GB2262238A

GB2262238A - Hollow needle for use in spinal anaesthesia

Info

Publication number: GB2262238A
Application number: GB9225886A
Authority: GB
Inventors: Magdy Yassin Aglan; Peter Kenneth Stansby
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-12-12
Filing date: 1992-12-11
Publication date: 1993-06-16
Anticipated expiration: 2012-12-11
Also published as: GB2262238B; GB9126396D0; GB9225886D0

Abstract

The needle has a closed tip 12 and a lateral orifice 14 close thereto communicating with an internal passage 11 through the shank, the orifice being equal in area to the internal cross-sectional area of the shank. This serves to maintain ideal flow characteristics through the needle and has the added advantages of increasing the strength of the needle tip and minimising the possibility of inadequate spinal anaesthesia. The needle has an atraumatic tip i.e. has no cutting edges, and will minimise damage to the dura, pushing the fibres aside so that they may close naturally with less spinal fluid leakage. <IMAGE>

Description

A EIOLLOW 1NEEDLE THIS INVENTION concerns a hollow needle for use in spinal anaesthesia which is a common technique used in various surgical procedures. Such a needle has a closed tip and a specifically designed lateral orifice close to the tip and communicating with the interior passage of the needle.

A problem experienced with spinal anaesthesia is the frequent occurrence of post-dural puncture headache. A closed (atraumatic) needle tip separates the dural fibres instead of cutting them, thus minimising cerebrospinal fluid (CSF) leakage and reducing the incidence of this headache.

A relatively new leind of atraurnatic spinal needle, known as the Sprotte needle, is now widely used but itself suffers from the disadvantage that the lateral orifice is elongate and of considerable size. When used for spinal anaesthesia, the Sprotte needle is inserted into the sub-arachnoid space and once CSF is released, spinal anaesthetic is introduced. The excessive length of the lateral orifice of such needles can result in bridging of the dura so that a significant proportion of the anaesthetic drug does not reach the CSF and is lost. Any deeper insertion of the needle to overcome this problem may result in unnecessary trauma to the spinal cord.

Another spinal needle, known as the Whitaire needle, has a much smaller hole which avoids the problem of bridging of the dura but suffers from the disadvantage that a very small lateral hole results in high velocity expulsion of the injected anaesthetic drug which causes an unpredictable spread of the drug and increases the occurrence of an excessive level of anaesthesia with consequent undesirable side effects.

An object of the present invention is to provide a hollow needle for use in spinal anaesthesia wherein the aforementioned problems are overcome.

Thus, according to the present invention there is provided a hollow needle for use in spinal anaesthesia, comprising a shank with an internal passage for fluid, a closed tip and a lateral orifice close to the tip and communicating with the internal passage, characterised in that the lateral orifice is equal to or substantially similar in area to the internal cross-sectional area of the internal passage of the shank.

Preferably, the wall of the needle reduces from the shank to the tip in the form of a convex curve.

Alternatively, the wall of the needle may reduce from the shank to the tip in the form of a cone with a shallow angle.

Furthermore, the wall of the needle reduces from a line which coincides with a point on the periphery of the lateral orifice closest to the tip.

The size of the lateral orifice must be such as to perform two functions, i.e. to determine the correct location of the needle tip by maintaining flow of CSF outwardly through the internal passage of the needle, and to permit injection of an anaesthetic drug into the sub-arachnoid space. When the anaesthetic drug is introduced under pressure from the syringe, the size of the orifice is generally insignificant except that too small a hole causes the injected fluid to issue at e Scessive velocity. Ho - ever, in the initial positioning of the needle which is determined by release of CSF, it is necessary to ensure that the fluid ;i!ill flow through the orifice and outwardly along the interior passage under natural bodily pressures.

The fls -.s characteristics through an orifice equal in size to the internal cross-sectional area of the internal passage was theoretically calculated and experimentally measured with reference to the "head" differential created by the bodily pressure. CSF must pass first through the orifice and then along the length of the needle to the distal end.

For laminar flow the formula for head loss (Hp) along the tube is given by equation number 1 below.

lIp = 32 /1 L V / 5 g D2 (equation 1) In the above equation "u" is the viscosity of the CSF, "L" is the tube length, "V" is the average flow velocity, "6" is the CSF density, "g" is gravitational acceleration and "D" is the internal diameter of the needle. For this to be valid for laminar flow the Reynolds number (R) must be less than 2,000. This is given by equation number 2 below.

R= & D/ji (equation 2) The formula for head loss through the orifice is given by equation number 3 below.

Hh = K v2 / 2 g (equation 3) In equation number 3, "K" is a constant of the order unity and "V" is the velocity through the orifice.

Considering a pressure head of 120mum which is average for normal bodily CSF pressure with a patient lying in a lateral position, and substituted in equation number 1, a flow velocity of 0.05 meters per second is obtained. This in turn is substituted in equation 2 giving a Reynolds number of 17.5. This confirms laminar flow.

Considering a pressure head of 450mm which is the average normal CSF pressure with the patient in a sitting position, a flow velocity of 0.18 metres per second is obtained and this in turn gives a Reynolds number of 63 which still confirms laminar flow.

The value of head drop across the orifice (equation 3) assuming that the orifice is the same diameter as the interior passage of the needle, is of order 0.1mum for a pressure head 120mm, and 1.6mm for a pressure head of 450mm. This is negligible in relation to the overall pressure head difference.

A comparative experiment was carried out using a standard 94-gauge Sprotte needle and a modified needle wherein the size of the lateral orifice was reduced to be substantially the same as the cross-sectional area of the internal passage of the needle. The experiment was set up in which normal saline which has the same viscosity and density as CSF, was allowed to drop through the needle into a container over a period of 4 to 5 minutes using an average head of 120mm. The container was then weighed and the flow rate calculated. This procedure was repeated 3 times, using three different needles of each type, i.e. standard and modified. In all cases the flow rate was 0.01 ~ 0.0001 ml/s. The experiment was repeated with a head of 450mm giving a flow rate of 0.024 + 0.001 ml/s.

It was therefore found by these experiments that while laminar flow was maintained through the needle the readiness of CSF to flow outwardly through the orifice is not diminished by reducing the size of the orifice, to be approximately equal to the internal cross-sectional area of the passage.

For CSF to be released and transferred from inside to outside of the body through the needle, the natural driving pressure head is absorbed by that required to overcome the friction along the internal passage of the needle and that required to cause the CSF to pass through the orifice into the passage. The latter is usually referred to as the minor head loss since it is generally small in relation to the head loss due to the friction along the tube, and this was confirmed by the experiments carried out. Calculating the Reynolds number, with pressure heads of 120 and 450mm, levels of 17.5 and 63 respectively can be obtained. These numbers are well below the generally accepted limit of 2,000 for turbulent flow, thus confirming laminar flow through the needle.As the diameter of the needle is the most important factor affecting the flow rate, there is no benefit in having a lateral hole larger than the cross-sectional area of the needle passage.

Thus, experiment, and by modifying a standard Sprotte needle to produce therein a smaller orifice, it was found that there was no difference in the measured flow rate through the original and modified needles. It follows that the orifice size may be reduced from a Sprotte 24-Gauge of about 1.7mm in length, to something like 0.32mm, without impeding the fluid flow rate to any measurable extent. Also, by reducing the length of the orifice the overall structural stability of the needle in the tip region, is enhanced.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a top view showing an end region of a spinal needle made in accordance with the invention; Fig. 2 is a side view thereof; and Fig. 3 is a cross-section taken on line Ill-Ill of Fig. 1.

Referring now to the drawings, the needle comprises a shank 10 which is hollow to define an internal passage 11. The wall of the needle extends from the shank 10 to the tip 12 in the form of a convex curved reducing wall portion 13.

The wall of the shank 10 immediately adjacent to the reducing wall portion 13 is cut away to define a lateral orifice 14 of circular form and having a diameter equal to or substantially equal to the internal passage 11 of the shank. It can be seen that the wall of the needle tapers from a tangent 15 to the circular orifice 14, i.e.

from a line which coincides with a point on the periphery of the orifice closest to the tip. As indicated by a line 16 in Fig. 2, the internal passage 11 sweeps up to the edge of the orifice 14 at a position thereon closest to the tip. The sweep may extend beyond the orifice if preferred for each of manufacture.

Accordingly, the orifice 14 is smaller than the elliptical orifice of a Sprotte needle which is some five times greater than the cross-section of the internal passage of the needle. The circular hole in accordance with the present invention is less susceptible to peripheral stresses, and the bending stiffness of the needle in the xicinitr of the orifice is greater also as a result of the reduced size of the orifice thus enhancing the structural stability of the needle in the region of the tip. To provide an orifice which is substantially equal to the cross-sectional area of the internal passage presents ideal flow characteristics commensurate with increased structural stability and minimal possibility of the needle bridging the Dura upon insertion.

The curved tip of the needle provides a gentle action which pushes the dural fibres aside during insertion and thus minimises the risk of a leak of CSF.

It is believed that failure of or at least incomplete spinal anaesthesia using the Sprotte needle was due to some of the anaesthetic substance being injected outside sub-arachnoid space owing to the excessively long lateral orifice. Whilst this problem may have been avoided by further inserting the needle by an extra 1 to 2mm after the CSF has been released, this could in many cases cause unnecessary trauma. Reducing the length of the orifice minimises the likelihood of trauma and avoids the possibility of failure of the anaesthetic drug.

It is not essential for the orifice to be circular in shape but if elongate then the cross-sectional area should remain substantially equal to that of the internal passage of the needle.

Further, it is not essential for the needle tip to have a shape in the form of a convex curve. For example it could be conical with a shallow angle but a gradual transition from tip to shank with no sudden changes of direction of the wall will assist in gently parting the dural fibres and minimising patient trauma.

Claims

1. A hollow needle for use in spinal anaesthesia, comprising a shank with an internal passage for fluid, a closed tip and a lateral orifice close to the tip and communicating with the internal passage, characterised in that the lateral orifice is equal to or substantially similar in area to the internal cross-sectional area of the internal passage of the shank.

2. A hollow needle as claimed in Claim 1, in which the wall of the needle reduces from the shank to the tip in the form of a convex curve.

3. A hollow needle according to Claim 1, in which the wall of the needle reduces from the shank to the tip in the form of a cone with a shallow angle.

4. A hollow needle according to any preceding claim, wherein the wall of the needle reduces from a line which coincides with a point on the periphery of the lateral orifice closest to the tip.

5. A hollow needle according to any preceding claim, wherein the internal passage sweeps up to the edge of the lateral orifice at a position thereon closest to the tip.

ss. A hollow needle according to any preceding claim, wherein said lateral orifice is circular.

7. A hollow needle according to any preceding claim, of 24 Gauge and having a circular lateral orifice with a diameter at or in the region of 0.32mm.

8. A hollow needle for use in spinal anaesthesia, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.