Medication can be administered into the epidural space to provide analgesia, anesthesia, and for a number of other reasons that we will not cover today (e.g., NSAIDs for their anti-inflammatory effects). In veterinary science, local anesthetic agents such as lidocaine and bupivacaine, and opioids such as morphine and alpha-2 agonists, are given into the epidural (or sometimes the subarachnoid space) to provide anesthesia and analgesia. These agents can be given as a single dose or as a continuous infusion via catheter. Placing an epidural catheter also allows for administration of the agent at the most efficacious site (e.g., thoracic or lumbar sites).
Local anesthetics can be administered to both the conscious and unconscious patient. In situations where general anesthesia is unadvisable, lidocaine or bupivacaine can be administered into the epidural space to provide anesthesia for surgery or other procedures of the hindlegs, perineum, and some laparotomy procedures. Lidocaine (1–2%) or bupivacaine (0.25%) is given at a rate of 0.5 ml per 10 cm crown to rump length or 1 ml per 4–5 kg. After local anesthetic administration, anesthesia occurs very rapidly with blockade of sensory, motor and sympathetic fibers. The animal is unable to move its limbs for a period determined by the specific agent. Blockade of sympathetic fibers causes vasodilation of the local area and the skin becomes warm and pink. In the hypovolaemic animal or in those with poor cardiovascular status, care must be taken to ensure that serious hypotension does not occur as a result of vasodilation. Placing the spinal needle is uncomfortable and most animals resent it unless they are profoundly depressed by their disease or are heavily sedated. It is often more useful to place the epidural drugs after induction of anesthesia. The benefit of epidural local anesthetics (and opioids) is that they can markedly reduce the quantity of subsequent depressant inhalation agent required and provide good muscle relaxation for some fracture repairs.
Epidural opioids are commonly administered for hindlimb and perineal procedures to provide improved surgical conditions, as well as reduce requirements for intra-operative inhalational agent. Some analgesia is also provided for forelimb and abdominal procedures, although the placement of an epidural catheter is more effective. The opioids are often used in preference to local anesthetic agents because the side effects are very much fewer. Motor function is not compromised, sympathetic blockade doesn’t occur, and in animals, many of the side effects noted in man, are not seen (respiratory depression, sedation, histamine release). Morphine is administered in preference to other agents as it has low lipid solubility and is slowly removed from the spinal space, thus providing long duration analgesia. The epidural morphine dosage is 0.1 mg/kg of preservative-free morphine diluted to 0.125 ml/kg for hindlimb and perineal surgery, and 0.25 ml/kg for abdominal surgery. Onset is slow (30–60 mins) and is best carried out immediately after induction of anesthesia.
Opioids and local anesthetic agents can be combined and administered into the epidural space to provide desensitization of the site during surgery, in combination with long-term post-operative analgesia. The incidence of untoward reactions may be increased when these two agents are combined.
EPIDURAL TECHNIQUE
The wings of the ilium are the starting point for finding the lumbosacral space when placing the needle for epidural injection. The spinal cord in the dog ends at about L6 and there is little chance of damaging the spinal cord during a lumbosacral puncture. It is also unlikely that CSF will be encountered in this space either, however, aspiration through the needle should be attempted before injection to ensure that the subarachnoid (intrathecal space) has not been breached. In the cat, the spinal cord ends more caudally so care is needed during needle placement, as it is possible to damage the cord.
On the midline, just cranial to the cranial-most point of the ilial wing, is the dorsal spinal process of L6. L7 is immediately caudal, followed by S1, which has a very small spinous process. The lumbosacral space is the hollow palpated between L7 and S1. In medium to fat dogs, it is possible to palpate only a hollow between the dorsal spinous processes; however, in thin dogs it is possible to palpate the lumbar muscles rising up on either side to create a “peak” with a very shallow “groove” in the center.
The site for needle placement should be clipped and prepped in a sterile manner as for surgery. Sterile gloves should be worn for the procedure. A spinal needle should be used because it contains a stylet of about 22–20 g x 1½”–2½” in length. It is placed exactly on midline in the center of the hollow palpated between the dorsal spinal processes of L7 and S1. A “pop” can be felt as the needle travels through the tough interspinous ligament and into the much less resistant epidural space. A sterile syringe containing air can be connected to the spinal needle and a small amount of air can be injected. It should be easily injected, almost as though “sucked” in. Once the position of the spinal needle is verified, the syringe containing the agent is connected and the agent injected slowly. If resistance occurs, withdraw the needle slightly and continue. The needle is withdrawn completely once the agent is administered. If, during aspiration CSF is detected, then one-third to one-half of the calculated dose can be administered. If blood is detected during aspiration, the needle should be withdrawn and the procedure started again.
Epidural administration of local anesthetic agents and opioids provides many advantages to the patient, which in turn can make anesthesia and the post-operative period much smoother.