Local Anesthesia for the Distal Extremity
Heidi L. Shafford, DVM, PhD, Diplomate ACVAA
Local anesthesia techniques are important analgesic tools. The author provides step-by-step instructions in providing IVRA and ring blocks to both dogs and cats.
Local anesthetics block nerve impulses caused by painful stimuli, which makes them effective analgesics. Regional and peripheral blocks are versatile and can be applied to most surgical and dental procedures. The objective of this article is to provide practical details that allow a clinician to implement these blocks immediately after assembling the necessary supplies.
DISTAL EXTREMITY BLOCKS
Indications for local anesthesia of the distal limb are listed in Table 1.
|Table 1. Indications for Local Anesthesia of the Distal Limb|
Common distal extremity blocks include:
- Intravenous regional anesthesia (IVRA)
- Metacarpal/metatarsal ring block
- Brachial plexus block
- Proximal radial—ulnar—median—musculoskeletal (RUMM) blocks.
This article discusses IVRA and ring blocks; see Table 2 for a comparison of their benefits. Although with practice RUMM blocks become easy, the latter 2 blocks—brachial plexus and proximal RUMM blocks—are used with greater success if a peripheral nerve locator is used. They require advanced technical skills, and are best learned in a wet laboratory setting before being applied in practice; instruction regarding their use is beyond the scope of this article.
|Table 2. Comparison of Benefits: IVRA & Ring Blocks|
|Efficacy||More effective than ring block for major procedures||Less effective than IVRA for major procedures|
|Location||Ideal for forelimb, distal to elbow||Forelimb, distal to carpus
Hindlimb, distal to tarsus
|Medication||Lidocaine||Bupivacaine, lidocaine, or mepivacaine|
|Duration||Analgesia persists approximately 20 min after tourniquet release||Dependent on duration of action of local anesthetic administered|
|Limitations||Restricted to procedures < 90 min due to tourniquet use|
|Additional benefits||Reduced blood at surgical site||Minimal supplies
Quick to perform
IVRA is a locoregional analgesic technique, which some may know as the Bier block, named for the physician who first described this technique nearly a century ago.
IVRA provides reliable, effective regional analgesia to facilitate fracture repair, toe amputation, and foreign body or mass removal, all under conditions of minimal blood loss. In my experience, it is most easily applied to the forelimb; however, this technique can be used in both forelimbs and hindlimbs of dogs and cats.
IVRA is a relatively simple, elegant technique that involves using the patient’s venous system to distribute local anesthetic medication. Essentially, the technique involves:
- Compressing the blood out of the distal limb
- Occluding arterial in-flow with a tourniquet
- Filling the venous system with an appropriate dose and volume of lidocaine.
The tourniquet allows the local anesthetic to remain in situ, with diffusion into the superficial and deep tissues and minimal leakage into systemic circulation.
IVRA is typically combined with general anesthesia and reduces inhalant requirements. For minor procedures, such as pad biopsy and foreign body removal, it may be possible to use this technique in sedated animals.
This patient has a malignancy affecting digit V, which requires toe amputation.
1. Clip hair as indicated for catheter and surgery. Using sterile technique, place a small-gauge IV catheter in the most distal vessel possible. Cap the catheter and secure with tape. Some advocate directing the catheter tip distally, but I place catheters for IVRA using standard proximal tip orientation.
2. Identify an arterial pulse distal to anticipated tourniquet location (eg, palmar artery). Later (Step 5), absence of a pulse will help verify that the tourniquet is occluding arterial and venous blood as intended.
3. Exsanguinate the distal limb by tightly wrapping self-adhesive elastic bandage material from the toes (distal) toward the elbow/tarsus (proximal), covering the catheter as you go (A). For large dogs, it may be helpful to wrap the leg a second time, starting again at the toes and wrapping proximally (B).
4. Apply a tourniquet just proximal to the top of the bandage, and note time of tourniquet application.
- Pneumatic tourniquet (recommended): Inflate cuff pressure to 50 to 100 mm Hg higher than the patient’s systolic arterial pressure; do not exceed 400 mm Hg.
- Nonpneumatic (elastic band) tourniquet: Consider placing a protective layer between the skin and band of tourniquet.
Surgical Insight: Pneumatic tourniquets are recommended because (1) the pressure is distributed over the wide surface of the cuff and (2) tourniquet pressure can be monitored.
5. Once the tourniquet is applied, remove the bandage and verify that the peripheral pulse is absent; do not proceed if an arterial pulse is present. If pulsatile blood is detected distal to the tourniquet, repeat steps 3 and 4. In the figure, the palmar artery is being used to verify absence of an arterial pulse.
Surgical Insight: At low tourniquet pressure, venous outflow is occluded; however, if arterial blood in-flow is not occluded—as indicated by presence of a peripheral pulse—then the distal limb swells due to venous occlusion.
6. Inject lidocaine slowly over 1 to 3 minutes (see IVRA: Materials & Medications for details on lidocaine dose and preparation); then remove the catheter.
7. Prep for and perform the procedure. During the procedure, monitor and maintain tourniquet pressure. Limit duration of tourniquet application to90 minutes or less.
8. Slowly and gradually release the tourniquet over a period of 2 to 4 minutes.
The ring block is another simple analgesic technique used for the paw and metacarpal/metatarsal region.
For the forelimbs, the goal is desensitization of the dorsal and palmar branches of the ulnar nerve, the superficial branches of the radial nerve, and the median nerve. For the hindlimbs, the goal is desensitization of the superficial and deep peroneal nerve and the tibial nerve and/or lateral and plantar nerves.
To create a ring of anesthetic that blocks superficial nerves, local anesthetic is delivered via SC injection in lines over the dorsum and ventrum of the distal limb.
- On the forelimb, injection sites are just proximal to the carpus.
- On the hindlimb, injection sites are on the dorsal and plantar surfaces, at the level of the tarsometatarsal joint.
In sedated animals, consider adding sodium bicarbonate to the local anesthetic medication to minimize the sting associated with SC administration (1 part sodium bicarbonate to 9 parts local anesthetic). In patients under general anesthesia, I do not add bicarbonate to the blocks.
|Ring Blocks: Materials & Medications|
||Up to 2 mg/kg (dogs and cats)
||4 to 12 H
||Up to 8 mg/kg (dogs)
Up to 6 mg/kg (cats)
|1 to 2 H
|Mepivacaine||5 mg/kg (dogs and cats)||2 to 4 H|
Medication Choices & Safety Measures
Choice of medication depends on desired duration of nerve blockade.
- For long duration analgesia, bupivacaine is recommended because lidocaine and mepivacaine are effective only for shorter periods of time.
- Onset and efficacy of blockade depend on (1) proximity to the nerves and (2) volume and concentration of medication (larger volume/higher concentration are associated with faster onset).
- Risk for inadvertent IV administration is minimal, particularly if the anesthetist aspirates before each injection; thus, the local anesthetics listed are appropriate for ring blocks in cats and dogs.
To ensure safe administration of medication:
- Calculate maximum total dose and label syringes, to prevent toxic adverse effects should inadvertent IV administration of the medication occur.
- Draw medications into syringes on the day of the procedure to prevent local anesthetics from adhering to plastic in the syringe.
STEP-BY-STEP APPROACH TO RING BLOCKS
Surgical Insight: The skin is often tightly adhered to the distal extremity, making injection challenging. Lift up the skin and slide the needle under it to help facilitate injection in this area.
2. Repeat SC needle insertion, aspiration of the syringe, and medication injection to create a circumferential ring of local anesthetic around the limb. The circumferential SC injections shown in the figures demonstrate local anesthetic blockade of superficial nerves in the forelimb and hindlimb of a cat.
Local anesthetic techniques, such as IVRA and ring blocks, should be incorporated as part of a multimodal approach to minimizing perioperative pain.
For IVRA, while lidocaine provides excellent in situ anesthesia and analgesia, once the tourniquet is released, the lidocaine’s effect diminishes after approximately 20 minutes. Therefore, plan to provide postoperative analgesia tailored to the patient’s anticipated pain.
Duration of local analgesia conferred by ring blocks is dependent upon the local anesthetic used (ie, bupivacaine confers longer duration analgesia). Additional postoperative analgesia benefits the patient, in anticipation of pain extending beyond the duration of the local block.
Visit tvpjournal.com/resources.asp#resources to find:
- Table 3. Minimizing Risks Associated with IVRA and Ring Blocks
- Local Anesthetic Calculation Cheat Sheet, which assists with calculating maximum doses of bupivacaine 0.5% solution.
|Table 3. Minimizing Risks Associated with IVRA & Ring Blocks|
|POTENTIAL ADVERSE EFFECT||IVRA||RING BLOCKS|
|Tissue or nerve damage
|Local anesthetic overdose
|Inadequate analgesia during procedure
Local anesthesia techniques are important analgesic “tools” because:
- Patients benefit from effective, preventive, site-specific analgesia
- Balanced anesthetic protocols allow reduced reliance on inhalants.
After reviewing the step-by-step instructions in this article, you should be able to incorporate IVRA and ring blocks into your anesthesia and pain management protocols with confidence.
IVRA = intravenous regional anesthesia; RUMM = radial—ulnar—median—musculoskeletal
Kushner LI, Fan B, Shofer FS. Intravenous regional anesthesia in isoflurane anesthetized cats: Lidocaine plasma concentrations and cardiovascular effects. Vet Anesthes Analges 2002; 29:140-149.
Skarda RT, Tranquilli WJ. Local and regional anesthetic and analgesic techniques: Cats. In Tranquilli WJ, Thurmon JC, Grimm KA (eds): Lumb and Jones’ Veterinary Anesthesia and Analgesia, 4th ed. Ames, Iowa: Blackwell Publishing, 2007, pp 595-603.
Staffieri F. Intravenous regional anesthesia. In Campy L, Read MR (eds): Small Animal Regional Anesthesia and Analgesia. Ames, Iowa: Wiley-Blackwell, 2014, pp 261-271.