Peripheral Nerve Blocks

▪Common Local Anesthetics

Lidocaine 0.5%, 0.75%, 1%, 1.5%, 2%, 4%, 5%
Chloroprocaine 1%, 2%, 3%
Ropivacaine 0.2%, 0.5%, 0.75%, 1%
Mepivacaine 1 %, 1.5%, 2%, 3%
Prilocaine 1%, 2%
Procaine 1%, 2%, 10%
Bupivacaine 0.25%, 0.375%, 0.5%, 0.75% (most toxic LA)
Ropivacaine 0.2% (least toxic)

Epinephrine 1:200,000 used with interscalene, femoral, and popliteal fossa blocks.

No epi used with distal peripheral blocks (i.e., ankle) due to vascular constriction.

Dermatome Levels

C2: top and occipit of head

C3: upper neck

C4: shoulders

C5: front of upper arm

C6: front of lower arm

C7: middle of dorsum of hand

C8: little finger

T1: back of arm up to middle of humerus

T2: back of arm from middle of humerus to back

T4: nipple line

T6: xiphoid

T10: navel

L1: waistband

L2: anterior upper thigh-lateral buttocks

L3: lower thigh and anterior knee

L4: anterior tibial

L5: anterior lower leg, ankle, and foot

S1: posterior foot, ankle

S2: posterior calf up to lower buttocks

S3-4: buttocks

S5: perineum

▪Peripheral Nerve Stimulating Technique

Electrical stimulator: Negative to Needle, Positive to Patient.

Attach the negative cathode to the needle with an alligator clip. The positive cathode attaches to the EKG electrode on the chest.

Single injection vs. continuous catheters.

Choosing a local anesthetic: Look at duration, dose needed, and toxicity, sensory vs.motor.

Need to confirm normal coagulation studies before placing block.

Contraindications: Patient refusal, the presence of an active infection at the site of the punture, and/or a true allergy to amide or ester local anesthetics. Complications: Intravascular injection, nerve damage, cardiac toxicity, post-op pain control.

**Chart pre-existing neuropathies, especially in diabetics.

Make sure to monitor vital signs throughout block procedure.

Set the PNS to 1.0-1.5 mA and advance needle until the muscle contraction at or distal to the needle placement is noted. Slowly reduce the current to a goal reduction of 0.5 mA, and slowly advance or withdraw the needle until maximum contractions are noted. It is a good sign if the twitch decreases as the mA decreases. If you still have a rigorous twitch with 0.3-0.4 mA, pull back as you are probably in the nerve. Aspiration is performed before injecting LA to check for intravascular placement (aspirating q 5 cc), then LA is injected. 2-3 cc of LA is injected and motor activity is observed for 15 seconds to observe for fade. If bone is encountered, the needle is redirected. Proximal pressure can be applied to favor spread of local distally.

▪pKa

pH at which 50% of solution is in ionized form and 50% in un-ionized form.

**The ionized form of a drug is water soluble; the un-ionized form is lipid soluble.

Acids (Induction Drugs)

Named after positively charged ions: Na⁺,Mg⁺⁺, Ca⁺⁺ (i.e., sodium thiopental).

Thiopental 7.6
Barbiturates

Bases (LA, Opioids, BZD)

Named after negatively charged ions: Chloride, sulfate (i.e., morphine sulfate, lidocaine hydrochloride).

Fentanyl 7.3
Lidocaine 7.9
Morphine 7.9
Cocaine 8.6
Ketamine 7.5

Local Anesthetics pKa

Bupivacaine 8.1
Chloroprocaine 9.1
Etidocaine 7.7
Lidocaine 7.9
Mepivacaine 7.6
Prilocaine 8.0
Procaine 8.9
Ropivacaine 8.1
Tetracaine 8.6

pKa

Local anesthetics with a pKa closest to physiological pH have a more rapid onset due to a higher concentration of non-ionized base that is able to pass through the nerve cell membrane.

Potency and duration of action directly correlate with lipid solubility.

Rate of absorption is directly proportional to the vascularity of the tissue and affected by the presence of a vasoconstrictor. (IV > tracheal > intercostals > caudal > paracervical > epidural > brachial plexus > sciatic > sub Q)

▪Nerve Fibers

Classification of Nerve Fibers

Nerve Fiber Type	Function	Blocked
A alpha	Proprioception—touch and pressure Motor	Blocked last	Bigger Faster Motor Myelinated Smaller Smaller Slower Unmyelinated
A beta	Proprioception Motor Pressure
A gamma	Muscle tone (motor)
A delta	Somatic pain (sharp)
B fibers	PRE-ganglionic autonomic	Blocked first
C fibers	POST-ganglionic sympathetic Temp, touch Pain (visceral)	Blocked second

Spinal levels.

High spinal affects these levels of spinal cord.

Peripheral nervous system.

Nerve roots of spinal cord become peripheral nerves. Peripheral nerve fibers are a mix of motor, sensory and autonomic fibers, and have individual sensitivities to LA. The Cm (minimal concentration that blocks nerve impulse conduction) is different for each fiber, as the Cm is higher for motor than sensory fibers and lowest for sympathetic fibers.

Sensitivity to LA: autonomic > sensory > motor. Sensitivity is also affected by axonal diameter, type of fiber,
degree of myelination, pH, frequency of stimulus, and electrolyte concentrations. Conduction is enhanced by a small diameter and lack of myelination. Conduction is antagonized by hypokalemia and hypercalcemia.

Complications of PNB are anatomic (organs/tissues surrounding nerve) and physiologic, which can be systemic or local. Potential allergic side effects from ester LA occur because they are derivatives of paraaminobenzoic acid.

Signs and symptoms of toxicity:

Plasma concentration of local anesthetic and effects:

1-5 mcg/ml: analgesia

5-10: lightheaded, tongue numb, tinnitus, muscle twitch

10-15: seizures, unconscious

15-25: coma, resp arrest

> 25: CV depression

Excitatory precedes depression

▪Airway Blocks

Indication

Decrease coughing, swallowing, or laryngospasm; for awake intubation.

LA

Lidocaine 0.5%, 1%, 2%, 2.5%
- Max lidocaine 3-5 mg/kg plain; lidocaine with epi 7 mg/kg max
Lidocaine nebulizer 2% or 4%
Lidocaine 2% viscous 2-4 mL×30 sec and swallow
Lidocaine 4% solution
Lidocaine lollipop 2%, 2.5%, 4%, 5% ointment or gel
Lidocaine 10% spray; each blast is 0.1 mL.
Cocaine 4%; max cocaine: 3 mg/kg with max dose 200 mg
Cetacaine made up of benzocaine, tetracaine, and butamben
- Max benzocaine: 3-4 mg/kg with max dose 250-300 mg

Cranial Nerves (CN) Affected in Airway Blocks

CN 5 (trigeminal)—Sensory: Nose, palate, oral cavity; upper teeth; skin over mandible, anterior 2/3 tongue, floor of mouth. Motor: Muscles of mastication for chewing.

CN 7 (facial)—Sensory: Taste, anterior 2/3 tongue.

CN 9 (glossopharyngeal)—Motor: Speak, gag, and swallowing. Sensory: Taste, posterior 1/3 tongue.

CN 10 (vagus)—Sensory: Pharynx, larynx. Motor: Pharynx and larynx, talks/swallows, lifts palate. SLN: Oral. RLN: Larynx glottis.

Note

Benzocaine toxicity → methemoglobinemia; treat with methylene blue 1-2 mg/kg.

▪Anesthesia of the Hypopharynx, Larynx, and Trachea

Recurrent Laryngeal Nerve Block: Translaryngeal and Transtracheal Blocks

Indications

Awake laryngoscopy and intubation; block laryngospasm; block sensory to larynx and trachea below
cords (supplied by recurrent laryngeal nerve (RLN) by providing topical anesthesia to the laryngotracheal mucosa innervated by vagus nerve. For thyroid surgery, block needs to be bilateral.

Blocks: Vagus nerve, CN X.

LA: Lidocaine 1 %, 2%, 4%; 3-4 mL with or without epinephrine.

Technique

Extend head. LA wheal over cricothyroid membrane. Place index and third finger on either side of thyroid cartilage (see figure). Insert 20-22 ga IV (cath over needle) into cricothyroid membrane (just below thyroid bone and above isthmus of thyroid gland). Aspirate air to confirm placement into trachea.

Remove needle, leaving plastic catheter in place. Have patient inhale just prior to injection and very rapidly remove plastic catheter. Pt will cough forcefully after injection and spray LA upward into the more superior laryngeal structures. Apply pressure to injection site to prevent subQ emphysema and hematoma.

Complications

Vascular/structural injury; IV injection; methemoglobinemia; gastric aspiration.

Translaryngeal technique.

▪Mouth: Glossopharyngeal Block

Indication

Abolish gag reflex or hemodynamic changes to laryngoscopy; anesthesia to mouth, oropharynx, and base of tongue.

Anatomy

CN 7, 9, 10; the glossopharyngeal nerve, which emerges from the skull through the jugular foramen, travels along the lateral wall of the pharynx.

LA

Lidocaine 2% 2-4 mL bilateral.

Technique

With the patient’s mouth wide open, a tongue blade held with the nondominant hand is introduced in the mouth to displace the tongue medially, creating a gutter between the tongue and the teeth. The gutter ends posteriorly in a cul-de-sac formed by the base of the palatoglossal arch. A 25-gauge spinal needle is inserted at the base of the cul-de-sac and advanced slightly (0.25-0.5 cm). After negative air and blood aspiration tests, 2 mL of 2% lidocaine is injected.The procedure is repeated on the other side.

Tips

If air is aspirated, the needle must be withdrawn until no air can be aspirated. If blood is aspirated, it is usually arterial (carotid artery) because the needle is too posterior and too lateral.The needle needs to be redirected medially.

▪Mouth: Superior Laryngeal Nerve Block

Injection: Invasive Technique

Indication

Abolish gag reflex or hemodynamic changes to laryngoscopy or bronchoscopy; helpful in awake intubations; anesthesia to mouth, oropharynx, or base of tongue.

Anatomy

Block CN 10.

LA

Lidocaine 1 %-2% 2-4 mL with or without epi.

Technique

Neck extended; cleanse skin with alcohol. Grasp and displace hyoid bone (only free floating bone in body) between index finger and thumb. Insert a 25-ga needle until contact with greater cornu of hyoid, walking needle off hyoid inferiorly; advance 2-3 mm thru thyrohyoid membrane. Withdraw injecting 1-cc LA.

Complications

Systemic toxicity, hematoma.

Notes

Successful block = hoarseness