Abstract
Objective
The basis for the transversus abdominis plane (TAP) block involves infiltration of a local anesthetic into the neurofascial plane between the internal oblique and the transversus abdominis muscles, causing a regional block that spreads between the L1 and T10 dermatomes. Thus, the TAP block is said to be suitable for lower abdominal surgery. This study was designed to compare the analgesic efficacy of two different concentrations of ropivacaine for TAP block in patients undergoing appendectomy.
Methods
Fifty-six patients with American Society of Anesthesiologists physical status I or II, aged 18 years and above, undergoing appendectomy were recruited in this prospective, randomized, double-blind study. They were divided into two groups: Group A patients who received 0.5 mL/kg of ropivacaine 0.5% and Group B patients who received 0.5 mL/kg of ropivacaine 0.2% via TAP block under ultrasound guidance. Postoperative pain was assessed using the visual analog scale upon arrival at the recovery room in the operating theatre, just prior to being discharged to the ward, and at 6 hours, 12 hours, 18 hours, and 24 hours postoperatively to compare the effectiveness of analgesia.
Results
Intraoperatively, patients in Group B required a significantly greater amount of additional intravenous fentanyl than those in Group A. There were no significant statistical differences in pain scores at rest and on movement at all assessment times as well as in the dose of 24-hour intravenous morphine consumption given via patient-controlled analgesia postoperatively between the two groups.
Conclusion
The effectiveness of two different concentrations of ropivacaine (0.5% versus 0.2%) given via TAP block was comparable in providing postoperative analgesia for patients undergoing appendectomy.
Keywords
analgesia; anesthetics, local: ropivacaine. appendectomy; nerve block: transversus abdominis plane; pain, postoperativepain measurement;
1. Introduction
Transversus abdominis plane (TAP) block is a new and rapidly evolving regional anesthetic technique that blocks abdominal neural afferents by introducing a local anesthetic into the neurofascial plane between the internal oblique and the transversus abdominis muscles.1 Innervation of the anterolateral abdominal wall arises from the anterior rami of spinal nerves T7–L1. Published reports agree that there is a reliable block spread between the L1 and T10 dermatomes following TAP block and that it is, therefore, better suited for lower abdominal surgery.1, 2
Two methods are used to perform TAP block: a blind technique, based on surface anatomy landmarks, via the “lumbar triangle of Petit”, and an ultrasound-guided technique performed under direct vision. The blind technique raises several important issues such as inaccurate placement of the needle tip, thus leading to a higher risk of potential damage to adjacent structures.3, 4 Ultrasound visualization of anatomical structures is currently the only method that offers potentially safe blocks of superior quality by allowing optimal needle positioning.5 Owing to its relatively low risk of complications and ease of performance, TAP block has become increasingly recognized as a treatment modality for postoperative pain management and has become more commonly used over the last decade.6 It not only provides effective postoperative analgesia, but also reduces opioid requirements and helps improve patient satisfaction in the early postoperative period.7, 8, 9, 10, 11, 12, 13, 14, 15
Most of the studies have shown that TAP block provides highly effective postoperative analgesia in the first 24–48 hours.12, 13, 14, 15 Despite the encouraging results, the ideal local anesthetic agent, its dose/concentration, and the volume of injection used for implementing the TAP block have not been well defined in the literature. Higher local anesthetic doses (and concentrations) have been used in order to provide prolonged postoperative analgesia. However, it has been demonstrated that high plasma concentrations of local anesthetics have been measured 15 minutes after administering the TAP block, and this raises concerns about the possibility of systemic toxicity.16, 17, 18
Ropivacaine was chosen as the local anesthetic for this study, as it can provide prolonged postoperative analgesia with a greater margin of safety for cardiotoxicity and neurotoxicity.19 We used 0.2% ropivacaine, as it is generally accepted as the optimal concentration required for postoperative analgesia in different regional anesthesia techniques.20 Thus, the aim of this study was to determine the effective concentration of ropivacaine needed for TAP block to provide effective and safe postoperative analgesia for appendectomy.
2. Methods
This prospective, double-blind, randomized clinical study was conducted after obtaining approval from the Medical Research & Ethics Committee, Universiti Kebangsaan Malaysia Medical Centre (Research code: FF-358-2012) and informed consent of patients. Fifty-six patients aged 18 years and above, with American Society of Anesthesiologists (ASA) physical status I or II and diagnosed with acute appendicitis, who required open, emergency appendectomy using a Lanz incision, were recruited in the study. Obstetric patients, patients known to have coagulopathy, and those with local skin infection over the abdominal wall as well as a history of allergy or contraindications to any drugs used in this study were excluded. Patients were also excluded from the study if the patient-controlled analgesia technique was unsuitable due to functional disability or inability to understand instructions. TAP blocks were performed by multiple operators who had the experience of administering more than 10 blocks under ultrasound guidance. Patients were randomized into two groups using computer-generated randomized numbers. Group A patients were given a TAP block using 0.5 mL/kg of ropivacaine 0.5%, whereas those in Group B was given 0.5 mL/kg of ropivacaine 0.2%, to a maximum volume of 30 mL for each patient.
2.1. Procedure of anesthesia
Standard monitoring with continuous electrocardiogram, noninvasive blood pressure, pulse oximetry, and capnograph was performed. Intravenous (IV) access was established, and baseline hemodynamic parameters were recorded. Patients were preoxygenated with 100% oxygen for 3 minutes, followed by rapid sequence induction using IV fentanyl 2 μg/kg, IV sodium thiopentone 3–5 mg/kg, and then IV suxamethonium 1–1.5 mg/kg. Patients were intubated, and anesthesia was maintained with sevoflurane/air at a minimum alveolar concentration of 1.0–1.2. Muscle relaxation was provided using IV rocuronium.
2.2. Ultrasound-guided right-sided TAP block
The study drug was prepared and diluted to 30 mL by an independent doctor who was not involved in the patient’s pain assessment. Unilateral right-sided TAP block was performed prior to skin incision under ultrasound guidance with an M-Turbo SonoSite device (SonoSite Inc., Bothwell, Washington, USA) and a linear 6–13 MHz transducer. The procedure was carried out under aseptic conditions. The external oblique, internal oblique, and transversus abdominis muscles were identified at the level of the midaxillary line between the 12th rib and the iliac crest, using ultrasound guidance. In the plane technique, an 80 mm short bevel needle was used and inserted in a sagittal plane approximately 3 cm medial to the ultrasound probe, and the needle was held parallel to the long axis of the ultrasound probe. After the tip of the needle was visualized in the transversus abdominis fascia and after negative aspiration for blood, a small volume (1–2 mL) of either study drug was injected to open the plane between the two muscles. This was followed by administration of the remaining volume of the study drug calculated which was injected and observed in real time.
Surgery was allowed to start after the TAP block procedure was completed. Patients were excluded from the study when intraoperative findings were other than acute appendicitis, e.g., perforated appendicitis/cecum or gynecological conditions. If the attending anesthetist, after ruling out other causes of hypertension and tachycardia, felt that the patient did not have adequate analgesia, additional boluses of IV fentanyl 0.5–1 μg/kg were administered. The total amount of IV fentanyl required was recorded. Other forms of analgesics were not given.
2.3. Postoperative pain
Postoperatively, patients were extubated and transferred to the recovery room. A patient-controlled analgesia machine containing morphine, with a bolus dose of 1 mg and a lock-out interval of 5 minutes without a background infusion, was assigned to each patient. Patients were assessed for pain scores at rest and on movement (with knee flexed) upon arrival at the recovery room, prior to being discharged from the recovery room, and in the ward at 6 hours, 12 hours, 18 hours, and 24 hours postoperatively. The pain score was assessed by the nurses who were blinded to the group allocation, using a 0–10 visual analogue scale, with 0 indicating no pain and 10 indicating the worst imaginable pain. Total IV morphine consumption during the postoperative 24-hour period was recorded. Any adverse events related to the TAP block or side effects of the drugs that were used in this study (such as visceral injury, local anesthetic toxicity, or incidence of postoperative nausea and vomiting) were also recorded. These adverse events were treated accordingly, following the standard institutional protocols.
2.4. Sample size calculation
The sample size was calculated using the PASS (Power Analysis and Sample Size System) software (NCSS, East Kaysville, Utah, USA). The sample size was estimated based on the 24-hour morphine requirement in a previous study by Niraj et al.13 We also considered the possibility that there would be a 15% absolute reduction in the 24-hour morphine requirement for patients in Group B compared to those in Group A. The power of this study was set at 80%, an α-value of 0.05, and a standard deviation of 18, whereby it was estimated that 24 patients would be required for each group. Anticipating a 15% dropout rate, 28 patients were eventually recruited in each group.
2.5. Statistical analysis
The data were analyzed using the SPSS version 21.0 software (SPSS Inc., Chicago, IL, USA). Chi-square test or Fisher exact test was used when appropriate to calculate any significant differences for categorical variables in demographic data, such as sex, ASA group, and race between the two groups. Independent t test was used to determine any significant differences for continuous variables such as age, weight, pain scores, total volume of ropivacaine, and total IV fentanyl used intraoperatively, as well as total IV morphine consumed postoperatively. A p value of less than 0.05 was considered statistically significant.
3. Results
A total of 56 patients were recruited in this study. However, three patients, two of whom had conversion to laparotomy and one required gynecological intervention, were excluded. Demographic data of the patients in both groups are shown in Table 1. There were no significant differences in the demographic data between the two groups.
Intraoperative data of the patients in both groups are shown in Table 2. There was no significant difference in the duration of anesthesia as well as total volume of ropivacaine for both groups. Out of 17 patients in this study who required additional IV fentanyl intraoperatively, five (29.4%) were in Group A and 12 (70.6%) were in Group B. The difference was statistically significant (p = 0.042). The total amount of IV fentanyl used intraoperatively was significantly higher for patients in Group B than that for patients in Group A (2.5 ± 0.5 μg/kg vs. 2.2 ± 0.5 μg/kg, p = 0.043).
There were no significant differences between the pain scores in both groups at all times of assessment, as shown in Table 3.
A comparison of the total 24-hour IV morphine requirement between patients of Group A and Group B is shown in Fig. 1 (21.6 ± 3.5 mg vs. 23.6 ± 4.4 mg, respectively). However, the difference was not statistically significant (p = 0.073).
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Two patients in Group A and one in Group B had postoperative nausea and vomiting. The patient in Group B vomited in the recovery room, whereas the other two patients had nausea and vomiting in the ward. They were treated with IV metoclopramide. No other adverse events were noted during this study.
4. Discussion
In this study, we compared two different concentrations of ropivacaine used for TAP block to see if different concentrations of local anesthetics would make a difference in terms of postoperative analgesia. We found no significant differences in the mean pain scores at all assessment times as well as in the total 24-hour postoperative IV morphine consumption for both groups. These findings were similar to a study conducted by Oliveira et al,21 comparing two different concentrations of ropivacaine for TAP block in patients undergoing gynecological laparoscopic surgery. In their study, they found that ropivacaine 0.25% had comparable effects on postoperative analgesia and quality of recovery to ropivacaine 0.5%. Another study by Ra et al22 also found no statistically significant differences in postoperative pain scores and the amount of rescue analgesic needed, when comparing levobupivacaine 0.25% and levobupivacaine 0.5% given via TAP block in patients undergoing laparoscopic cholecystectomy. In a study, Bharti et al23 found that patients who received bupivacaine 0.25% via TAP block had significantly less total IV morphine consumption and lower pain scores over a 24-hour period postoperatively, compared to the placebo group.
In this study we found that ropivacaine 0.2% was as effective as ropivacaine 0.5% in providing postoperative analgesia; however, intraoperatively, we found that patients who received ropivacaine 0.2% required a significantly higher amount of total IV fentanyl. In this study, the difference in additional IV fentanyl used intraoperatively was found to be statistically significant; however, significant statistical findings do not necessarily equate to a clinical significance. Patients who received ropivacaine 0.2% only needed an average difference of 0.3 μg/kg more fentanyl intraoperatively compared to the patients who received ropivacaine 0.5%, which is not clinically significant in terms of difference in amount given. El-Dawlatly et al,15 in their study, found that TAP block using bupivacaine 0.5% significantly reduced intraoperative sufentanil usage in patients undergoing laparoscopic cholecystectomy. Chen and Phui24 performed a case series of 10 patients and found that TAP block using ropivacaine 0.375% provided effective intraoperative analgesia even with the conversion from laparoscopic to open cholecystectomy, with minimal additional fentanyl requirement intraoperatively.
Although one may assume that a higher concentration of a local anesthetic given via TAP block may provide better-quality postoperative analgesia, this raises the issue of potentially toxic plasma concentrations of the local anesthetic, as the TAP block generally involves injection of a single large dose of local anesthetic into a relatively vascular plane.16 Griffiths et al17 reported that the mean peak total venous ropivacaine concentrations exceeded a potentially neurotoxic threshold value (2.2 μg/mL) after bilateral TAP block with 3 mg/kg ropivacaine at 15 minutes, 30 minutes, 60 minutes, and 90 minutes. In 2013, Griffiths et al18 again reported plasma ropivacaine levels that exceeded the neurotoxic threshold, in a pregnant mother undergoing cesarean section, who became symptomatic after being given a total dose of 2.5 mg/kg of ropivacaine. Copeland et al25 found that general anesthesia was able to change whole-body and regional pharmacokinetics of injected local anesthetics, as well as their systemic effects. They found that blood concentrations of local anesthetics were doubled in anesthetized patients when compared to conscious patients. Thus, it would be safer if we can determine and use the minimum local anesthetic dose or concentration necessary to be given via TAP block to achieve the desired clinical effect, without causing toxic effects. Based on our findings, we would like to suggest that ropivacaine 0.2% would provide a better margin of safety, especially when used for bilateral TAP block where a higher volume of a local anesthetic is required. Furthermore, ropivacaine 0.2% is available as a prepared solution and is readily used, hence minimizing errors in dilution during drug preparation.
Charlton et al26 reported no significant differences between TAP group and non-TAP group patients for postoperative nausea and vomiting. However, a recent meta-analysis demonstrated clinically significant reductions in postoperative nausea and vomiting for TAP group patients as compared to non-TAP group patients.10 TAP blocks reduce the need for postoperative opioid used, thus avoiding their side effects such as nausea and vomiting. This probably explains the low incidence of postoperative nausea and vomiting in our study. TAP block is a relatively safe technique, with only a few reported cases of serious adverse events such as liver injury and bowel hematoma due to needle displacement.1, 8 In this study, as in most TAP block studies carried out previously, no major adverse events such as visceral organ injury or local anesthetic toxicity were observed.10
Moeschler et al5 studied the spread of various volumes of contrast given via TAP block for cadavers. They found that an increased volume of injectate correlated with an increased cranial–caudal spread, and demonstrated a good spread around the midaxillary line where the iliohypogastric, subcostal, and intercostal nerves have been shown to course in the TAP. Their study suggested that 15 mL of injectate provided an additional cranial–caudal spread and may be an optimal volume for anesthesia. However, it was also noted that the degree of injectate spread may differ between live patients and cadavers. In our study, the total volume of ropivacaine used ranged from 23 mL to 30 mL, based on the patients’ bodyweight.6 Therefore, further studies should perhaps look into the optimal volume rather than the concentration of ultrasound-guided local anesthetics required to produce an effective sensory blockade for patients undergoing surgery of the anterior abdominal wall.
There were a number of limitations to this study. The TAP blocks were performed by multiple operators who had variable skills, depending on their experience of performing the block, and this could lead to performance bias. Furthermore, even though the TAP block was performed under direct vision using ultrasound guidance, there was still a possibility that the injected drugs could spread intramuscularly due to incomplete separation of the fascial planes under pressurization, which could affect the overall outcome.5 In this study, all blocks were performed after induction of anesthesia; thus, we were unable to assess and confirm the anatomical distribution and adequacy of analgesia prior to surgical incision. We also did not obtain serum ropivacaine levels to compare the safety profile, regarding the potential for local anesthetic toxicity, between the higher and lower concentrations used. There were several results that were close to statistical significance such as duration of anesthesia, pain score upon arrival to the recovery room, and morphine consumption. This may be due to an inadequate sample size, and by increasing the sample size, a more significant difference may be observed in the parameters mentioned.
5. Conclusion
This study shows that 0.5 mL/kg of ropivacaine 0.5% was comparable to 0.5 mL/kg of ropivacaine 0.2% given via TAP block in providing postoperative analgesia in patients undergoing appendectomy.