A new technique of epidural and intrathecal catheterization to evaluate pharmacokinetics of epidural administration in dogs: a prospective study

Background: Experimental research on animals should be performed before applying new treatment modalities in humans. Among experimental animal models, canine model research is a commonly used method. Injecting drugs into the epidural has also been commonly used in the canine animal model. In canine models, epidural catheters are often inserted into the lumbosacral area during epidural drug injection. Unlike the existing canine model, we describe a new and alternative technique of epidural and intrathecal catheterization to investigate methods of epidural drug administration in dogs. Methods: Twelve adult dogs were used in this study. The epidural procedures were performed with dogs in sternal recumbency under deep sedation. Epidural catheterization was perforemed at the T1-T2 intervertebral space with C-arm fluoroscopy guidance. After confirming the loss of resistance, a flexible epidural catheter was passed cranially to the C2-C3 level. The intrathecal procedures were performed with dogs in lateral recumbency under deep sedation. Intrathecal catheterization was performed through the cisterna magna with the neck slightly flexed. An 18-gauge Tuohy needle was inserted into the subarachnoid space through the atlanto-occipital space. After confirming cerebrospinal fluid leakage without bleeding, a flexible intrathecal catheter was passed caudally to the C2-C3 level. Results: All epidural and intrathecal catheterizations were successfully performed under deep sedation without any complications. Conclusion: The new technique of epidural and intrathecal catheterization in canines is an alternative procedure for investigating epidurally administered drugs.

Conclusion: The new technique of epidural and intrathecal catheterization in canines is an alternative procedure for investigating epidurally administered drugs.

Background
As neuraxial techniques have been extensively used in the past century, considerable 3 progress has been made in epidural drug administration. The epidural technique is not only used commonly in regional anesthesia and postoperative pain control but is also used for the perioperative care of high-risk surgical patients [1][2][3][4]. Local anesthetics and opioids are the most commonly used epidural agents, and different pharmacological classes of drugs have been investigated for epidural administration. Experimental research on animals should be conducted to determine whether the newly-developed agents can be used safely and effectively in humans and to evaluate their pharmacology and neurotoxicity. We considered that the neuraxial administration of antiviral agents to the spinal structures with the highest viral burden may be more effective than the other routes of drug administration in herpes zoster and conducted a study to observe neurological safety after intrathecal acyclovir injections in rats. There was no evidence of neurological or histopathological abnormality following intrathecal acyclovir injection in rats [5]. We also compared intrathecal concentrations of acyclovir following epidural and intravenous (IV) administration in rats to investigate the pharmacokinetics of acyclovir.
We found that the epidural administration of acyclovir provided superior drug concentrations in the intrathecal space compared with IV administration [6]. We concluded that further experimental research using other animal species should be conducted to evaluate the safety, efficacy, and toxicity of acyclovir before clinical application in humans.
A canine model has been developed and used because it shows a response similar to that observed in humans and is comparable to humans in size [7]. Epidural injections in dogs are mostly performed in the lumbosacral space because the space is especially wide in the vertebral structure of dogs [8][9][10][11]. In the existing canine model, it was difficult to advance the catheter over a long distance and locate the tip of the intrathecal catheter immediately adjacent to the tip of the epidural catheter in order to investigate the 4 pharmacokinetics of a new epidurally administered drug. Hence, we attempted a new technique of inserting the catheter at the upper thoracic (T1-T2) epidural space under deep sedation.
The present paper describes a new canine model for catheterization to epidurally administer drugs.

Animal Preparation and Procedure
The study was conducted after obtaining institutional approval for the use of animals by Twelve adult mongrel dogs with a mean weight of 21 ± 2.77 kg were used in this study.
Every dog was cared for by well-trained men from the Laboratory Animal Program of the Korea University Medical Center. To prevent distress, experiments were performed in a quiet and comfortable environment; dogs were heavily sedated by intramuscular injection of 1 mg/kg xylazine hydrochloride (100 mg/mL; Rompun® Bayer, Seoul, Korea) and 3 mg/kg ketamine. Oxygen (100%) was supplied through a facial mask at a flow rate of 5 L/min. After sedation, the dog was placed in a sternal recumbency position during the epidural procedure and a lateral recumbency position during the intrathecal procedure for maximum flexion of the cervical spine. The cephalic or accessory cephalic veins were used for fluid administration and blood sampling. The procedures were performed after aseptic preparation.

Epidural Catheterization
Epidural catheterization was performed at the first thoracic (T1)-second thoracic (T2) intervertebral space with the C-arm fluoroscopy-guided midline approach. Before the procedure, the fur of the lower cervical and upper thoracic areas were shaved, and the 5 skin disinfected. The needle entry point was confirmed by palpation of T1 and T2 spinous processes and by C-arm fluoroscopy. An 18-gauge, 90 mm Tuohy epidural needle (B. Braun Medical, Inc., PA, USA) was inserted in the line between T1 and T2 with the bevel facing cephalad. As the needle was advanced slowly through the supraspinous and interspinous ligaments and pierced the interarcuate ligament, a distinct popping sensation was felt.
Then, the epidural space was confirmed by "loss of resistance" test ( Fig. 1). A flexible epidural catheter (Perifix TM ; B-Brown, Melsungen AG, Germany) was advanced cranially to the second cervical (C2)-third cervical (C3) level through the needle. The location of the catheter tip was verified fluoroscopically by injecting contrast medium into the catheter ( Fig. 2). Once the correct position of the catheter was confirmed, it was fixed in place with sterile dressing. We measured the depth from the skin to the epidural space at the T1-T2 level every time. We also measured the length of the epidural catheter inserted at the C2-C3 level.

Intrathecal Catheterization
Intrathecal catheterization was performed through the cisterna magna (CM) in the sternal recumbency position with the neck slightly flexed. Before the procedure, the fur of the occipital and posterior neck was shaved, and the skin was disinfected. The needle entry point was the center of a virtual triangle comprising the external protuberance and the two tips of transverse processes of the first cervical vertebra (C1) (Fig. 3). After the bony landmarks were palpated, the 18-gauge Tuohy needle was inserted perpendicularly to the skin in the midline until resistance was no longer felt. After confirming cerebrospinal fluid (CSF) leakage without bleeding, a flexible intrathecal catheter (Perifix TM ; B-Brown, Melsungen AG, Germany) was inserted and advanced caudally to the C2-C3 level so that the tip of the catheter was located immediately adjacent to the tip of the epidural catheter ( Fig. 4-A, B, C). The location of the catheter tip was verified fluoroscopically by injecting contrast medium through the catheter (Fig. 5). Once the correct position was confirmed, the catheter was fixed on the occiput with sterile dressing. We measured the depth from the center of the virtual triangle to the intrathecal space (cm) each time. We also measured the length of the intrathecal catheter inserted at the C2-C3 level.
After acquiring the data, the dogs were euthanized with potassium chloride. After the intramuscular injection of 5 mg/kg ketamine and 1 mg/kg xylazine hydrochloride prior to the euthanasia, the dogs were confirmed to be unconscious. Potassium chloride was then intravascularly injected at a dose of 150 mg/kg. Fortunately, clonic spasm did not occur after the administration of potassium chloride. Death of dogs was confirmed by lack of pulse, breathing, corneal reflex, response to a firm toe pinch, and respiratory sounds and heartbeat with stethoscope and by the graying of the mucous membranes and rigor mortis. The euthanasia methods employed in this study followed the American Veterinary Medical Association (AVMA) guidelines for the euthanasia of dogs.

Results
We performed epidural catheterization at T1-T2 and intrathecal catheterization at CM and located it at the expected level (C2-C3) under deep sedation in the twelve dogs. The tips of the epidural and intrathecal catheter were successfully located at the C2-C3 level intervertebral space. All of these procedures were successful at one time and had no side effects, such as difficulty in catheter progression or dural puncture during epidural insertion. The depth from the skin to the epidural space at T1-T2 level was 7.64 ± 0.63 cm (mean ± SD), and the depth from the center of the virtual triangle to the intrathecal space was 3.16 ± 0.47 cm (mean ± SD). The length of the epidural catheter from the T1-T2 to the C2-C3 intervertebral space was 19.36 ± 2.58 cm (mean ± SD). The length of the intrathecal catheter from the CM to the C2-C3 intervertebral space was 5.59 ± 1.86 cm (mean ± SD).

Discussion
Heath et al. reported that analgesia was absent in 12% of dogs in which lumbosacral epidural analgesia technique was attempted [12]. One of the major complications of epidural analgesia in dogs and cats is technical failure to locate the lumbosacral space and/or insert a needle into the epidural space; however, reports of individual technical failures are rare [12]. Epidural catheterization of the cervical spine of the dog is difficult because the superior lamina overrides the inferior lamina, creating a small interlaminar space. In contrast, epidural catheterization of the thoracic spine is not difficult since the lamina does not cover the intervertebral space [13]. It is also easy to detect the spinous processes of the thoracic vertebrae by palpation because they are considerably longer than those in the cervical vertebrae. Based on the anatomy of the canine spine, we chose to insert the Touhy needle at the T1-T2 intervertebral space. Indeed, we also initially attempted to place the epidural catheter into the lumbosacral area. However, the epidural catheter did not progress in the first two dogs. Thus, we then performed epidural catheter insertion into the thoracic area in two dogs. When epidural catheter insertion was performed at the thoracic level, the Tuohy epidural needle was easily inserted into the epidural space using the "loss of resistance" technique, and the epidural catheter was easily advanced to the C2-C3 level. Therefore, the other dogs also performed epidural catheter insertion into the thoracic area. During the procedure, the epidural catheter could be placed in the cervical area without difficulty in catheter progression or dural puncture.
Similar to Yaksh's model, intrathecal catheterization was performed between the foramen magnum and the first cervical vertebra (C1); however, we performed the procedure using 8 a Touhy needle without dissection, and the catheter was caudally advanced to the C2-C3 level (5.59 ± 1.86 cm). In this way, we were able to easily advance the catheter to the spinal level intrathecally. Then, the catheter was maintained firmly in place. This technique was less stressful for the dogs because of the non-invasiveness of the procedures. We expect this technique to be useful in surgery or pain control in the forelimb and upper part of dogs.
However, this study has limitations. First, a more appropriate evaluation of this new technique would have been made if we were able to compare the results of this study with the results of the previous study using a lumbosacral epidural insertion. Second, the study involved only 12 dogs, and we did not confirm movement after recovery; therefore, we could not confirm the safety of the technique used in this study. Additionally, a further limitation may be the omission of recording procedure lengths as a measure of time while reporting a new technique.

Conclusions
The new technique of epidural and intrathecal catheterization used in dogs in this study is an alternative procedure for studying the toxicity and pharmacology of epidurally administered drugs.   ARRIVE Chesklist.pdf supporting information(data).xlsx