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Year : 2017  |  Volume : 11  |  Issue : 1  |  Page : 32-36

Acoustic puncture assist device™ versus conventional loss of resistance technique for thoracic paravertebral space identification: Clinical and ultrasound evaluation

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

Correspondence Address:
Ashraf Abualhasan Abdellatif
Security Forces Hospital, Riyadh
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/1658-354X.197368

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Date of Web Publication2-Jan-2017


Background: Acoustic puncture assist device (APAD™) is a pressure measurement combined with a related acoustic signal that has been successfully used to facilitate epidural punctures. The principal of loss of resistance (LOR) is similar when performing paravertebral block (PVB). We investigated the usefulness of APAD™ by comparing it with the conventional LOR techniques for identifying paravertebral space (PVS).
Subjects and Methods: A total of 100 women who were scheduled for elective breast surgery under general anesthesia with PVB were randomized into two equal groups. The first group (APAD group) was scheduled for PVB using APAD™. The second group (C group) was scheduled for PVB using conventional LOR technique. We recorded the success rate assessed by clinical and ultrasound findings, the time required to identify the PVS, the depth of the PVS and the number of attempts. The attending anesthesiologist was also questioned about the usefulness of the acoustic signal for detection of the PVS.
Results: The incidence of successful PVB was (49) in APAD group compared to (42) in C group P < 0.05. The time required to do PVB was significantly shorter in APAD group than in C group (3.5 ± 1.35 vs. 4.1 ± 1.42) minutes. Two patients in APAD group needed two or more attempts compared to four patients in C group. The attending anesthesiologist found the acoustic signal valuable in all patients in APAD group.
Conclusion: Using APAD™ compared to the conventional LOR technique showed a lower failure rate and a shorter time to identify the PVS.

Keywords: Acoustic puncture assist device; loss of resistance; paravertebral space

How to cite this article:
Ali MA, Abdellatif AA. Acoustic puncture assist device™ versus conventional loss of resistance technique for thoracic paravertebral space identification: Clinical and ultrasound evaluation. Saudi J Anaesth 2017;11:32-6

How to cite this URL:
Ali MA, Abdellatif AA. Acoustic puncture assist device™ versus conventional loss of resistance technique for thoracic paravertebral space identification: Clinical and ultrasound evaluation. Saudi J Anaesth [serial online] 2017 [cited 2021 Mar 5];11:32-6. Available from:

  Introduction Top

The thoracic paravertebral space (PVS) is a wedge-shaped area between the head and neck of the ribs. The posterior boundary is the superior costotransverse ligament (SCTL), anteriorly is the parietal pleura, laterally the posterior intercostal membrane and the base of the triangle medially is the postero-lateral aspect of the vertebra, the intervertebral disc, and the intervertebral foramina. Within this space lies the intercostal (spinal) nerve, the dorsal ramus, the intercostal vessels, the rami communicates, and the sympathetic chain. Local anesthetic (LA) injected into this space results in a dense unilateral sensory, motor, and sympathetic blockade.[1]

The identified advantages of thoracic paravertebral block (PVB) include reduced postoperative pain, analgesic consumption, opioid-related side effects, and improved patient satisfaction.[2] The classic approach in performing PVB is by inserting a needle 2.5 cm lateral to the spinous process of the thoracic vertebra and penetrating the SCTL using a loss of resistance (LOR). One potential difficulty with this technique is the reliance on the subjective feel of LOR as the needle passes into the PVS.[3],[4]

The acoustic puncture assist device (APAD™) is designed to help the identification of the epidural space.[5],[6],[7] The APAD™ records the pressure changes during epidural puncture and interprets this signal as a sound, such that the higher the pressure, the higher the pitch tone and vice versa. The device also demonstrates a diagram of the pressure on the monitor and stores the pressure data on a SD card.

However, the principle of LOR is similar when performing PVB.[8] There have been no reports concerning the identification of the PVS using the APAD™. Therefore, we investigated the usefulness of APAD™ as compared to the conventional LOR technique for identifying the PVS. Confirmation of the position of the needle was done using ultrasound (US) imaging of the tip of the needle in the PVS and LA injection under real-time US imaging.

  Subjects and Methods Top

After approval of the local medical ethics committee (Security Force Hospital, Riyadh, KSA), and with written informed consent of the patients, 100 women, American Society of Anesthesiologists (ASA) physical Status I–III, between the ages of 18 and 75 years old undergoing unilateral elective breast surgery under general anesthesia with PVB were enrolled in this study. They were randomized into two equal groups, a group to receive PVB using APAD™ (APAD group) and a group to receive PVB by the conventional LOR techniques (C group). Patients with known coagulation disorders, body mass index >35 kg/m 2, hypersensitivity to amide LA, skin lesions at the puncture site and patients with spinal deformity were excluded from the study.

PVB was performed in the lateral position with the block side up. Standard ASA monitors were applied, and supplemental oxygen at 6 L/min was administered via facemask. The patient was given divided doses of intravenous midazolam 0.05 mg/kg. Under full aseptic conditions, local infiltration of the skin was performed with 3 ml of 2% lidocaine, 2.5 cm lateral to the most cephalad aspect of the T3 spinous process, seeking contact with transverse process (TP) of the fourth thoracic vertebra as proposed by Eason and Wyatt.[3] All blocks were conducted with an 18 gauge epidural needle (Arrow International, Reading, PA, USA).

In APAD group, the disposable of the APAD™ (Biosensors BV, Hillegom, The Netherlands) was flushed with sterile saline solution, using a 20 ml syringe (Terumo, Laguna, Philippines), the syringe was connected to a syringe pump with an obligatory occlusion alarm set at 900 mmHg and an infusion rate at 100 ml/h. The disposable consists of a sterile extension line (2 m), outfitted with a thin membrane that will expand when the pressure in the tubing rises. The anesthesiologist connected the epidural needle without a stylet to of the extension line and waited until there was a constant stream of saline at the tip of the needle.

Holding the epidural needle with both hands, the anesthesiologist started insertion of the needle perpendicular to the skin in all planes toward the PVS, while concentrating on alterations in sound and the tactile sensation of resistance to the needle. When the TP was contacted, it was accompanied with a high-frequency pitch tone. The needle was withdrawn 1–2 cm then “walked off” caudally and advanced until the SCTL was penetrated.

Since infusion rate was kept constant, the pressure in the system was solely caused by the density of the tissues in which the needle tip was at any given moment, in this way the passage of the needle tip from tissue of high resistance (SCTL) to those of low resistance (PVS) was made visible and audible.

The moment there was a sudden drop in pitch tone, needle insertion was ceased promptly, and the actual pressure level was checked on the monitor. When the pressure remained at this level regardless of a running infusion, it could be presumed that the PVS had been reached. The pressure reading of every puncture procedure was recorded. For all patients, the peak pressure just before identification of the PVS and the plateau pressure with the tip of the needle in the PVS were recorded.

In C group, the anesthesiologist continuously advanced the needle with the constant pressure exerted on the plunger of a saline filled syringe. The needle was advanced perpendicular to the skin. Once the TP was contacted the needle was withdrawn 1–2 cm then “walked off” caudally until the SCTL was penetrated.

After LOR had been achieved, US evaluation was performed using a low-frequency (5–2 MHz) curved array transducer (C60x MicroMaxx; SonoSite Inc., Bothell., Washington, USA) covered with a sterile adhesive covering. The US probe was positioned in a vertical plane medial to the needle with the needle in the out-of-plane position. The TP, SCTL, PVS, and parietal pleura were identified in a parasagittal view. After repeated negative aspiration for blood or cerebrospinal fluid, 20 ml of ropivacaine (0.5% in 5 ml increments) were injected under real-time US imaging to detect both the position of the needle tip and the spread of LA.

The quality of sensory block was assessed by bilateral application of ice and pinprick every 5 min after that up to 30 min after block placement. The attending anesthesiologist measured and recorded the time taken to identify the PVS defined as the time from the first skin perforation until the needle penetration into the PVS, i.e., LOR to saline in C group or the decrease in the acoustic signal in APAD group, furthermore, the attending anesthesiologist was questioned regarding the usefulness of the acoustic signal (indicative or misleading). The needle depth was noted when the TP was contacted and on identification of the PVS. LOR should occur after approximately 1 cm from the TP. If it is not, the needle will be withdrawn from the skin and reinserted after checking landmarks and patient position. After assessment of the block, patients were transferred to the operating room and received General anesthesia (GA).

All blocks were performed by one anesthesiologist with experience in performing PVB, the second anesthesiologist performed observation, recording, and US evaluation. The primary outcome was the number of successful blocks assessed by US and clinical findings. US findings include visualization of either the needle tip or proper spread of the LA in the PVS (observing anterior displacement of the parietal pleura on injection). The clinical finding is adequate sensory block (T2–T6) within a maximum time of 30 min. Secondary outcomes were the time required to identify the PVS, the number of attempts, the depth of PVS, and usefulness of the acoustic signal.

Each patient was examined for acute block complications such as LA toxicity, hypotension, bradycardia, vascular puncture or pneumothorax. Sensory blocks were assessed bilaterally to rule out the spinal or epidural spread. At the end of the surgery, patients were followed up for 12 h, for the 1st h in the PACU then in the general ward later. Chest X-ray was requested if US showed that the needle tip underneath the pleura (intrapleural), the patient had any difficulty in breathing, desaturated or had diminished air entry at any time after the block. Because the focus of the study was the US investigation, postoperative pain scores were not assessed.

  Results Top

Totally 100 women were studied. The characteristics of the patients and types of surgeries are presented in [Table 1]. There was no statistically significant difference between both groups.
Table 1: Patient characteristics and type of surgeries

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In APAD group, successful PVB has been achieved in 49 patients. In one patient, despite accepted US criteria, the block was clinically unsuccessful. Conversely in C group, the correct PVS identification was confirmed by the US in 42 patients, all these blocks were clinically successful. In the remaining eight patients, the LA was seen in the surrounding tissues, these patients did not meet the clinical criteria. A shorter time to identify the PVS was noticed in APAD group. There was no statistically significant difference in the number of attempts and depth of TP or PVS between the studied groups [Table 2].
Table 2: Procedure characteristics

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In APAD group, the mean value of the peak pressure was 45.3 ± 11.6 kpa and the mean pressure in the PVS was 11 ± 3.8 kpa. The attending anesthesiologist found the changes in the acoustic signal in APAD group remarkable in all patients.

Statistical analysis

The sample size was calculated assuming that the APAD™ would improve the success rate of PVS identification by 25% compared to that of the control group, with an alpha error of 0.05 and a power of 80%. Forty-five patients per group were needed to demonstrate statistical significance. Therefore, we enrolled fifty patients in each group to allow for possible protocol violations during the study. Statistical analysis was performed using SPSS 17.0 software (SPSS Inc., Chicago, IL, USA). The data are expressed as mean ± SD or numbers of patients. Normality of numerical data distribution was tested using the Shapiro–Wilk test. Independent sample t-test was used to compare continuous variables exhibiting a normal distribution, and Chi-squared or Fisher's exact test for noncontinuous variables. The value of P < 0.05 is considered statistically significant.

  Discussion Top

“Regional anesthesia always works provided that you put the right dose of the right drug in the right place.” This adage from an editorial by Denny and Harrop-Griffiths [9] sounds simple but reflects the reality of regional anesthesia.

Several attempts have been made to improve or facilitate PVS detection by the LOR technique by adding a visual signal.[8],[10],[11]

The Episure™ Autodetect™ syringe is an LOR syringe with an internal coaxial compression spring that applies constant pressure on the plunger. The plunger automatically depresses when the needle tip enters PVS. A case report has shown that it is valuable for identification of the PVS and has also aided in identifying false LOR.[8] It is currently indicated by the United States Food and Drug Administration for use with an epidural needle for the verification of the needle tip placement in the epidural space.[12]

Epidrum™ is a recently developed air operated, LOR device for identification of the epidural space. It is placed between the epidural needle and the syringe and has a thin diaphragm on the top. The diaphragm is rapidly deflated when the epidural needle tip is located in the epidural space, which allows the operator to interpret the position of the needle tip.[13] Hanley et al.,[10] in a recent study, published a case series of a successful use of Epidrum™ for PVB.

Richardson et al.[11] used pressure monitoring as a supplementary technique for detection of the PVS, a transducer is connected to the end of toughy needle via a three-way tap and a pressure waveform is displayed during needle advancement, there is a sudden lowering of the pressure when the PVS is reached.

In APAD™, using a running infusion pump during pressure measurement prevents obstruction of the needle and exaggerates differences in pressure measurements between tissues. The pressure information obtained on the tip of the needle is translated into an acoustic and visual signal.[14],[15]

In C group, US evaluations showed the tip of the needle to be in the PVS in 42 patients, the blocks were clinically successful in all of them. In seven patients, the tip of the needle and LA injection were visualized above the SCTL in the paraspinal muscles. In these patients, the blocks were not clinically successful. Lonnqvist and Hesser [16] had found that false positive LOR may occur when the needle tip is radiologically imbedded in the posterior spinal muscles. In one patient, the tip of the needle and the LA were seen below the pleura (intrapleural), this patient has partial sensory block.

In APAD group, US evaluations showed either the tip of the needle or the displacement of the pleura during LA injection in all patients. The blocks were clinically successful in 49 patients. In one patient PVB was clinically unsuccessful, the sensory block was restricted to T4 dermatome, an intercostal spread of LA was suspected.

The “pop” sensation described when penetrating the SCTL is not a consistent finding and may not be appreciated as a definitive give, which may be one of the factors leading to the reported failure rates of 6.1% and 10.1% of PVBs.[17],[18] Hadzic and Vloka [19] also stated that such a change in resistance is subtle and nonspecific at best. They do not utilize LOR and rely on the anatomical relationship of the advancement of the needle tip 1 cm past TP. In our study, we noticed that the pressure difference between the SCTL and PVS was small compared to the pressure difference between the ligamentum flavum and epidural space in previous studies.[5],[6],[7],[13] This may explain the indistinct LOR when performing the PVB compared to epidural block.

The attending anesthesiologist distinguished the change in the acoustic signal in all patients. It is well-known that the auditory stimulus is better suited to detect small changes than tactile ones.[20]

In C group, two or more attempts were required to locate the PVS in four patients compared to two patients in APAD group. The anesthesiologist could not get LOR up to 1 cm past TP in three patients in C group and one patient in APAD group. Accidental vascular puncture and aspiration of blood from toughy needle occurred in one patient in each group.

In one patient in APAD group, there was an initial drop in acoustic signals shortly after insertion of the needle, however, it was not like the typical one so the needle advancement had been stopped and pressure tracing was observed. There was a decrease in pressure followed by a progressive increase in plateau pressure. The anesthesiologist continued the needle insertion until there was a typical fall in the acoustic signal, and identification of the PVS was confirmed by the pressure reading.

The cause of the fewer attempts in APAD group compared to C group is thought to result from that acoustic signal replacing the subjective detection of resistance change by the operator's thumb. The pressure reading provides an objective endpoint of the identification of the PVS, and one is no longer restricted to the interpretation of the tactile input alone.

Although there was no statistically significant difference in the depth of the PVS, the time from the skin penetration to the PVS identification was shorter in the APAD group compared with the C group. The APAD™ enabled the anesthesiologist to control the Tuohy needle with both hands which improved handling and passage of the needle through the SCTL.

Accidental epidural or intrathecal puncture did not occur in any patient. Vascular puncture occurred in one patient in each group. There was one incident of a pleural puncture in C group, the patient had no complaint. He had been followed by a postoperative Chest X-ray which was free. One patient in C group and two patients in APAD group developed hypotension (myelin basic protein decreased >20% of its basal value) and responded to ephedrine 5 mg increments and lactated Ringer's solution. No LA toxicity was recorded. In a multiple center, prospective study of 367 pediatric and adult patients, the frequency of complications was hypotension in 4.6%, vascular puncture in 3.8%, pleural puncture in 1.1%, and pneumothorax in 0.5%.[18]

We concluded that identifying the PVS with the aid of APAD™ is reliable, simple, and safe. Sensitivity and specificity are improved, and correct needle placement becomes objective and reproducible. APAD™ offered several advantages over conventional LOR techniques regarding increased success rate, the time required for successful detection, and greater Tuohy needle stability. A limitation of this study is that because we could not blind the equipment per se, results might possibly have been affected.

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Conflicts of interest

There are no conflicts of interest.

  References Top

Macintosh RR, Bryce-Smith R. Local Analgesia-Abdominal Surgery. 2nd ed. Edinburgh: Livingstone; 1962.  Back to cited text no. 1
Najarian MM, Johnson JM, Landercasper J, Havlik P, Lambert PJ, McCarthy D. Paravertebral block: An alternative to general anesthesia in breast cancer surgery. Am Surg 2003;69:213-8.  Back to cited text no. 2
Eason MJ, Wyatt R. Paravertebral thoracic block-a reappraisal. Anaesthesia 1979;34:638-42.  Back to cited text no. 3
Boezaart AP, Raw RM. Continuous thoracic paravertebral block for major breast surgery. Reg Anesth Pain Med 2006;31:470-6.  Back to cited text no. 4
Lechner TJ, van Wijk MG, Maas AJ, van Dorsten FR, Drost RA, Langenberg CJ, et al. Clinical results with the acoustic puncture assist device, a new acoustic device to identify the epidural space. Anesth Analg 2003;96:1183-7.  Back to cited text no. 5
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[PUBMED]  Medknow Journal  
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Hanley C, Sweeney KJ, Kerin MJ, McDonnell JG. Successful use of 'Epidrum' loss-of-resistance device for thoracic paravertebral blockade in patients undergoing breast cancer surgery: A case series. Eur J Anaesthesiol 2014;31:648-9.  Back to cited text no. 10
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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;66:568-73.  Back to cited text no. 14
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  [Table 1], [Table 2]


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