Elastomeric driver for epidural resistance syringe

A syringe, particularly for epidural anesthesia, has an elastomeric driver in the form of a single elastic band perforated for mounting onto the tip of the syringe between the barrel and the needle hub. The elastic band is stretched into engagement with the thumbrest of the plunger to drive the plunger to full depression into the syringe barrel. The elastic band extends symmetrically on either side of the barrel to apply balanced force. Various configurations for the elastic band are disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention pertains to the field of medical syringes and more 
particularly concerns elastomeric drivers for depressing the plunger of 
resistance syringes used for accurately positioning an epidural needle in 
preparation for injection of anesthetic into the epidural space of the 
spinal cord. 
2. State of the prior Art 
Epidural anesthesia involves the injection of a liquid anesthetic substance 
into the epidural space which surrounds the dura mater which in turn 
surrounds the spinal cord proper. The epidural space is defined between 
the ligamentum flavum on the posterior or back side of the spinal cord, 
and the anterior longitudinal ligament on the anterior or frontal side of 
the spinal cord. These flexible but tough ligaments interconnect the bony 
vertebrae which enclose and protect the spinal cord and spinal canal. 
A dose of an anesthetic such as lidocaine or bupivacaine, by way of 
example, produces a regional nerve block suitable for surgical procedures 
to be performed on portions of the anatomy affected by the nerve block, 
primarily but not limited to portions of the anatomy below the level of 
the spinal column at which the anesthetic is injected. 
In the preferred midline technique the epidural needle passes through the 
supraspinous, interspinous and ligamentum flavum structures before 
entering the epidural space. Insertion of the needle into the epidural 
space is complicated by the lack of feedback as to the position of the 
needle tip, coupled with the imperative need to avoid puncturing the dura 
mater which surrounds the spinal cord, since there is potential for 
catastrophic trauma to the spinal cord with the epidural needle. Extreme 
caution must therefore be exercised in the positioning of the needle tip, 
which must pierce through the tough, resilient, leather-like ligamentum 
flavum and then stop immediately within the narrow epidural space, short 
of puncturing the dura mater. 
The needle must be moved through the ligamentum flavum very slowly and in a 
carefully controlled fashion. At the same time, pressure is applied to the 
plunger of the attached syringe which is filled either with air or saline 
solution. The object is to continuously test for loss of resistance to 
injection, experienced when the needle lumen enters the epidural space 
after clearing the ligamentum flavum. This loss of resistance is 
experienced by little if any resistance to injected air or fluid, and a 
negative aspiration test then indicates that the needle lumen is properly 
positioned in the epidural space. Special syringes, known as loss of 
resistance syringes and characterized by very low friction between the 
plunger and the barrel of the syringe, are used for positioning the needle 
lumen in the epidural space. Once correct positioning of the needle is 
achieved, the resistance syringe is separated from the epidural needle and 
another syringe, loaded with the anesthetic is attached, after which the 
anesthetic is injected. 
In order to appreciate the contribution being made by the present 
invention, it is important to understand the demands placed upon the 
anesthesiologist's dexterity by this procedure. It is of critical 
importance that the needle traverse the ligamentum flavum in a carefully 
measured and controlled manner. Typically, this is achieved by applying 
resistance to the advancing needle with the anesthesiologist's 
non-dominant hand (the left-hand if the anesthesiologist is right-handed) 
while the dominant hand applies pressure to the plunger of the resistance 
syringe to test for resistance to injection while at the same time slowly 
advances the needle. Variations of this technique may be adopted according 
to personal preference, for example the needle may be advanced 
continuously while pressure on the syringe barrel is also maintained 
continuously to test for resistance. In the alternative, the needle is 
advanced in very small increments, e.g. 1 millimeter, testing for 
resistance to injection after each advance. 
The difficulty of correctly positioning the needle lumen in the epidural 
space has spurred many attempts to develop methods and devices for 
detecting and indicating correct needle placement. These expedients have 
generally exploited the low resistance to injection and subatmospheric 
pressure characteristic of the epidural space. One such technique involves 
placement of a drop of saline solution on the open hub of a epidural 
needle. The drop will be "sucked-in" as the needle lumen enters the 
epidural space where, for reasons not well understood, prevails 
sub-atmospheric pressure. Other means used for this purpose include 
capillary attachments with fluid indicators developed by Odom, or inflated 
balloons by Macintosh, which deflate upon entering the epidural space. It 
is also known to use spring loading devices to facilitate the loss of 
resistance phenomena which occur as the epidural needle passes from the 
dense ligamentum flavum into the lesser resistance of the epidural space. 
U.S. Pat. No. 5,024,662, in which this applicant is a co-inventor, 
describes an attachment for a resistance syringe for aiding the 
anesthesiologist in correct placement of the epidural needle. The patented 
attachment has an elastomeric band retained to the syringe barrel by a 
ring which slides onto the syringe barrel against the finger flange of the 
syringe to anchor the ends of the elastic band to the barrel while a 
midportion of the band is pulled by the plunger of the syringe. 
Consequently, the plunger is urged by elastic force into the syringe 
barrel, but is held back by fluid, air or liquid in the barrel, until the 
needle lumen enters the epidural space. At that point the contents of the 
syringe are injected into the epidural space under the force of the 
stretched band, providing the anesthesiologist with immediate kinesthetic 
indication of correct needle placement. 
While this arrangement works well, disposable elastomeric drivers have been 
developed by this applicant which are of still greater simplicity and very 
low cost. 
SUMMARY OF THE INVENTION 
The elastomeric drivers of this invention, described below in several 
alternative forms, are for use with a syringe of the type which has a 
syringe barrel, a tip at one end of the barrel to which can be mounted the 
hub of an epidural needle, a plunger which makes a sliding fit into the 
opposite end of the barrel, and a thumbrest at an exterior end of the 
plunger. 
In general, the elastomeric driver of this invention in its various forms 
has a first portion which is engageable to the tip of the syringe barrel 
and a second portion engageable to the thumbrest of the plunger. In each 
embodiment the elastomeric driver has a width dimension and a thickness 
dimension. The elastomeric driver is apertured for admitting the tip of 
the syringe barrel. The aperture is in the form of one or more holes. Each 
such hole passes through the thickness dimension and is positioned within 
the width dimension of the driver. As installed on the syringe, the length 
of the elastomeric driver is such as to normally urge the plunger to full 
depression into the barrel. That is, the effective installed length of the 
elastomeric driver is smaller than the distance between the barrel tip and 
the plunger thumbrest with the plunger fully depressed into the barrel, 
and the elastomeric driver remains somewhat stretched between the tip and 
thumbrest with the plunger fully depressed in the barrel. 
More specifically, the elastomeric driver according to this invention may 
take several distinct forms. 
In a first, closed loop variant of the invention, the elastomeric driver is 
a continuous elastic band in which the first and second portions, 
engageable respectively to the barrel tip and the thumbrest, are at 
diametrically opposed points of the band. The band may be an annular flat 
band of elastic material and of uniform width. The first portion of the 
elastomeric driver may be a portion of the band adjacent to a single small 
hole in the band. The perforated portion of the band is fitted onto the 
syringe tip and the diametrically opposite portion of the band is 
stretched over the thumbrest. The band may have two diametrically opposite 
holes, either of which can receive the barrel tip, so that the elastomeric 
band can be reversibly installed on the syringe. One or both of the first 
and second portions of the band may be enlarged in width, for example in 
the form of circular pads with a hole centered in the pad, for easier 
handling during installation and greater frictional contact with the 
thumbrest. One or both of the pads may be coated with adhesive for 
positive retention to the thumbrest. 
In a second, open-ended variant of the invention, the elastomeric driver is 
a unitary band of elastic material with two opposite ends, each end being 
perforated for receiving the barrel tip. The elastomeric driver is 
installed on the syringe by sliding the two perforated ends one over the 
other onto the barrel tip, and then stretching a midportion of the 
elastomeric driver over the thumbrest of the plunger, for urging the 
plunger into the barrel under elastic tension of the elastic band. The two 
ends of the band are retained to the syringe tip between the hub of the 
epidural needle and the syringe barrel. The elastic band may be of uniform 
width with a small hole near each end. Alternatively, the ends of the band 
may be enlarged in the form of circular pads perforated by a small hole in 
the center of each end pad. The midportion of the band may be enlarged in 
width, for example, to form a circular pad midway between the ends of the 
band, for improved frictional contact with and retention to the thumbrest 
of the syringe. The midportion of the elastomeric driver, whether enlarged 
or not, may have an adhesive for positive retention of the midportion to 
the thumbrest. 
In any of the various forms of the elastomeric driver, the elastic band may 
have a tab integral with the band and extending transversely therefrom 
adjacent to the thumbrest engaging portion of the band. The tab provides a 
convenient finger hold to facilitate handling and stretching of the band 
during installation on the syringe, after the perforated portion of the 
band has been engaged to the tip. 
It is preferred that the elastic material of the elastomeric driver provide 
sufficient elastic tension for positively driving the plunger into the 
barrel as soon as the needle lumen is unoccluded upon entry into the 
epidural space, but without excessive impact of the plunger against the 
tip end of the barrel. In particular, the elastic force applied by the 
elastomeric driver should be such as to avoid shattering the syringe when 
used with a glass syringe, yet provide positive kinesthetic indication of 
entry into the epidural space. 
These and other features, improvements and advantages of the present 
invention will be better understood by reference to the following detailed 
description of the preferred embodiments and the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a typical disposable plastic resistance syringe generally 
designated by the letter S and which has a cylindrical hollow syringe 
barrel B with a tip T at one end. The tip T has an axial bore opening into 
the interior of barrel B, as shown in FIG. 1a, where the hub H of an 
epidural needle N is shown mounted onto the tip T to place the hollow 
needle in fluidic communication with the interior of the barrel B. A 
syringe plunger P makes a sliding seal L inside the barrel B and is 
movable from the drawn condition shown in solid lining in FIG. 1 to a 
depressed condition L' shown in phantom lining. The plunger has a 
thumbrest R at its exterior, free end. 
FIG. 1 shows an elastomeric driver, generally designated by the numeral 10, 
installed on the syringe S. The elastomeric driver is a unitary 
elastomeric element which has a tip engaging portion 12 and a thumbrest 
engaging portion 16, the connecting portions 18 of the elastomeric element 
10 being stretched between the barrel tip T and thumbrest R so as to urge 
the plunger P into the barrel B, from the solid-lined, drawn position of 
the plunger to the phantom lined, depressed position in FIG. 1. 
The elastomeric driver 10 can take different forms, five of which are 
illustrated in FIGS. 2 through 6. The embodiments illustrated in FIGS. 2, 
3 and 4 are characterized in that the elastomeric element is a continuous 
band of elastic material. The embodiments of FIGS. 5 and 6 on the other 
hand are open-ended bands of elastic material, i.e, which have two 
opposite ends. All of the illustrated embodiments, however, can be 
installed on the syringe S in the manner illustrated in FIG. 1, as will 
now be described. 
The elastomeric driver 10a of FIG. 2 is a continuous elastic band of 
uniform width with a small hole 14 which is of sufficient size to admit 
the barrel tip T of the syringe S. The hole 14 may be slightly undersized 
with respect to the diameter of the tip T, so that the tip engaging 
portion 12a of the elastic band 10a is press-fitted onto the syringe tip 
T. The diametrically opposite thumbrest engaging portion 16a of the band 
10a is stretched over and diametrically across the thumbrest R of the 
syringe S. The length of the band 10a is such that the connecting portions 
18a are elastically stretched between the thumbrest R and the barrel tip T 
even with the plunger T in the fully depressed condition suggested at L' 
in FIG. 1. 
The embodiment of FIG. 3 is a continuous band 10b of elastic material which 
is wider along the connecting portions 18b than the width of the band 10a 
in FIG. 2. The band 10b has a hole 14 sized to receive the syringe tip T, 
and is enlarged in width to form a circular pad 12b around the hole 14 
which receives the barrel tip T. A second circular pad 16b is the 
thumbrest engaging portion of the elastomeric driver 10b and is situated 
diametrically opposite to the tip engaging pad 12b along the closed band 
10b, i.e., the two pads 16a and 16b are connected by two portions 18b of 
equal length. A tab 20 extends from the thumbrest engaging pad 16b 
transversely to the band 10b, and serves as a convenient finger hold when 
stretching the band 10b away from the syringe tip T in order to engage the 
pad 16b to the thumbrest R after fitting the pad 12b onto the syringe tip 
T. 
The embodiment 10c of FIG. 4 differs from that of FIG. 3 only in that the 
circular pad 16b also has a center hole 14, and pad 12b has a tab 20 
similar to the top 20 of pad 16b in FIG. 3. The elastomeric driver 10c of 
FIG. 4 is consequently reversible in that either pad 12b, 16b may be 
fitted to the syringe tip T as the tip engaging portion of the elastomeric 
driver, the other of the pads 16b and 12b being engageable to the 
thumbrest R by stretching the connecting portions 18b as shown in FIG. 1. 
The embodiment of FIG. 5 is an open ended band of uniform width in which 
the tip engaging portion of the elastomeric driver 10c includes two 
opposite ends 22, each end being perforated with a hole 24 sized to 
receive the syringe tip T. The ends 22 of the band 10d are fitted onto the 
syringe tip one over the other as shown in FIG. 1a, stacked one over the 
other along the length of tip T. The thumbrest engaging portion of the 
band 10d is a midportion 26 of the band equidistant between the band ends 
22. The midportion 26 is frictionally engaged to the thumbrest R by 
stretching the connecting portions 28 of the band along diametrically 
opposed sides of the syringe S, analogously to the portions 18 in FIG. 1, 
so that the midportion 26 lies diametrically across the thumbrest R, 
similarly to portion 16 in FIG. 1. Installation of the elastomeric driver 
10d and engagement to the syringe thumbrest R is facilitated by a tab 30 
integral to the band 10d which extends transversely to the midportion 26 
as seen in FIG. 5. The ends 22 are secured to the tip T because the two 
ends 22 are held under elastic tension transversely to the length of the 
tip T by the stretched connecting portions 28 of the band. The tension of 
the portions 28 keeps the two ends 22 against the end of the syringe 
barrel B so that installation of the elastomeric driver 10d does not 
interfere with mounting of the needle hub H on the tip T. 
The embodiment 10e shown in FIG. 6 differs from the elastomeric driver 10d 
of FIG. 5 in that the band 10e is somewhat wider along connecting portions 
28d than band 10d, the tip engaging ends have been enlarged to form 
circular end pads with the tip receiving hole 24 centered in each end pad 
22b, and the thumbrest engaging midportion 26b is of enlarged width to 
form a circular pad integral with the band 10e and equidistant to the end 
pads 22b. The elastomeric driver 10e is fitted to the syringe S by 
pressing the two ends 22b onto the syringe tip T, so that the tip passes 
consecutively through the hole 24 in each pad 22b, and the end pads 22b 
are stacked one over the other on the tip T as just explained in 
connection with the band 10d in FIG. 1a. The connecting portions 28b are 
then stretched by pulling on the midportion 26b over and onto the 
thumbrest R where it is retained by frictional engagement under the 
elastic tension of the stretched portions 28b extending on either side of 
the syringe S, as shown for the side portions 18 in FIG. 1. 
The enlarged width of the elastic band about the holes 14, 24 in the 
several embodiments described above substantially reinforces the strength 
of the material surrounding the holes and ensures against tearing of the 
material during installation of the elastomeric driver to the syringe S, 
as might occur by possible mishandling of the elastomeric driver, such as 
excessive stretching of the elastic band. The enlarged midportion of the 
elastomeric driver, as at 16b in FIG. 3, and 26b in FIG. 6, provides a 
substantially increased area of contact and a consequent increase in 
frictional engagement between the band midportion and the thumbrest R, 
better ensuring retention of the elastomeric driver while handling the 
syringe S. The enlarged midportions 16b and 26b may also be somewhat 
easier to grasp and handle by some users during installation of the 
elastomeric driver onto the resistance syringe S. Positive retention of 
the band midportion to the thumbrest R can be ensured by providing a layer 
32 of adhesive material on an inner surface of the elastomeric driver 
which makes contact with the thumbrest R, as shown by way of example for 
the elastomeric driver 10e in FIG. 6. This adhesive may be a suitable 
pressure activated adhesive,and may be initially covered with a release 
sheet which is removed just prior to installation of the elastomeric 
driver onto the resistance syringe S. The use of adhesive in this fashion 
is appropriate for any of the embodiments shown in FIGS. 2 through 6. 
Installation of the elastomeric driver on the resistance syringe S, in any 
of the described embodiments, does not interfere in any way with the 
normal functioning of the syringe. The epidural needle hub H fits onto the 
syringe tip T in the normal manner, as illustrated in FIG. 1a. The tip 
engaging portion or portions of the elastomeric driver fit onto the tip T 
between the needle hub H and the bottom end of the syringe barrel B, 
without prejudice to normal mounting of the epidural needle to the 
syringe. The needle hub H also acts as a stop for the tip engaging portion 
or portions of the elastomeric driver, which can be beneficial in the 
occasional case where the elastomeric driver 10 might slip off of the 
thumbrest R, in which case the needle hub may help retain the elastomeric 
driver to the syringe S. 
The elastomeric drivers 10a through 10e are unitary elastomeric elements 
which can be produced in large quantities from thin elastic sheet material 
or thin walled elastic tubing, by dye cutting or other well known methods. 
The choice of a particular elastomeric material is not critical and may be 
selected from among the numerous commonly available elastic materials, 
including those employed in the manufacture of ordinary commercially 
available rubber bands of the type sold in office supplies stores and the 
like. The thickness of the elastic material and the width of the elastic 
band is selected so as to provide a positive elastic force driving the 
syringe plunger P into the syringe barrel B when there is no resistance to 
fluid flow through the epidural needle N, thereby to provide a positive 
kinesthetic indication to the anesthesiologist of proper needle placement 
in the epidural space. This general criterion allows a wide and 
non-critical choice of elastomeric band parameters including band 
dimensions and material. A suitable material for the elastomeric drivers 
in any of the embodiments described above is SBR natural rubber in 0.040 
inch thickness for use with plastic disposable resistance syringes. The 
open-ended elastic band 10d may be 6 inches between the end holes 24. The 
continuous bands 10a, 10b and 10c ay be about 3 to 3.5 inches in flattened 
diameter, i.e. approximately 6 to 7 inches in circumference. SBR natural 
rubber in 0.010 inch thickness may be used with glass resistance syringes, 
in which case the length of the elastomeric drivers may be about 1 inch 
shorter than for the 0.040 thickness to partly adjust for the lower 
elastic strength of the thin band. However, many different elastic 
materials may be found suitable for making the elastomeric drivers of this 
invention, with proper adjustment of band thickness and length in 
accordance with the inherent elastic strength of the particular material, 
as will be evident to those having ordinary skill with such materials. 
While certain preferred embodiments of the invention have been described 
and illustrated for purposes of clarity and example, it must be understood 
that many changes, substitutions, and modifications to the described 
embodiments will become obvious to those possessed of ordinary skill in 
the art without their thereby departing from the scope and spirit of the 
present invention which is defined by the following claims.