Epidural analgesia (or more anatomically, extradural) is used to provide analgesia to the pelvic limbs but will also afford analgesia to the caudal abdomen, perineal region and at appropriate doses, the thoracic region.
The technique described is that adopted by the author. There are several techniques and these are widely described in the literature. The transverse section below is at the level of L5. In the sagittal section the spinal cord actually ends a little further forward – you can perform an epidural between L6/L7 without danger of hitting the spinal cord in an adult animal. Note that in young animals the spinal cord can extend to the sacrum. It is therefore often more likely to perform an intrathecal injection in younger animals (or if you aspirate CSF redirect (withdraw) the needle to inject extradurally.)
Site of action of epidurally administered drugs
For local anaesthetics the site of action is the spinal nerves. For pelvic limb blockade the area from L3 to S1 must be covered. For the abdominal wall, this extends forwards to T11-L3. Solutions injected at L7 flow primarily cephalad but there is some loss of solution through intervertebral foraminae.
Technique
The area should be aseptically prepared, the operator gloved and the site draped. Animal positioned in sternal with the limbs drawn forwards. A spinal needle is be used, which is advanced perpendicular to the skin until a popping sensation is felt as the ligamentum flavum is penetrated. The stylet is then removed and a 2mL syringe is attached to aspirate and check for inadvertent intra-thecal injection (denoted by the presence of CSF). A test injectate of saline is injected to ensure minimal pressure required to inject. The drug solution to be injected is attached and with very gentle pressure the injection begins. Administration should be over 2 minutes. The needle is then withdrawn. You may prefer to position the animal in lateral recumbency.
Confirming correct needle placement:
No CSF aspirated
Popping sensation felt once ligamentum flavum is penetrated
Lack of resistance to injection (a test injectate of saline may be used)
Hanging drop technique (epidural space is under slight negative pressure). Only works in sternal/difficult to appreciate from lateral)
Loss of resistance (LOR) technique. A low resistance syringe is attached to the spinal needle. Pressure is applied as the needle is advanced. LOR is noted when the needle enters the epidural space. LOR may occur (false positive) if the needle is in fat. If the needle is blocked false negatives can occur.
Electrolocation using a nerve locator – useful in obese patients
If you aspirate CSF reduce dose by 1/5-1/2 this is therefore an intrathecal technique.
Contraindications
Absolute
Infection at injection site
Coagulopathies
Hypovolaemia/hypotension – avoid local anaesthetics – opioids ok.
Relative
Distortion of anatomy- ie pelvic fractures
Obesity – unable to locate landmarks
Risks of epidurals
hypotension
motor blockade (only w local, not opioids)
urinary retention – always express bladder post sx)
slow hair regrowth (11%)
What to use for epidural analgesia
Local anaesthetics will provide total sensory (and motor) blockade. Opioids used epidurally reduce sensation however do not totally block sensation. The combination of local anaesthetics and opioids provides the advantage of excellent analgesia during the procedure with a long lasting effect.
Some authors advocate using a limited volume (1ml/5kg) whilst others use the calculated dose. Limiting the bupivacaine 0.5% to 5ml maximum volume will provide anaesthesia for the pelvic limbs Doses may be reduced with older (fibrous tissue in epidural space), pregnant (engorged vasculature in epidural space) and obese(more fat in epidural space) patients. Preparations should be preservative-free and local anaesthetics adrenaline-free.
**Sensory blockade tends to last longer than motor blockade.
What affects spread?
Volume & concentration
Actual mass (dose) of drug is the most important factor influencing spread rather that absolute volume.
Injection speed/pressure
Administer slowly over 1-2mins. Rapid injection can cause bradycardia/apnoea/asystole thought to be due to changes in CSF pressure.
Site of injection
More cranial injection gives more cranial spread. Drug volumes quoted given assume lumbosacral injection.
Direction of needle bevel
Most relevant when a Tuohy needle is used – less concern with a Quinke needle.
Patient position
Most local solutions are hypobaric so a head up position encourages cranial spread. Hyperbaric bupivacaine is used in humans (spinals are performed seated) to encourage the local to ‘sit’ at the base of the spine where its effects are desired. The solution is made ‘heavy’ (hyperbaric) by addition of glucose 8%. This is only relevant with spinal injections, not epidurals.
Fat in the epidural space
May affect spread giving a patchy block.
Venous plexuses
Venous engorgement is reported to affect spread by decreasing the epidural space. This is the case in pregnant patients plus those with increased intra-abdo pressure. During pregnancy in animals it is reported that onset of blockade is more rapid – use lower volumes in these cases.
Age
The dura mater is more permeable to local anaesthetics in old age – use lower doses – there are more arachnoid villi in the dura. Decreased numbers of myelinated fibres may allow local to penetrate the nerves more readily.
Cardiovascular Effects
Negative CV effects are usually the result of preganglionic SNS blockade.
Vasodilation produces hypotension. These fibres maintain vascular tone in blood vessels and blocking them causes vasodilation. Blood pools in the venous circulation, reduces venous return, this cardiac output, hence hypotension. Fluid therapy should be provided to counteract this. Some authors advocate a bolus of 10ml/kg Hartmann’s when performing an epidural. I tend to monitor BP and if MAP drops below 60mmHg give a 10ml/kg fluid bolus (but first check whether we can decreased the volatile agent %.
If cardioaccelerator fibres serving the heart (T1-T4) are blocked bradycardia can occur. Treatment is with atropine 0.01mg/kg IV or glycopyrrolate 0.01mg/kg IV (slower onset than atropine though).
Spinal Analgesia
-onset more rapid than epidural
-placement confirmation more obvious – CSF issues from the needle hub
-profound anaesthesia
-risk of cardioaccelerator blockade much higher than epidural administration
Doses
Morphine 0.01-0.03mg/kg
Bupi 0.5% 0.05ml/kg blocks up to L3 dermatome
Inadvertent spinal injection of an epidural dose can have profound, fatal consequences so care with confirmation of needle placement and dose calculation is advised
Note – nurses are not permitted to perform epidurals as this constitutes entering a body cavity.