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ANALGESIA

The Episure^TM Syringe: A Novel Loss of Resistance Syringe for Locating the Epidural Space

From the Department of Anesthesia, Stanford University, Stanford, California.

Address correspondence and reprint requests to Edward T. Riley, MD, Department of Anesthesia, Stanford University Medical Center, 300 Pasteur Ave., Stanford, CA 94305. Address e-mail to edriley{at}stanford.edu.

Abstract

INTRODUCTION: The Episure syringe^TM is a unique spring-loaded loss-of-resistance (LOR) syringe with a coaxial compression spring within a Portex Pulsator^TM LOR syringe. This syringe supplies a constant pressure while the operator is advancing the Tuohy needle.

METHODS: We evaluated the syringe using an artificial model of the ligamentum flavum, an anesthetized pig, and women who desired epidural analgesia for labor.

RESULTS: The operator, using the spring-loaded syringe, was able to stop the forward movement of the needle, so that compared with a standard LOR syringe less of the needle protruded out the back of the laboratory model. Satisfactory labor analgesia in the human study and radiograph analyses in the porcine model confirmed epidural placement in all attempts.

CONCLUSION: The spring-loaded syringe is a potentially useful LOR syringe that provides a reliable, objective end-point for identification of the epidural space.

The loss-of-resistance (LOR) technique, the most commonly used technique for locating the epidural space (1), is subjective and operator-dependent. To standardize the force, we modified the Portex Pulsator^TM LOR syringe (Smiths Medical MD, St. Paul, MN), so that the intrasyringe pressure during needle placement is provided by a compression spring, not by the operator’s thumb (Fig. 1). The Episure syringe^TM (Indigo Orb, Santa Clara, CA) is a spring-loaded syringe with a coaxial compression spring that provides a restoring force to the syringe plunger. The aim of this syringe design is to provide a more objective sign that the epidural space has been entered compared with the traditional subjective, operator-dependent LOR techniques. We now report on how this syringe functioned on a laboratory and animal model as well as in a clinical setting.

View larger version (27K): [in this window] [in a new window]   Figure 1. An illustration of the spring-loaded loss-of-resistance syringe. The components of the syringe are named in the drawing.

METHODS

Experiment 1: Laboratory Model
A 17-gauge Tuohy needle was introduced into 2-cm thick, closed-cell foam (secured to a wood block). The experimental spring-loaded syringe was filled with air and attached to the hub of the Tuohy needle. The needle was advanced until there was LOR, indicating that the needle-tip had passed through the back end of the foam. Calibrated rods of a known diameter were used to measure how far the needle tip protruded from the back of the foam.

A total of 60 replications (20 replications per volume) were performed with the spring-loaded syringe containing varying volumes of air (2, 4, and 6 mL). After testing with the spring-loaded syringe, a standard LOR syringe (Epilor^TM syringe, BD Medical Systems, Franklin Lakes, NJ) filled with air was used to find the LOR through the foam model. Ten replications were performed using a standard LOR syringe filled with air, and the distance the needle tip protruded from the back of the foam was measured with calibrated rods. An experienced obstetric anesthesiologist conducted all the laboratory experiments.

Experiment 2: Animal Model
After IRB approval, a pig was anesthetized and placed in the prone position. The epidural space was located with a 17-gauge Tuohy needle using the spring-loaded syringe filled with air or saline. Three spring-loaded syringes were tested. The syringes were identical except for the springs that had different constants and preloads (Table 1). When we used air in the spring-loaded syringe, we experienced a number of false LORs, and after five attempts, we changed to saline. Fifteen consecutive attempts to identify the epidural space with the spring-loaded syringe containing saline were made (five attempts with each spring-loaded syringe of different constants and preloads outlined in Table 1). Placement of the needle in the epidural space after LOR was confirmed by radiograph after injection of contrast dye through the Tuohy needle. Although all three syringes with different constants and preloads performed equally well, we chose the syringe with the lowest spring constant (Syringe 1 in Table 1) to evaluate clinically as the investigators felt that this spring-loaded syringe would be more sensitive in determining LOR.

Experiment 3: Clinical Study
After obtaining IRB approval and written informed consent, 30 ASA 1 or 2 parturients of mixed parity requesting epidural analgesia during active labor were enrolled in this unblinded prospective study. With the patient in the sitting position, local anesthetic was infiltrated, and a 17-gauge Tuohy needle was inserted into the L2/3 or L3/4 interspace until the operator felt the needle was imbedded into the interspinous ligament. The spring-loaded syringe filled with normal saline was attached to the hub of the Tuohy needle and advanced with both hands on the wings of the Tuohy needle until a LOR was noted. Once the epidural space was located, a 19-gauge, wire reinforced, single-orifice epidural catheter (B. Braun Medical, Bethlehem, PA) was inserted 5-cm into the epidural space and aspirated for blood or cerebrospinal fluid. Initial analgesia was provided with 15 mL of 0.125% bupivacaine plus 10-µg sufentanil.

The primary end-point was initial satisfactory analgesia, defined as reported pain scores less than or equal to 1 obtained within 30 min of the initial bolus (Verbal Pain Score 0–10 with 0 = no pain and 10 = worst imaginable pain). Successful epidural placement was assumed if initial satisfactory analgesia was obtained.

Demographic and obstetric data, including the age, height, weight, and parity of the patient, were recorded along with the depth to the epidural space, the number of passes required to enter the space, whether any false LOR or insertion difficulties were encountered and if any accidental dural puncture occurred. The two authors performed all the epidural insertions (15 epidural placements each).

Data are reported as the mean ± sd and median (range), unless otherwise stated. Descriptive statistics were used to summarize demographic, obstetric and outcome data. Normal distribution was determined using Quantile-Quantile plots and the Kolmogorov-Smirnov test and Student’s t tests and analysis of variance were used as appropriate. Analyses were performed with SPSS 11.0 statistical package (Chicago, IL), and Microsoft Excel with P < 0.05 was considered statistically significant.

RESULTS

Experiment 1: Laboratory Model
In all 60 replications using the spring-loaded syringe, there was no movement of the spring-loaded syringe barrel until the tip of the epidural needle passed through the back end of the closed-cell foam. The mean distance the needle protruded from the back of the foam was 1.2 ± 0.1 mm when using the spring-loaded syringe compared with 1.6 ± 0.3 mm when using a standard LOR syringe (P < 0.0001). There was no difference in the length of the needle protrusion in the spring-loaded syringe groups containing 2, 4, or 6 mL of air (P = 0.13).

Experiment 2: Animal Model
When the spring-loaded syringe was filled with saline, there were no false LORs, and the barrel moved forward quickly when the epidural space was entered. All three of the springs performed well with saline and we experienced no false LORs with the saline-filled spring-loaded syringe. The epidural space was easily located in all 15 attempts with the spring-loaded syringe (five attempts per different constants and preloads outlined in Table 1).

Experiment 3: Clinical Study
The patients’ demographic, obstetric, and epidural placement data are outlined in Table 2. In all 30 patients, the epidural space was correctly identified using the spring-loaded syringe as evidenced by satisfactory analgesia (Verbal Pain Score 1 within 30 min after the initial epidural bolus). There were no accidental dural punctures or false LORs encountered. However, on one occasion, the plunger moved forward about 1 mm and stopped. The procedure was continued without incident. In another patient, there were numerous attempts to locate the epidural space and the number of passes was not counted.

DISCUSSION

This study shows that this unique spring-loaded syringe reliably detected the epidural space in both a porcine model and in a clinical setting. The advantage of the spring-loaded syringe is that both hands can be used to advance and steady the epidural needle. When the spring-loaded syringe was used with air in the porcine model, there were many false LORs because the tissues did not supply enough resistance to keep the plunger from advancing. This may explain why Evron et al. (2) found a higher accidental dural puncture rate when using air compared with saline in their LOR technique.

In the laboratory model, the operator stopped advancing the needle sooner with the spring-loaded syringe than with a standard LOR syringe. Theoretically, this may lead to fewer accidental dural punctures; however, whether this would hold true in the clinical situation is unclear. Demonstrating that this spring-loaded syringe will be associated with fewer accidental dural punctures than a standard LOR syringe will be a difficult task because of the low incidence of this outcome and the large number of patients required to have adequate power in the study (3).

One application for this syringe may be to facilitate teaching of the epidural technique to clinicians. Both the student and the teacher will get an objective, visual signal when the needle tip enters the epidural space. The spring-loaded syringe may assist attending physicians in more closely supervising residents doing an essentially "blind," subjective procedure.

Whether experienced practitioners will use this syringe is unclear. Anesthesia care providers proficient in a particular technique are reluctant to change. In an epidural technique survey, only 48% of anesthesiologists said they would try an alternative if they experienced difficulty with their preferred technique (1).

In conclusion, the spring-loaded syringe provides a unique and potentially useful method of detecting Tuohy needle placement in the epidural space. Filling the spring-loaded syringe with saline was preferable to using air in the porcine model. Future studies are required to determine if this spring-loaded syringe leads to fewer accidental dural punctures or if it reduces the time for residents to learn the epidural technique.

ACKNOWLEDGMENTS

The porcine model was supplied by Indigo Orb, Inc., Irvine, CA. Indigo Orb is a private, for profit, company that develops medical devices and the Episure^TM syringe is one of their products. Indigo Orb had no input in the data analyses or manuscript preparation.

Footnotes

Accepted for publication June 18, 2007.

Presented in part at the 36th annual meeting of the Society of Obstetric Anesthesiology and Perinatology, Fort Meyers, FL, May 2004, and at the 37th annual meeting of the Society of Obstetric Anesthesiology and Perinatology, Palm Springs, CA, May 2005.

The study was conducted at Lucile Packard Children’s Hospital and Stanford University School of Medicine, Stanford, California, and at the LyChron laboratory, Mountain View, California.

Edward T. Riley is currently a shareholder in Indigo Orb.

REFERENCES

1. Wantman A, Hancox N, Howell PR. Techniques for identifying the epidural space: a survey of practice amongst anaesthetists in the UK. Anaesthesia 2006;61:370–5[Web of Science][Medline]
2. Evron S, Sessler D, Sadan O, Boaz M, Glezeman M, Ezri T. Identification of the epidural space: loss of resistance with air, lidocaine, or the combination of air and lidocaine. Anesth Analg 2004;99:245–50[Abstract/Free Full Text]
3. Pan PH, Bogard TD, Owen MD. Incidence and characteristics of failures in obstetric neuraxial analgesia and anesthesia: a retrospective analysis of 19,259 deliveries. Int J Obstet Anesth 2004; 13:227–33[Web of Science][Medline]

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