Old but Gold: Updates and Myth-Busting of Epidural Anesthesia
Gianluca Bini, DVM MRCVS DACVAA
The use of locoregional techniques in veterinary anesthesia is on the rise. Although this has been largely fueled by the past opioid shortage, veterinarians are recognizing the increased value of this analgesic modality. Blocking pain in the transmission phase of the nociceptive pathway prevents the possible trigger of central sensitization and chronic pain 1 .
When compared to systemic analgesia these techniques provide many other benefits including superior intraoperative anesthetic stability and better postoperative pain scores, as well as a lower requirement for postoperative opioids, greater food intake and the potential for cost savings in larger dogs 2 .
Locoregional techniques can be categorized as peripheral nerve blocks and neuraxial techniques; epidural anesthesia is the most used neuraxial technique in veterinary medicine.
When successful, epidurals are a great tool to provide analgesia in dogs and cats undergoing a wide range of abdominal and orthopedic surgical procedures 3,4,5,6,7 , providing good perioperative and postoperative analgesic coverage 8,9 .
Anatomy/Landmarks
During this procedure the drugs are injected into the epidural space, a virtual space inside the spinal canal between the vertebral bones and the dura mater, the most exterior of the meninges. The epidural space contains spinal nerve roots, connective and adipose tissue, as well as arteries, veins and lymphatics 10 .
Although drugs could be injected at different epidural locations, most commonly in dogs and cats the injection would be performed in the lumbosacral epidural space between L7 and S1. This space can be identified by placing thumb and middle finger of one hand on the cranial aspect of each ileal wing, while palpating the spinal processes with the index finger. The sacrum can be identified by palpating its fused spinous processes, forming the median sacral crest.
Moving the index finger cranially the operator will identify a moderately wide intervertebral space (L7-S1) and, continuing more cranially, the last lumbar spinous process (L7) which, compared to the median sacral crest, is very prominent.
A valid alternative could be a sacrococcygeal epidural; this is favored by some authors 11 due to the absence of the dural sac at this location. In this case the S3-Cd1 space can be identified by palpating the dorsal midline caudal to the spinous process of the third sacral vertebra and cranial to the first caudal vertebra. To facilitate this the tail can be repeatedly moved dorsoventrally; this will open and close the intervertebral space and facilitate its identification 11 .
Once the appropriate landmarks have been identified, using proper sterile technique, the needle is inserted in the intervertebral space perpendicular to the skin. In order to reach the epidural space the needle is advanced, in this order, through the skin, subcutaneous tissue, fat, interspinous ligament which runs between the spinal processes, and ultimately through the more rigid ligamentum flavum 7 . The passage of the needle through the ligamentum flavum is the point where the classic “pop” is felt. Humans also have a supraspinous ligament, between the spinal processes, but this is absent in dogs and cats 7 .
Once the needle is inserted in the interspinous ligament, the stylet is removed and the hub of the needle is filled with sterile saline, leaving a dome shaped bubble above the hub. This will be used to confirm placement with the hanging drop technique.
Positioning of the patient and ergonomics may play a role in the success of the procedure. Epidural injections can be performed either in sternal or in lateral recumbency. Regardless of recumbency, extending the hind limbs cranially will expand the caudal intervertebral spaces, facilitating the procedure 7 . The operator performing the injection should stand to either the right or to the left of patient, depending on if they are right- or left-hand dominant, and ideally use the dominant hand to insert the needle.
Confirming Placement
Hanging drop
The hanging drop is a non-invasive technique often used to confirm placement. As mentioned in the section above, we place a small amount of sterile saline into the needle hub. Once the needle pierces the ligamentum flavum, a “pop” will be felt and the sub-atmospheric pressure inside the epidural space may suck the saline from the hub into the epidural space 12 . This technique is associated with low reliability; a tissue plug could occlude the needle, and the respiratory cycle volume status of the patient, abdominal pressure, and position of the patient all affect success of the hanging drop, especially if the patient is in lateral recumbency 12,13,14,7 .
Loss of resistance syringe
The loss of resistance is another non-invasive technique performed by filling a purpose-built plastic syringe with saline and an air bubble. The loss of resistance syringe is then connected to the hub of the needle and the operator injects; if there is resistance to injection the operator has to adjust the needle position and retry. Once there is no resistance to injection and the shape of the air bubble does not change, this confirms the needle positioning in the epidural space. This technique can be used alone or in conjunction with the hanging drop and is particularly useful in lateral recumbency where the hanging drop technique is not reliable. It is important to realize that the use of syringes not designed for loss of resistance technique, because of the intrinsic resistance applied by the plunger against the syringe walls, could lead to false negative results; therefore, it is strongly suggested to use a specialized syringe.
In the past, air injection was used instead of an air bubble in saline, but this has been demonstrated to cause less cranial spread and uneven distribution of drugs, and spinal cord compression 15 , and could lead to air embolism if the injected air is absorbed in the epidural veins 16,17,18 .
Alternative methods
Other methods have been described to aid the identification of the epidural space.
Neurostimulation has been described involves the use of an insulated needle connected to a nerve stimulator instead of an epidural needle. The anode is placed on a pelvic limb, and a current of 1-3 mA with a frequency of 1-2Hz and 0.1ms of pulse duration is used. Once the needle reaches the epidural space, the operator will be able to identify rhythmic twitches of the pelvic limbs as well as the tail. This could be used in conjunction with the LOR technique; a study found an increase in sensitivity but a decrease in specificity compared to either of the two techniques performed separately 19 .
Ultrasound guided epidural injection is another possibility; several approaches have been described both for lumbosacral and sacrococcygeal epidural 20,21 and once the operator becomes familiar with the anatomy and the landmarks for the desired approach, this could be easily performed.
Needle selection
Historically spinal needles (also known as Quincke needles) have been used to perform epidural injections but it is important to recognize the tip of these needles is extremely sharp because, as the name suggests, this needle is designed to perform spinal injections. The goal when performing spinal injections is to reach the subarachnoid space, which is filled with cerebrospinal fluid (CSF). This is deeper than the epidural space, and to perform this the needle has to penetrate the arachnoid and deposit the drugs between the arachnoid and the pia mater.
Instead, an epidural needle (e.g., a Tuohy needle) should be selected when performing epidural injections. The tip of a Tuohy needle is curved and slightly blunt, with the bevel designed to be directed cranially. The bluntness of Tuohy needles allows the operator to get a better feeling of each different tissue layer encountered by the needle 22 . Contrary to what some literature suggests, the dural sac in dogs continues caudal to L7-S1, therefore it is important to recognize that there is a chance of accidental intrathecal injection when performing epidural injections at that intervertebral space 11 .
Spinal needles are considered traumatic needles and have a higher chance to damage important structures. Tuohy needles, in comparison, are atraumatic and their use can decrease the incidence of complications such as accidental dural puncture and puncture of the vessels present in the epidural space, while facilitating better drug spread 23,24 .
Drug and volume selection
Drug choice for epidural anesthesia is mostly affected by the required length of analgesia.
An opioid or an alpha-2 agonist alone can be used to limit motor blockade but can also be paired with a local anesthetic to prolong and enhance depth of the blockade. A few examples of drug choices are presented in table 1. The maximum suggested doses are reported.
| Drug name | Maximum dose | Onset (min) | Duration (h) |
|---|---|---|---|
| Lidocaine | 4mg/kg | 5.15 | 1.5-2 |
| Bupivacaine | 2mg/kg | 10-20 | 4-6 |
| Ropivacaine | 3mg/kg | 10-20 | 2-4 |
| Morphine |
0.1mg/kg | 30-60 | 6-24 |
| Buprenorphine | 0.005mg/kg | 60 | 16-24 |
| Medetomidine | 0.005mg/kg | 20-30 | 2-6 |
| Dexmedetomidine | 0.001mg/kg | 20-30 | 4-6 |
| Bupivacaine + Morphine | 1mg/kg + 0.1mg/kg | 10-15 | 16-24 |
| Morphine + Medetomidine | 0.1mg/kg + 0.005mg/kg | 20-60 | 10-20 |
When deciding how much volume to inject in the epidural space, the goal is to provide adequate analgesic coverage based on the type of procedure; this will affect the spinal cord length that needs to be blocked.
Ideally, the maximum total injected volume should be calculated first, then the dose of the opioid/adjunct (if used) and ultimately the dose of local anesthetic. If the volume of the opioid/adjunct plus local anesthetic exceeds the total injected volume desired, the amount of local anesthetic can be decreased to fit within the volume limit. Otherwise, if the volume of the opioid/adjunct plus local anesthetic is below the maximum total injected volume, sterile saline (0.9% NaCl) can be added to the solution to reach the desired volume.
A total injected volume of 0.22 mL/kg would allow drug spread to the first lumbar vertebra while a 0.31 mL/kg would allow spread up to the 11 th thoracic vertebra in dogs 26,27 . In cats, a volume of 0.3 mL/kg would spread up to the 7 th thoracic vertebra, while a volume of 0.2 mL/kg would spread up to the first lumbar vertebra, making it a safer choice 28 . The spinal canal does not increase in size proportionally to the animal weight, therefore some literature suggest to not exceed 6mL of total volume for larger dogs 29 , although it is important to recognize that there is no evidence backing that precise volume.
A potentially more accurate volume calculation method is the occipito-coccygeal length nomogram 11 ; this allows the choice of a total volume based on the length (in cm) between the occipital condyle and the first coccygeal vertebrae and the desired epidural block extension.
The operator then selects what percentage of the spinal cord to block, and the nomogram provides the volume needed to perform such block.
When attempting an epidural injection cerebrospinal fluid (CSF) could be obtained if the needle reaches the subarachnoid space, and the CSF will be noted flowing upwards from the needle hub. In this case the operator could slowly withdraw the needle and test again with a LOR syringe whether the needle has been retracted in the epidural space or ideally remove the needle and retry the epidural placement of the needle. Some literature suggests proceeding with spinal injection, halving the doses of either local anesthetic or morphine calculated for epidural administration. Caution should be taken because this could result in an overdose; in fact, the morphine and local anesthetics doses for spinal injection are about 30 and 10 times lower than the ones for epidural injection, respectively 7 .
Side effects
One of the most common side effects of epidural anesthesia is hypotension. This occurs due to sympathetic nervous system preganglionic blockade 30 . The sympathetic chain fibers arise from T1 to L4, making it a relatively easy target of the injected drugs. Another common side effect of epidurals is urinary retention due to partial parasympathetic blockade; the detrusor dysfunction 31 . Both hypotension and urinary retention have been reported with local anesthetics, morphine, or their combination 31,32,33,6 .
Other potential side effects of epidurals include spinal cord, nerve root and dura mater iatrogenic lesions, local anesthetic toxicity, pruritus, and allergic reactions to the local anesthetics 11 . Intravascular injection is also a possibility; if blood is identified in the needle hub the operator should remove the needle and retry. Inadvertent intravascular injection could lead to a spike in local anesthetic plasma concentration and toxicity.
Contraindications
Although epidural anesthesia is a very valuable analgesic technique, there are some absolute contraindications for this procedure, including lack of appropriate equipment, lack of expertise, sepsis, coagulopathies, and infection at the injection site 34 . Relative contraindications include increased intracranial pressure, thrombocytopenia, infection remote from site of lumbar puncture, neurologic diseases, and hypovolemia 34 .
Failure rate
As for every procedure, there is a chance of failure. There is minimal published literature on this subject; a failure rate between 11% and 32% has been reported 35,36 . Rescue systemic analgesia should be readily available if needed.
References
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