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 Table of Contents  
Year : 2018  |  Volume : 5  |  Issue : 2  |  Page : 141-145

Acoustic puncture assist device versus ultrasound imaging technique for thoracic epidural space identification in obese patients

Anesthesia Department, Faculty of Medicine, Alexandria University, Alexandria, Egypt

Date of Submission20-Apr-2017
Date of Acceptance23-Jan-2018
Date of Web Publication28-Jun-2018

Correspondence Address:
Yasser M.M Osman
Anesthesia Department, Faculty of Medicine, Alexandria University, Alexandria
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/roaic.roaic_49_17

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Introduction Newer techniques have been used lately such as the ultrasonography (US) and the acoustic puncture assist device (APAD) to identify the epidural space (EDS). The difference between using these techniques in obese patients has not been studied enough yet. The primary aim of this study is to evaluate the ease of placement of thoracic epidural catheter in obese patients using either APAD versus US imaging assisted technique. The secondary aim is to compare the incidence of complications between both techniques.
Patients and methods The institutional review board at the University of Alexandria approved the study protocol. One hundred obese patients (BMI >30 kg/m2) were randomly enrolled into one of the two study groups. The placement of thoracic epidural catheter was done using an US-assisted technique in cases of group I, while epidural catheter was inserted into the thoracic EDS using APAD in group II patients.
Results First attempt success rate for EDS localization was higher in group II (APAD) as compared with group I (US group) (83 vs. 79%), but there was no significant statistical difference between both groups (P=0.461). Mean time for EDS localization was statistically significantly longer in group I (US-guided group) than in group II (APAD) (78.44±23.6 vs. 58.78±22.2 s; P<0.00001). Patients of group II were statistically more comfortable during the procedure (P=0.001). The mean visual analogue scale score for discomfort postprocedure was 2.5±1.18 in cases of group I versus 1.74±0.85 in patients of group II. There was no statistical difference as regards the complication in both groups.
Conclusion The previous findings of the shorter time of EDS localization and more patient comfort in APAD-guided epidural analgesia than those of US-guided technique has ended in the conclusion that the use of APAD for thoracic epidural anesthesia in obese patients is a better choice than using US.

Keywords: acoustic puncture assist device, obese patients, thoracic epidural, ultrasound imaging technique

How to cite this article:
Osman YM, El-Gamal N. Acoustic puncture assist device versus ultrasound imaging technique for thoracic epidural space identification in obese patients. Res Opin Anesth Intensive Care 2018;5:141-5

How to cite this URL:
Osman YM, El-Gamal N. Acoustic puncture assist device versus ultrasound imaging technique for thoracic epidural space identification in obese patients. Res Opin Anesth Intensive Care [serial online] 2018 [cited 2021 Jun 15];5:141-5. Available from: http://www.roaic.eg.net/text.asp?2018/5/2/141/235488

  Introduction Top

The incidence of morbid obesity has tripled over the past three decades throughout the world [1],[2]. The World Health Organization estimated that in 2015, 2.3 billion people are overweight (BMI 25–30 kg/m2) and 700 million are obese (BMI>30 kg/m2) [3].

Epidural anesthesia has many advantages over general anesthesia for obese patients. The use of regional anesthesia avoids potential cardiovascular and pulmonary complications of general anesthesia and is associated with no airway manipulation. Epidural anesthesia also offers better postoperative pain control and reduces the perioperative and postoperative opioid requirements, which can cause postoperative pulmonary complications in obese patients. However, regional anesthesia has its limitations and technical difficulties in this patient population [3],[4].

Thoracic epidural analgesia is commonly used for postoperative pain control after abdominal surgery [3],[4]. The technical difficulties in performing epidural anesthesia especially in obese patients is mainly attributed to excess body fat that may conceal the landmarks and increase the distance between the skin and the epidural space (EDS) [3]. The increased distance between the skin and the EDS makes the manipulation of the needle to reach the space more difficult and may mask the clinical sense of loss of resistance (LOR), which is traditionally used to identify the EDS. LOR is a relatively subjective technique, hence can lead to incorrect placement of epidural catheters, patchy, or inadequate analgesia as well as high incidence of epidural failure [3],[4].

Newer techniques have been used lately, as the ultrasound (US)-assisted placement and the APAD to identify the EDS. The use of sonography to identify the anatomy of the thoracic spine and the distance between the skin and the EDS is well documented in the literature to increase the success of the procedure [5],[6],[7].

Lechner, the founder of APAD, used it for more than 5000 interventions till 2011. The principle of LOR is the basis of APAD and it works on with the integration of audio signals and visual graphics for objective confirmation of EDS. Lechner has concluded that the device is reliable, safe, simple, and facilitates the epidural analgesia procedure [8].

This study aims to evaluate and compare the ease and safety of insertion of an epidural catheter in the thoracic space of obese patients using APAD versus US-guided technique.

  Patients and methods Top

The study protocol was reviewed and approved by the Ethics Committee of the Alexandria Faculty of Medicine. A written consent was obtained from all patients participating in this study. One hundred obese patients with BMI 30 or more undergoing abdominal surgery, under general anesthesia, with a planned insertion of thoracic epidural catheter were eligible for enrolment. The patients were randomly assigned into one of the two study groups using a computer-generated randomization table.

Group I consists of 50 patients scheduled for epidural catheter insertion using US imaging-assisted technique.

Group II consists of 50 patients scheduled for APAD-guided epidural catheter insertion.

The sample size was calculated by taking the success rate of conventional LOR to be 98% in the patients, assuming 80% power, with 95% confidence interval of the two techniques.

The exclusion criteria included patients refusing to participate in the study or any contraindication to perform epidural analgesia such as infection at the site of needle insertion, previous vertebral column surgery or deformity, coagulopathy, neuromuscular disorders, and allergy to local anaesthetics.

The study was performed half an hour before surgery in a separate room dedicated to the performance of regional block. All patients were connected to standard monitors (ECG, pulse oximetry, and noninvasive blood pressure) after establishing intravenous access, 1–3 mg intravenous midazolam was administered for anxious patients.

Patients were placed in a sitting position. The pertinent surface anatomic landmarks (iliac crests, spinous processes, interspinous gaps) were determined by palpation.

After proper sterilization and draping of the patient’s back, epidural anesthesia was done using an 18-G Tuohy epidural needle (modified for the use in obese patients, that is, longer than the regular 80 mm needle). Maximal three attempts were allowed; an attempt was defined as redirection in the same space or choosing a different space for EDS localization. For more than three attempts, the procedure was considered as failure, the epidural catheter was not placed, and the patient was withdrawn from the study.

Constant pressure graph trace and acoustic dip in the APAD-guided cases and LOR in US technique were the end points for successful localization of space.

In group I, a SonoSite M-Turbo (Sonosite, Bothell, Washington, USA) US machine and a low-frequency (2–5 MHz) curved array probe were used to do an US-assisted epidural analgesia. The probe was oriented longitudinally to obtain a parasagittal oblique view of the thoracic spine, in which the interlaminar spaces were identified and marked. The probe was then rotated 90° to obtain a transverse view of the thoracic spine. The interspinous and interlaminar spaces were identified by visualizing the intrathecal space between the ligamentum flavum–dura mater complex and the posterior aspect of the vertebral body. The midline and the location of each interlaminar space were marked on the skin. The Tuohy needle was introduced in the midline under vision by the US just to be sure that the needle is in the right track. Then the needle was advanced slowly till it reaches the EDS without the use of the US. Space localization was accomplished using LOR to saline.

In group II, APAD (Medky equipment’s Schansestraat; The ) device was used to locate the EDS. The anatomical landmarks were used to identify the point of entry of the needle. The epidural needle is connected to the APAD device through a transducer. The diaphragm senses the pressure changes as the needle is advanced through the ligaments. A sensor in the device records the change in pressure, this pressure changes are represented as audible acoustic signal and visual graphics displayed on the device monitor. The disposable kit of the APAD has two ends. One end is connected to a syringe filled with 50 ml saline mounted on an infusion pump (delivers fluid at 50 ml/h); the other end is the one connected to the epidural needle through the transducer. The proper function of the kit was tested before starting the procedure. First, the end of the tube that will be connected to the needle was occluded by the thumb, and then the pressure is permitted to rise in the tubes. The rise in the pitch of acoustic signal was noted; the higher the pressure, the higher is the pitch tone. If the release of occlusion is followed by the sudden drop in the tone, then the device is cleared for the procedure.

A research coordinator recorded the depth of the EDS from the skin, the number of attempts to perform the epidural anesthesia, the repositioning of the epidural needle either in the same space or in different space, the time taken for space localization (it is defined as the time taken in seconds from skin puncture until successful space localization). Any complications such as paresthesia during catheter insertion, dural puncture, blood in catheter, and root irritation were recorded. One attempt is defined as inserting the needle into the patient’s back till it is removed completely whether or not it has reached the EDS.

At the end of performing epidural anesthesia, the patient’s discomfort during the procedure was assessed by visual analogue scale (VAS) score 0–10 (<3=mild discomfort, 4–6=moderate discomfort, and 7–10=severe discomfort).

In both groups, a 3 ml test dose of 2% lignocaine with epinephrine was administered to test the success of catheter placement.

  Results Top

One hundred patients enrolled in this study were divided into two equal groups. The demographic data of patients of the two groups (age, sex, and BMI) were recorded and analyzed. No statistical differences were found between these data in both groups ([Table 1]).
Table 1 Demographic data and BMI in cases of both groups

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The success rate of EDS localization in the first attempt was higher in group II (APAD cases) as compared with group I (US-guided group) (83 vs. 79%), but there was no significant statistical difference between both groups (P=0.461) ([Table 2]).
Table 2 Epidural analgesia procedure data in both groups and P value

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Mean time for EDS localization was statistically significantly longer in group I (US group) than in group II (APAD) (78.44±23.6 vs. 58.78±22.2 s; P<0.00001) ([Table 2]).

A sensory level (after injecting the test dose through catheter) was elicited in all patients of both groups.

Patients in group II were statistically more comfortable during the procedure than patients of group I as the mean VAS score for discomfort postprocedure was 1.74±0.85 and 2.5±1.18, respectively, P<0.001).

No statistical difference was elicited as regards the depth of the EDS between groups I and group II (6.38±0.98 vs. 6.42±1.14 cm; P=0.425).

Dural puncture was observed in one patient in group I versus two patients in group II.

  Discussion Top

This study showed a statistically significant longer mean time for EDS localization in group I (US group) than in group II (APAD), but there was no difference in the success rate. It also demonstrated that using the APAD was statistically associated with more comfort for the patients as shown by the VAS score.

In 2014, Nishiyama [9] demonstrated the feasibility of using US in obese patients for the placement of thoracic epidural catheters. The success rate of placement of the catheter during the first attempt was 14/16 (87.5%), which is comparable to the success rate in the current study (79% in the first attempt). The difference may be attributed to the larger sample size, in the present study 50 patients compared with only 16 patients in the Nishiyama study.

On the other hand, Qian et al. [10] demonstrated a lower success rate of 63.3% in the first attempt. The lower success rate of the study by Qian et al. [10] might be because their study was performed on pregnant patients.

Objective confirmation of EDS and correct catheter placement using acoustic dip in pitch tone along with visual graph sketching in group II 50/56 were noticed in the current study; however, Lechner et al. [8] demonstrated 100% success rate. The obese patients with a BMI of 30 or more enrolled in this study may have contributed to the lower success rate.

Mean time for EDS localization was statistically significantly longer in group I (US assisted group) than in group II (APAD). The continuous and fast progression of Tuohy needle in patients of APAD till it reaches the EDS compared with the intermittent advancement of needle in the US group could explain the shorter time needed to localize the space in APAD group than in patients of US-guided group (58.78±22.2 vs. 78.44±23.6 s; P<0.00001). The uninterrupted and relatively faster movement of needle in group II (APAD technique) is due to the fact that the needle advancement is guided by the change in pitch tone. The sense of hearing can detect small changes in pitch tone, while the US may need some time to get a most clear view of the needle and its progress through the different layers especially in obese patients [11].

The objective confirmation of EDS obtained by using the APAD is due to the integration of the basic principle of LOR technique with audiovisual aids. The transducer sensed the changes in the pressure, which were displayed graphically and represented by augmented audible signals. Depending on the pressure changes as the needle progresses through different tissue layers, pitch tone variations help in better needle handling (the needle is held and advanced swiftly with both hands, guided by changes in acoustic signals) and thus the success, the relative ease of the procedure. The use of APAD in performing thoracic epidural anesthesia was also addressed by another study by Lechner in 2004 who stated that it is possible to perform thoracic epidural anesthesia using APAD with greater safety and accuracy than with the traditional method of LOR, but it was performed on 100 average patients not obese ones as in this study [12].

The lesser tissue handling in group II (no ultra sound probe being manipulated on the patient back) and shorter time of localization of the EDS with continuous and steady movement of needle also explain significantly lower patient’s discomfort during the procedure as shown by the VAS score in the APAD group as compared with the US group (1.74±0.85 vs. 2.5±1.18; P<0.00001). The incidence of dural puncture was statistically insignificant between both groups.

  Conclusion Top

From the previous findings of shorter time for EDS localization and more patient comfort when using the APAD than in US-guided procedure, we concluded that using APAD for performing thoracic epidural anesthesia in obese patients is a better choice than using US.


APAD-guided thoracic epidural anesthesia is recommended in obese patients as it is a quick, safe, and effective procedure.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Flegal KM, Carroll MD, Ogden CL, Johnson CL. Prevalence and trends in obesity among US adults, 1999–2000. JAMA 2002; 288:1723–1727.  Back to cited text no. 1
Adams JP, Murphy PG. Obesity in anaesthesia and intensive care. Br J Anaesth 2000; 85:91–108.  Back to cited text no. 2
Ingrande J, Brodsky JB, Lemmens HJM. Regional anesthesia and obesity. Curr Opin Anaesthesiol 2009; 22:683–686.  Back to cited text no. 3
Motamed C, Farhat F, Rémérand F, Stéphanazzi J, Laplanche A, Jayr C. An analysis of postoperative epidural analgesia failure by computed tomography epidurography. Anesth Analg 2006; 103:1026–1032.  Back to cited text no. 4
Balki M, Lee Y, Halpern S, Carvalho JC. Ultrasound imaging of the lumbar spine in the transverse plane: the correlation between estimated and actual depth to the epidural space in obese parturients. Anesth Analg 2009; 108:1876–1881.  Back to cited text no. 5
Chin KJ, Karmakar MK, Peng P. Ultrasonography of the adult thoracic and lumbar spine for central neuraxial blockade. Anesthesiology 2011; 114:1459–1485.  Back to cited text no. 6
Arzola C, Davies S, Rofaeel A, Carvalho JC. Ultrasound using the transverse approach to the lumbar spine provides reliable landmarks for labor epidurals. Anesth Analg 2007; 104:1188–1192.  Back to cited text no. 7
Lechner TJ, van Wijk MG, Jongenelis AA, Rybak M, van Niekerk J, Langenberg CJ. The use of a sound-enabled device to measure pressure during insertion of an epidural catheter in women in labour. Anaesthesia 2011; 66:568–573.  Back to cited text no. 8
Nishiyama T. Thoracic epidural catheterization using ultrasound in obese patients for bariatric surgery. J Res Obes 2014; 2014. Article ID 538833, DOI: 10.5171/2014.538833.  Back to cited text no. 9
Qian W, Cheng Y, Tian-long W. Ultrasound facilitates identification of combined spinal-epidural puncture in obese parturients. Chin Med J (Engl) 2012; 125:3840–3843.  Back to cited text no. 10
Blamey PJ, Cowan RS, Alcantara JI, Whitford LA, Clark GM. Speech perception using combinations of auditory, visual, and tactile information. J Rehabil Res Dev 1989; 26:15–24.  Back to cited text no. 11
Lechner TJ, van Wijk MG, Maas AJ, van Dorsten FR. Thoracic epidural puncture guided by an acoustic signal: clinical results. Eur J Anaesthesiol 2004; 21:694–699.  Back to cited text no. 12


  [Table 1], [Table 2]


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