The administration of drugs with analgesic properties via the epidural route provides effective anaesthesia and analgesia for procedures and painful pathologies involving the pelvis and pelvic limbs, the tail, perineum and abdomen. Furthermore, epidural administration can be used to provide analgesia of the thorax. The advantages of epidural administration of analgesics and local anaesthetics are well known: it reduces the need for other anaesthetics, improves haemodynamic function and muscle relaxation, facilitates intestinal surgery and accelerates recovery, reduces the likelihood of adverse effects associated with systemic administration of opioids.

Contraindications

Infection or neoplasia at the injection site; bleeding disorders (coagulopathy, thrombocytopenia); hypovolemia/hypotension (if using local anaesthetics); septicaemia; (controversial); traumatic or congenital anatomical abnormalities – if landmarks cannot be identified.

Equipment

• Medium to large dogs: 20-gauge spinal/Tuohy needles in varying lengths (1.5–3.5 inches)
• Small dogs and cats: 22-gauge spinal/Tuohy needles in varying lengths (1.5–3.5 inches)

Positioning

Patients may be positioned in either sternal or lateral recumbency depending on the patient’s medical condition or clinician’s preference. Sternal recumbency has several advantages as it is easier for the veterinarian to keep the spinal needle in the correct planes and orientation, it also allows use of the “hanging drop” technique for confirmation of the correct needle placement. The pelvic limbs may be positioned in one of two ways while the patient is in sternal recumbency:

• Rostral extension of the pelvic limbs – increases lumbosacral (LS) and L6–L7 distance.
• “Frog-legged” with the pelvic limbs resting on the stifle and the feet extended posteriorly. This position may allow easier palpation of anatomical landmarks in obese patients.

A lumbosacral epidural may be administered with patients positioned in lateral recumbency, with the pelvic limbs taped or held in a rostral position by an assistant. This position may be preferable in cases not amenable to sternal positioning, such as femoral fracture or severe pelvic fractures.

Anatomic Landmarks

• Lumbosacral epidural: The LS intervertebral space is located by palpation of the cranial border of the iliac crests using the the thumb and third finger of the non-dominant hand; an imaginary line between them runs over the L6-to-L7 space, which is palpated with the index finger. The index finger is placed on the patient’s midline and palpates the LS space as a depression on the midline. The index finger is also used to confirm the proper space by palpating cranially (the dorsal spinous process of L6 is larger than L7) and caudally (the sacrum does not have intervertebral spaces).
• Caudal epidural: Palpate the space between the sacrum-Cd1 or Cd1-Cd2, which can be easily identified by having a team member move the tail up and down.

Preparation

The injection site should be clipped to ensure hairless margins laterally to the aspects of the ilial wings, cranially to L3-4 and caudally to S3-Co1. The skin is prepared aseptically in a standard presurgical fashion to avoid infection/abscess of the epidural space and discospondylitis.

Technique

In most instances, the preemptive approach to pain management in surgical patients results in epidural injections performed after induction to general anaesthesia before the start of surgery. Epidural injections and catheter placement may under certain circumstances be performed on sedated patients (most commonly in the intensive care unit), with superficial infiltration of local anaesthetic to facilitate the procedure. Strict aseptic technique should be followed throughout the epidural injection or epidural catheter placement. Sterile gloves as well as a facemask and hair covering should be worn. A sterile fenestrated drape should be placed over the intended needle insertion site. A sterile table drape can assist in maintaining a sterile field with which to place all required equipment and drugs. Under sterile conditions, the needle is introduced perpendicular to the skin (with the bevel of the needle directed cranially) while the index finger of the palpating hand remains in the L-S intervertebral space to ensure accurate positioning. Adjustments to the angle of insertion can be made, as required, to facilitate correct placement in the epidural space. Once the needle has traversed through the skin and subcutaneous tissue, the stylet is removed. The hub of the needle is then filled with sterile saline to facilitate the “hanging drop” confirmation technique. The needle is then advanced further. As the needle advances, a ‘‘pop’’ may be felt when it pierces the ligamentum flavum, and the needle is introduced into the epidural space. Once the ligament is penetrated, the “hanging drop” solution in the fluid is aspirated into the needle shaft by the subatmospheric epidural pressure. In dogs, I recommend advancing the needle all the way to the floor of the epidural space and then withdrawing 1 to 2 mm; in this way, the position of the needle is ensured in the epidural space and being off midline can be ruled out. In cats, the presence of the spinal dura mater beyond L7 makes it likely that CSF is obtained if the needle is advanced to the floor; therefore, it is best avoided. Instead, flicking of the tail, movement of the hind limbs, or twitching of the skin in the area of the L-S intervertebral space is commonly observed in cats when the needle enters the epidural space and pricks the spinal cord or cauda equina, without subsequent adverse effects; however, for this reason, smaller-gauge spinal needles are recommended in cats. To verify correct placement of the needle, several tests can be performed. A plastic or glass syringe, specifically designed to offer minimal resistance, can be attached to the needle, and air can be injected to detect “loss of resistance” on injection because of the subatmospheric pressure of the epidural space.

Advanced Confirmation Techniques

Epidural Pulse Wave Measurement

For the epidural pressure waveform method, the epidural needle or catheter is connected to a pressure transducer, volume is injected into the space, and waveforms are observed on the monitor. The presence of the injected fluid in the epidural space facilitates transmission from CSF pressures and allows arterial pulsations to be visible.

Electrolocation

A shielded nerve stimulator or Tuohy needle is primed with 0.2 to 1 mL of saline, and connected to a peripheral nerve stimulator set to deliver a current at 1 Hz, with a pulse width of 0.2 m sec. Initially the current is set at 1.2 mA as the needle is advanced into position. Confirmation of epidural needle placement is confirmed when twitches were observed in the pelvic limbs and/or tail. The lowest mean (range) current reported to elicit pelvic limb twitches is 0.72 mA (0.4–1.0 mA); lowest mean (range) current reported to elicit tail twitches is 0.58 mA (0.4–1.0 mA); tail twitches were reliably lost at a mean current of 0.37 mA (0.2–0.8).

Ultrasound

Ultrasound-guided epidural injection has been described in dogs.¹

Injection

Prior to injection, the hub of the needle should be observed for the presence of CSF or blood. If CSF is obtained during epidural attempts, withdrawing the needle slowly may reposition the needle back into the epidural space. It is not recommended to inject an epidural dose intrathecally. It is recommended to reduce the dose by 25% to 50%. If blood is obtained during epidural attempts, withdraw the needle and redirect your approach or consider an alternative location (e.g., L6–L7, between the sacrum - Cd1 or Cd1-Cd2) using a new needle. Do not inject drugs intended for epidural administration if blood is observed as inadvertent IV injection of LA can cause systemic toxicity (convulsions, cardiovascular depression). Bupivacaine (and to a lesser extent ropivacaine), may cause cardiac arrest due to increased cardiotoxicity. Therefore, it is recommended to always aspirate before injection.

Size Differences

The spinal cord typically ends at the level of the L6 vertebra in medium/large adult dogs; however, in cats and in small or young dogs, the spinal cord and meninges may extend to the level of the L7, making piercing of meninges and leakage of CSF more likely compared to medium/large dogs.

Drugs

A detailed review of current drugs used for epidural anaesthesia and analgesia by Steagall and colleagues (2017)² is freely available on-line: www.frontiersin.org/articles/10.3389/fvets.2017.00068/full

The most commonly used drugs for epidural administration are opioids, local anaesthetics or a combination of the twodrugs.

• Local anaesthetics: The site of action for local anaesthetics administered in the epidural space is primarily the spinal nerve roots. Local anaesthetics result in autonomic, sensory and motor blockade. Bupivacaine is the most commonly used local anaesthetic drugs due to their longer duration of analgesia of two to four hours. Ropivacaine has the advantage of being less arrhythmogenic and toxic for the CNS and cardiovascular system.
• Bupivicaine 0.5% dosing: 0.5–1.0 mg/kg
• Ropivicaine 0.75% dosing: 1.0–1.65 mg/kg
• Opioids: The site of action for epidural-administered opioids is the opioid receptors in the dorsal horn of the spinal cord. They provide segmental analgesia without sensory, sympathetic or motor blockade. Morphine is the most widely used as it is the least lipid-soluble of the commonly used opioids and, therefore, has the slowest onset of action (30 to 60 minutes) but the longest duration of action (up to 24 hours). Preservative-free morphine is recommended.
• Morphine dosing: 0.1 mg/kg (diluted with saline to 0.3 mL/kg)
• Local anaesthetic/opioid combinations: The combination of opioids with local anaesthetics may be beneficial because affinity of opioid drugs for their receptors in the spinal cord is increased by local anaesthetics.

Volume

In small animals, a total epidural volume of injectate that approximates 0.2 mL/kg but does not exceed 6 mL for animals has been recommended.³ For drugs that do not cause sympathetic or motor blockade, such as the opioids, it is not necessary to adhere to this rule. Volumes greater than 0.2 ml/kg which contain local anaesthetic may spread cranially and potentiate motor blockade of the diaphragm (phrenic nerves).

Volume guide:

• Anaesthesia/analgesia caudal to the diaphragm: 0.2 ml/kg (combined volume opioid+LA)
• Opioid ONLY analgesia to the thorax: 0.3 ml/kg (do not combine with LA at this dose)

Complications

Reported complications with the epidural administration of opioids and local anaesthetic agents are rare, but may include respiratory depression, pruritus (reported in humans), hypotension, nausea, vomiting, delayed hair regrowth and urinary retention. Urinary bladder management should include emptying the bladder at the time of surgery (expression or urinary catheterization) and monitoring bladder size every four to six hours postoperatively until the patient is able to urinate.

Indwelling Epidural Catheter Placement Technique

Catheterization of the epidural space provides the opportunity for repeated or constant delivery of analgesics to the spinal cord, and is usually accomplished by using commercial kits.

Steps

• Anatomical landmarks are confirmed and the patient is prepared as previously described.
• The length of epidural catheter to be placed inside the patient is then pre-measured by carefully placing the catheter over the sterile drape against the patient. For severe cranial abdominal pain, the tip of the catheter should be advanced to the level of L1–2 or L2–3. For pelvic origin pain, the catheter is advanced only to the level of L5–6. Remember to include the distance from the skin surface to the epidural space in the estimation of catheter length needed. Mark the catheter with a sterile pen (often included in the catheter kit) or utilise the reference markings, if present on the catheter (brand specific).
• The mark just created will not be visible during placement, so to assist in accurate placement a second mark exactly the same length of the Tuohy spinal needle is placed on the catheter. The tip of Tuohy spinal needle is placed at the mark created above and a second mark is placed on the catheter at the level of the catheter hub.
• The epidural space that has been chosen for insertion is then carefully palpated again and the thumb of the non-needle placing hand is firmly embedded into the depression between L7–S1 or L6–7. The Tuohy spinal needle is then inserted into the desired epidural space. Correct placement is then confirmed using the previously mentioned techniques.
• Once a positive placement has been confirmed, the catheter guide is placed on the epidural catheter and the appropriate tip is inserted into the Tuohy needle hub. The catheter guide is then gently seated into the Tuohy needle hub and the catheter is gently advanced to the tip of the needle. Be careful not to disrupt the placement of the tip of the Tuohy needle. Gentle resistance should be felt as the catheter tip makes the turn at the tip of the Tuohy needle on its way cranially. After this the catheter should advance smoothly with minimal resistance. If difficulty is encountered in attempting to advance the catheter past the tip of the needle, then withdraw the catheter to ensure the tip is within the barrel of the Tuohy needle once again. Then gently put cranial pressure on the hub of the Tuohy needle and move it cranially 5 to 25 mm and attempt to advance the catheter once again. If resistance is encountered again, then the needle placement is likely incorrect. The catheter should be withdrawn, and needle placement rechecked. If a positive site for advancement of the catheter with minimal resistance is not found after 2–3 successive attempts, then needle placement should take place with fluoroscopic guidance.
• Once the catheter advances with minimal resistance, then the catheter should be advanced until the second mark on the catheter is entering the catheter hub. Advance the catheter 10–25 mm further and then gently withdraw the Tuohy needle, the catheter guidewire, and the catheter guide over the catheter and completely remove them from the catheter.
• The second mark on the catheter should now be visible outside the patient. If the first mark is not visible, then withdraw the catheter gently until the first mark is visible outside the patient.
• The catheter is then cut to allow 10 to 20 cm of catheter to remain outside the patient.
• The catheter tip is then gently inserted into the tapered end of the injection adapter until resistance is met, withdrawn 1–2 mm and then the hub is tightened.
• The floating Luer end of the 0.22-micron filter device is then attached firmly to the hub of the catheter adapter after removing the plastic coverings of both devices. The Luer tip catheter injection port is then firmly attached to the other end of the 0.22-micron filter device after removing the plastic coverings of both devices.
• Catheter placement is then verified by attempting to inject a small quantity of preservative-free 0.9% sterile saline through the filter device and into the catheter. Because of the small inside diameter of the catheter, the resistance to injection is large. As long as injectate continues to flow, then catheter placement is confirmed. Radiographic verification is recommended to confirm correct placement of the epidural catheter tip.
  Carefully attach a clean tape butterfly to the catheter near the skin-catheter interface and suture or staple the tape butterfly to the patient. A second clean tape butterfly is attached to the filter device and stapled or sutured to the patient.
• The skin around the catheter-skin interface is wiped again with an antimicrobial skin preparation and a small amount of sterile antimicrobial ointment (Betadine or chlorhexidine) is placed at the catheter-skin interface. An occlusive plastic skin drape is then placed over the entire area to secure the catheter and prevent accidental removal.

References

1.  Liotta A, Busoni V, Carrozzo M, Sandersen C, Gabriel A, Bolen G. Feasibility of ultrasound-guided access at the lumbo-sacral space in dogs. Veterinary Radiology & Ultrasound. 2015;56(2):220–8.

2.  Steagall PVM. An update on drugs used for lumbosacral epidural anesthesia and analgesia in dogs. Frontiers in Veterinary Science. 2017;4:68.

3.  Torske KE, Dyson DH. Epidural analgesia and anesthesia. The Veterinary Clinics of North America Small Animal Practice. 2000;30(4):859–74.