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OBTURATOR NERVE BLOCK: AN EVALUATION OF TECHNIQUE*
Brit. J. Anaesth. (1969), 41, 695
OBTURATOR NERVE BLOCK: AN EVALUATION OF TECHNIQUE* BY FLORELLA MAGORA, R. ROZIN, Y. BEN-MENACHEM AND A. MAGORA
SUMMARY
Obturator nerve block has become an accepted method in the treatment of pain in the hip area or of adductor spasm, such as that seen in multiple sclerosis, and as a diagnostic tool in the evaluation of adductor spasticity and mobility of the hip joint. The usual technique is one of a blind anatomical approach by inserting a needle into the obturator foramen, with injection of various substances into the nerve, or adjacent to it, just before it divides into its anterior and posterior branches (Bonica, 1968, 1953; Collins, 1966; Moore, 1965; and Parks and Kennedy, 1967). With this technique alone, however, failure to localize the exact site of the nerve "even in the most expert hands" is not infrequent (Moore, 1965). It has been suggested (Bonica, 1968) that advanced radiographic control methods should be used in nerve blocks in order to avoid the use of large quantities of agent that may spill in the adjacent tissues. Bonica (1953) has also recommended electrical stimulation as a further means of ensuring proper nerve location. In the present report, the results of a comparison of the blind anatomical, radiographic and electric stimulation techniques of obturator nerve block will be presented and assessed. MATERIAL, METHOD AND TECHNIQUES
Fourteen obturator nerve blocks were carried out in three male and in five female patients, whose ages ranged between 46 and 76 years. Two of these patients had multiple sclerosis with severe adductor spasm (3 blocks) and two had severe
rheumatoid arthritis with intractable pain and limitation of hip joint movements. In one of the latter patients block was performed after a unilateral Moore's arthroplasty following a fracture of the neck of the femur. The remaining 4 patients had degenerative coxarthrosis with severe pain and limitation of motion. One of these patients had previously undergone Smith-Petersen nailing of one hip because of a fracture of the neck of the femur. In each of the obturator nerve blocks the blind anatomical technique was used initially and then controlled by radiography and electrical stimulation of the nerve, in order to determine the accuracy of the needle location. For the duration of the block, the patient was placed in the supine position, with the hips slightly flexed and abducted. The accepted blind anatomical technique was used (Bonica, 1953; Collins, 1966; Moore, 1965). After identification of the pubic tubercle, a site 2 cm inferior and lateral to it was marked and an 18 or 20 gauge needle, 7 cm long, was inserted perpendicularly until contact with the inferior ramus of the pubic bone was made. The needle was then slightly withdrawn FLORELLA MAGORA, M.D.|; R. ROZIN, M.D4; Y. BENMENACHEM, M.D.**; A. MAGORA, M.D4; from the
Hadasah University Hospital, Jerusalem, Israel. * This study was supported by Grant VRA-ISR-30-67 from the Social Rehabilitation Service, U.S., Department of Health, Education and Welfare, Washington, D.C. t From the Department of Anaesthetics. i From the Department of Physical Medicine and Rehabilitation. ** From the Department of Radiology.
Downloaded from http://bja.oxfordjournals.org/ at Carleton University on July 8, 2015
Fourteen obturator nerve blocks, in 8 patients, were carried out by blind anatomical approach, as controlled by image-intensifier fluoroscopy and electrical stimulation. After description of techniques and analysis of results, it is concluded that the anatomical approach, as controlled by electrical stimulation of the nerve, is the most accurate method. 0.5 mA was found to be the pertinent rheobase with regard to exact nerve location. The advantages and disadvantages of the radiographic and electrical stimulation techniques are discussed.
BRITISH JOURNAL OF ANAESTHESIA
6PS
I FIG. 1 Antero-posterior radiography of position of needles in obturator nerve block. Tip of needles located beneath the middle third of superior pubic ramus. Site considered correct.
A smaller gauge needle than usual was considered necessary for these experiments in order to avoid blunting of the tip by the bony surfaces, or bending, which might crack the delicate teflon coating, and thereby cause an undue spread of electrical current during the stimulation. Care was taken to hold the needle by the insulated part and not by the connecting cables, so as to avoid spread of the current to the examiner's hand. Five per cent phenol solution 6-10 ml was injected in 5 of the nerve blocks and 1.5 per cent lignocaine solution 5-15 ml in 9. RESULTS
Observation of the accuracy of the needle localization using the three techniques of obturator nerve block is presented in table I. The blind anatomical technique was accurate in only two of the fourteen trials, while the needle had to be re-located in the other twelve. It should be stressed that, in most of the cases, the corrective manoeuvres required were few, slight, and mainly in respect of depth. The radiographic technique, as controlled by electrical stimulation, was exact in 6 cases (table I). In other words, in 8 instances, in spite of
FIG. 2 Tip of needle observed beneath the outer third of the superior pubic ramus, but site considered radiographically incorrect.
Downloaded from http://bja.oxfordjournals.org/ at Carleton University on July 8, 2015
and re-directed laterally into the obturator foramen and then advanced an additional 2-3 cm. When the needle was thought to be properly located, its exact position was ascertained by means of image-intensifier fluoroscopy carried out in one plane (antero-posterior) only. Location was considered accurate if the tip of the needle was observed beneath the mid-point of the superior pubic ramus, in the obturator foramen (figs. 1, 2). When a correct anatomical position was established by both blind and radiographic techniques, electrical stimulation of the nerve was carried out. This was done by using teflon insulated needles, with only their tips free, connected to a square pulse galvanic stimulator with constant voltage. The technique employed was to try to obtain the lowest possible rheobase at the direct nerve motor point, as observed by either a visible or palpable contraction of the adductor muscles. The needle was considered correctly placed if the necessary rheobase was 0.5 mA or less; a higher rheobase was taken to indicate incorrect location, and the needle was moved slightly in all directions, until the proper rheobase was obtained.
OBTURATOR NERVE BLOCK: AN EVALUATION OF TECHNIQUE
697
TABLE I
Relation of results of obturator nerve block by anatomical method, as controlled by radiography and electrical stimulation. Electrical Result of Block Anatomical No. block stimulation Radiography approach
proper blind anatomical insertion and correct radiographic needle location, the rheobase was higher than 0.6 mA and the needle had, therefore, to be re-located. The electrical stimulaton was within the limits of our preset criteria (rheobase values up to 0.5 mA) in 11 cases. In the remaining 3, although the needle was anatomically and radiographically correctly placed, the rheobase was found to be between 1 and 3 mA. In spite of this, lignocaine was injected, but without apparent clinical effect upon the nerve. In contrast, in the other 11 cases, relatively small quantities of the agent caused observable signs of nerve block, such as decrease of spasticity or pain. DISCUSSION
The best technique for peripheral nerve block is the one which ensures the most accurate location of the needle, causes the least discomfort to the patient, has the greatest ease of execution and requires the least elaborate and expensive equipment. Exact location of the nerve site is necessary in order to be able to introduce minimal quantities of agent with maximal clinical effects and the least unwarranted consequences. With a peripheral nerve such as the obturator certain technical difficulties may arise, related to its anatomical location and early proximal branching.
Maximal effectt Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect No effect** No effect No effect
The blind anatomical technique, using well defined anatomical landmarks, is adequate for approximate but not exact location of the nerve trunk (Bonica, 1953). As a result, it may often happen that larger quantities of agents than are necessary are injected, sometimes without effect or with an adverse influence upon the surrounding tissues. In addition, we believe that too much reliance is sometimes placed on the reaction of the patient with regard to the relationship of the needle to the nerve; paraesthesia, dysaesthesia and pain along the nerve are unreliable indicators because most of these patients in any case, have pain over all the area involved, and some of them may be unco-operative or prone to suggestion. Radiographic control of the needle position is a technique with certain obvious advantages, such as immediately available information about the location of the needle in the obturator foramen, and the patient's comfort. Its main drawback in the obturator nerve block is that fluoroscopy may be used only in one plane because of the position of die patient and the impossibility of moving his position; on the other hand, if negatives in two planes are taken, their developing would require a few minutes. Furthermore, even if the latter method is used, the exact radiographic location of the needle within the obturator foramen does not imply that the nerve will be found there. The radiographic control was, in our studies, accurate
Downloaded from http://bja.oxfordjournals.org/ at Carleton University on July 8, 2015
Correct* Correct Correct Correct Correct Correct Correct Correct Incorrect Correct Correct Incorrect Correct Correct 5 Incorrect 6 Correct Correct Incorrect 7 Correct Incorrect Incorrect 8 Correct Incorrect Incorrect 9 Correct Incorrect Incorrect 10 Correct Incorrect Incorrect 11 Correct Incorrect Incorrect 12 Incorrect Incorrect Incorrectt 13 Incorrect Incorrect Incorrect 14 Incorrect Incorrect Incorrect * Rheobase 0.5 mA or less, t Rheobase above 0.5 mA. j Immediate decrease of spasticity or pain with minimal quantity of agent. ** Very little or no effect in spite of injection of larger quantities of agent. 1 2 3 4
698
The rheobase level that we established as a standard requirement is not painful to the patient (even though he may feel some discomfort). The best indicator of effective electrical stimulation is the visually observed, or palpated, contraction of adductors which may be noticed even in very spastic or contracted muscles. The technique requires inexpensive equipment and has the additional advantages of not needing highly skilled technical staff and of being mobile. Last but not least, the combination of blind anatomical technique as controlled by electrical stimulation, is not time consuming; at the most, 2-3 minutes are
required from the time of insertion of the needle to the moment of exact location of the nerve. In conclusion, we strongly recommend the use of electrical nerve stimulation as an easy and reliable aid for the location of the obturator nerve. When the injection of more toxic agents, such as phenol, is contemplated the use of radiography seems to us warranted provided no facilities for electrical stimulation are available. REFERENCES
Bonica, J. J. (1953). The Management of Pain. With Special Emphasis on the Use of Analgesic Block in Diagnosis, Prognosis and Therapy, pp. 345-348. Philadelphia: Lea & Febiger. (1968). Autonomic innervation of the viscera in relation to nerve block. Anesthesiology, 29, 793. Collins, V. J. (1966). Principles of Anesthesiology, pp. 769-787. Philadelphia: Lea & Febiger. Moore, D. C. (1965). Regional Block. A Handbook for Use HI the Clinical Practice of Medicine and Surgery, 4th ed. Springfield: Thomas. Parks, C. R., and Kennedy, W. F. (1967). Obturator nerve block: A simplified approach. Anesthesiology, 28, 775.
BLOQUAGE DU NERF OBTURATEUR: UNE EVALUATION DE LA TECHNIQUE SOMMAIRE
Quatorze bloquages du nerf obturateur ont ete faits chez huit patients par une technique anatomiquemem aveugle, sous controle par fluoroscopie a intensificateur d'images et stimulation electrique. La description des techniques et 1'analyse des resultats menent a la conclusion que la technique anatomique, controlee par la stimulation Electrique du nerf, est la methode la plus exacte. On a note que 0,5 mA est la rheobase adequate par rapport a la localisation exacte nu nerf. Les avantages et desavantages des techniques de stimulation radiographique et Electrique sont discutes. NERVENBLOCKADE DURCH LEITUNGSUNTERBRECHUNG: EINE BEWERTUNG DER METHODE ZUSAMMENFASSUNG
Vierzehn Nervenblockaden durch Leitungsunterbrechung wurden an acht Patienten durchgefuhrt, indem der anatomische Zugang ohne Hilfe des Auges durch Bildverstarker-Fluoroskopie und elektrische Stimulation kontrolliert wurde. Nach Beschreibung der Methoden und Analyse der Resultate wird geschlossen, dafi der durch elektrische Stimulation des Nerven kontrollierte anatomische Zugang die genaueste Methode ist. 0,5 mA wurde als die entsprechende Rheobase im Hinblick auf die exakte Lokalisation des Nerven ermittelt. Die Vorziige und Nachteile der radiographischen und elektrischen Stimulations-Methoden werden diskutiert.
Downloaded from http://bja.oxfordjournals.org/ at Carleton University on July 8, 2015
in less than half the cases, although the number of blocks was admittedly small. In 8 cases, in spite of the fact that the needle was in the proper location radiographically, it had to be relocated, and in 5 of them the rheobase became 0.5 mA or less, implying that a better needle-nerve relationship was obtained. Radiographic control alone would have meant the injection of larger amounts of the substances and diminished clinical effects. Additional general disadvantages are the necessity to carry out the procedure in the X-ray department, the high cost of the elaborate equipment and the need for highly trained staff. Electrical stimulation has proved to be the most accurate technique of nerve location. The choice of rheobase at 0.5 mA or less was based on our experience that this is the electrical current intensity required to stimulate the direct motor point of the obturator provided the needle site is accurate. We obtained this value in 11 cases, in all of whom the quantity of injected agents needed was minimal, and the clinical effect maximal, indicating the correctness of the chosen rheobase value. The three cases in which the exact location of the nerve could not be determined by electrical stimulation (rheobase values 1-3 mA), and in which the block was ineffective, could be explained by a congenital anomaly of the course of the nerve. With further exploration, the nerve could probably have been located, but the procedure would have caused undue pain to the patient. These three failures, however, prove the value of electrical determinations of the nerve location and, moreover, of our arbitrarily chosen rheobase; whenever this is higher than 0.5 mA it indicates inaccurate location and no agent should be injected.
BRITISH JOURNAL OF ANAESTHESIA
OBTURATOR NERVE BLOCK: AN EVALUATION OF TECHNIQUE* BY FLORELLA MAGORA, R. ROZIN, Y. BEN-MENACHEM AND A. MAGORA
SUMMARY
Obturator nerve block has become an accepted method in the treatment of pain in the hip area or of adductor spasm, such as that seen in multiple sclerosis, and as a diagnostic tool in the evaluation of adductor spasticity and mobility of the hip joint. The usual technique is one of a blind anatomical approach by inserting a needle into the obturator foramen, with injection of various substances into the nerve, or adjacent to it, just before it divides into its anterior and posterior branches (Bonica, 1968, 1953; Collins, 1966; Moore, 1965; and Parks and Kennedy, 1967). With this technique alone, however, failure to localize the exact site of the nerve "even in the most expert hands" is not infrequent (Moore, 1965). It has been suggested (Bonica, 1968) that advanced radiographic control methods should be used in nerve blocks in order to avoid the use of large quantities of agent that may spill in the adjacent tissues. Bonica (1953) has also recommended electrical stimulation as a further means of ensuring proper nerve location. In the present report, the results of a comparison of the blind anatomical, radiographic and electric stimulation techniques of obturator nerve block will be presented and assessed. MATERIAL, METHOD AND TECHNIQUES
Fourteen obturator nerve blocks were carried out in three male and in five female patients, whose ages ranged between 46 and 76 years. Two of these patients had multiple sclerosis with severe adductor spasm (3 blocks) and two had severe
rheumatoid arthritis with intractable pain and limitation of hip joint movements. In one of the latter patients block was performed after a unilateral Moore's arthroplasty following a fracture of the neck of the femur. The remaining 4 patients had degenerative coxarthrosis with severe pain and limitation of motion. One of these patients had previously undergone Smith-Petersen nailing of one hip because of a fracture of the neck of the femur. In each of the obturator nerve blocks the blind anatomical technique was used initially and then controlled by radiography and electrical stimulation of the nerve, in order to determine the accuracy of the needle location. For the duration of the block, the patient was placed in the supine position, with the hips slightly flexed and abducted. The accepted blind anatomical technique was used (Bonica, 1953; Collins, 1966; Moore, 1965). After identification of the pubic tubercle, a site 2 cm inferior and lateral to it was marked and an 18 or 20 gauge needle, 7 cm long, was inserted perpendicularly until contact with the inferior ramus of the pubic bone was made. The needle was then slightly withdrawn FLORELLA MAGORA, M.D.|; R. ROZIN, M.D4; Y. BENMENACHEM, M.D.**; A. MAGORA, M.D4; from the
Hadasah University Hospital, Jerusalem, Israel. * This study was supported by Grant VRA-ISR-30-67 from the Social Rehabilitation Service, U.S., Department of Health, Education and Welfare, Washington, D.C. t From the Department of Anaesthetics. i From the Department of Physical Medicine and Rehabilitation. ** From the Department of Radiology.
Downloaded from http://bja.oxfordjournals.org/ at Carleton University on July 8, 2015
Fourteen obturator nerve blocks, in 8 patients, were carried out by blind anatomical approach, as controlled by image-intensifier fluoroscopy and electrical stimulation. After description of techniques and analysis of results, it is concluded that the anatomical approach, as controlled by electrical stimulation of the nerve, is the most accurate method. 0.5 mA was found to be the pertinent rheobase with regard to exact nerve location. The advantages and disadvantages of the radiographic and electrical stimulation techniques are discussed.
BRITISH JOURNAL OF ANAESTHESIA
6PS
I FIG. 1 Antero-posterior radiography of position of needles in obturator nerve block. Tip of needles located beneath the middle third of superior pubic ramus. Site considered correct.
A smaller gauge needle than usual was considered necessary for these experiments in order to avoid blunting of the tip by the bony surfaces, or bending, which might crack the delicate teflon coating, and thereby cause an undue spread of electrical current during the stimulation. Care was taken to hold the needle by the insulated part and not by the connecting cables, so as to avoid spread of the current to the examiner's hand. Five per cent phenol solution 6-10 ml was injected in 5 of the nerve blocks and 1.5 per cent lignocaine solution 5-15 ml in 9. RESULTS
Observation of the accuracy of the needle localization using the three techniques of obturator nerve block is presented in table I. The blind anatomical technique was accurate in only two of the fourteen trials, while the needle had to be re-located in the other twelve. It should be stressed that, in most of the cases, the corrective manoeuvres required were few, slight, and mainly in respect of depth. The radiographic technique, as controlled by electrical stimulation, was exact in 6 cases (table I). In other words, in 8 instances, in spite of
FIG. 2 Tip of needle observed beneath the outer third of the superior pubic ramus, but site considered radiographically incorrect.
Downloaded from http://bja.oxfordjournals.org/ at Carleton University on July 8, 2015
and re-directed laterally into the obturator foramen and then advanced an additional 2-3 cm. When the needle was thought to be properly located, its exact position was ascertained by means of image-intensifier fluoroscopy carried out in one plane (antero-posterior) only. Location was considered accurate if the tip of the needle was observed beneath the mid-point of the superior pubic ramus, in the obturator foramen (figs. 1, 2). When a correct anatomical position was established by both blind and radiographic techniques, electrical stimulation of the nerve was carried out. This was done by using teflon insulated needles, with only their tips free, connected to a square pulse galvanic stimulator with constant voltage. The technique employed was to try to obtain the lowest possible rheobase at the direct nerve motor point, as observed by either a visible or palpable contraction of the adductor muscles. The needle was considered correctly placed if the necessary rheobase was 0.5 mA or less; a higher rheobase was taken to indicate incorrect location, and the needle was moved slightly in all directions, until the proper rheobase was obtained.
OBTURATOR NERVE BLOCK: AN EVALUATION OF TECHNIQUE
697
TABLE I
Relation of results of obturator nerve block by anatomical method, as controlled by radiography and electrical stimulation. Electrical Result of Block Anatomical No. block stimulation Radiography approach
proper blind anatomical insertion and correct radiographic needle location, the rheobase was higher than 0.6 mA and the needle had, therefore, to be re-located. The electrical stimulaton was within the limits of our preset criteria (rheobase values up to 0.5 mA) in 11 cases. In the remaining 3, although the needle was anatomically and radiographically correctly placed, the rheobase was found to be between 1 and 3 mA. In spite of this, lignocaine was injected, but without apparent clinical effect upon the nerve. In contrast, in the other 11 cases, relatively small quantities of the agent caused observable signs of nerve block, such as decrease of spasticity or pain. DISCUSSION
The best technique for peripheral nerve block is the one which ensures the most accurate location of the needle, causes the least discomfort to the patient, has the greatest ease of execution and requires the least elaborate and expensive equipment. Exact location of the nerve site is necessary in order to be able to introduce minimal quantities of agent with maximal clinical effects and the least unwarranted consequences. With a peripheral nerve such as the obturator certain technical difficulties may arise, related to its anatomical location and early proximal branching.
Maximal effectt Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect Maximal effect No effect** No effect No effect
The blind anatomical technique, using well defined anatomical landmarks, is adequate for approximate but not exact location of the nerve trunk (Bonica, 1953). As a result, it may often happen that larger quantities of agents than are necessary are injected, sometimes without effect or with an adverse influence upon the surrounding tissues. In addition, we believe that too much reliance is sometimes placed on the reaction of the patient with regard to the relationship of the needle to the nerve; paraesthesia, dysaesthesia and pain along the nerve are unreliable indicators because most of these patients in any case, have pain over all the area involved, and some of them may be unco-operative or prone to suggestion. Radiographic control of the needle position is a technique with certain obvious advantages, such as immediately available information about the location of the needle in the obturator foramen, and the patient's comfort. Its main drawback in the obturator nerve block is that fluoroscopy may be used only in one plane because of the position of die patient and the impossibility of moving his position; on the other hand, if negatives in two planes are taken, their developing would require a few minutes. Furthermore, even if the latter method is used, the exact radiographic location of the needle within the obturator foramen does not imply that the nerve will be found there. The radiographic control was, in our studies, accurate
Downloaded from http://bja.oxfordjournals.org/ at Carleton University on July 8, 2015
Correct* Correct Correct Correct Correct Correct Correct Correct Incorrect Correct Correct Incorrect Correct Correct 5 Incorrect 6 Correct Correct Incorrect 7 Correct Incorrect Incorrect 8 Correct Incorrect Incorrect 9 Correct Incorrect Incorrect 10 Correct Incorrect Incorrect 11 Correct Incorrect Incorrect 12 Incorrect Incorrect Incorrectt 13 Incorrect Incorrect Incorrect 14 Incorrect Incorrect Incorrect * Rheobase 0.5 mA or less, t Rheobase above 0.5 mA. j Immediate decrease of spasticity or pain with minimal quantity of agent. ** Very little or no effect in spite of injection of larger quantities of agent. 1 2 3 4
698
The rheobase level that we established as a standard requirement is not painful to the patient (even though he may feel some discomfort). The best indicator of effective electrical stimulation is the visually observed, or palpated, contraction of adductors which may be noticed even in very spastic or contracted muscles. The technique requires inexpensive equipment and has the additional advantages of not needing highly skilled technical staff and of being mobile. Last but not least, the combination of blind anatomical technique as controlled by electrical stimulation, is not time consuming; at the most, 2-3 minutes are
required from the time of insertion of the needle to the moment of exact location of the nerve. In conclusion, we strongly recommend the use of electrical nerve stimulation as an easy and reliable aid for the location of the obturator nerve. When the injection of more toxic agents, such as phenol, is contemplated the use of radiography seems to us warranted provided no facilities for electrical stimulation are available. REFERENCES
Bonica, J. J. (1953). The Management of Pain. With Special Emphasis on the Use of Analgesic Block in Diagnosis, Prognosis and Therapy, pp. 345-348. Philadelphia: Lea & Febiger. (1968). Autonomic innervation of the viscera in relation to nerve block. Anesthesiology, 29, 793. Collins, V. J. (1966). Principles of Anesthesiology, pp. 769-787. Philadelphia: Lea & Febiger. Moore, D. C. (1965). Regional Block. A Handbook for Use HI the Clinical Practice of Medicine and Surgery, 4th ed. Springfield: Thomas. Parks, C. R., and Kennedy, W. F. (1967). Obturator nerve block: A simplified approach. Anesthesiology, 28, 775.
BLOQUAGE DU NERF OBTURATEUR: UNE EVALUATION DE LA TECHNIQUE SOMMAIRE
Quatorze bloquages du nerf obturateur ont ete faits chez huit patients par une technique anatomiquemem aveugle, sous controle par fluoroscopie a intensificateur d'images et stimulation electrique. La description des techniques et 1'analyse des resultats menent a la conclusion que la technique anatomique, controlee par la stimulation Electrique du nerf, est la methode la plus exacte. On a note que 0,5 mA est la rheobase adequate par rapport a la localisation exacte nu nerf. Les avantages et desavantages des techniques de stimulation radiographique et Electrique sont discutes. NERVENBLOCKADE DURCH LEITUNGSUNTERBRECHUNG: EINE BEWERTUNG DER METHODE ZUSAMMENFASSUNG
Vierzehn Nervenblockaden durch Leitungsunterbrechung wurden an acht Patienten durchgefuhrt, indem der anatomische Zugang ohne Hilfe des Auges durch Bildverstarker-Fluoroskopie und elektrische Stimulation kontrolliert wurde. Nach Beschreibung der Methoden und Analyse der Resultate wird geschlossen, dafi der durch elektrische Stimulation des Nerven kontrollierte anatomische Zugang die genaueste Methode ist. 0,5 mA wurde als die entsprechende Rheobase im Hinblick auf die exakte Lokalisation des Nerven ermittelt. Die Vorziige und Nachteile der radiographischen und elektrischen Stimulations-Methoden werden diskutiert.
Downloaded from http://bja.oxfordjournals.org/ at Carleton University on July 8, 2015
in less than half the cases, although the number of blocks was admittedly small. In 8 cases, in spite of the fact that the needle was in the proper location radiographically, it had to be relocated, and in 5 of them the rheobase became 0.5 mA or less, implying that a better needle-nerve relationship was obtained. Radiographic control alone would have meant the injection of larger amounts of the substances and diminished clinical effects. Additional general disadvantages are the necessity to carry out the procedure in the X-ray department, the high cost of the elaborate equipment and the need for highly trained staff. Electrical stimulation has proved to be the most accurate technique of nerve location. The choice of rheobase at 0.5 mA or less was based on our experience that this is the electrical current intensity required to stimulate the direct motor point of the obturator provided the needle site is accurate. We obtained this value in 11 cases, in all of whom the quantity of injected agents needed was minimal, and the clinical effect maximal, indicating the correctness of the chosen rheobase value. The three cases in which the exact location of the nerve could not be determined by electrical stimulation (rheobase values 1-3 mA), and in which the block was ineffective, could be explained by a congenital anomaly of the course of the nerve. With further exploration, the nerve could probably have been located, but the procedure would have caused undue pain to the patient. These three failures, however, prove the value of electrical determinations of the nerve location and, moreover, of our arbitrarily chosen rheobase; whenever this is higher than 0.5 mA it indicates inaccurate location and no agent should be injected.
BRITISH JOURNAL OF ANAESTHESIA