- Email: [email protected]
Effect of nitrous oxide on cerebral blood flow
British Journal of Anaesthesia 1994; 72: 250-252
CORRESPONDENCE
Interpleural analgesia
J. RICHARDSON
Department of Anaesthesia S. SABANATHAN
Department of Cardio- Thoracic Surgery Bradford Royal Infirmary Bradford P . A. LONNQUIST
St Gdrans Paediatric Hospital Stockholm 1. Murphy DF. Interpleural analgesia. British Journal of Anaesthesia 1993; 71: 426-434. 2. Broome IJ, Sherry KM, Reilly CS. A combined chest drain and intrapleural catheter for post-thoracotomy pain relief. Anaesthesia 1993; 48: 724-762. 3. Rosenberg PH, Scheinin BM-A, Lepantalo MJA, Lindfors O. Continuous intrapleural infusion of bupivacaine for analgesia after ihoracotomy. Anesthesiology 1987; 67:811-813. 4. Ferrante FM, Chan VWS, Arthur GR, Rocco AG. Intrapleural analgesia afteT thoracotomy. Anesthesia and Analgesia 1991; 72: 105-109. 5. Lonnqvist PA, Hildinsson U. The caudal boundary of the paravertebral space. A study in human cadavers. Anaesthesia 1992; 47: 1051-1052. 6. EngJ, Sabanathan S. Site of action of continuous extrapleural intercostal nerve block. Annals of Thoracic Surgery 1991; 51: 387-389. 7. Sabanathan S, Richardson J, Mearns AJ. Management of pain in thoracic surgery. British Journal of Hospital Medicine 1993; 50: 114-120.
Sir,—While it is fair to say that local anaesthetic injected into the pleural or paravertebral spaces induces analgesia by inhibiting the same group of nerves, possibly by the same mechanism, clearly these are not identical tissue planes and some differences might be found between them. For all other indications, it is likely that interpleural, intercostal and paravcrtebral blocks may be used interchangeably, but at thoracotomy the tissue planes which would otherwise allow spread of local anaesthetic to reach distant intercostal nerves are disrupted and much of the local anaesthetic is lost. By locating specifically the intact tissue planes in the paravertebral gutter and
inserting the extradural catheter into this space under direct vision, Sabanathan and colleagues [1] have shown this technique to be consistently effective. This is likely to be the reason why these authors have achieved considerable success using this block for post-thoracotomy pain relief while others have found it to be inconsistent. I agree that in this situation it is simplistic to include both paravertebral and interpleural analgesia together. In my review of interpleural analgesia, this difference was cited specifically in the summary and conclusion of the article. D. F. MURPHY
Sir Charles Gairdner Hospital Perth, Western Australia 1. Sabanathan S, Mearns AJ, Bickford-Smith PJ, Eng J, Berrisford RG, Bibby SR, Majid MR. Efficacy of continuous extrapleural intercostal nerve block on postthoracotomy pain and pulmonary mechanics. British Journal of Surgery 1990; 77: 221-225.
Effect of nitrous oxide on cerebral blood flow Sir,—We read with interest the article by Field and colleagues [1], addressing the important issue of the effect of nitrous oxide on cerebral blood flow. The authors have shown that there is an increase in cerebral blood flow (CBF) after inhalation of 30-60 % nitrous oxide in oxygen. However, in their study, "baseline measurements" of CBF were made using 100% oxygen and not air. This is rather unfortunate and makes the conclusions drawn questionable. Hyperoxia (FiOt 0.85-1.0) has been known to cause vasoconstriction of the cerebral vessels [2], resulting in a decrease in global CBF by 11-14% of control values [3]. In our laboratory, Eintrei, Odman and Lund have shown a reduction of 40 % in cortical blood flow in pigs after inhalation of 100 % oxygen compared with air [4]. We have also found a significant decrease in capillary blood flow in the brain cortex after inhalation of increasing concentrations of oxygen using the hydrogen clearance technique [5]. Assuming that the normal global CBF level is in the range 50 ml/100 g min"1, the increase in CBF seen by Field and colleagues (8.6-9.8 ml/100 g min"') was less than 20% when nitrous oxide was added (and consequently the Fl Oi was reduced). In other words, the increase in flow seen after the addition of nitrous oxide may be explained, at least in part, by the decrease in inspired oxygen concentration and, the baseline values of CBF obtained by Field and colleagues using 100% oxygen, may have been falsely low. Therefore, could the increase in CBF after inhalation of nitrous oxide be the result of vasodilatation following a decrease in the inspired fraction of oxygen? F. SJOBERG
P. KjELLGREN A. GuTTA Department of Anaesthesiology and the Bums Unit University Hospital Linkoping, Sweden 1. Field LM, Dorrancc DE, Krzeminska EK, Barsoum LZ. Effect of nitrous oxide on blood flow in normal humans. British Journal of Anaesthesia 1993; 70: 154-159. 2. McDowall DG. Interrelationship between blood oxygen tension and cerebral blood flow. In: Payne JP, Hill DW, eds. Oxygen Measurements in Blood and Tissue. London: Churchill, 1966; 205-214. 3. Edvinsson L, MacKenzie ET, McCulloch J, eds. Changes in arterial gas tensions. In: Cerebral Blood Flow and Metabolism. New York: Raven Press, 1992: 524-552. 4. Eintrei C, Odman S, Lund N. Effects of increases in the inspired oxygen fraction on brain surface oxygen pressure fields and regional cerebral blood flow. Advances m Experimental and Medical Biology 1985; 191: 131-13*. 5. Eintrei C, Bergsten B, Gustafsson U, Malmqvist L-A, Sjoberg F. Selective reduction of high cerebral capillary
Downloaded from http://bja.oxfordjournals.org/ at University of Chicago on July 12, 2015
Sir,—We are concerned that the review by Murphy [1], which grouped together paravertebral analgesia and intrapleural analgesia and reviewed both subjects under the heading" interpleural analgesia" may worsen existing confusion between these different techniques. Instillation of local anaesthetic into the paravertebral or cxtrapleural space behaves differently from local anaesthetic injected between the parietal and visceral pleura and this is apparent in thoracic surgery. Post-thoracotomy intrapleural administration results in loss of up to 60% of anaesthetic solution dirough chest drains [2], mixing of the local anaesthetic with blood and fluid in the pleural space [3,4] and unimpeded gravitational pooling of solution in the dependent parts of the chest [3], all to the detriment of efficacy. We and others have demonstrated that solutions administered extrapleurally, on the other hand, remain confined but well distributed within the paravertebral space in close proximity to the intercostal nerve roots, their posterior primary rami and the sympathetic chain [5, 6]. Not only is post-thoracotomy pain relief reliably excellent, but blood concentrations of local anaesthetic are predictable and side effects are minimal [7], By combining research references from these two techniques, inconsistent results are explained adequately. These differences are much too important to be referred to as "variations on the technique of interpleural analgesia (with)... little to choose between them".
CORRESPONDENCE
251
bloodflowduring hyperoxemia. Swedish Society of Anaesthesiology and Intensive Care 1991; 5: A5.
L. M. FIELD
St George's Hospital London 1. Field LM, Dorrance DE, Krzeminska EK, Barsoum LZ. Effect of nitrous oxide on cerebral blood flow in normal humans. British Journal of Anaesthesia 1993; 70: 154-159. 2. Kety SS, Schmidt CF. The effect of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men. Journal of Clinical Investigation 1945; 27: 484-492. 3. Lambertsen CJ, Kough RH, Cooper DY, Emmel GL, Loeschke HH, Schmidt CF. Oxygen toxicity: effects in man of oxygen inhalation at 1 and 3.5 atmospheres upon blood gas transport, cerebral circulation and cerebral metabolism. Journal of Applied Physiology 1953; 5: 471-486.
Pressure in the extradural space Sir,—I read with interest the letter by Guedj [1] describing an ingenious device for detecting negative (subatmospheric) pressure within the extradural space. However, Telford and Holloway [2] have clearly shown that, in the lumbar region at least, the pressure in the extradural space is always positive (supra-atmospheric). An artefactual negative pressure is produced only as a result of tenting of the dura by the blunt Tuohy needle. This would suggest that techniques which rely on eliciting a negative pressure to locate the extradural space are more likely than those using loss of resistance to result in accidental dural puncture. S. D. PAWSEY
Middlesex Hospital London 1. Guedj P. A simple indicator for identifying the extradural space. British Journal of Anaesthesia 1993; 71: 726. 2. Telford RJ, Holloway I t . Observations on deliberate dural puncture with a Tuohy needle: pressure measurements. Anaesthesia 1991; 46: 725-772.
Sir,—The question of the pressure in the extradural space, first raised by Janzen in 1926, has been a subject of much controversy, especially regarding negative pressure. Does it really exist, and if so, docs it occur naturally or is it caused as an artefact by the devices used to identify the space? Bromage, after a comprehensive study of the literature, concluded in 1978 that the pressure in the extradural space in the lumbar region is positive unless the patient lies head-down in the
P. GUEDJ
Misgav Ladach Hospital Jerusalem 1. Bromage PR. Epidural Analgesia. Philadelphia: W. B. Saunders, 1978; 160-171.
Respiratory depression after extradural fentanyl Sir,—Following earlier correspondence [1], we wish to comment further on respiratory depression after extradural fentanyl. Three cases [2-i] have now been documented in the obstetric anaesthetic literature and although it is tempting to speculate on common factors and the mechanisms involved, the important message is that it can occur and it can occur relatively late after extradural injection. Drs Chrubasik, Chrubasik and Black were mistaken when they stated that one of the patients we reported received diamorphine in addition to extradural fentanyl [1]. In our opinion the events described in these three obstetric patients are not explained on the basis of additional systemic drug administration and direct access offentanyl to the brain stem caused by rostral spread in cerebrospinal fluid is the likely mechanism. D. W. NOBLE
Aberdeen L. M. MORRISON
Livingston M. S. BROCKWAY
Livingston J. H. MCCLURE
Edinburgh 1. Chrubasik J, Chrubasik S, Black A. Respiratory depression after extradural fentanyl. British Journal of Anaesthesia 1993; 71: 164-165. 2. Brockway MS, Noble DW, Sharwood-Smith GH, McClure JH. Profound respiratory depression after extradural fentanyl. British Journal of Anaesthesia 1990; 64: 243-245. 3. Noble DW, Morrison LM, Brockway MS, McClure JH. Adrenaline, fentanyl or adrenaline and fentanyl as adjuncts to bupivacaine for extradural anaesthesia in elective Caesarean section. British Journal of Anaesthesia 1991; 66: 645-650. 4. Wang CY. Respiratory depression after cxtradural fentanyl. British Journal of Anaesthesia 1992; 69: 544. Sir,—We apologise to Dr Noble and colleagues for misrepresenting the treatment of one of their reported cases of ventilatory depression after extradural fentanyl. I.v. supplements of diamorphine (or other opioid) were given to some of the patients reported in their study and, in particular, to two in the group in which two patients needed naloxone. We clearly put two and two
Downloaded from http://bja.oxfordjournals.org/ at University of Chicago on July 12, 2015
Sir,—We thank Professor Sjoberg, Mr Kjellgren and Dr Gupta for their comments. Our study [1] was conducted in normal human volunteers. The intention was to mimic, as closely as possible, the clinical situation in which nitrous oxide is administered in oxygen, without the introduction of a third gas, for example nitrogen. Therefore, we feel that our results reflect accurately what occurs in routine clinical practice. In studies using normal human volunteers, Kety and Schmidt [2] and Lambertsen and colleagues [3] did indeed show a decrease in cerebral blood flow when 100% oxygen was inspired. However, the accompanying hypervenrilation and hypocapnia could alone have been responsible for the documented changes in cerebral haemodynamics [2]. Our volunteers did not hyperventilate. While we accept that greater concentrations of oxygen may cause global cerebral vasoconstriction and certainly cortical vasoconstriction in animal studies, the situation in normal humans is far less clear and warrants further investigation.
lateral position; whereas negative respiratory pressure is transmitted to the thoracic extradural space [1]. Many methods of identifying the extradural space have been described, some based on the existence of negative pressure (such as the hanging-drop technique of Gurtierez); others depend upon recognition of sudden release of resistance to injection of fluid (such as the "technique du mandrin liquide" proposed by Sicard, later popularized by Dogliotti and called the "loss of resistance test"). However, every technique has advantages and drawbacks. For example, in the Guttierez technique, the sucking of the drop of liquid is elicited in 82 % and the hanging-drop technique lacks reliability in the lumbar region. On the other hand, the loss of resistance test is difficult to use in the thoracic and cervical regions. The Macintosh balloon indicator, on which our device is based, seems to resolve most problems. As the pressure in the indicator balloon is always greater than that of the extradural space, regardless of the spinal level (thoracic, cervical or lumbar) or of the patient's position, abrupt deflation indicates entry into the extradural space in all patients. A further advantage of this technique is that both hands of the anaesthetist can be used for careful needle advancement. Finally, we do think that a small percentage of failures (i.e. accidental dural puncture) is less related to the technique, than to the skills and experience of the practitioner.
CORRESPONDENCE
Interpleural analgesia
J. RICHARDSON
Department of Anaesthesia S. SABANATHAN
Department of Cardio- Thoracic Surgery Bradford Royal Infirmary Bradford P . A. LONNQUIST
St Gdrans Paediatric Hospital Stockholm 1. Murphy DF. Interpleural analgesia. British Journal of Anaesthesia 1993; 71: 426-434. 2. Broome IJ, Sherry KM, Reilly CS. A combined chest drain and intrapleural catheter for post-thoracotomy pain relief. Anaesthesia 1993; 48: 724-762. 3. Rosenberg PH, Scheinin BM-A, Lepantalo MJA, Lindfors O. Continuous intrapleural infusion of bupivacaine for analgesia after ihoracotomy. Anesthesiology 1987; 67:811-813. 4. Ferrante FM, Chan VWS, Arthur GR, Rocco AG. Intrapleural analgesia afteT thoracotomy. Anesthesia and Analgesia 1991; 72: 105-109. 5. Lonnqvist PA, Hildinsson U. The caudal boundary of the paravertebral space. A study in human cadavers. Anaesthesia 1992; 47: 1051-1052. 6. EngJ, Sabanathan S. Site of action of continuous extrapleural intercostal nerve block. Annals of Thoracic Surgery 1991; 51: 387-389. 7. Sabanathan S, Richardson J, Mearns AJ. Management of pain in thoracic surgery. British Journal of Hospital Medicine 1993; 50: 114-120.
Sir,—While it is fair to say that local anaesthetic injected into the pleural or paravertebral spaces induces analgesia by inhibiting the same group of nerves, possibly by the same mechanism, clearly these are not identical tissue planes and some differences might be found between them. For all other indications, it is likely that interpleural, intercostal and paravcrtebral blocks may be used interchangeably, but at thoracotomy the tissue planes which would otherwise allow spread of local anaesthetic to reach distant intercostal nerves are disrupted and much of the local anaesthetic is lost. By locating specifically the intact tissue planes in the paravertebral gutter and
inserting the extradural catheter into this space under direct vision, Sabanathan and colleagues [1] have shown this technique to be consistently effective. This is likely to be the reason why these authors have achieved considerable success using this block for post-thoracotomy pain relief while others have found it to be inconsistent. I agree that in this situation it is simplistic to include both paravertebral and interpleural analgesia together. In my review of interpleural analgesia, this difference was cited specifically in the summary and conclusion of the article. D. F. MURPHY
Sir Charles Gairdner Hospital Perth, Western Australia 1. Sabanathan S, Mearns AJ, Bickford-Smith PJ, Eng J, Berrisford RG, Bibby SR, Majid MR. Efficacy of continuous extrapleural intercostal nerve block on postthoracotomy pain and pulmonary mechanics. British Journal of Surgery 1990; 77: 221-225.
Effect of nitrous oxide on cerebral blood flow Sir,—We read with interest the article by Field and colleagues [1], addressing the important issue of the effect of nitrous oxide on cerebral blood flow. The authors have shown that there is an increase in cerebral blood flow (CBF) after inhalation of 30-60 % nitrous oxide in oxygen. However, in their study, "baseline measurements" of CBF were made using 100% oxygen and not air. This is rather unfortunate and makes the conclusions drawn questionable. Hyperoxia (FiOt 0.85-1.0) has been known to cause vasoconstriction of the cerebral vessels [2], resulting in a decrease in global CBF by 11-14% of control values [3]. In our laboratory, Eintrei, Odman and Lund have shown a reduction of 40 % in cortical blood flow in pigs after inhalation of 100 % oxygen compared with air [4]. We have also found a significant decrease in capillary blood flow in the brain cortex after inhalation of increasing concentrations of oxygen using the hydrogen clearance technique [5]. Assuming that the normal global CBF level is in the range 50 ml/100 g min"1, the increase in CBF seen by Field and colleagues (8.6-9.8 ml/100 g min"') was less than 20% when nitrous oxide was added (and consequently the Fl Oi was reduced). In other words, the increase in flow seen after the addition of nitrous oxide may be explained, at least in part, by the decrease in inspired oxygen concentration and, the baseline values of CBF obtained by Field and colleagues using 100% oxygen, may have been falsely low. Therefore, could the increase in CBF after inhalation of nitrous oxide be the result of vasodilatation following a decrease in the inspired fraction of oxygen? F. SJOBERG
P. KjELLGREN A. GuTTA Department of Anaesthesiology and the Bums Unit University Hospital Linkoping, Sweden 1. Field LM, Dorrancc DE, Krzeminska EK, Barsoum LZ. Effect of nitrous oxide on blood flow in normal humans. British Journal of Anaesthesia 1993; 70: 154-159. 2. McDowall DG. Interrelationship between blood oxygen tension and cerebral blood flow. In: Payne JP, Hill DW, eds. Oxygen Measurements in Blood and Tissue. London: Churchill, 1966; 205-214. 3. Edvinsson L, MacKenzie ET, McCulloch J, eds. Changes in arterial gas tensions. In: Cerebral Blood Flow and Metabolism. New York: Raven Press, 1992: 524-552. 4. Eintrei C, Odman S, Lund N. Effects of increases in the inspired oxygen fraction on brain surface oxygen pressure fields and regional cerebral blood flow. Advances m Experimental and Medical Biology 1985; 191: 131-13*. 5. Eintrei C, Bergsten B, Gustafsson U, Malmqvist L-A, Sjoberg F. Selective reduction of high cerebral capillary
Downloaded from http://bja.oxfordjournals.org/ at University of Chicago on July 12, 2015
Sir,—We are concerned that the review by Murphy [1], which grouped together paravertebral analgesia and intrapleural analgesia and reviewed both subjects under the heading" interpleural analgesia" may worsen existing confusion between these different techniques. Instillation of local anaesthetic into the paravertebral or cxtrapleural space behaves differently from local anaesthetic injected between the parietal and visceral pleura and this is apparent in thoracic surgery. Post-thoracotomy intrapleural administration results in loss of up to 60% of anaesthetic solution dirough chest drains [2], mixing of the local anaesthetic with blood and fluid in the pleural space [3,4] and unimpeded gravitational pooling of solution in the dependent parts of the chest [3], all to the detriment of efficacy. We and others have demonstrated that solutions administered extrapleurally, on the other hand, remain confined but well distributed within the paravertebral space in close proximity to the intercostal nerve roots, their posterior primary rami and the sympathetic chain [5, 6]. Not only is post-thoracotomy pain relief reliably excellent, but blood concentrations of local anaesthetic are predictable and side effects are minimal [7], By combining research references from these two techniques, inconsistent results are explained adequately. These differences are much too important to be referred to as "variations on the technique of interpleural analgesia (with)... little to choose between them".
CORRESPONDENCE
251
bloodflowduring hyperoxemia. Swedish Society of Anaesthesiology and Intensive Care 1991; 5: A5.
L. M. FIELD
St George's Hospital London 1. Field LM, Dorrance DE, Krzeminska EK, Barsoum LZ. Effect of nitrous oxide on cerebral blood flow in normal humans. British Journal of Anaesthesia 1993; 70: 154-159. 2. Kety SS, Schmidt CF. The effect of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men. Journal of Clinical Investigation 1945; 27: 484-492. 3. Lambertsen CJ, Kough RH, Cooper DY, Emmel GL, Loeschke HH, Schmidt CF. Oxygen toxicity: effects in man of oxygen inhalation at 1 and 3.5 atmospheres upon blood gas transport, cerebral circulation and cerebral metabolism. Journal of Applied Physiology 1953; 5: 471-486.
Pressure in the extradural space Sir,—I read with interest the letter by Guedj [1] describing an ingenious device for detecting negative (subatmospheric) pressure within the extradural space. However, Telford and Holloway [2] have clearly shown that, in the lumbar region at least, the pressure in the extradural space is always positive (supra-atmospheric). An artefactual negative pressure is produced only as a result of tenting of the dura by the blunt Tuohy needle. This would suggest that techniques which rely on eliciting a negative pressure to locate the extradural space are more likely than those using loss of resistance to result in accidental dural puncture. S. D. PAWSEY
Middlesex Hospital London 1. Guedj P. A simple indicator for identifying the extradural space. British Journal of Anaesthesia 1993; 71: 726. 2. Telford RJ, Holloway I t . Observations on deliberate dural puncture with a Tuohy needle: pressure measurements. Anaesthesia 1991; 46: 725-772.
Sir,—The question of the pressure in the extradural space, first raised by Janzen in 1926, has been a subject of much controversy, especially regarding negative pressure. Does it really exist, and if so, docs it occur naturally or is it caused as an artefact by the devices used to identify the space? Bromage, after a comprehensive study of the literature, concluded in 1978 that the pressure in the extradural space in the lumbar region is positive unless the patient lies head-down in the
P. GUEDJ
Misgav Ladach Hospital Jerusalem 1. Bromage PR. Epidural Analgesia. Philadelphia: W. B. Saunders, 1978; 160-171.
Respiratory depression after extradural fentanyl Sir,—Following earlier correspondence [1], we wish to comment further on respiratory depression after extradural fentanyl. Three cases [2-i] have now been documented in the obstetric anaesthetic literature and although it is tempting to speculate on common factors and the mechanisms involved, the important message is that it can occur and it can occur relatively late after extradural injection. Drs Chrubasik, Chrubasik and Black were mistaken when they stated that one of the patients we reported received diamorphine in addition to extradural fentanyl [1]. In our opinion the events described in these three obstetric patients are not explained on the basis of additional systemic drug administration and direct access offentanyl to the brain stem caused by rostral spread in cerebrospinal fluid is the likely mechanism. D. W. NOBLE
Aberdeen L. M. MORRISON
Livingston M. S. BROCKWAY
Livingston J. H. MCCLURE
Edinburgh 1. Chrubasik J, Chrubasik S, Black A. Respiratory depression after extradural fentanyl. British Journal of Anaesthesia 1993; 71: 164-165. 2. Brockway MS, Noble DW, Sharwood-Smith GH, McClure JH. Profound respiratory depression after extradural fentanyl. British Journal of Anaesthesia 1990; 64: 243-245. 3. Noble DW, Morrison LM, Brockway MS, McClure JH. Adrenaline, fentanyl or adrenaline and fentanyl as adjuncts to bupivacaine for extradural anaesthesia in elective Caesarean section. British Journal of Anaesthesia 1991; 66: 645-650. 4. Wang CY. Respiratory depression after cxtradural fentanyl. British Journal of Anaesthesia 1992; 69: 544. Sir,—We apologise to Dr Noble and colleagues for misrepresenting the treatment of one of their reported cases of ventilatory depression after extradural fentanyl. I.v. supplements of diamorphine (or other opioid) were given to some of the patients reported in their study and, in particular, to two in the group in which two patients needed naloxone. We clearly put two and two
Downloaded from http://bja.oxfordjournals.org/ at University of Chicago on July 12, 2015
Sir,—We thank Professor Sjoberg, Mr Kjellgren and Dr Gupta for their comments. Our study [1] was conducted in normal human volunteers. The intention was to mimic, as closely as possible, the clinical situation in which nitrous oxide is administered in oxygen, without the introduction of a third gas, for example nitrogen. Therefore, we feel that our results reflect accurately what occurs in routine clinical practice. In studies using normal human volunteers, Kety and Schmidt [2] and Lambertsen and colleagues [3] did indeed show a decrease in cerebral blood flow when 100% oxygen was inspired. However, the accompanying hypervenrilation and hypocapnia could alone have been responsible for the documented changes in cerebral haemodynamics [2]. Our volunteers did not hyperventilate. While we accept that greater concentrations of oxygen may cause global cerebral vasoconstriction and certainly cortical vasoconstriction in animal studies, the situation in normal humans is far less clear and warrants further investigation.
lateral position; whereas negative respiratory pressure is transmitted to the thoracic extradural space [1]. Many methods of identifying the extradural space have been described, some based on the existence of negative pressure (such as the hanging-drop technique of Gurtierez); others depend upon recognition of sudden release of resistance to injection of fluid (such as the "technique du mandrin liquide" proposed by Sicard, later popularized by Dogliotti and called the "loss of resistance test"). However, every technique has advantages and drawbacks. For example, in the Guttierez technique, the sucking of the drop of liquid is elicited in 82 % and the hanging-drop technique lacks reliability in the lumbar region. On the other hand, the loss of resistance test is difficult to use in the thoracic and cervical regions. The Macintosh balloon indicator, on which our device is based, seems to resolve most problems. As the pressure in the indicator balloon is always greater than that of the extradural space, regardless of the spinal level (thoracic, cervical or lumbar) or of the patient's position, abrupt deflation indicates entry into the extradural space in all patients. A further advantage of this technique is that both hands of the anaesthetist can be used for careful needle advancement. Finally, we do think that a small percentage of failures (i.e. accidental dural puncture) is less related to the technique, than to the skills and experience of the practitioner.