This lesson can be saved to your computer by going to the file menu and clicking on save as. Save this lesson as Spinal Anesthesia Lesson 2.
Spinal Anesthesia or A Point Well Taken
Approaches to the Subarachnoid Space
Okay, let’s get down to brass tacks (Whatever that means?)There are basically three approaches to entering the subarachnoid space using a spinal needle. The most common way is the midline approach. With this technique, the needle is directed into the midline between the spinous processes and toward the umbilicus on a slight cephalad angle.
Midline, Lateral or Paramedian
If bone is encountered early, the needle is withdrawn to the subcutaneous tissue and redirected upward. If bone is encountered when the needle is deeply inserted, the needle should be withdrawn and redirected downward.
The second variation is the lateral or paramedian approach. I think this is a great approach because it not only reduces the incidence of PDPH but it is also ideal for patients with arthritic or deformed spines. Using this technique the puncture is performed 1.5 cm. lateral to the midline opposite the center of the interspace selected. A normal 3 inch spinal needle is inserted approximately 10º to 15º off the midline without deviation cephalad or caudad. With the needle to the center of the interspace very little resistance will be encountered until the needle enters the tough ligamentum flavum. At this point the needle is advanced ever so slightly until the subarachoid space is entered. Using this technique, the needle essentially avoids the supra and interspinous ligaments. You will find that this technique is quite simple and that needle deviation is less likely to occur. Normally when a Quinke needle is inserted using a midline approach with the bevel facing parallel to the longitudinal axis of the spine (notice I said spine, not dural fibers), the needle tends to deviate approximately 2.3 mm off the midline in a direction opposite the needle’s opening. This often causes the needle to encounter bone. The clinician is forced to redirect the needle and in some instances this redirection of the needle proceeds repeatedly. It sort of reminds me of someone looking for water using a divining rod. This deviation of the needle, I believe, occurs because the needle must pass through the supra and interspinous liagaments which, of course, are avoided using the lateral approach. A modification of the paramedian approach comes from our friends from the Department of Anatomy at the University of Pretoria, South Africa. Although their technique is described for the placement of an epidural needle, it is just as appropo for the insertion of a spinal needle. Using their technique, the needle is inserted through the skin just lateral to the dorsal spine making sure that the needle is perpendicular to the skin (no upward direction). The needle is then advanced towards and onto the lamina of the vertebra parallel to the dorsal spine and thereafter it is walked cephalad on the lamina (maintaining contact with the lamina) to its superior ridge (needle walked along the lamina in a superior direction). The needle is withdrawn slightly to allow for more superior direction as it is walked cephalad along the lamina. The change in angle between attempts is not more than 10º, with no inward direction of the needle.
The third technique is called the Taylor approach. Althouhgh I’ve attempted to do this technique only once during my carreer (and unsuccessfully might I add), those who have been sucessful in employing the technique tell me that its quite useful in patients who have had lumbar spine fusions.
Taylor approach
In this technique, a 12 cm (5 inch) spinal needle is directed upward, medially and forward at an angle of about 50 degrees, 1 cm. medial and 1 cm. above the lowest prominence of the posterior- superior spine. The angle of the needle approximates the angle the dorsal aspect of the sacrum makes with the skin. The needle is then advanced so that its point enters the lumbosacral space between the sacrum and the last lumbar vertebra.
Specific Gravity, Density and Baricity
Now comes the next critical step in giving a spinal anesthetic, the actual injection of the local anesthetic. For the novice, this step is often delayed not because he or she is unprepared; it’s usually because the anesthetist is awe-struck at the realization that he, indeed, was able to intentionally poke a hole in the dura. You can’t mistake this individual. He’s the one that after 35 minutes of poking and jabbing, fills himself with pride as he stares at the spinal needle from which spinal fluid continuously drips. But when he emerges from his reverie, he is faced with thta fact that he now has to give one of three types or categories of local anesthetics. The anesthetic solutions are usually categorized as hyperbaric, isobaric or hypobaric. But what do we actually mean by these terms?
The classical definition for density is "mass per unit volume". I always cringe when I read this definition because I have a hard time relating to it. A mass of what? And what the heck is a unit volume? Well, there is web site which I had the good fortune to stumble upon while squandering my time surfing the net, that very nicely explained the concepts of density and specific gravity. The site is called, “AskGeoMan” and the following is the verbatim transcript which helps define the terms density and specific gravity:
“What are density and specific gravity?
When the density of a substance is recorded a small excript representing the temperature at which measurments are made is often placed next to the figures. For example the density of distilled water at 37ºC would be written as 0.9934 (37ºC) or the density of cerebral spinal fluid (CSF) at 37ºC would be indicated 1.00010 (37ºC). Since specific gravity compares the density of one substance at a specific temperature to the density of another substance at a certain temperature, the calculated number would have two excripted numbers. For example the specific gravity of CSF (which is calculated by comparing the density of CSF to the density of water) would be written as 1.0076 (37º)/(37ºC) if the density measurements of both CSF and water were made at 37ºC.
In anesthesia, we don’t compare the density of a local anesthetic to water. Rather we compare the density of a local anesthetic to the density of CSF. This comparative ratio is termed baric gravity or baricity. Like specific gravity the derived number is portrayed with two temperature excripts. Lypholized (powdered) tetracaine, for example, can be prepared using sterile water, a dextrose solution, or with CSF. When prepared with sterile water to a concentration of 0.1%, its baricity is 0.9933 (37ºC)/(37ºC) and is termed hypobaric because its baricity is less than CSF(i.e. 1.0000 (37ºC)/(37ºC)). Similarly, a 0.5% concentration of tetracaine prepared with 0.45% saline and 5% dextrose has a baricity of 1.0127 and thus is termed hyperbaric because its baricity is greater than that of CSF. And if tertacaine is prepared with CSF to a final concentarion of 1.0% then it will have a baricity very similar to CSF.
At this point you should be asking yourself two questions: What’s on television and is there anything good to eat in the refridgerator? Although these are very good questions, which I frequently ask myself while trying to study this stuff, they are certainly not germain to the issue at hand. The two questions you should be asking yourself are: 1) What’s the importance of baricity of the local anesthetic to the dermatomal level? and 2) How does the concentration relate to the level of block?
Let’s suppose you wanted to provide a spinal anesthetic to male patient who requires an inguinal hernia repair. To achieve adequate anesthesia for this surgery you must extend the height of your block to at least the T6 level because the spermatic cord receives neural innervation from this level. Since you are injecting the local anesthetic into the subarachnoid space of the lumbar spine, you need to assure that the anesthetic will spread cephalad. This means that once the patient is positioned supine the anesthetic must be heavy enough to traverse the highest point in the lumbar curvature which in the adult male is at about the third lumbar vertebra and make its way cephalad toward the lowest point of the thoracic vertebrae (T5).
A hyperbaric solution will help assure that the level of the block will be achieved since it will easily disseminate through CSF. On the hand, if this patient needed a knee arthroscopy an isobraic local anesthetic solution, which is usually prepared by mixing the local anesthetic with the patient’s CSF, is usually sufficient to block the nerves innervating the knee. In this instance the need to extend the block greater than the lumbar area is not needed although hyperbaric local anesthetics are commonly used and blocks of at least the level of T10 are often achieved. Similarly blocks of the anal-rectal area can be achieved by giving a hypobaric local anesthetic solution with the patient positioned prone and in a head down jack-knife position. In each case, the baricity of the local anesthetic is determined in large part by
the osmolality of the solution.
Osmolality
The density or baric gravity of a solution depends on the amount of solute that is suspended in a certain volume of solution. Once the solute is suspended in solution, the solution exerts a pressure which we call its osmolality. Most spinal anesthetics are hypertonic which means that their osmolality (expressed in milliosmoles) falls outside the range of CSF osmolality (i.e. 257-305 milliosmoles). However if we sequentially dilute the local anesthetic so that the ratio of solute to solvent becomes less (that is we reduce its concentration) the osmolality of the solution and hence its baricity is reduced. To this degree, 5% lidocaine in 7.5% dextrose has a osmolality of 825 milliosmoles; but if we were to remove the dextrose and dilute the lidocaine to 2% and then 1%, the osmolality of the lidocaine solution falls to 321 and 294 milliosmoles respectively. This also means that the solution becomes less hyperbaric. Reducing the concentration of a local anesthetic has some noteable effects. On the positive side it tends to reduce neural damage since all local anesthetics produce reversible histiologic changes in nerve tissue. An example of this is the phenomenon referred to as transient radicular irritation (TRI) that has recently been reported in the anesthesia literature. It is manifested by complaints of aching or cramping of the thighs, calfs or buttocks which seem to worsen at night.The incidence of this problem has been reported to range from about 10% to 37% in the non-pregnant population and the five percent concentration of lidocaine has been implicated in the genesis of the problem. Admittedly the cause and effect is really not so clear because pencil point needles and the lithotomy position have also been implicated. Even the< reduction in concentration to 2% does not seem to ameliorate the problem. Anyway, a reduction in the local anesthetic’s osmolality usually tends to reduce the damage it can potentially cause to neural tissue. Reducing the concentration of a local anesthetic also has the effect of limiting the spread of the anesthetic within the subarachnoid space.
Today, the most commonly used local anesthetics for spinal anesthesia are lidocaine, bupivicaine and tetracaine. Lidocaine and bupivicaine usually come prepared with a dextrose solution thus making them hyperbaric. Tetracaine enjoys greater flexibility of use since it can be prepared with a glucose solution, CSF or sterile water. When prepared as a 1% solution, its osmolality can lie within the range of CSF osmolality or slightly exceed it. However, when prepared with sterile water to a final concentration of 0.1%, tetracaine’s baricity is below that of water. This is an important concept partcularly if you had decided to administer a hypobaric solution for the patient who might be requiring a hemorrhoidectomy. Concentrations greater than 0.1% may not necessarily assure that your solution is hypobaric and an anesthetic of greater baricity injected in the CSF with the patient in a head-down, jack-knife position will only invite dissaster.
What I have been intimating is that we have the ability to control the level and duration of the spinal anesthetic by not only knowing the chemical properties of the local anesthetic (i.e. ionization, protein binding, etc) we use, but also by understanding the physical nature of the administered drug. What I mean by this, is that the level as well the duration of the block can be controlled not only by assuring that the apporpriate baricity of the drug is administered but also by giving a certain amount or volume at a ceratin rate of injection. Lets briefly examine these latter concepts.
Amount of Drug.
The extent of anesthesia varies accordling to the amount of drug. With greater amounts of agent there is an increase in the duration, in the height and in the intensity of spinal anesthesia. For example 5 mg of tetracaine will give a 45 minute block but 15 mg will give a block lasting 3 hours. There is an upper limit to the total amount of agent that may be employed regardless of the volume and this is determined by that amount of the drug which may produce neurological damage. If the volume is fixed, the amount of agent employed is limited by that concentration known to produce central nervous system damage.
Volume of Solution
If the amount of drug remains constant, then the extent of the anesthesia is increased by increasing the volume. In other words a large volume may produce a high level of block. But a large volume with a reduced solute, that is, a weak concentration, may produce an extended block but the intensity and duration of the block will be reduced. When large volumes of spinal anesthetic solutions are used, a simple physical displacement of the spinal fluid may take place. The extent of the anesthesia may then closely correspond to the volume of the subarachnoid space occupied. Investigations on glass models indicate that volumetric displacement actually limits the extent of the anesthesia to the area filled by the agent.
The Rate of Injection.
This is the most important factor in determining the height of anesthesia. Slow injections do not cause marked diffusion, and hence, the levels are low. On the other hand, very rapid injections may cause anesthesia to reach well up into the thoracic area. For example, with procaine a slow injection will give anesthesia to the umbilicus; a moderate rate of injection will give anesthesia to the xiphoid process - a rapid injection will give anesthesia to the nipple line or above.
Barbotage is a method of mixing anesthetic agents in the subarachnoid space. It is derived from the French meaning tame duck and implies a dabbling or puddling. The technique was described in 1907 by our good friend, Bier. The procedure consists of injecting a solution is into the CSF and then immediately withdrawing it only to again be reinjected. This to-and-fro action may be repeated. The distribution of the injected solution can be extensive and unpredictable and is not recommended.
THE FATE OF INJECTED AGENTS
There is a fall in concentration immediately after injection of anesthetic agents into the subarachnoid space. This is due to four factors:
Dilution
Diffusion
Fixation
Absorption by two routes:
Through arachnoid villi
Directly from capillary bed of parenchyma.
Dilution.
Dilution, which occurs within the first 1 to 2 minutes following the injection of the local anesthetic, is due to mixing of the anesthestic with CSF. This process is governed not only by the force with which the solution is injected, but also the amount of the CSF in the subarachnoid space. With rapid rates of injection, turbulent currents in the spinal fluid are produced and mixing and dilution of the local anesthetic is facilitated.
Diffusion.
In the following 2 to 6 minutes there is a second phase of decreasing concentrations. This decrease is primarily due to diffusion of the agent in the spinal fluid by virtue of its molecular motion.
Fixation
Fixation may be considered the process wherein the anesthetic solution becomes adsorbed to nervous tissue. It begins after the anesthetic has been completely distributed.
Fixation is a surface phenomenon. Molecules of the anesthetic agent provide a large, free surface and these come in contact with the nerve fibers as free base. As you may recall (hopefully) local anesthetics have polar groups with a lipophilic aromatic group which has a negative charge and hydrophilic amino group with a positive charge. By polar association the anesthetic molecule orients itself with the nerve cytoplasm in the following manner: the hydrophilic amino group is directed into the axoplasm and lipophilic negative part is oriented in the lipid phase of the surface plasma membrane. Once oriented in this way the molecule undergoes dissociation so that the hydrophilic amino group takes on a hydrogen and a positive charge. This catonic form, it is noted, is a quaternary amino derivative and combines with cellular constituents. The net effect is to prevent depolarization possibly by closure of surface pores.
Absorption
Uptake of local anesthetics from the spinal fluid and from the nerve fibers into the vascular compartment accounts for the third phase of the slow decrease in total concentration of agent in the spinal fluid. Disappearance from the subarachnoid space is via two routes through the arachnoid villi and directly into the capillary or lymphatic channels of the nerve bundles and into capillaries of the nerve tissue parenchyma. The greater portion of the drug leaves the subarachnoid space via the venous drainage while a smaller portion leaves via small lymphatic channels.
The Nitty Gritty
This part of my monograph is what students frequently search for because all the other stuff, although interesting (Yeah, right!) doesn’t tell them how much of a local anesthetic is needed to acheive an adequate level of block for certain procedures. When I was a very junior officer in the Army, my boss was a Colonel who gave 100 mg of lidocaine (I think that’s the only agent he ever gave) to all adults regardless of the patient’s height or procedure. And the truth is, except for a few high blocks, all his patients did remarkably well. Of course,the patients, mostly GIs, were usually strapping, young healthy people. I don’t think a 100mg dosage to a debilitated patient or to a woman attempting to have a vaginal delivery is appropriate. So based on years of experience and on the ability to plagerize the following table is enclosed. Students often copy and reduce this table to keep in their pockets as a quick reference.
This table was reproduced without the expressed permission from the textbook, Nurse Anesthesia 2nd ed, Naglehout and Zaglaniczny (Eds). Philadelphia, WB Saunders Co. p 1004
Well, congratulations. You made it through this pile of verbiage. It wasn’t so painful now, was it? Don’t answer this question; it’ rhetorical.
Click here for Spinal Quiz 2 your final quiz.
References:
Greene HM. A technique to reduce the incidence of headache following lumbar puncture in ambulatory patients, with aplea for more frequent examination of cerebrospinal fluids. Northwest Med 1923:22:240-241.
Greene HM. Lumbar puncture and the prevention of postpuncture headache. JAMA 1926: 6: 391-392
Reina MA., Dittmann D.Garcia AL and van Zundert A. New Perspectives in the Microscopic Structure of Human Dura Mater in the Dorsolumbar Region. Regional Anesthesia 22(2):161-166, 1997
Bijttner J, Wresch KP, Klose R. Postdural puncture headache: Comparison of 25-gauge Whitacre and Quincke needles. Reg Anesth 1993: 18: 166-169.
Ross BK, Benedetti C, Chadwick HS, Mancuso JJ. Sprotte needle for obstetric anesthesia: Decreased incidence of post dural puncture headache, Reg Anesth 1992; 17: 29 -33
Richardson MG, Wissler RN:The Effects of Needle Bevel Orientation During Epidural Catheter Insertion in Laboring Parturients.Anesth Analg 1999;88:352-35
Corbey MP, Bach AB, Lech K, et al Grading of Severity of Postdural Puncture Headache After 27-gaugeQuincke and Whitacre Needles Acta Anaesthesiol Scand 1997; 41 :779-784
Ross AW, Greenhalgh C, McGlade DP, Balson IG, Chester SC, Hutchinson RC, Ashley JE The Sprotte Needle and Post Dural Puncture Headache Following Caesarean Section Anaesth Intensive Care 1993; 21:280-283
Öncel S, Hasegeli L, Uguz MZ, Savaci S, Önal K, Oyman S The Effect of Epidural Anaesthesia and Size of Spinal Needle on Post-Operative Hearing Loss J Laryngol Otol 1992; 106:783-787
Wang LP, Magnusson M, Lundberg J, Törnebrandt K Auditory Function After Spinal Anesthesia Reg Anesth 1993; 18:162-165
Campbell DC, Douglas MJ, Pavy TJG, Merrick P, Flanagan ML, McMorland GH Comparison of the 25-Gauge Whitacre With the 24-Gauge Sprotte Spinal Needle for Elective Caesarean Section: Cost Implications Can J Anaesth 1993; 40:1131-
1135
Lynch J, Krings-Ernst I, Strick K, Topalidis K, Schaaf H, Fiebig M Use of a 25-Gauge Whitacre Needle to Reduce the Incidence of Postdural Puncture Headache Br J Anaesth 1991; 67:690-693
Ready LB, Cuplin S, Haschke RH, Nessly M Westbrook JL, Uncles DR, Sitzman BT, Carrie LES. Spinal Needle Determinants of Rate of Transdural Fluid Leak Anesth Analg 1989; 69:457-460
Iqbal J, Davis LE, Orrison WW Jr, An MRI study of lumbar puncture headaches., Headache 1995; 35 (7):, 420-422)
Hogan QH. Lumbar epidural anatomy. Anesthesiology 1991: 75: 767-775 186. Fink BR, Walker S. Orientation of fibers in human dorsal dura mater in relation to lumbar puncture. Anesth Analg 1989; 69: 768-772.
Dittmann M, Schifer JU, Bond-Taylor W. Anatomical re-evaluation of lumbar dura mater in regard to postspinal headache. Anesthesia 1988; 43: 635-637
Wong CA, Slavenas P. The incidence of transient Radicular Irritation Afewt Spoinsal Anesthesia in Obstetric Patients. Reg Anesth Pain Med 1999;24 (1): 55-58
Tarkkila P, Huhtala J et al. Transient Radicular Irritation Afetr Spinal Anaesthesia with Hyperbaric LignocaineBr J Anaesth 1995; 74:328-329
Hampl KF, Schneider MC, et al. A similar Incidence of transient Neurologic Symptoms after Spinal Anesthesia with 2% and 5% Lidocaine. Anesth Analg 1996; 83: 1051-1054
Kempen PM, and Mocek CK.Bevel Direction, Dura Geometry, and Hole Size in Membrane Puncture.Regional Anesthesia 1997, 22(3):267-272
Nurse Anesthesia 2nd ed, Naglehout and Zaglaniczny (Eds). Philadelphia, WB Saunders Co. p 1004
Principles of Anesthesiology Vincent J. Collins (ED). Philadelphia, Lea & Febiger, pp518-539
Boon JM, Prinsloo E, and Raath RP. Paramedian Approach for Epidural Block: An Anatomic and Radiologic Description. Regional Anesthesia and Pain Medicine. Vol 28. NO.3, 2003: pp 221-227
