.1%. The specificity of the sphincter tone test (TAS) given an unsuccessful caudal blockade was 94.4%. The PPV of this test in predicting intraoperative adequate caudal block given a positive test (LAS) seems excellent (99.4%). The NPV of this test to predict intraoperative inadequate blockade was 85%.

Combining the presence of a TAS with the anesthesiologist’s assessment of the adequacy of the block would have been an important observation. However, because of the small number (n = 17) of patients who had a TAS and unsuccessful caudal blockade at the end of surgery, this small sample size lacked adequate statistical power, and we were unable to test this hypothesis.

We conclude that if the anal sphincter tone is lax at the end of surgery, one can assume a successful block of at least the sacral segments. If one encounters a TAS tone in the patient at the end of surgery, it may be assumed that the caudal block has worn off and/or that the sacral segments were inadequately blocked and that the provisions of analgesia in the postoperative period may be inadequate. The latter would suggest the need to repeat the caudal injection to have a successful block before the patient awakens and leaves the operating room, or consideration should be given to an alternative approach to postoperative analgesia.

The authors thank Raafat S. Hannallah, MD, Professor of Anesthesiology and Pediatrics, Children’s National Medical Center, for critically reviewing the manuscript, Mrs. Deirdre Savoy for technical assistance, and the PACU nurses for the observation that prompted this study.

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References

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