Introduction

The ability to achieve adequate anesthesia or analgesia anywhere in the body by blocking nerves has helped to reduce morbidity and mortality associated with surgery. While the use of regional anesthesia has been part of the therapeutic strategy in recent decades, incorporating neurolocalization techniques, either by peripheral nerve stimulation or ultrasound visualization of the nerve, has expanded the scope of the art. In recent years, veterinary anesthesiology has accepted the challenge of exploring this area, describing techniques, testing doses and defining both the indications and contraindications of each procedure. This is a review of the general principles for the use of the peripheral nerve stimulator (PNS) as an assistant for neurolocalization in limbs' blockades.

Fundamentals of Nerve Stimulator Guidance in Peripheral Nerve Blockade

The rationale behind electrical nerve stimulation was to guide the needle close to the nerve to facilitate a successful nerve blockade. Current clinical practice utilizes a stepwise approach: to locate a target nerve, initially, a higher output current is applied to guide the needle in the direction of the target nerve. After a motor response is elicited, the nerve stimulator output and the needle position have to be adjusted to achieve a minimal but sustained motor response at a lower output current (usually below 0.5 mA). On reaching this final needle position local anesthetics are applied.

 Peripheral nerve stimulator is a constant direct current generator, which produces a square wave current that does not suffer variations compared to the impedance change of the various tissues.

 The pulse frequency can be set to 1 or 2 Hz (number of stimuli per second).

 The duration of each electrical pulse can be adjusted between 0.1 and 0.3 msec.

 Electrical pulse durations of less than 0.15 msec stimulate single motor nerve fibers.

 The current varies between 0.1 and 5 mA.

 The negative electrode (black connector) connects to the neurostimulator, the injection needle.

 The positive (red connector) to the skin of the patient by ECG connector or crocodile.

 The current required to produce a motor response depends on the distance between the tip of the needle and the nerve.

 A motor response obtained with a low current intensity indicates that the needle tip is in proximity to the nerve trunk.

 In clinical practice, the initial stimulating current for a motor response is 1 to 2 mA.

 Then the current intensity is gradually reduced as the needle electrode is advanced in the direction of the nerve trunk seeking to maintain the same motor response.

 In the moment in which the effector muscle contractions are obtained with a current of 0.5 mA (duration of the stimulus of 0.1 mS) the needle-electrode is at the desired distance for injection.

 Muscle responses obtained with a lower current intensity may indicate too close to the nerve at the risk of injecting local anesthetic into the perineural sheaths with the consequent possibility of causing damage to nerve structures.

 As a general rule the absence of muscle clonia should be checked at 0.2 mA.

Thoracic Limb Regional Anaesthesia

Brachial Plexus Blockade

Brachial plexus blockade provides analgesia to the thoracic limb and is useful in the management of pain in patients with fractures or injuries.

 The technique is easily performed in patients with low height and lean but can be complicated in obese patients.

 The brachial plexus is formed by the ventral branches of the cervical spinal (C6, C7, C8) and the first thoracic (T1) nerves.

 After they have passed through the intervertebral foramina and intertransversus musculature, they emerge through and cross the ventral border of the scalenus muscle where they spread out over a fairly large area and extend to the thoracic limb by traversing the axillary space.

 Specific nerves exit the plexus. These include the suprascapular, subscapular, musculocutaneous, radial, median, ulnar and axillary nerves.

 The axillary artery and vein lie ventromedial to the caudal portion of the brachial plexus.

Technique (Campoy 2006)

 The patient should lie in lateral recumbency with the limb to be blocked uppermost.

 Landmarks include the scapula-humeral joint, trachea, jugular vein and the axillary artery.

 The puncture point is located at the level of the scapulohumeral joint in a plane medial to the scapula.

 The pulse of the axillary artery can be palpated.

 Care should be taken to protect the external jugular vein, which is to be found in this region.

 The direction of insertion of the stimulating needle should be caudal, with a moderate dorsal orientation relative to the body axis.

 Muscular responses

 Musculocutaneous: Flexion and supination of elbow

 Radial: Extends elbow

 Ulnar: Flexes carpus

 Median: Flexes carpus, pronates limb

 Dose

 0.3 ml/kg of 0.5% bupivacaine equates to a total dose of 1.5 mg/kg.

 No resistance should be noted during injection.

Paravertebral Brachial Plexus Blockade

 For this technique the transverse process of the sixth cervical (C6) vertebra should be identified by palpation.

 An insulated needle (22-gauge 5-cm for dogs < 13 kg, 21-gauge 10-cm for dogs > 13 kg) connected to a nerve stimulator is inserted just dorsal to the transverse process (2–5 cm from the spinous process depending on body weight) aiming dorsoventrally at a 30- to 45-degree angle with respect to the sagittal plane of the patient until the needle contacts the transverse process.

 The needle is then reoriented to become parallel to the sagittal plane and then advanced cranially to the transverse process. In order to block C6, injection is made when the correct muscular response is observed at a current intensity of 0.5 mA (0.1 mS, 2 Hz).

 The tip of the needle is then reoriented caudally to the transverse process and a second injection should be made to block C7.

 For blockade of C8 and T1, the first rib has to be palpated and the needle is inserted parallel and cranial to it, slightly dorsal to the spine of the scapula, and is directed ventrally at a 30° angle with respect to the sagittal plane of the patient.

 Injection should be made when the correct muscular response is observed at a current intensity of 0.5 mA (0.1 mS, 2 Hz).

 Muscular responses

 C6: Contraction of supra and infraspinatus muscles. Flexion, extension, rotation of the shoulder.

 C7: Flexion and supination of elbow. Flexion of the shoulder.

 C8: Flexes carpus

 T1: Flexes carpus, pronates limb

 Dose: 0.05 mL/kg of 0.25–0.5% bupivacaine or 0.2–0.5% ropivacaine for each nerve.

 Useful to improve analgesia and muscular relaxation of the shoulder and elbow.

Pelvic Limb Regional Anaesthesia

The combination of the lumbar plexus and the sacral plexus forms the lumbosacral plexus and in the dog it originates from the ventral roots of the fourth lumbar to the second sacral nerves.

 The lumbar plexus provides the roots for:

 The lateral cutaneous femoral nerve (L4)

 The femoral nerve (L4, L5 and L6)

 The genitofemoral nerve (L3 and L4)

 The obturator nerve (L4, L5 and L6)

 The sacral plexus provides the roots for:

 The pudendal nerve

 The caudal cutaneous femoral nerve

 The gluteal nerves

 The sciatic nerve arises from the last two lumbar nerves and the first

 two sacral nerves (L6 to S2)

Femoral Nerve Blockade (Inguinal Approach)

Used in combination with a sciatic nerve block, anesthesia of the pelvic limb can be achieved.

 The patient should lie in lateral recumbency with the limb to be blocked uppermost-abducted 90 degrees and extended caudally.

 The femoral triangle is delimited by the sartorius muscle cranially, the pectineus muscle caudally, and the iliopsoas proximally.

 The puncture site is located within the femoral triangle, cranial to the femoral artery.

 Dose: 0.05 to 0.1 mL/kg of 0.25–0.5% bupivacaine or 0.2–0.5% ropivacaine.

Paravertebral Lumbar Plexus Blockade

This blockade can be applied to a wide range of surgical procedures. Used in combination with a sciatic nerve block by parasacral approach, anesthesia of the coxofemoral joint can be achieved.

Technique

 The patient should lie in lateral recumbency with the limb to be blocked uppermost.

 To block the lumbar plexus a total of four injection points should be used.

 Three for the lumbar plexus block (fourth, fifth and sixth lumbar nerves) and one for the sacral plexus block.

 The total dose of local anesthetic is divided in four equal parts, one per injection point.

 Dose: 0.05 mL/kg per injection point of 0.25–0.5% bupivacaine or 0.2–0.5% ropivacaine.

 An insulated needle connected to a nerve stimulator is employed for the nerve location.

 The roots of the lumbar plexus are blocked at three injection points using a paravertebral approach.

 For the fourth lumbar nerve block, the injection site is located in the L4–L5 intervertebral space, 1–2 cm lateral to the midline. The stimulating needle is introduced with a sagittal direction until contractions of the sartorius or quadriceps muscle are evoked.

 For the fifth lumbar nerve block the stimulating needle is introduced at the L5–L6 intervertebral space, 1–2 cm lateral to the midline until the contractions of the quadriceps muscle are evoked, with a clear extension of the knee joint.

 For the sixth lumbar nerve block the stimulating needle is introduced at the L6–L7 intervertebral space, in this case the contractions of the gluteal or biceps femoris muscle are evoked with extension of the hip.

 The nerve stimulator should be set on a pulse duration of 0.1 mseconds, a frequency of 2 Hz and a current of 2 mA.

 The stimulation current is progressively decreased until a clear contraction of the muscular target is elicited with 0.5 mA, and thus the local anesthetic or saline solution is injected.

 To avoid an intraneural injection the absence of muscular contraction at 0.2 mA should be checked.

 A negative aspiration test should be performed to exclude accidental intravascular injection.

Sciatic Nerve Blockade (Parasacral Approach)

 In order to perform the parasacral plexus block a line is drawn between the cranial dorsal iliac crest and the ischiatic tuberosity.

 This line is divided into three equal parts and the injection site is located at the junction of the cranial third with the middle third.

 The stimulating needle is introduced in this site, deep enough until the stimulation of the sciatic nerve roots are detected observing the contractions of the gastrocnemius muscle or even digital (and/or tarsus) flexion or extension.

 With this approach the local anesthetic solution is injected in the area where the seventh lumbar nerve joins the first and second sacral nerves to originate the sciatic nerve.

Sciatic Nerve Blockade (Lateral Approach)

This blockade will result in anesthesia of the stifle and the structures distal to it. Used in combination with a femoral nerve block, anesthesia of the pelvic limb can be achieved.

Technique

 The patient should lie in lateral recumbency with the limb to be blocked uppermost.

 The puncture site is located between greater trochanter and ischiatic tuberosity, nearer to the greater trochanter.

 The stimulation needle is inserted perpendicular to the skin.

 Advancing the needle may result at first in contractions of the gluteal musculature by means of direct muscle stimulation.

 In the event of bone contact, the needle should be withdrawn and redirected.

 Dorsiflexion (peroneal component) or plantar flexion (tibial component) of the foot with a stimulating current of 0.5 mA will be considered as positive response.

 The nerve stimulator should be set on a pulse duration of 0.1 mseconds, a frequency of 2 Hz and a current of 2 mA.

 The stimulation current is progressively decreased until a clear contraction of the muscular target is elicited with 0.5 mA, and thus the local anesthetic or saline solution is injected.

 To avoid an intraneural injection the absence of muscular contraction at 0.2 mA should be checked.

 A negative aspiration test should be performed to exclude accidental intravascular injection.

 Dose: 0.05 mL/kg of 0.25–0.5% bupivacaine or 0.2–0.5% ropivacaine.

References

1.  Campoy L, Martin-Flores M, et al. Distribution of a lidocaine-methylene blue solution staining in brachial plexus, lumbar plexus and sciatic nerve blocks in the dog. Vet Anaesth Analg. 2008;35:348–354.

2.  Portela DA, Otero PE, et al. Combined paravertebral plexus block and parasacral sciatic block in healthy dogs. Vet Anaesth Analg. 2010;37:531–541.

3.  Rioja E, Sinclair M, et al. Comparison of three techniques for paravertebral brachial plexus blockade in dogs. Vet Anaesth Analg. 2011;39(2):190–200.

4.  Campoy L. Fundamentals of regional anesthesia using nerve stimulation in the dog. In: Gleed RD, Ludders JW, eds. Recent Advances in Veterinary Anesthesia and Analgesia: Companion Animals. Ithaca, NY: International Veterinary Information Service (www.ivis.org).