- Email: [email protected]
The effect of body position on the ‘hanging drop’ method for identifying the extradural space in anaesthetized dogs
Veterinary Anaesthesia and Analgesia, 2007, 34, 59–62
doi:10.1111/j.1467-2995.2006.00290.x
RESEARCH PAPER
The effect of body position on the ‘hanging drop’ method for identifying the extradural space in anaesthetized dogs Kiyokazu Naganobu
BVSc, PhD
& Mitsuyoshi Hagio
DVM, PhD
Faculty of Agriculture, Veterinary Teaching Hospital, University of Miyazaki, Miyazaki 889-2192, Japan
Correspondence: Kiyokazu Naganobu, BVSc, PhD, Veterinary Teaching Hospital, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan. E-mail: [email protected]
Abstract Objectives To assess the accuracy of the ‘hanging drop method’ for identifying the extradural space in anaesthetized dogs positioned in sternal or lateral recumbency. Study design Prospective randomized-experimental study. Animals Seventeen clinically healthy adult dogs, 10 females and seven males weighing 8.4–26.2 kg. Methods Dogs were positioned in either sternal (n ¼ 8) or lateral (n ¼ 9) recumbency under general anaesthesia. A 20 SWG spinal needle pre-filled with 0.9% saline was advanced through the skin into the lumbosacral extradural space and the response of the saline drop recorded, i.e. whether it: 1) was aspirated from the hub into the needle; 2) remained within the hub, or 3) moved synchronously with i) spontaneous respiration, ii) heart beat or iii) manual lung inflation. The position of the needle tip was ultimately determined by positive contrast radiography. Results One dog positioned in lateral recumbency was excluded from the study because bleeding occurred from the needle hub. Saline was aspirated into the needle in seven of eight dogs held in sternal recumbency but in none of the dogs positioned in lateral recumbency. Accurate needle tip placement in the extradural space was confirmed by positive contrast radiography in all dogs.
Conclusion and clinical relevance The ‘hanging drop’ method, when performed with a spinal needle, appears to be a useful technique for identifying the location of the extradural space in anaesthetized medium-sized dogs positioned in sternal, but not in lateral recumbency. The technique may yield ‘false negative’ results when performed in dogs positioned in sternal recumbency. Keywords anaesthesia, analgesia, dogs, extradural, hanging drop technique.
Introduction The ‘hanging drop method’ is a technique used to identify the accurate position of a spinal needle tip within the extradural space, and is characterized by the movement of fluid from the hub into the shaft of the spinal needle as a result of the needle tip entering the extradural space, where the pressure is sub-ambient. One disadvantage of the technique in humans is that it yields ‘false negative’ results, i.e. aspiration does not occur even when the needle tip is present in the extradural space. It was reported that a positive ‘hanging drop’ response was elicited in only 82% of cases (Bromage 1978). Furthermore, the socalled ‘false drop’ response – in which liquid movement is observed when the extradural space remains unpunctured – arises in 2% of cases (Bromage 1978). It seems probable that ‘false negative’ and, or ‘falsepositive’ results could occur in dogs under certain conditions: previous reports have indicated that the hanging drop method is effective when used in 59
‘Hanging drop’ method in dogs K Naganobu and M Hagio
animals positioned in sternal recumbency (Torske & Dyson 2000; Tranquilli et al. 2000; Gaynor & Mama 2002). It is also said that the test is most easily applied using Tuohy needles (Hardie & Kyles 1996; Hansen 1998) although spinal, rather than Tuohy needles would appear to be more commonly used for providing extradural analgesia and anaesthesia in dogs. Skarda (1996) considered that accurate extradural space location was best achieved using the ‘loss of resistance’ method. Although the ‘hanging drop’ method has been used in dogs, we have not, despite an extensive literature search, been able to find information examining the effects of subject position on its accuracy. In this study, we attempted to compare the accuracy of the ‘hanging drop’ technique in identifying the extradural space of dogs positioned in either sternal or lateral recumbency. Materials and methods This study was approved by the Committee of Ethics for Animal Experiments in the Faculty of Agriculture, Miyazaki University. Seventeen clinically healthy dogs, 10 females and 7 males weighing 8.4–26.2 kg, were randomly assigned to two study groups: nine dogs were studied in lateral (five in right lateral and four in left lateral) recumbency and eight dogs in sternal recumbency where the dogs’ pelvic limbs were pulled cranially. Five Beagle and four mixedbreed dogs were included in the lateral recumbency group, and four Beagle and four mixed-breed dogs were included in the sternal recumbency group. Pre-anaesthetic medication in all cases was acepromazine (0.02 mg kg)1, IM) (PromAce )1 10 mg mL ; Fort Dodge Laboratories Inc., Fort Dodge, IA, USA) and buprenorphine (5 lg kg)1, IM) (Lepetan 0.2 mg mL)1; Otsuka Pharmaceutical Co. Ltd, Tokyo, Japan) and anaesthesia was induced with thiamylal sodium (Isozol 25 mg mL)1; Nichiiko, Toyama, Japan) prepared at a dose of 20 mg kg)1, which was administered slowly intravenously (IV) until endotracheal intubation was possible. After endotracheal intubation, anaesthesia was maintained with isoflurane (Forane; Abbott Laboratories, Chicago, IL, USA) delivered in 100% oxygen. Intravenous Ringer’s lactate solution was infused at 10 mL kg)1 hour)1 . Dogs were allowed to breathe spontaneously during the procedure and capnography (Capnox, Colin, Komaki, Japan), noninvasive arterial blood pressure, rectal temperature and lead II ECG (BSM-3101; Nihon Kohden, Tokyo, 60
Japan) were monitored. The body temperature was maintained between 36.5 and 38.3 °C, using an electrical heating pad. After each dog was placed in either sternal or lateral recumbency, the lumbosacral area was clipped and prepared for surgery. A small skin incision was made in the midline lumbosacral area, and a 20 SWG spinal needle with stylet (SN-2070; Terumo, Tokyo, Japan) was advanced through the incision. Before the needle encountered the ligamentum flavum or bone, the stylet was removed and a drop of sterile physiological (0.9%) saline placed in the hub. The needle was then advanced again until either a feeling of increased resistance, a distinct ‘pop’ upon penetrating the ligament, or movement of the saline from the hub into the needle was detected. If the needle struck bone before these sign(s) were identified, it was withdrawn, redirected and advanced in a different direction. Once the aforementioned sign(s) had been observed, saline remaining within the hub was observed for movement synchronous with either breathing, heart beat, or the manual compression of the reservoir bag of the anaesthetic breathing system. After this, a 2.5 mL plastic syringe filled with 1.0 mL of 0.9% physiological saline was connected and the plunger withdrawn gently in an attempt to aspirate fluid. If this proved unsuccessful, 0.5–1.0 mL saline was injected to determine the presence or absence of resistance on injection. Thereafter, the syringe was disconnected and a second drop of saline placed in the hub to determine the absence or presence of a pressure difference between the needle tip and hub. Finally, iohexol (Omnipaque 240; Daiichi, Tokyo, Japan) was injected through the needle and a radiograph taken to identify the position of the needle tip. Dogs in which blood or CSF were aspirated were excluded from further evaluation and statistical analysis. The dogs were allowed to recover from anaesthesia once the final reading had been taken. The data are expressed as the mean ± SD. The data were examined using Mann–Whitney U-tests or v2 tests. A value of p < 0.05 was considered significant. Results Systolic arterial blood pressure and end-tidal CO2 partial pressures were between 99 and 163 mmHg and 3.86 and 8.13 kPa (29 and 61 mmHg) respectively, throughout the experiment. These values were maintained by changing the isoflurane
Ó 2006 The Authors. Journal compilation Ó 2006 Association of Veterinary Anaesthetists, 34, 59–62
‘Hanging drop’ method in dogs K Naganobu and M Hagio
vaporizer setting and, or the rate of fluid administration. The heart rate decreased to 54 beats minute)1 in one dog, to which atropine (0.015 mg kg)1, IV) (Atropine sulfate 0.5 mg mL)1; Fuso Pharmaceutical Industries Ltd, Osaka, Japan) was administered. Blood issued from the needle hub in one dog positioned in lateral recumbency, which was subsequently excluded from further evaluation. Neither CSF nor blood were aspirated from any of the remaining dogs. There were no significant differences in the body mass, age and sex between the two study groups (Table 1). Positive contrast radiography showed that the needle tip was in the extradural space in all animals. Movement of the saline drop from the needle hub into its shaft was observed in seven of eight dogs held in sternal recumbency but in none of the dogs in lateral recumbency (Table 1). In one dog positioned in sternal recumbency and in which a positive ‘hanging drop’ response had been observed, the small amount of saline remaining in the hub oscillated in response to both the heart beat and respiration and was displaced from the shaft to the hub upon manual compression of the reservoir bag. In the one dog positioned in sternal recumbency in which a positive ‘hanging drop’ sign had not been observed, the saline remained in the hub yet moved upon reservoir bag compression. In this case, saline moved into the needle from the hub once the second aliquot of saline had been injected. For dogs in lateral recumbency, saline in the hub did not move synchronously with the heart beat, the spontaneous breathing rate or upon squeezing the reservoir bag. Movement of the saline into the needle was not observed in any of the dogs in lateral recumbency even after saline had been injected into the needle. No resistance was felt during saline injection into the needle in any of the dogs in either position. Table 1 Effect of body position on the ‘hanging drop’ method
n Body mass (kg) Age (years) Female/male Positive hanging drop test* [number of dogs (%)]
Sternal recumbency
Lateral recumbency
8 13.1 ± 6.2 2.7 ± 1.5 5/3 7 (88)
8 12.6 ± 3.0 3.1 ± 2.0 4/4 0 (0)
*Significantly different between positions (p < 0.05).
Discussion In the ‘hanging drop’ method, a drop of fluid in the needle hub is aspirated into the needle shaft by the negative extradural pressure. Extradural pressure is negative in the lumbar region in standing cattle (Lee et al. 2002) but appears to be variable in humans and dependent upon conditions, e.g. puncture site or posture (Bromage 1978; Cousins & Veering 1998). We could not find any information on extradural pressure in dogs. In humans, a transient negative extradural pressure can be created by ‘dimpling’ the dura using the needle, even when the extradural pressure is not negative. Irrespective of this, actual or artifactual sub-ambient extradural pressures aspirate fluid from the needle’s hub first into the needle and then into the extradural space (Bromage 1978; Cousins & Veering 1998). In the present study, needle tip location in the extradural space was demonstrated by the ‘hanging drop’ method in 88% (7/8) of the dogs in sternal, and in none (0/8) of the dogs in lateral recumbency. This confirms previous views that the ‘hanging drop’ method is most useful in dogs positioned in sternal recumbency (Torske & Dyson 2000; Tranquilli et al. 2000; Gaynor & Mama 2002). Shah (1984) demonstrated posture-dependent changes in extradural pressure in pregnant humans, with the mean extradural pressures being 22.6, 14.8 and 2.2 cmH2O in the supine, lateral and prone positions, respectively. It seems likely that the extradural pressure of dogs in sternal recumbency may also be lower than those in lateral recumbency, which would account for the positional effects on the results on the current study. However, the effect of position on extradural pressure was not examined. It is possible that tissue cores plug the spinal needle during attempts at extradural needle insertion, with the result of producing ‘false negative’ responses with the ‘hanging drop’ method (Cousins & Veering 1998). In one dog positioned in sternal recumbency in the current study, a movement of saline into the needle was observed only after the needle had been ‘flushed’. It is likely that saline movement was initially prevented by a tissue plug, which was flushed from the needle by the second saline injection. If this was the case, then it indicates the possibility of injecting tissues and, or microorganisms into the extradural space. To our knowledge, however, no complications specific to the ‘hanging drop’ or to other methods have been
Ó 2006 The Authors. Journal compilation Ó 2006 Association of Veterinary Anaesthetists, 34, 59–62
61
‘Hanging drop’ method in dogs K Naganobu and M Hagio
reported in either animals or humans, which may be the result of the importance attached to aseptic technique and skin preparation, and the avoidance of injections at infected sites. In view of the potential for tissue cores to develop, it may be more advisable for the needle to be as close as possible to the ligamentum flavum before removal of the stylet. As no resistance was felt during saline injection in the present study, the tissue plug, if present, would probably not have prevented the subsequent injection of more saline or drug. However, a normal syringe that was relatively insensitive to resistance was used; the use of a proprietary ‘loss of resistance’ syringe may have more readily detected the presence of a plug. Needle tip location in the extradural space was not always identified by the ‘hanging drop’ method in the current study – even for dogs placed in sternal recumbency – in which case an alternative technique must be sought. We did not compare the efficacy of the ‘hanging drop’ method with any other techniques, although the sensation of resistance, or ‘popping’ as the needle passes through the ligamentum flavum can be appreciated as the ‘hanging drop’ technique is performed, while the absence of resistance during fluid or air injection, the identification of subcutaneous crepitation, or a ‘whoosh’ sound heard during air injection can be used to confirm extradural space puncture after the hanging drop method has been tried. In other words, the ‘hanging-drop’ method can be used in conjunction with other techniques. Bromage (1978) described a ‘false drop sign’ occurring in about 2% of human cases when the ‘hanging drop method’ is used. It may, therefore, be desirable to double- or triple-check for correct needle placement using several techniques. Twenty to 22 SWG needles have been recommended for lumbosacral extradural injection in medium-sized dogs (Skarda 1996; Dobromylskyj et al. 2000). In the present study, the hanging drop method was evaluated using 20 SWG, 7-cm long spinal needles in dogs weighing 8.4–26.2 kg. Spinal needles tend to cut through tissue fibres, whereas the Tuohy needle tip is relatively dull and more likely to cause dimpling of the dura and augment the generation of negative extradural pressures. Therefore, it would be of value to further study the ‘hanging drop’ method in dogs of various sizes using a range of needle sizes and patterns. While the present study involved only a small number of healthy dogs, our results show that the 62
‘hanging drop’ method is more useful in dogs placed in sternal recumbency. Some conditions, such as aging, pregnancy, lung disease or trauma affect extradural pressure in humans, and may, in theory, influence the results of the ‘hanging drop’ method in dogs, though this has not been investigated. Furthermore, none of the dogs studied were very obese or very thin, it may have been useful to examine the usefulness of the ‘hanging drop’ technique in dogs with extremes of bodily condition. References Bromage PR. (1978) Epidural Analgesia. W.B. Saunders, Philaderphia, PA, USA, pp. 160–214. Cousins MJ, Veering BT. (1998) Epidural neural blockade. In: Neural Blockade in Clinical Anesthesia and Management of Pain, 3rd edn. Cousins MJ, Bridenbaugh PO (eds.) Lippincott-Raven, Philadelphia, PA, USA, pp. 243–321. Dobromylskyj P, Flecknell PA, Lascelles BD, et al. (2000) Management of postoperative and other acute pain. In: Pain Management in Animals. Flecknell PA, WatermanPearson A (eds.) W.B. Saunders, London, UK, pp. 81–145. Gaynor JS, Mama KR. (2002) Local and regional anesthetic techniques for alleviation of perioperative pain. In: Handbook of Veterinary Pain Management. Gaynor JS, Muir WW (ed.) Mosby, St Louis, MO, USA, pp. 261–280. Hansen B. (1998) Epidural analgesia. In: Kirk’s Current Veterinary Therapy XIII. Small Animal Practice. Bonagura JD (ed.) W.B. Saunders, Philadelphia, PA, pp. 126–131. Hardie EM, Kyles A. (1996) Pain management in the small animal patient. In: Current Techniques in Small Animal Surgery, 4th edn. Bojrab MJ (ed.), Williams& Wilkins, Baltimore, MD, USA, pp. 3–17. Lee I, Yamagishi N, Oboshi K et al. (2002) Multivariate regression analysis of epidural pressure in cattle. Am J Vet Res 63, 954–957. Shah JL. (1984) Effect of posture on extradural pressure. Br J Anaesth 56, 1373–1377. Skarda RT. (1996) Local and regional anesthetic and analgesic techniques: dogs. In: Lumb and Jones’ Veterinary Anesthesia, 3rd edn. Thurmon JC, Tranquilli WJ, Benson GJ (eds.) Williams & Wilkins, Baltimore, MD, USA, pp. 426–447. Torske KE, Dyson DH. (2000) Epidural analgsia and anesthesia. Vet Clin North Am: Small Anim Pract 30, 859– 874. Tranquilli WJ, Grimm KA, Lamont LA. (2000) Pain Management for the Small Animal Practitioner. Teton New Media, Jackson, WY, USA. (Japanese translation). Received 28 July 2005; accepted 9 January 2006
Ó 2006 The Authors. Journal compilation Ó 2006 Association of Veterinary Anaesthetists, 34, 59–62
doi:10.1111/j.1467-2995.2006.00290.x
RESEARCH PAPER
The effect of body position on the ‘hanging drop’ method for identifying the extradural space in anaesthetized dogs Kiyokazu Naganobu
BVSc, PhD
& Mitsuyoshi Hagio
DVM, PhD
Faculty of Agriculture, Veterinary Teaching Hospital, University of Miyazaki, Miyazaki 889-2192, Japan
Correspondence: Kiyokazu Naganobu, BVSc, PhD, Veterinary Teaching Hospital, Faculty of Agriculture, University of Miyazaki, Miyazaki 889-2192, Japan. E-mail: [email protected]
Abstract Objectives To assess the accuracy of the ‘hanging drop method’ for identifying the extradural space in anaesthetized dogs positioned in sternal or lateral recumbency. Study design Prospective randomized-experimental study. Animals Seventeen clinically healthy adult dogs, 10 females and seven males weighing 8.4–26.2 kg. Methods Dogs were positioned in either sternal (n ¼ 8) or lateral (n ¼ 9) recumbency under general anaesthesia. A 20 SWG spinal needle pre-filled with 0.9% saline was advanced through the skin into the lumbosacral extradural space and the response of the saline drop recorded, i.e. whether it: 1) was aspirated from the hub into the needle; 2) remained within the hub, or 3) moved synchronously with i) spontaneous respiration, ii) heart beat or iii) manual lung inflation. The position of the needle tip was ultimately determined by positive contrast radiography. Results One dog positioned in lateral recumbency was excluded from the study because bleeding occurred from the needle hub. Saline was aspirated into the needle in seven of eight dogs held in sternal recumbency but in none of the dogs positioned in lateral recumbency. Accurate needle tip placement in the extradural space was confirmed by positive contrast radiography in all dogs.
Conclusion and clinical relevance The ‘hanging drop’ method, when performed with a spinal needle, appears to be a useful technique for identifying the location of the extradural space in anaesthetized medium-sized dogs positioned in sternal, but not in lateral recumbency. The technique may yield ‘false negative’ results when performed in dogs positioned in sternal recumbency. Keywords anaesthesia, analgesia, dogs, extradural, hanging drop technique.
Introduction The ‘hanging drop method’ is a technique used to identify the accurate position of a spinal needle tip within the extradural space, and is characterized by the movement of fluid from the hub into the shaft of the spinal needle as a result of the needle tip entering the extradural space, where the pressure is sub-ambient. One disadvantage of the technique in humans is that it yields ‘false negative’ results, i.e. aspiration does not occur even when the needle tip is present in the extradural space. It was reported that a positive ‘hanging drop’ response was elicited in only 82% of cases (Bromage 1978). Furthermore, the socalled ‘false drop’ response – in which liquid movement is observed when the extradural space remains unpunctured – arises in 2% of cases (Bromage 1978). It seems probable that ‘false negative’ and, or ‘falsepositive’ results could occur in dogs under certain conditions: previous reports have indicated that the hanging drop method is effective when used in 59
‘Hanging drop’ method in dogs K Naganobu and M Hagio
animals positioned in sternal recumbency (Torske & Dyson 2000; Tranquilli et al. 2000; Gaynor & Mama 2002). It is also said that the test is most easily applied using Tuohy needles (Hardie & Kyles 1996; Hansen 1998) although spinal, rather than Tuohy needles would appear to be more commonly used for providing extradural analgesia and anaesthesia in dogs. Skarda (1996) considered that accurate extradural space location was best achieved using the ‘loss of resistance’ method. Although the ‘hanging drop’ method has been used in dogs, we have not, despite an extensive literature search, been able to find information examining the effects of subject position on its accuracy. In this study, we attempted to compare the accuracy of the ‘hanging drop’ technique in identifying the extradural space of dogs positioned in either sternal or lateral recumbency. Materials and methods This study was approved by the Committee of Ethics for Animal Experiments in the Faculty of Agriculture, Miyazaki University. Seventeen clinically healthy dogs, 10 females and 7 males weighing 8.4–26.2 kg, were randomly assigned to two study groups: nine dogs were studied in lateral (five in right lateral and four in left lateral) recumbency and eight dogs in sternal recumbency where the dogs’ pelvic limbs were pulled cranially. Five Beagle and four mixedbreed dogs were included in the lateral recumbency group, and four Beagle and four mixed-breed dogs were included in the sternal recumbency group. Pre-anaesthetic medication in all cases was acepromazine (0.02 mg kg)1, IM) (PromAce )1 10 mg mL ; Fort Dodge Laboratories Inc., Fort Dodge, IA, USA) and buprenorphine (5 lg kg)1, IM) (Lepetan 0.2 mg mL)1; Otsuka Pharmaceutical Co. Ltd, Tokyo, Japan) and anaesthesia was induced with thiamylal sodium (Isozol 25 mg mL)1; Nichiiko, Toyama, Japan) prepared at a dose of 20 mg kg)1, which was administered slowly intravenously (IV) until endotracheal intubation was possible. After endotracheal intubation, anaesthesia was maintained with isoflurane (Forane; Abbott Laboratories, Chicago, IL, USA) delivered in 100% oxygen. Intravenous Ringer’s lactate solution was infused at 10 mL kg)1 hour)1 . Dogs were allowed to breathe spontaneously during the procedure and capnography (Capnox, Colin, Komaki, Japan), noninvasive arterial blood pressure, rectal temperature and lead II ECG (BSM-3101; Nihon Kohden, Tokyo, 60
Japan) were monitored. The body temperature was maintained between 36.5 and 38.3 °C, using an electrical heating pad. After each dog was placed in either sternal or lateral recumbency, the lumbosacral area was clipped and prepared for surgery. A small skin incision was made in the midline lumbosacral area, and a 20 SWG spinal needle with stylet (SN-2070; Terumo, Tokyo, Japan) was advanced through the incision. Before the needle encountered the ligamentum flavum or bone, the stylet was removed and a drop of sterile physiological (0.9%) saline placed in the hub. The needle was then advanced again until either a feeling of increased resistance, a distinct ‘pop’ upon penetrating the ligament, or movement of the saline from the hub into the needle was detected. If the needle struck bone before these sign(s) were identified, it was withdrawn, redirected and advanced in a different direction. Once the aforementioned sign(s) had been observed, saline remaining within the hub was observed for movement synchronous with either breathing, heart beat, or the manual compression of the reservoir bag of the anaesthetic breathing system. After this, a 2.5 mL plastic syringe filled with 1.0 mL of 0.9% physiological saline was connected and the plunger withdrawn gently in an attempt to aspirate fluid. If this proved unsuccessful, 0.5–1.0 mL saline was injected to determine the presence or absence of resistance on injection. Thereafter, the syringe was disconnected and a second drop of saline placed in the hub to determine the absence or presence of a pressure difference between the needle tip and hub. Finally, iohexol (Omnipaque 240; Daiichi, Tokyo, Japan) was injected through the needle and a radiograph taken to identify the position of the needle tip. Dogs in which blood or CSF were aspirated were excluded from further evaluation and statistical analysis. The dogs were allowed to recover from anaesthesia once the final reading had been taken. The data are expressed as the mean ± SD. The data were examined using Mann–Whitney U-tests or v2 tests. A value of p < 0.05 was considered significant. Results Systolic arterial blood pressure and end-tidal CO2 partial pressures were between 99 and 163 mmHg and 3.86 and 8.13 kPa (29 and 61 mmHg) respectively, throughout the experiment. These values were maintained by changing the isoflurane
Ó 2006 The Authors. Journal compilation Ó 2006 Association of Veterinary Anaesthetists, 34, 59–62
‘Hanging drop’ method in dogs K Naganobu and M Hagio
vaporizer setting and, or the rate of fluid administration. The heart rate decreased to 54 beats minute)1 in one dog, to which atropine (0.015 mg kg)1, IV) (Atropine sulfate 0.5 mg mL)1; Fuso Pharmaceutical Industries Ltd, Osaka, Japan) was administered. Blood issued from the needle hub in one dog positioned in lateral recumbency, which was subsequently excluded from further evaluation. Neither CSF nor blood were aspirated from any of the remaining dogs. There were no significant differences in the body mass, age and sex between the two study groups (Table 1). Positive contrast radiography showed that the needle tip was in the extradural space in all animals. Movement of the saline drop from the needle hub into its shaft was observed in seven of eight dogs held in sternal recumbency but in none of the dogs in lateral recumbency (Table 1). In one dog positioned in sternal recumbency and in which a positive ‘hanging drop’ response had been observed, the small amount of saline remaining in the hub oscillated in response to both the heart beat and respiration and was displaced from the shaft to the hub upon manual compression of the reservoir bag. In the one dog positioned in sternal recumbency in which a positive ‘hanging drop’ sign had not been observed, the saline remained in the hub yet moved upon reservoir bag compression. In this case, saline moved into the needle from the hub once the second aliquot of saline had been injected. For dogs in lateral recumbency, saline in the hub did not move synchronously with the heart beat, the spontaneous breathing rate or upon squeezing the reservoir bag. Movement of the saline into the needle was not observed in any of the dogs in lateral recumbency even after saline had been injected into the needle. No resistance was felt during saline injection into the needle in any of the dogs in either position. Table 1 Effect of body position on the ‘hanging drop’ method
n Body mass (kg) Age (years) Female/male Positive hanging drop test* [number of dogs (%)]
Sternal recumbency
Lateral recumbency
8 13.1 ± 6.2 2.7 ± 1.5 5/3 7 (88)
8 12.6 ± 3.0 3.1 ± 2.0 4/4 0 (0)
*Significantly different between positions (p < 0.05).
Discussion In the ‘hanging drop’ method, a drop of fluid in the needle hub is aspirated into the needle shaft by the negative extradural pressure. Extradural pressure is negative in the lumbar region in standing cattle (Lee et al. 2002) but appears to be variable in humans and dependent upon conditions, e.g. puncture site or posture (Bromage 1978; Cousins & Veering 1998). We could not find any information on extradural pressure in dogs. In humans, a transient negative extradural pressure can be created by ‘dimpling’ the dura using the needle, even when the extradural pressure is not negative. Irrespective of this, actual or artifactual sub-ambient extradural pressures aspirate fluid from the needle’s hub first into the needle and then into the extradural space (Bromage 1978; Cousins & Veering 1998). In the present study, needle tip location in the extradural space was demonstrated by the ‘hanging drop’ method in 88% (7/8) of the dogs in sternal, and in none (0/8) of the dogs in lateral recumbency. This confirms previous views that the ‘hanging drop’ method is most useful in dogs positioned in sternal recumbency (Torske & Dyson 2000; Tranquilli et al. 2000; Gaynor & Mama 2002). Shah (1984) demonstrated posture-dependent changes in extradural pressure in pregnant humans, with the mean extradural pressures being 22.6, 14.8 and 2.2 cmH2O in the supine, lateral and prone positions, respectively. It seems likely that the extradural pressure of dogs in sternal recumbency may also be lower than those in lateral recumbency, which would account for the positional effects on the results on the current study. However, the effect of position on extradural pressure was not examined. It is possible that tissue cores plug the spinal needle during attempts at extradural needle insertion, with the result of producing ‘false negative’ responses with the ‘hanging drop’ method (Cousins & Veering 1998). In one dog positioned in sternal recumbency in the current study, a movement of saline into the needle was observed only after the needle had been ‘flushed’. It is likely that saline movement was initially prevented by a tissue plug, which was flushed from the needle by the second saline injection. If this was the case, then it indicates the possibility of injecting tissues and, or microorganisms into the extradural space. To our knowledge, however, no complications specific to the ‘hanging drop’ or to other methods have been
Ó 2006 The Authors. Journal compilation Ó 2006 Association of Veterinary Anaesthetists, 34, 59–62
61
‘Hanging drop’ method in dogs K Naganobu and M Hagio
reported in either animals or humans, which may be the result of the importance attached to aseptic technique and skin preparation, and the avoidance of injections at infected sites. In view of the potential for tissue cores to develop, it may be more advisable for the needle to be as close as possible to the ligamentum flavum before removal of the stylet. As no resistance was felt during saline injection in the present study, the tissue plug, if present, would probably not have prevented the subsequent injection of more saline or drug. However, a normal syringe that was relatively insensitive to resistance was used; the use of a proprietary ‘loss of resistance’ syringe may have more readily detected the presence of a plug. Needle tip location in the extradural space was not always identified by the ‘hanging drop’ method in the current study – even for dogs placed in sternal recumbency – in which case an alternative technique must be sought. We did not compare the efficacy of the ‘hanging drop’ method with any other techniques, although the sensation of resistance, or ‘popping’ as the needle passes through the ligamentum flavum can be appreciated as the ‘hanging drop’ technique is performed, while the absence of resistance during fluid or air injection, the identification of subcutaneous crepitation, or a ‘whoosh’ sound heard during air injection can be used to confirm extradural space puncture after the hanging drop method has been tried. In other words, the ‘hanging-drop’ method can be used in conjunction with other techniques. Bromage (1978) described a ‘false drop sign’ occurring in about 2% of human cases when the ‘hanging drop method’ is used. It may, therefore, be desirable to double- or triple-check for correct needle placement using several techniques. Twenty to 22 SWG needles have been recommended for lumbosacral extradural injection in medium-sized dogs (Skarda 1996; Dobromylskyj et al. 2000). In the present study, the hanging drop method was evaluated using 20 SWG, 7-cm long spinal needles in dogs weighing 8.4–26.2 kg. Spinal needles tend to cut through tissue fibres, whereas the Tuohy needle tip is relatively dull and more likely to cause dimpling of the dura and augment the generation of negative extradural pressures. Therefore, it would be of value to further study the ‘hanging drop’ method in dogs of various sizes using a range of needle sizes and patterns. While the present study involved only a small number of healthy dogs, our results show that the 62
‘hanging drop’ method is more useful in dogs placed in sternal recumbency. Some conditions, such as aging, pregnancy, lung disease or trauma affect extradural pressure in humans, and may, in theory, influence the results of the ‘hanging drop’ method in dogs, though this has not been investigated. Furthermore, none of the dogs studied were very obese or very thin, it may have been useful to examine the usefulness of the ‘hanging drop’ technique in dogs with extremes of bodily condition. References Bromage PR. (1978) Epidural Analgesia. W.B. Saunders, Philaderphia, PA, USA, pp. 160–214. Cousins MJ, Veering BT. (1998) Epidural neural blockade. In: Neural Blockade in Clinical Anesthesia and Management of Pain, 3rd edn. Cousins MJ, Bridenbaugh PO (eds.) Lippincott-Raven, Philadelphia, PA, USA, pp. 243–321. Dobromylskyj P, Flecknell PA, Lascelles BD, et al. (2000) Management of postoperative and other acute pain. In: Pain Management in Animals. Flecknell PA, WatermanPearson A (eds.) W.B. Saunders, London, UK, pp. 81–145. Gaynor JS, Mama KR. (2002) Local and regional anesthetic techniques for alleviation of perioperative pain. In: Handbook of Veterinary Pain Management. Gaynor JS, Muir WW (ed.) Mosby, St Louis, MO, USA, pp. 261–280. Hansen B. (1998) Epidural analgesia. In: Kirk’s Current Veterinary Therapy XIII. Small Animal Practice. Bonagura JD (ed.) W.B. Saunders, Philadelphia, PA, pp. 126–131. Hardie EM, Kyles A. (1996) Pain management in the small animal patient. In: Current Techniques in Small Animal Surgery, 4th edn. Bojrab MJ (ed.), Williams& Wilkins, Baltimore, MD, USA, pp. 3–17. Lee I, Yamagishi N, Oboshi K et al. (2002) Multivariate regression analysis of epidural pressure in cattle. Am J Vet Res 63, 954–957. Shah JL. (1984) Effect of posture on extradural pressure. Br J Anaesth 56, 1373–1377. Skarda RT. (1996) Local and regional anesthetic and analgesic techniques: dogs. In: Lumb and Jones’ Veterinary Anesthesia, 3rd edn. Thurmon JC, Tranquilli WJ, Benson GJ (eds.) Williams & Wilkins, Baltimore, MD, USA, pp. 426–447. Torske KE, Dyson DH. (2000) Epidural analgsia and anesthesia. Vet Clin North Am: Small Anim Pract 30, 859– 874. Tranquilli WJ, Grimm KA, Lamont LA. (2000) Pain Management for the Small Animal Practitioner. Teton New Media, Jackson, WY, USA. (Japanese translation). Received 28 July 2005; accepted 9 January 2006
Ó 2006 The Authors. Journal compilation Ó 2006 Association of Veterinary Anaesthetists, 34, 59–62