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Abstract
Introduction The transversus abdominis plane block is widely used in postoperative pain management after abdominal surgery. However, large interindividual variation in the cutaneous distribution area of the block has been demonstrated. The purpose of the present study was to explore the reproducibility of the block by determining the intraindividual variation when repeating the block on two separate days.
Methods Ultrasound-guided posterior transversus abdominis plane blocks were performed in 16 healthy volunteers and repeated after at least 2 days. Cutaneous sensory block areas and distributions, thresholds for mechanical stimulation, abdominal muscle thicknesses at rest and during maximal contraction, waist circumferences and block duration times were measured on both days. Outcome measurements from the 2 days were compared using a one-sample t-test and intraclass correlation coefficients were calculated for each parameter. Agreement was evaluated visually using Bland-Altman plots.
Results None of the mean values of the outcome measurements differed significantly between the 2 days. Intraclass correlation coefficient was 0.75 (moderate-to-good reliability) for lateral abdominal muscle thickness during maximal contraction, but ranges from −0.07 to 0.67 were found for all other outcome measurements (poor-to-moderate reliability).
Conclusion We found a moderate-to-good reproducibility for lateral abdominal muscle thickness during maximal voluntary contraction, but a moderate-to-poor reproducibility for all other block characteristics. However, the cutaneous distribution of the block was still located primarily inferior to a horizontal line through the umbilicus and lateral relative to a vertical line through the anterior superior iliac spine.
- truncal blocks
- postoperative pain
- pain medicine
- interventional pain management
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Introduction
The transversus abdominis plane (TAP) block is widely used in postoperative pain management after laparoscopic and open abdominal surgery such as cholecystectomy, hysterectomy, prostatectomy, cesarean section, colorectal resection, appendectomy and inguinal hernia repair.1–8 A review showed a reduction in postoperative morphine requirement, nausea and vomiting in patients who received TAP block after abdominal surgery, but no difference in visual analog pain scale score.9 However, the existing literature regarding TAP block is characterized by substantial differences in TAP block techniques, surgical procedures, local anesthetic (LA) types and LA volumes being investigated.
When using a nerve block, it is important to know the block characteristics in terms of the blocked structures, block duration and the variability of these parameters, both on the interindividual as well as on the intraindividual level. The TAP block, introduced by Rafi in 2001, is a fascial plane block where LA is injected into the neurovascular plane between the transversus abdominis muscle and the internal oblique muscle.10 11 In theory, this would block the 7th to the 11th intercostal nerves, the subcostal nerve, the iliohypogastric nerve and the ilioinguinal nerve. However, there is a substantial variation in how these nerves divide and lie in this plane.12 Further, the nerves are not visualized with ultrasound when the block is performed as opposed to peripheral nerve blocks, and the extent of LA spread may also be subject to substantial variation. Consequently, variations in block characteristics may be expected to be large for a fascial plane block.
We have previously described some of the block characteristics of the posterior TAP block.13 We found a non-dermatomal distribution of the cutaneous sensory block area (CSBA), a large interindividual variation of some of the other block characteristics: size and location of CSBA and sensory and motor block duration. By contrast, motor block of the three abdominal muscle layers showed little variation. In the present study, the aim was to explore the intraindividual variation in the block characteristics of the posterior ultrasound-guided TAP block (nomenclature and approach of the TAP block as described by Tran et al 8) when repeated in the same individuals at least 2 days apart. We hypothesized that there would be a large intraindividual variation in the measured block characteristics, that is, CSBA, muscle relaxing effect and the duration of sensory and motor block. We used the same study population as in our previous study, but the data on intraindividual variation have not been published before.13
Methods
Approvals and monitoring
The Regional Committee on Health Research Ethics (H-1-2011-030) and the Danish Health and Medicines Authority (2011-004719-22) approved the study protocol. The study was conducted in accordance with the Declaration of Helsinki and Good Clinical Practice and monitored by the Copenhagen University Good Clinical Practice Unit. The trial was registered at ClinicalTrials.gov (identifier NCT01679392, September 2012), including this secondary analysis.13
Study population
Ultrasound-guided TAP blocks were performed in 16 healthy volunteers on two separate study days after giving verbal and written informed consent. The study was conducted between August 2012 and March 2013. Exclusion criteria were age younger than 18 years, American Society of Anesthesiologist classification≥3, body weight≤50 kg, body mass index≥35 kg/m2, use of analgesics 24 hours prior to examination, prior surgery to the abdominal wall, or allergy to any of the study drugs.
Randomization, allocation concealment and blinding
On the first study day, the volunteers were randomized to a unilateral TAP block with LA on one side and placebo on the contralateral side (control). Based on a computer-generated list, health personnel with no relation to the study made sealed opaque envelopes containing information on the side of injection of LA and placebo. A nurse with no relation to the study prepared the study medication in two syringes marked “right” and “left.” A second person delivered the syringes to the anesthesiologist who performed the TAP blocks. On the second study day, the volunteers were allocated to the same intervention as on the first study day (same side of LA and saline injection). The randomization list was not released until all patients were included, and all data were collected. All volunteers and investigators were blinded to treatment allocation.
Intervention
A detailed description of the methods used on day 1 as well as the data regarding cutaneous distribution, muscle relaxing effect and block duration has previously been published.13 In short, after a secure intravenous access and monitoring with non-invasive blood pressure, continuous electrocardiography and pulse oximetry, the TAP block was performed using ultrasound guidance and in-plane needle orientation. The skin entry site was approximately at the anterior axillary line between the 12th rib and the iliac crest. The needle tip was advanced in-plane and positioned in the TAP approximately 1 cm anterior to the tapering of the fascial layers of the transversus abdominis and the internal oblique muscle into the middle thoracolumbar fascia that lies posterior the quadratus lumborum muscle. 20 mL ropivacaine (7.5 mg/mL) was injected unilaterally and 20 mL isotonic saline (placebo) on the control side. The end point of the injection was to visualize the LA/saline dissect the fascial layer between the internal oblique and transversus abdominis muscles in both posterior and anterior direction, filling the TAP. On day 2, the TAP block was performed using the same technique as described for day 1. The two blocks were separated by a minimum of 48 hours to ensure no residual blockade from the first block. All blocks were performed by one of two anesthesiologists, each with more than 150 TAP block experience prior to the study.
Outcome measures
On the days of both the first and the second TAP blocks, the volunteers were assessed before performing the blocks (T0) and 90 min after end of injection (T90). We evaluated the reproducibility of the two TAP blocks concerning the following outcome measures:
CSBA: both the size and location of the blocked area were evaluated.
Threshold for mechanical stimulation: evaluated by “mechanical detection threshold” and “mechanical pain threshold.”
Muscle thickness of the lateral abdominal wall: evaluated at rest and during maximal voluntary contraction.
Waist circumference: evaluated at rest and during maximal voluntary contraction.
Block duration: evaluation of sensory and motor block duration.
CSBA was determined using a cool roller and marked on the skin. The area was transferred to transparencies before location and area size were calculated from photographs. The relative lateral–medial as well as the superior–inferior distributions of the CSBA were defined according to (1) a vertical reference line through the anterior superior iliac spine and (2) a horizontal reference line at the umbilical cord level. Mechanical detection threshold and mechanical pain threshold were measured using an ascending series of 20 von Frey filaments (0.008–300 g) in the center of the CSBA. “Muscle thickness at rest” and “muscle thickness during maximal voluntary contraction” (basic abdominal crunch) were measured using ultrasound imaging by placing the ultrasound probe in the transverse plane approximately 2 cm lateral to the anterior aponeurosis of the three lateral abdominal muscles, capturing images in the end-expiratory phase. The thicknesses of each of the three abdominal muscles were measured. “Waist circumference at rest” and “waist circumference during maximal voluntary contraction” were measured at end-expiratory respiration at the umbilical level. Sensory and motor block duration was determined by equipping the volunteers with diaries and instructions to test sensory and motor effect every hour at home by simple touch and by performing a basic abdominal crunch, respectively.
Statistical analyses
No power calculation was performed in the present study. The number of volunteers included was based on a convenience sample determined by a previous study.13 The intraindividual variation of the TAP block was estimated for each parameter. The mean difference between day 1 and day 2 was calculated for each measured parameter, and compared using one-sample t-tests. Agreement was visualized in Bland-Altman plots. The Bland-Altman plots were also inspected for heteroscedasticity, which refers to the circumstance in which the variability of a variable is unequal across the range of values of a second variable that predicts it. To determine the proportion of the variation in the measured parameters that was due to the differences between day 1 and 2, we calculated the intraclass correlation coefficient (ICC) from a two-way mixed effects model using an absolute agreement definition. We followed guidelines by Koo et al for interpretation of ICC reliability measures as: below 0.50: poor; between 0.50 and 0.75: moderate; between 0.75 and 0.90: good; above 0.90: excellent.14 Calculations were made using IBM SPSS statistics V.22. A two tailed p value of 0.05 was considered statistically significant.
Results
We included eight women and eight men. The median age was 23 years (range 19–32), median height 173 cm (range 150–193) and median body weight 68 kg (range 50–86). We were able to obtain all the planned measurements from the included volunteers, except for one volunteer where the three outcome measurements that involved CSBA calculated from photographs were not available (CSBA size and locations).
None of the mean differences differed significantly between day 1 and day 2 (table 1). The highest ICC was found for “lateral abdominal muscle wall thickness at maximum voluntary contraction” which indicates moderate-to-good reliability. All other ICC values were in the range of −0.07 to 0.67 indicating “poor”-to-“moderate” reliability (table 1). Only for lateral abdominal muscle wall thickness at maximum voluntary contraction and “block duration, motor” did the CIs for the ICCs not include zero (table 1).
Comparison of block characteristics after a repeated unilateral transversus abdominis plane block
The agreements for all nine outcomes were inspected visually for outliers and heteroscedasticity in Bland-Altman plots (figure 1A). In the Bland-Altman plot for mechanical pain threshold using an ascending series of von Frey filaments, we found a funnel effect (figure 1B). For all other outcomes, we found no signs of heteroscedasticity.
Bland-Altman plots for (A) cutaneous sensory block area with no heteroscedasticity, and (B) mean pain threshold (MPT) with a funnel effect. Lines indicate mean ±2 SD.
The percentage of the CSBA located laterally ranged between 58% and 100% (figure 2), and the percentage of the CSBA located infraumbilically ranged between 86% and 100% (figure 3).
Percentage of cutaneous sensory block area (CSBA) located lateral to a vertical line through the anterior superior iliac spine. Error bars indicate difference between days 1 and 2 for each volunteer. Volunteers are arranged according to percentage on day 1.
Percentage of cutaneous sensory block area (CSBA) located inferior to the umbilical level. Error bars indicate difference between days 1 and 2 for each volunteer. Volunteers are arranged according to percentage on day 1.
Even though the distribution of the CSBA was not significantly different between day 1 and day 2, we observed a variation between the patients in the visual reproducibility of the distribution of the CSBA (figure 4).
Examples of good and poor reproducibility of a repeated transversus abdominis plane block. Images A (A1 (frontal view) and A2 (lateral view)) and B (B1 and B2) show one volunteer on day 1 and day 2. Images C (C1 and C2) and D (D1 and D2) show another volunteer on days 1 and 2.
Discussion
The present trial aimed to explore the intraindividual variation of the TAP block, using the same technique, on the same person, on two separate days. For all of our outcomes, there were no mean significant differences between the 2 days of observation. We found a moderate-to-good reproducibility for lateral abdominal muscle thickness during maximal voluntary contraction, but only a moderate-to-poor reproducibility for all other block characteristics.
We found the CSBAs to be consistently located inferior lateral on the abdomen, flank and hip for all participants on both days, even for subjects for whom the CSBA sizes varied between the 2 days (figures 2–4). However, on an intraindividual level, the reproducibility of both the size of the blocked area and the specific location was moderate to poor.
The reproducibility of the thresholds for mechanical detection and pain detection was also low. This is in accordance with a previous study using quantitative sensory testing to detect sensory dysfunction after breast cancer surgery, that found the method to be a useful tool on a group-to-group comparison level, but less so on an individual level.15
As postoperative pain originates not only from the superficial surgical site, but also from deep tissue, it is relevant to explore the reproducibility of other parameters than the CSBA.16 Out of the four outcome measures describing the tension of the abdominal muscles (waist circumferences at rest and during maximal contraction as well as lateral abdominal muscle wall thickness at rest and during maximal contraction), the lateral abdominal muscle thickness during maximal contraction had the highest ICC of 0.75. It therefore seems that the muscle relaxing effect of the TAP block is the most reproducible parameter. This is an interesting finding since pain from deep tissue such as skeletal muscle may produce both referred pain (due to central convergence of afferent nerves) and projected pain (mechanical stimulation of afferent nerves that is interpreted as originating from the innervation areas of these nerves), as well as affect the cutaneous nociception.16
Heteroscedasticity was seen in only one of the explored parameters when visualized in Bland-Altman plots. The funnel effect seen for mechanical pain threshold suggests that the size of the day-to-day variation is dependent on the size of the mean value for each subject. This heteroscedasticity was not seen for mechanical detection threshold and may therefore simply be a random finding.
There are not many data in the literature on the intraindividual variation in nerve block duration. In the present study, we investigated a fascial plane block and found a moderate-to-poor reproducibility in the duration of the fascial plane block. Whether this is true for peripheral nerve blocks in general remains to be investigated. A recent study on the common peroneal nerve found that the intraindividual variation in nerve block duration contributed less than the interindividual variation to overall nerve block duration.17
Different block techniques contribute to the heterogeneity of the literature on TAP block.18 As different techniques may result in large interindividual variations in the CSBA, a strength of the present study is the strictly identical conditions during which the TAP blocks were performed on the two test days.19 20 This enabled us to explore the true intraindividual variation and limited confounding factors such as blocks performed using different techniques or by physicians with different levels of technical expertise.
We chose healthy volunteers to avoid confounding by prior surgery. However, despite the strictly controlled conditions, attaining 100% identical block procedures on the two test days may be difficult, and even though all TAP blocks were performed by one of two physicians, both experienced in performing TAP blocks, some variation between block techniques may have influenced the estimated intraindividual variations observed.
Further limitations include the lack of power calculation, and the study may therefore have been underpowered to detect differences in outcome measures. The present study should therefore be characterized as a pilot study. In addition, the volunteers were pseudoblinded on day 2 because they would be biased to expect blockage of the same side as on day 1.
When implementing the present findings to clinical practice, both the intraindividual and interindividual variation of the TAP block will undoubtedly increase when performed under more difficult conditions, for example, in patients with obesity or in patients with tissue edema. Furthermore, when performed in patients postoperatively, the reproducibility can be affected by an altered cutaneous nociception caused by referred and projected pain.16
The present study contributes to our understanding of the TAP block as part of a multimodal approach to postoperative analgesia. Due to the heterogeneity of the literature, the exact mechanism behind the analgesic effect of the TAP block, the optimal technique, dosing and effect on different surgical procedures is still uncertain. The day-to-day variation of the TAP block, when performed under identical conditions, therefore helps us to understand which effects are more consistent. Future studies of the TAP block reproducibility should focus on postoperative effects, including analgesic efficacy, even though achieving identical postoperative conditions on two separate test days is a challenge.
Conclusion
We found a moderate-to-good reproducibility for lateral abdominal muscle thickness during maximal voluntary contraction, but a moderate-to-poor reproducibility for all other block characteristics. Further research is needed to determine the reproducibility of the analgesic effect of the TAP block in patients undergoing surgery.
References
Footnotes
Presented at Part of this work was presented in ‘Cutaneous Sensory Block Area, Muscle-Relaxing Effect, and Block Duration of the Transversus Abdominis Plane Block: A Randomized, Blinded, and Placebo-Controlled Study in Healthy Volunteers. Støving K, Rothe C, Rosenstock CV, Aasvang EK, Lundstrøm LH, Lange KH. Reg Anesth Pain Med 2015;40(4):355-62.
Contributors CR, KHWL, KS, LHL, EAK and CVR planned and designed the study. CR, KS and KHWL conducted the experiments. CSJ and LHL analysed and interpreted the data and drafted the first version of the manuscript. All authors critically revised the manuscript. All authors approved the final version of the manuscript.
Funding This work was locally funded by grants from Nordsjællands Hospital, Hillerød, Denmark.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement Data are available upon reasonable request to the corresponding author. The corresponding author can be contacted by the e-mail stated in the article. Data include deidentified data and deidentified clinical photos. Reuse of data will be permitted upon request if the authors find the request meaningful and appropriate.