Injection Pressure as a Marker of Intraneural Injection in Procedures of Peripheral Nerves Blockade

Th e blockade of peripheral nerves carries a certain risk of unwanted complications, which can follow after unintentional intraneural injection of a local anesthetic. Up till today, the research of measuring injection pressure has been based on animal models, even though the histological structure of periphery nerve is diff erent for animal and human population, so the application pressure which presages intraneural injection in human population is still unknown. As material in performing this study there have been used  Wistar rats and  delivered stillborns. After bilateral access to the median nerve, we applied  ml of  lidocaine with epinephrine, with the help of automatic syringe charger. Th e needle was at fi rst placed perineural on one side, and then intraneural on the other side of both examination groups. During every application the pressure values were monitored using the manometer, and then they were analyzed by special software program BioBench. All perineural injections resulted with the pressure ≤ , kPa, while the majority of intraneural injections were combined with the injection pressure ≥ , kPa. Th e diff erence between intraneural and perineural injection pressures for the two diff erent examination groups (rats and delivered stillborns) was not statistically signifi cant (P>,). As prevention from intraneural injections today are in use two methods: the method of causing paresthesia or the method of using the peripheral nerve stimulator. However the nerve injury can still occur, independent from the technique used. If our results are used in clinical practice on human population, than the high injection pressure could be the marker of intraneural lodging of a needle.


Introduction
In the last few years there has been a great leap forward in the development of regional anesthesia.Since  the concept of ambulatory surgery and the concept of acute pain is developing, the equipment for regional anesthesia is improving, the new pharmacological resources and new techniques of regional anesthesia are being found ().At the same time, safer methods of access and outcome for patients are developing, and the advantages of regional anesthesia (more physiological and economical) are taken in consideration, while the general anesthesia is being pushed into the second plan.However, the blockade of peripheral nerves, as any other medical procedure, carries a certain risk of unwanted complications (,,,,).Perioperative neural injury after the blockade of peripheral nerves can be a result of several factors.Th ese factors can be generally divided into two categories: .Ones which are unconnected with the techniques of regional anesthesia and .Ones which are directly connected with the techniques of regional anesthesia.
Factors that are unconnected to the techniques of regional anesthesia are: improper patient positioning or surgical retractor, surgical trauma, ischemia caused by bandaging, improperly applied casts or already existing neurological diseases which now become clinically visible.
In contrast to that, the factors which directly facilitate perioperative neural injuries include: . mechanical trauma caused by needle or catheter . ischemic injury due to vasoconstrictors or neural edema and . chemical injury which can be a result of neurotoxic eff ect of local anesthetic.
It is known, from the researches so far, that the unwanted complications during nerve blockade can happen after unintentional intraneural injection of local anesthetic in the surrounding neural structures (,,).How to prevent mentioned complications are the themes that are most frequently discussed on the congresses of anesthesiologists.As prevention from intraneural injections and consequential complications today are in use two methods of locating nerves.Th ose are: the method of causing paresthesia or the method of using the peripheral nerve stimulator.However the injury can still occur, independent from used techniques.Relatively new method in detection of nerve structures is ultrasound, which beside certain advantages has signifi cant defects.All this presented the need for development new, better and safer method in prevention of unwanted complications.Injection application in diff erent tissues results in diff erent values of injection pressures, which depends on structure, compactness and elasticity of tissue.Up till today, the research of measuring injection pressure has been based on animal models, even though the histological structure of peripheral nerve is diff erent for animal and human population, so the application pressure which presages intanueral injection in human population is still unknown.In experimental animals, like a rat or a rabbit, the peripheral nerves consist of - fasciculi with the diameter of , to , mm, with approximately , axons ( are myelinated motor axons,  and  are myelinated and unmyelinated sensitive axons, and  are unmyelinated sympathetic axons), ().Human peripheral nerves consist of  to  fasciculi with coeffi cient of variation around , and the diameter of , to  mm ().In other words, humans mainly have polyfascicular nerves, unlike the experimental animals in whom dominates the oligofascicular type of nerves.

Materials and Methods
As material in performing this study there have been used:  grown up Wistar rats and  delivered stillborns.-Storing data -BioBench simplifi es getting the data and their adjustment using built-in base of data, which also has all information for majority of physiological monitors and amplifi ers.
-Automatic creation of record -BioBench automatically generates incoming information which then saves to record in order to show the change of stimulated levels of voltage by the user or whether the data is entered or not.It can also input its own data into the record.
-Analysis of files -BioBench analysis can be used to open and analyze already entered data.
-Exporting of files -BioBench can inscribe in ASCII document fi le the data which can be used for further analysis in other software applications.
-Recording graphics -BioBench graphics have the form of traditional physio-graphics and lined tables which are routinely used.
-Confi guration settings -BioBench can record, save and reset diff erent settings.Th is saves time when there are more users that have diff erent confi guration settings.
-Examples of fi les with data for the purpose of learning -BioBench has many examples of fi les with the data that can be looked at without using and additional hardware or equipment for physiological monitoring.
In this study we used BioBench program in order to register and analyze the values of pressures (expressed in psi -pound per square inch;  psi = , kPa) , during intraneural and perineural application, registering also the time interval needed for the application.

Results
Statistical analysis was executed using SPSS program, version ,.Maximum value of pressure (psi) during intraneural and perineural injection is compared using paired t-test.P value < , is considered signifi cant.
Generally speaking, all injections were characterized by small increase of pressure in the beginning of application (fi rst - seconds), resulting in maximum pressure, which was then followed by significantly lower pressure during the remaining part of application.Even though all perineural injections resulted with the pressure ≤ , kPa, the majority of intraneural injections were combined with the injection pressure ≥ , kPa.In rats, during intraneural applications, the maximum pressure was , kPa, while the minimum pressure was , kPa, achieved in peak eff ect.Maximum pres-sure reached in all perineural applications was , kPa and minimum was , kPa, also achieved in peak effect (Graphic . and .).The average value of maximum pressure achieved in peak eff ect for intraneural injection was ,±, kPa (the average value ± standard deviation), in comparison to ,±, kPa for perineural injection (P≤,).Th e diff erence between average values of intra and perineural injections (with  safe interval) was signifi cant ( t=,; df=; P=,).
In delivered stillborns, during intraneural applications, the maximum pressure was , kPa, while the minimum pressure was , kPa, achieved in peak effect.Maximum pressure in all perineural applications was , kPa and minimum was , kPa, also achieved in peak effect.(Graphic . and .).The average value of maximum pressure of intraneural injection achieved in peak eff ect for delivered stillborns was ,±, kPa (the average value ± standard deviation), in comparison to ,±, kPa for perineural injection (P≤,).Th e diff erence between average values of intra and perineural injections (with  safe interval) was significant ( t=,; df=; P=,).Th e values of maximum pressure during intraneural and perneural applications show separation between minimum pressure at peak for intraneural and maximum pressure at peak for perineural injections (Graphic .).Th e diff erence of intraneural and perineural injection pressures for the two diff erent examination groups (rats and delivered stillborns) was not statistically signifi cant (P>,), (Graphic .).Th e pressure gradient slope from the beginning of the procedure to the attainment of the peak pressure was different between intraneural and perineural groups (. + . versus . + .), (p < .).Th e lowest pressure attained in the period following the peak pressure was signifi cantly higher in the intraneural group than in perineural group (.

Discussion
In present clinical practice there is no consensus in technique or method which reduces risk from intraneural injection.Th is produced the need for development of objective monitoring and reliable prevention of intraneural injections and consecutive neurological injuries.Th ere are many discussions about how to prevent intraneural injection and nerve injury combined with peripheral nerve block, and all these discussions are focused on the methods of nerve localization (paresthesia versus nerve stimulator).However, there is still no evidence that one method is safer than the other because neurological sequels follow both methods (,,,, ,,,,).Th e oldest method in detection of nerve structures during peripheral nerve blockade is method of paresthesia.Whether causing paresthesia presents direct trauma with a needle, which increases the risk of nerve injury, still remains unknown, although today exists the tendency towards abandoning this method in many centers.Th is comes from the fact that paresthesia can be compromised in cases of sedated or anesthetized patients, which can potentially expose them to failure of recognizing intraneural injection.Given that causing paresthesia can be lessened in partially anesthetized nerve, for example in incompletely attained anesthesia, than the supplementary block can theoretically increase the risk of nerve injury.Th is method is unacceptable for pediatric patients, because a child is not able to precisely report paresthesia or to distinguish it from other discomforts during the block execution.Children under  years of age distinguish pain with principle all or nothing «either it hurts or it doesn't».In present clinical practice for the detection of nerve structures most often are used periphery nerve stimulators.However, it should be pointed out that the nerve stimulators used in blockade of peripheral nerves quite vary in their characteristics, like stimulating frequencies, maximal production of voltage, duration of stimulus and their preciseness (,,).Because of this the nerve stimulators undergo the tests of preciseness.Unfortunately, the majority of manufacturers make the tests using the current of , mA.It would be much more effi cient if they would do these tests with clinically relevant current range from , to , mA.In present clinical practice there is no consensus about standard strength of current for peripheral nerve stimulator used by the executors.While one group of anesthesiologists uses higher current in the beginning, which is then lowered as the needle advances into the tissue, the other group uses the opposite procedure.Also, using the current for peripheral nerve stimulator that is too low comes with increased risk of intraneural injection.Today's progress of ultrasound technology enables visualization of nerve before the insertion of a needle, which represents one new, not invasive method in localization of nerve structures in procedures of regional anesthesia.Observing the advancement of a needle in real time under ultrasound navigation improves the preciseness and safety of the procedure of peripheral nerve block.Ultrasound apparatus sends sound waves with the frequency greater than , cycles per second (kHz).Ultrasound controls the beam under the laws of refl ection and refraction.However, the quantity of ultrasound refl ection depends on acoustic mismatch.Propagation through dense objects, like bone for example which is filled with almost all refl ected rays of ultrasound, produces hyperechoic (bright) image, as a strong signal returned to the emitter.In the contrary, fatty tissue and tendons have low refl ection, therefore they produce hypoechoic (dark) images.Th e contours of structures are best delineated when the ultrasound beam is used under the angle of  degrees.Generally speaking, in transversal presentation the nerves can be seen as round or oval structures which are nodular and hypoechoic, usually with centrally located hyperechoic shadow (,,,,).So far the experience of using ultrasound in procedures of regional anesthesia showed to be useful for the following: -visualization of nerves which helps in defi ning the best place for the insertion of a needle; -placement and advancement of a needle securing the real time navigation of the needle towards the targeted nerve, which avoids or at least minimizes unnecessary randomized movements by executor in trying to achieve wanted level of anesthesia; -observation of spreading of local anesthetic during the injection securing its deposit around the nerve; Ultrasound is successfully being used for defi ning anatomical structures of brachial plexus in interscalene, supraclavicular, infraclavicular and axillary access.Although the method is new, so far many experiences of successful procedures and use of ultrasound method in anesthesia of brachial plexus have been published.Generally speaking, the picture of high resolution is achieved using high frequency (>MHz), when brachial plexus takes superfi cial localization, - cm from the skin in interscalene, supraclavicular and axillary access.For deeper localization, in infraclavicular access, it is necessary to use lower frequency (<MHz), which is signifi cantly reflected on the picture resolution.Contrary to high successfulness in achieving wanted level of anesthesia and even higher safety during the procedure of regional anesthesia, the use of ultrasound method has also some important disadvantages (high price of ultrasound apparatus, making it less accessible, and its big size, making it less portable).Th is is exactly what distinguishes our methods, detection of nerve structures using application pressure.Also, presently available ultrasound technology does not diff erentiate between peripheral nerves and tendon fi bers, which with sometimes poor picture resolution presents additional disadvantage of this method.Anesthesiologists often rely on subjective estimate of abnormal resistance to injection during the performance of peripheral nerve block, knowing that intraneural injection results with bigger resistance to injection and consecutive mechanical damage of the nerve.However, Hadžić and associates showed that the perception of certain resistance can rather vary among the anesthesiologists and that this method is inconsistent and can be aff ected by diff erent designs of needles ().Th e failures of this procedure are shown in one experimental study in which  of testers executed the application of local anesthetic under the pressure of , kPa, un-able to evaluate this pressure as abnormally high and not even one stopped the injection ().Ability of diff erent performers to assess and control the injection pressure is made more diffi cult by the diff erence in the strength of the hand, experience among the operators, as well as by the diff erence in resistance of injection trough diff erent types of needles (diff erent length and the caliber of the lumen).Resistance to injection is greater for needles with a smaller diameter, and such needles are used for peripheral blockades.Even more importantly, the achieved pressure during injection signifi cantly varies among the needles of same length and diameter but of diff erent manufacturers.Th is is probably due to the difference in inner diameter of tested needles, even though the diameter and the length of the needles are similar.
Th is has a big clinical implication for anesthesiologists who use needles from diff erent manufacturers.In earlier preformed study that was carried out on rabbits it was shown that generally higher pressure (higher than , kPa) is needed in order to inject local anesthetic and other solutions into the fasciculus of sciatic nerve of a rabbit, in comparison to perineural application ().Also the injection of local anesthetic into sciatic nerve of a dog or median nerve of a rat resulted in high application pressure (,).In our study the majority of intraneural injections into median nerve of rats and delivered stillborns were combined with injection pressure greater then , kPa, while not even one perineural injection resulted in pressure greater then , kPa.Peak pressure and slope-to-peak eff ect provided good separation between intraneural and perineural groups of pressures.However, since the slope-to peak was  seconds in rats, or  seconds in delivered stillborns, the rate of pressure rise did not off er information of great advantage over simple monitoring of absolute injection pressure.Inability of attaining statistically signifi cant values between intraneural and perineural pressures for two diff erent tested groups (rats and delivered stillborns) is not the result of diff erent numbers of fasciculi, but of compliancy and compatibility of nerve tissue in which the application is conducted.Th is means that diff erences in pressures during intraneural and perineural applications can be expected in other species as well as in other age groups.
Th is study shows that a certain pressure level must be overcome in order to allow fl uid to fl ow out from the needle and go into relatively noncompliant nerve tissue.Th ere are two phases of fl uid administration: First is isostatic phase during which there is no fl ow at the tip of the needle.A certain opening pressure must be reached within the syringe-tubing-needle system in order to initiate the injection into a tissue compartment.During this phase the pressure is equal throughout the entire closed system (including the tip of the open needle; Pascal's law).Second phase is dynamic phase.Once the opening pressure is overcome and the injection is initiated, the injection pressure becomes aff ected by the rate of injection and the fl ow characteristics of the fl uid which passes through (Bernoulli's principle).Th erefore intraneural injection results with signifi cantly higher injection pressures prior to penetration of the internal bundle of fasciculi.Th e pressure continues to be high probably due to the restricted diff usion space within the bundle.

Conclusion
. Intraneural application of local anesthetic into median nerve of human or animal origin in most cases results in high injection pressure ( > , kPa)..Perineural application of local anesthetic into median nerve of rats or delivered stillborns results in low application pressure ( < , kPa)..Th e condition for avoiding intraneural application in clinical practice and consecutive damaging of periphery nerve is diff erentiating nerve structures based on pressures registered by manometer, but also good knowledge of anatomical relations in the region of application.
The pressure information displayed by manometar reliably indicate the pressure at the tip of the needle, regardless to the size of the needle or the rate of injection applied.As long as the injection pressure is low, injection into poorly compliant tissue can be avoided and the neurological damage can be prevented.Based on our research it is obvious that the measuring of pressure during the nerve blockade is very important in order to decrease the risk of neurological complications.If our results are used in clinical practice on human population, than the high injection pressure could be the marker of intraneural (intrafascicular) lodging of a needle, which carries a risk of neural injury.Also, very small manometer, which is easily portable, and financially quite available apparatus for measuring the pressure, can help in differentiation between perineural and intraneural injection.
SUPPORTED BY: Ministry of Education and Science, Bosnia and Herzegovina; Bosnalijek Pharmaceuticals, Sarajevo, Bosnia and Herzegovina; Department of Anesthesiology at St. Luke´s Roosevelt Hospital, New York, USA  + ILVANA VUČKOVIĆ ET AL.: INJECTION PRESSURE AS A MARKER OF INTRANEURAL INJECTION IN PROCEDURES OF PERIPHERAL NERVES BLOCKADE.versus . + .), (p < .).Discriminant analysis was used to determine which aspect of recorded injection pressure provided separation between perineural and intraneural injections.It has been shown that the peak pressure and slope-to-peak both provided  sensitivity and  specifi city.Examination of means and  safe interval around the average value of perineural and intraneural groups of pressures showed that the earliest time at which the lowest pressure of intraneural group exceeds the highest pressure of perineural group was  seconds in rats, or  seconds in stillborns.