ANAESTHESIOLOGY
DRUGS FOR EPIDURAL INJECTION
Epidural administration of local anaesthetics ( Table 1 ) provides complete anaesthesia , sufficient to perform surgery , to the caudal half of the body . Administering opioids epidurally provides additional analgesia often of longer duration and with fewer adverse effects than systemic administration ( Table 1 ). If morphine is used in the epidural injection , premedication with another mu agonist opioid is recommended .
Other drugs , such as alpha-2-agonists , nonsteroidal antiinflammatory drugs , and ketamine , may be administered epidurally as well , though this is not common in small animals .
Local anaesthetics Lumbosacral epidural anaesthesia with local anaesthetics provides complete anaesthesia to the caudal half of the body by blocking the intradural spinal nerve roots and the peripheral layer of the spinal cord . 1 This technique is useful for pelvic and hindlimb orthopaedic procedures , perineal and anal surgeries , exploratory laparotomy , and cesarean section . In addition to complete anaesthesia , at least some sympathetic and motor blockade is produced with local anaesthetics . Lidocaine , mepivacaine , and bupivacaine consistently cause motor blockade , while motor blockade is less intense and of shorter duration with levobupivacaine and ropivacaine . 1 The duration of motor blockade is generally shorter than the duration of analgesia , and , depending on the procedure and local anaesthetic chosen , motor function usually returns by the time a patient recovers from anaesthesia . 1-3
Doses and precautions . The type and volume of local anaesthetic chosen depends on the desired result . Lidocaine ( 2 %) administered at 1 ml / 6 kg completely anaesthetises the pelvic limbs and posterior abdomen , caudal to L1 , within
10 to 15 minutes and lasts 60 to 120 minutes . With a comparable volume of 0.5 % to 0.75 % bupivacaine , the onset is 20 to 30 minutes with a duration of four to six hours . Volumes of 1 ml / 7.5 kg are adequate for pelvic , perineal , and hindlimb procedures . 2 , 3
If the total epidural injection volume of local anaesthetic and other adjunct analgesics exceeds 1 ml / 5 kg , high blockade of the sympathetic nerve roots can occur and Horner ' s syndrome , vasodilation , and hypotension can result . 2
Total drug volume should not exceed 8 ml , and doses of epidural drugs should be calculated by using the ideal lean body weight of the patient . Doses should be further reduced by 25 % in pediatric , geriatric , and pregnant animals since the epidural space may be smaller than expected . 2 , 4 Hypotension is more likely in animals with hypovolemia or cardiovascular compromise , so correct hypovolemia before administering local anaesthetics epidurally . Hypotension is treated with intravenous crystalloids ( 20 to 30 ml / kg ), colloids such as hetastarch ( 5 to 20 ml / kg ), or vasopressors or inotropes such as ephedrine , vasopressin , or dopamine . 1 , 2
In addition to sympathetic blockade , local anaesthetic overdosage can also result in blockade of the motor nerves innervating intercostal muscles and even paralysis of the diaphragm should the local anaesthetic reach the level of C5 ; hypoventilation , hypercapnia , and hypoxia can result . 1 , 2 Ventilatory support , using positive pressure ventilation , may be required . Careful attention to total volume injected will reduce the incidence of these adverse effects .
Controversy exists as to whether a patient should
Drug Dose * Time to onset ** Duration of action 3-5 mg / kg or
10 – 15 min 1-2 hours
Lidocaine 2 %**
1 ml / 7kg for analgesia of pelvis or hindlimbs
1ml / 6kg for abdominal anaesthesia Lidocaine 2 %** with adrenalin ( 5mg / ml ) As above 10 – 15 min 2-4 hours Mepivicane 3 % **
Bupivicane 0.5 % **
3-4.5 mg / kg or 1 ml / 7kg for analgesia of pelvis or hindlimbs 1ml / 6kg for abdominal anaesthesia 3-4.5 mg / kg or 1 ml / 7kg for analgesia of pelvis or hindlimbs 1ml / 6kg for abdominal anaesthesia
10 – 15 min 1.5 – 2 hours
20 - 30 min 4 - 6 hours
Morphine |
0.05 -0.15mg/ kg |
30-60 min |
12 -24 hours |
Meperidine |
0.5 -1.5 mg / kg |
5 – 10 min |
1-4 hours |
Oxymorphone |
0.025-0.15 mg / kg |
20 – 40 min |
7 – 10 hours |
Fentanyl |
0.001-0.01 mg / kg |
5 - 20 min |
2 – 4 hours |
* Total drug volume should not exceed 8 ml |
** Base dose on lean body weight and reduce by further 25 % for geriatric , paediatric and pregnant animals |
Issue 06 | FEBRUARY 2017 | 13