anesthesia bag size for dogs. epidural anesthesia
Epidural anesthesia (EA) consists of injecting a solution of local anesthetic and/or analgesic into the epidural space (EP) of the spinal cord and is an alternative method of analgesia of the caudal half of the body.
The main indications for epidural anesthesia:
- Surgical interventions on the pelvic extremities, abdominal organs, and perineum;
- Presence of pain caudal to the diaphragm (e.g., in peritonitis, pancreatitis, severe trauma of pelvic bones and pelvic extremities);
- Management of postoperative pain;
- Patients for whom general anesthesia is undesirable.
Epidural anesthesia technique
EP puncture can be performed at the level of the cervical, thoracic, lumbar and caudal spine. Puncture in the cervical and thoracic spine is technically more difficult than in the lumbar and caudal spines, requires more experience and involves many risks. Because of this, the most common site for epidural injections is the lumbosacral space.
The dural sac in most dogs ends at the L6 level, but in cats it extends to the S2. Therefore, in cats there is a greater risk of accidental subarachnoid injection.
The patient may be in the ventrodorsal position (on the chest) with the pelvic limbs extended cranially or in the lateral position (on the side).
The injection area is prepared according to aseptic rules (the hair is shaved, the skin is treated with an antiseptic solution) (Fig. 1).
The injection site must be identified. When puncturing into the lumbosacral space the place for injection is the intersection of lines between the wings of the iliac bone and spinous processes of the 7th lumbar and 1st sacral vertebrae (Fig. 2, 3, 4).
The spinal needle is inserted at a 90° angle to the skin surface. The cut of the needle should be directed cranially. In addition to the resistance when inserting the needle, hitting the yellow ligament causes a characteristic sensation of something hard, rough. A sudden loss of resistance as you progress through the yellow ligament means that the needle has entered the EA. At this point, you need to remove the mandrel and check for blood and CSF in the needle sleeve (Figures 5, 6, 7).
There are two main methods for identifying EAs:
Figure 8. Injection of local anesthetic.
1. The “loss of resistance” technique – the needle is advanced in reciprocating motion, advancing a few millimeters at a time, then stopped and gently pressed on the syringe piston with physiological solution, trying to feel whether the needle is still in the ligament thickness, or whether resistance has been lost and it has entered the EA. 2. The “hanging drop” technique – before entering the EP, a drop of saline is suspended from the needle pavilion, which disappears into the needle lumen under negative pressure when the end of the needle enters the EP.
Local anesthetics are administered fractionally and slowly (Fig. 8).
Why is EP puncture done exclusively with spinal needles?
Spinal needles have 3 features:
- The short bevel of the needle makes them “relatively blunt,” whereby the needle tends to advance between the nerves, pushing them apart rather than dissecting them. “Relative bluntness” also gives a greater “pop” sensation when passing through the yellow ligament (Fig. 9).
- The step in the needle sleeve tells you which way the needle is beveled.
- The mandrel reduces the likelihood of needle obturation by skin, subcutaneous tissue, ligaments, bones, and the intervertebral disc.
Figure 9. Spinal needles.
EA is recommended to be performed only if equipment and medications necessary to correct possible complications (from mild arterial hypotension to circulatory arrest) are fully provided.
To perform EA, most patients are sedated, since even a slight movement of the animal increases the risk of unintentional puncture of the dura mater and, consequently, of subarachnoid penetration of the anesthetic. If general anesthesia is not recommended for the patient, the EA puncture is performed after infiltration blockade of the injection site up to the yellow ligament with 1-2% lidocaine. Intravenous catheters are placed and resuscitation equipment is ensured prior to EA (Fig. 10).
During EA there is minimal monitoring of basic hemodynamic parameters (electrocardiogram is recorded, blood pressure and oxygen saturation of arterial blood are measured) (Fig. 11).
Fig. 10. providing general anesthesia for the puncture. Fig. 11. hemodynamic monitoring during the puncture.
Physiology of epidural anesthesia
The EP is filled with loose connective tissue which surrounds the epidural veins and spinal nerve roots. When the local anesthetic solution is injected into the EA, there is diffusion through the dura muffs of the spinal nerves into the cerebrospinal fluid.
In EA there are variants from analgesia with weak motor blockade to deep anesthesia with complete motor blockade. The required intensity of anesthesia is achieved by selecting the concentration and dose of the anesthetic.
Nerve roots are made up of different types of fibers, so the onset of anesthesia will not be a one-step process. There are 3 types of fibers: A (α, β, γ, δ), B and C.
Type B and C fibers are blocked first (since they are thinner), and then A fibers. Myelinated fibers are blocked earlier than unmyelinated ones.
Since there is diffusion and dilution of the local anesthetic, complete blockade of the most resistant fibers may not occur.
As a result, the border of sympathetic blockade (judged by temperature sensitivity) may be 2 segments higher than the border of sensory blockade (pain and tactile sensitivity), which, in turn, is 2 segments higher than the border of motor blockade.
|Fiber type||Function||Diameter, μm||Myelination||Sensitivity to blockade|
|Аα||Proprioceptive sensitivity, motor activity||12-20||full||+|
|Аδ||Temperature sensitivity, rapid conduction of pain||2-5||full||+++|
|В||Sympathetic preganglionic fibers||1-3||weak||++++|
|С||Sympathetic postganglionic fibers, slow conduction of pain||0,3-1,3||none||++++|
Preparations for epidural anesthesia
The choice of anesthetic primarily depends on the purpose of the epidural block – whether it is the primary method of anesthesia, used as an adjunct to general anesthesia, or used for analgesia. The most common drugs for EA: local anesthetics, opioids, alpha-2 agonists.
In our clinic alpha-2 agonists are not used, since even with epidural administration they lead to significant systemic effects, in particular, they cause hypotension.
Depending on the duration of the surgical intervention short-acting or long-acting anesthetics are used. Short-acting anesthetics include lidocaine, novocaine, long-acting – bupivacaine, ropivacaine.
Low concentrations of local anesthetics provide analgesia without motor blockade. Higher concentrations provide complete sensory and motor blockade.
Adding auxiliary drugs, especially opioids, to local anesthetics has a greater effect on the quality of the blockade than on its duration. Adrenaline, in turn, prolongs the effect of local anesthetics (in particular, lidocaine and novocaine).
Doses of local anesthetics for dogs
|Drug||Dose||Distribution in the spinal canal||Onset of action (min)||Duration of action (hours)|
|Lidocaine 2%||3.0-5.0 mg/kg||L1||5-15||1.0-1.5 1.5-2.0 in combination with adrenaline|
|Lidocaine 2%||1.0 ml/3.5 kg||Т9||5-15||1.0-1.5 1.5-2.0 in combination with adrenaline|
|Lidocaine 2%||0.31 ml/kg||Т12||5-15||1.0-1.5 1.5-2.0 in combination with adrenaline|
|Bupivacaine 0.5%||1.0-2.5 mg/kg||L1||10-20||4,0-6,0|
|Bupivacaine 0.5%||1.0 ml/3.5 kg||Т9||10-20||4,0-6,0|
|Bupivacaine 0.5%||0.31 ml/kg||Т12||10-20||4,0-6,0|
|Bupivacaine 0.25%||1.0 ml/10 cm*||Т10-9||10-20||4,0-5,0|
|Bupivacaine 0.25%||1.5 ml/10 cm*||Т5-2||10-20||4,0-5,0|
|Bupivacaine 0.1%||1.5 ml/10 cm*||Т5-2||10-15||2,0-4,0|
|Ropivacaine 0.5%||0.8 ml/10 cm*||L1||10-20||2,0-4,0|
|Ropivacaine 0.5%||1.2 ml/10 cm*||Т9-5||10-20||2,0-4,0|
|Ropivacaine 0.2%||1.0 ml/10 cm*||Т10-9||10-20||1,0-1,5|
|Ropivacaine 0.2%||1.5 ml/10 cm*||Т5-2||10-20||1,0-1,5|
|Ropivacaine 0.1%||1.5 ml/10 cm*||Т5-2||10-15||2,0-4,0|
* The dose is calculated according to the length of the patient’s back, measured from the occipital bone to the first caudal vertebra.
Doses of local anesthetics for cats
|Drug||Dose||Distribution in the spinal canal||Onset of action (min)||Duration of action (hours)|
|Novocaine 2%||1.0-2.0 ml||L1-T4||5-10||20-30 min|
|Lidocaine 2%||1.0-1.5 ml||L1-T9||5-15||1,5-2,0|
|Bupivacaine 0.5%||1.0-1.5 ml||L1-T9||10-20||4,0-6,0|
|Ropivacaine 0.5%||1.0-1.5 ml||L1-T9||10-20||2,0-4,0|
Our clinic uses single injections of 1-2% lidocaine or 2% novocaine in the EP for short-term pelvic surgeries. For longer surgeries, ropivacaine 0.2 to 1% is used because it is less toxic than bupivacaine.
Opioid Doses for Dogs and Cats
|Drug||Dose||Volume of saline (ml/kg)||Onset of action (min)||Duration of action (hours)|
|Morphine + Bupivacaine 0.5%||0.1 mg/kg 1.0 mg/kg||Volume of local anesthetic||10-15||16-24|
In epidural analgesia, morphine is the drug of choice. Opioids can be administered epidurally without local anesthetics, affecting well on type C fibers (sensitive) and poorly on type A fibers (motor), they do not cause motor block. In this EA, animals can walk, but slight ataxia of the pelvic limbs may be present. Sometimes local anesthetics in the spinal canal spread more cranially than expected.
Factors affecting cranial spread :
- The size, age, and physique of the patient (including obesity);
- increased intra-abdominal pressure (pregnancy, gastric dilation);
- volume of drugs administered;
- dosage of drugs administered; speed of drug administration; direction of the needle cut;
- position of the patient.
In ageing animals the volume and extensibility of the EP decreases, so the anesthetic spreads more cranially in older patients than in younger ones when the same dose is administered. To prevent too much blockage, older patients are given a smaller dose of anesthetic.
Obese patients have more fatty tissue in the EA, which leads to a decrease in the volume of the EA. Reduced space volume requires a lower dose of anesthetic.
Due to increased intra-abdominal pressure, the volume of the epidural venous plexus increases, which leads to a decrease in the volume and extensibility of the EP and, consequently, to a higher level of blockage than expected.
A head-down position of the patient should be avoided. Block with local anesthetics up to the T1 level leads to paralysis of the intercostal muscles and up to C5-C7 to paralysis of the diaphragm.
Possible complications of epidural anesthesia:
- Bleeding and hematoma formation in the spinal canal. Trauma to the epidural veins with a needle often causes minor bleeding in the spinal canal; it is usually benign and resolves without treatment. Clinically significant hematoma may occur after EA in case of blood clotting disorders.
- Improper positioning of the needle in the spinal canal may result in:
- a) inadequate anesthesia or analgesia (no blockade, unilateral or mosaic blockade);
- b) intravascular injection of local anesthetic (CNS complications – seizures, loss of consciousness; from the cardiovascular system – arterial hypotension, arrhythmia, shock);
- c) unintentional subarachnoid block (in this case the anesthetic dose should be reduced by 50-75% of the initial one).
- Penetration of infection into the spinal canal. Meningitis and epidural abscesses are extremely rare complications. It is necessary to follow the rules of asepsis when performing a blockade.
- High blockade. Causes: an excessive dose of anesthetic; use of a standard dose of anesthetic in those patients for whom it should be reduced; unusually high sensitivity to the anesthetic; excessive spread of the drug in the cranial direction. High blockade can cause hypotension and hypoventilation due to visceral sympathetic blockade and intercostal muscle paralysis. Patients with this complication are likely to require ventilation and hemodynamic support, which includes oxygen therapy, artificial lung ventilation, infusion therapy, and administration of vasopressors.
- Urinary retention. Epidural blockade at the lumbosacral level reduces bladder tone and inhibits reflex urination. In the postoperative period the presence of the act of urination should be monitored. In the absence of the latter, it is recommended to divert the urine by pressing or catheterization of the bladder.
Contraindications to epidural anesthesia:
- Coagulopathy and treatment with anticoagulants.
- Severe hypovolemia.
- Presence of infection (sepsis, skin infection at the puncture site).
- Neurological dysfunctions.
- CNS diseases.
- Anatomical disorders that complicate the technique of performing a puncture.
Catheterization of the epidural space
The most important advantage of EP catheterization at the lumbar level is prolonged analgesia of abdominal organs, pelvic bones and pelvic extremities without repeated punctures.
Most often epidural catheters are inserted into animals to provide adequate prolonged analgesia of the caudal half of the body in the postoperative period.
The puncture technique for catheter placement is identical to the single epidural injection. The exception is the use of a special Tuohy needle (Fig. 12). As a rule, special kits for EA are used (such a kit includes: Tuohy needle, epidural catheter, bacterioviral filter, aseptic plaster) (Fig. 13).
Fig. 12. Tuohy needle. Figure 13. epidural anesthesia kit.
The epidural catheter is advanced cranially into the EP, usually 1-2 vertebrae. The catheter together with the filter is sutured to the skin. The correct positioning of the catheter in the EP is confirmed radiologically. If the catheter is not radiopaque, a radiopaque substance should be injected into it (Fig. 14, 15, 16).
|Fig. 14. insertion of the radiopaque contrast agent (Omnipaque) into the epidural catheter. Fig. 15. Radiological confirmation of the correct location of the catheter. Figure 16. Distribution of radiopaque contrast agent in the epidural space.|
Catheters are placed under sedation or anesthesia and kept under aseptic conditions. Under the right conditions can be used up to 14 days. The puncture site is evaluated at least twice a day for swelling and bleeding.
The most frequent complication of epidural catheterization is catheter dislocation. It is recommended to periodically check the correct positioning of the catheter radiologically.
During EP catheterization the same doses of anesthetics and opioids are used as in single puncture. But most often in the postoperative period, infusion of local anesthetics and opioids is performed at a constant rate (IPS). Bupivacaine – 0.25% 0.2 mg/kg/hr IPS. Ropivacaine – 0.2% 0.2 mg/kg/hr IPS. Morphine – 0.3 mg/kg/day. IPS 3.0 ml/hour. Morphine – 0.3 mg/kg/day. + Bupivacaine – 0.5% 0.75 mg/kg/day.
EA is more and more widely used in veterinary practice due to an uncomplicated technique of puncture, a good level of anesthesia, low risk of complications for patients, and the possibility of not using general anesthesia.