Local Anesthetics: History and Types

• Local anesthetics interrupt pain impulses in a specific region of the body without loss of consciousness and are completely reversible with no residual effect on the nerve fiber.

History

• Cocaine was the first local anesthetic isolated from coca leaves by Albert Niemann in Germany in the 1860s.
• The first clinical use of Cocaine was in 1884 by Sigmund Freud to wean a patient from morphine addiction.
• Freud's colleague Karl Kollar first noticed Cocaine's anesthetic effect.
• Kollar introduced cocaine to clinical ophthalmology as a topical ocular anesthetic.
• In 1884, Dr. William Stewart Halsted first described the injection of cocaine into a sensory nerve trunk to create surgical anesthesia.

Types of Local Anesthetics

• Local anesthetics are divided into two groups: esters and amides.

Esters

• Esters are hydrolyzed in plasma by pseudo-cholinesterase.
• Para-aminobenzoic acid, a byproduct of ester metabolism, is a common cause of allergic reactions.

Classification of Esters

• Esters of P-amine benzoic acid include procaine.
• Esters of benzoic acid include benzocaine and cocaine.
• Other esters include chloroprocaine, cyclomethycaine, dimethocaine/larocaine, propoxycaine, procaine/novocaine, proparacaine, and tetracaine/amethocaine.

Amides

• Amides are metabolized in the liver to inactive agents.
• True allergic reactions to amides are rare, especially with lidocaine.

Classification of Amides

• Common amides include lidocaine.
• Other amides include articaine, bupivacaine, cinchocaine/dibucaine, etidocaine, levobupivacaine, lidocaine/lignocaine, mepivacaine, piperocaine, prilocaine, ropivacaine, and trimecaine.

Mechanism of Action

• Local anesthetics block nerve conduction by reducing the influx of sodium ions into the nerve cytoplasm.
• Sodium ions are unable to flow into the neuron, and potassium ions cannot flow out, inhibiting the depolarization of the nerve.
• Nerve impulses generated downstream from blocked nodes cannot propagate to the ganglion if a few Nodes of Ranvier are inhibited.
• Local anesthetics slow the rate of depolarization.
• Local anesthetics reduce the height and rate of rise of action potential.
• Local anesthetics slow axonal conduction and increase the threshold potential.
• Local anesthetics ultimately prevent the propagation of action potential without altering the resting membrane potential.

Order of Sensory Function Block

• The order of sensory function block is: pain, cold, warmth, touch, deep pressure, and motor function.
• Recovery occurs in the reverse order.

Absorption

• Absorption depends on blood flow (vascularity).
• The absorption rate from fastest to slowest is: interpleural, intercostal, lumbar epidural, brachial plexus, subcutaneous, sciatic, and femoral.

Distribution

• Distribution occurs rapidly to high blood supply organs such as the heart, brain, liver, and lungs.
• Equilibration with fat is slow but has a high affinity.
• Lungs sequester and may metabolize local anesthetics.

Metabolism

• Esters are rapidly metabolized by plasma cholinesterase to para-amino benzoic acid (PABA), which can cause allergic reactions.
• Amides are metabolized by the liver and are dependent on hepatic blood flow. Caution should be used in elderly patients and those with congestive heart failure (CHF) or hypovolemia.

Amides vs. Esters

Amides

• Stable in solution
• Slowly metabolized by hepatic amidases
• Hypersensitivity reactions are extremely rare
• Routinely used in clinical practice

Esters

• Unstable in solution
• Rapidly metabolized by plasma cholinesterase
• Metabolite para-amino benzoate (PABA) is associated with hypersensitivity reactions
• Superseded by amides

Adjuvants to Local Anesthetics

• Common adjuvants include: vasoconstrictors, dextrans, dextrose, hyaluronidase, carbon dioxide/carbonated salt, and preservatives such as methyl hydroxybenzoate in multiple-dose vials.

Vasoconstrictors

• Most local anesthetics produce some degree of vasodilation and may be rapidly absorbed after local injection.
• Vasoconstrictors decrease systemic toxicity and increase the safety margin of local anesthetics by reducing their rate of absorption.
• Vasoconstrictors are frequently added to local anesthetics to enhance their potency and prolong their effects.
• Adrenaline is the most commonly used vasoconstrictor.
• Absolute contraindication for adrenaline containing local anesthetic agents exists for infiltration around end-arteries, such as in ring blocks of fingers or the penis. Intense vasoconstriction in these areas with terminal vascular supply may lead to severe ischemia and necrosis.

Toxicity from Local Anesthetic Drugs

• Toxicity occurs when excessive blood levels arise, usually due to accidental rapid intravenous injection, rapid absorption from highly vascular sites (e.g., mucous membranes), or absolute overdose.
• Intercostal nerve blocks result in higher blood levels than subcutaneous infiltration, while plexus blocks are associated with the slowest absorption rates and lowest blood levels.

Signs and Symptoms of Local Anesthetic Toxicity

• Toxicity involves the central nervous system (CNS) and cardiovascular system (CVS), with the CNS being more sensitive. CNS manifestations tend to occur earlier.
• Brain excitatory effects occur before the depressant effects.

CNS Signs & Symptoms

• Early or mild toxicity includes: light-headedness, dizziness, tinnitus, circumoral numbness, confusion, and drowsiness. Patients often do not volunteer this information unless asked.
• Severe toxicity includes: tonic-clonic convulsions leading to progressive loss of consciousness, coma, respiratory depression, and respiratory arrest.

CVS Signs & Symptoms

• Early or mild toxicity, if with adrenaline, includes tachycardia and hypertension.
• If no adrenaline is present, early symptoms include bradycardia with hypotension.
• Severe toxicity usually requires 4-7 times the convulsant dose before CV collapse occurs.
• Collapse is due to the depressant effect of the local anesthetic on the myocardium, such as with bupivacaine.
• Severe and intractable arrhythmias can occur with accidental IV injection.

Essential Precautions and Treatment

• Secure intravenous access before injecting any dose. Always have adequate resuscitation equipment and drugs available.
• Stop the injection and assess the patient. Call for help while treating the patient.
• Ensure an adequate airway, give O2 via facemask, and ventilate the patient if there is inadequate spontaneous respiration.
• Intubate if the patient is unconscious and unable to maintain an airway.
• Administer IV fluids and vasopressors such as ephedrine to correct hypotension; use adrenaline if ephedrine is ineffective.
• Treat arrhythmias and start chest compressions if cardiac arrest occurs.
• Treat convulsions with anticonvulsant drugs such as diazepam at 0.2-0.4mg/kg or thiopentone at 1-4 mg/kg.

Lidocaine (Lignocaine, Xylocaine)

• Lidocaine is the most commonly used local anesthetic.
• It is used at 0.5% for infiltration +/- adrenaline.
• It is used at 4% for topical analgesia of the throat and larynx.
• It is used at 1.5-2% for nerve and extradural blocks.
• It is used at 4% for corneal analgesia.
• It is used at 1-2% jelly for urethral analgesia.
• It is used at 5% ointment for tracheal tubes.

Bupivacaine (“Marcaine”)

• Commonly used in central blocks and peripheral nerve blockade.
• Available in 0.5% in plain or hyperbaric formulation.
• It has a more toxic effect on the myocardium.

Compounding Local Anesthetics

• This involves mixing agents to get the advantages of both.
• Lidocaine + Bupivacaine = rapid onset + prolonged action.
• Note the eutectic mixture of local anesthetics (EMLA), which is a mixture of unionized forms of lidocaine and prilocaine in cream form

Toxicities of Local Anesthetics

• Systemic toxic reactions associated with local anesthetics are generally the result of over-dosage leading to high blood levels of the agent. It is important to administer the smallest effective amount of the most dilute solution to reduce pain.
• Hypersensitivity is rare. A careful patient history should be taken to identify the presence of allergies. Patients allergic to one type of anesthetic may or may not be allergic to the other type.
• CNS toxicity includes excitement characterized by tremors, shivering, and convulsions. CNS depression is characterized by respiratory depression and, if enough drug is absorbed, respiratory arrest.
• Cardiovascular toxicities can cause depression of the cardiovascular system, peripheral vascular action arteriolar dilation (except cocaine which is vasoconstrictive). Hypotension and abnormal heartbeat characterize such depression, and may result in both cardiac and respiratory arrest.
• Signs of toxicity occur on a continuum from early to late stages shown as the following: oral and tongue numbness, lightheadedness, tinnitus, visual disturbances, muscular twitching, convulsions, unconsciousness, coma, respiratory arrest, then cardiovascular collapse.

Methods of Administering Local Anesthesia

• Include local infiltration, surface anesthesia, nerve and plexus block, epidural block, and spinal (subarachnoid) block.

Local Infiltration

• This occurs when nerve endings in the skin and subcutaneous tissues are blocked by direct contact with a local anesthetic injected into the tissue.
• Primary use is for surgical procedures involving a small area of tissue.

Topical Block

• This is accomplished by applying the anesthetic agent to mucous membrane surfaces to block nerve terminals in the mucosa. The anesthetic agent is rapidly absorbed into the bloodstream.
• It is often used during examination procedures involving the respiratory tract. For topical application, the local anesthetic is always used without epinephrine.
• The topical block easily anesthetizes the surface of the cornea of the eye and the oral mucosa.

Surface Anesthesia

• This is accomplished by the application of a local anesthetic to the skin or mucous membranes in places such as the mouth, nose, pharynx, eye, and urethra.
• Surface anesthesia relieves itching, burning, and surface pain, and is used to perform endoscopies.

Nerve Block

• A local anesthetic is injected around a nerve that leads to the operative site. More concentrated forms of local anesthetic solutions are used for this type of anesthesia.
• Plexus blocks apply the same principle as nerve blocks, except the plexus is a collection of nerves, and a larger area is anesthetized.

Penile Blocks

• This are used for penile surgery such as circumcision.
• Sensation of most of the shaft and the glans of the penis is transmitted by the dorsal penile nerves. The nerves with the accompanying arteries emerge under the pubis symphysis, close to the midline, and transverse the dorsum of the penis.
• The method utilizes Bupivacaine 0.5% administered using a 25G needle, and adrenaline must not be used.
• The amount used varies with age.

Method

• Clean the area with hibitane in spirit.
• Use the 2nd and 3rd fingers of the left hand to palpate the lower border of pubic symphysis.
• Insert the 25G needle attached to the syringe at right angles to the skin between the 2nd and 3rd fingers of the left hand until bony contact is made. Redirect the needle to pass just inferior to the lower border of the arch of the pubic symphysis but not deep to. Direct the needle either side of the midline about 2 mm below the inferior border as the blood vessels run in the midline. Aspirate and inject half of the local anesthetic on either side.
• Potential complications include direct I.V injection and toxicity of the drug and hematoma due to puncture of the blood vessels.

Digital Nerve Block

• There are two dorsal and two palmar/planter branches of digital nerve blocks for each finger and toe.
• Inject a ring of local anesthetic at the base of the digit. A tourniquet can be applied to localize the anesthetic and reduce bleeding, but should not be in place for more than 15 minutes. Do not use adrenaline. Maximum volume is 4 ml of 0.1% lignocaine.

Anatomy of the Spinal and Epidural Area

• The spinal cord extends to L2, and the dural sac extends to S2.
• Nerve roots (cauda equina) are from L2-S2
• During spinal anesthesia, a needle is inserted below L2, thus the L3-L4 or L4-L5 interspace is commonly used.

Structures Penetrated

• Skin, subcutaneous fat
• Supraspinous ligament, interspinous ligament
• Ligamentum flavum (last layer before epidural)
• Dura + arachnoid for spinal

Epidural Block

• The local anesthetic is deposited outside the dura mater, acting on the nerve roots.

Spinal Blocks

• A local anesthetic solution is injected into the subarachnoid space. Nerve roots in the subarachnoid space contain sensory (posterior) and motor (anterior) fibers.

Technique of Spinal Anesthesia

• Wash the patient’s back and shave it if necessary. Start an IV infusion, and apply ECG and pulse oximetry monitoring if available. Check the blood pressure and leave the curve on. Vasopressors is needed.

Spinal Anaesthesia Procedure

• Scrub and wear a sterile gown and gloves. Swab the site of the procedure 3 times using a different swab each time. Open the spinal pack, draw up the drugs, and drape the patient preferably with a single "window" drape. Sit on a stool so that the site of lumbar puncture is at eye level. Select either the sitting or the lateral position for the pt.
• Choose the site of injection by feeling the highest point of the iliac crest, which passes through the body of the 4th lumbar vertebra or the interspace between 4th and 5th lumbar vertebrae. Choose the widest space between the 2 spines with the above points as guide. Infiltrate the skin with a local anesthetic.
• Insert the spinal needle in the interspace at right angles to the skin. Once through the ligament, there is distinct loss of resistance. Advance the needle slightly to pierce the dura mater and remove the stilette to check for CSF flow through the needle.
• Replace the stilette, rotate the needle gently, and push it forward or withdraw it slightly if the flow is poor. When there is free flow of CSF inject the drug, place a sterile swab over the site, and hold it in place with strapping.

Lateral Position

• Place the patient on their side, right or left, with buttocks and shoulders parallel to the edge of the table. The nurse stands in front of the patient placing one hand behind the patient's neck and the other behind the patient's knees. The back should be arched to open the intervertebral and interlaminar spaces.

Sitting Position

• The patient sits on the table, instructed to arch or push out their back. The feet are placed on a stool, and arms are folded across the chest with the head flexed. A nurse supports the patient.

Drugs and Dosages for Spinal Anesthesia

• Bupivacaine 0.5% (macaine) is used in 2.5-3 ml doses.
• Bupivacaine 0.5% hyperbaric is a 0.5% solution mixed with 80mg/ml dextrose. For perineal surgery (saddle block), use 1-1.5ml; for prostatectomy and lower limb surgery, use 2-2.5ml; for hysterectomy and appendectomy, use 3-3.5ml
• Lignocaine 5% heavy is used in 1.5-2ml doses.

Care of Patients Under Spinal Anesthesia in the Operating Room

• Check blood pressure every 5 minutes and record on the anesthetic chart.
• Monitor pulse rate every 5 minutes.
• Observe the depth and rate of respiration.
• Monitor color/oximetry; pallor or cyanosis requires attention.

In the Recovery Area and Ward

• Observe the general condition of the patient carefully. Note sweating, nervousness, and nausea and treat appropriately. Monitor vital signs for 4 hours after the return of the patient to the ward. Record blood pressure and pulse every 15 minutes for 1 hour and half hourly thereafter. -Record SPO2 or observe color every 15 minutes. Observe Respiration rate every 15 minutes.
• The position is lying flat for the first 6-12 hours, and if the patient ambulates, a fine needle had to have been used.

Immediate Complications of Spinal Anesthesia

• Hypotension, respiratory depression, nausea and vomiting, broken needle, and total spinal

Late Complications of Spinal Anesthesia: Headache

• Headache thought to be due to leakage of CSF through the hole in the dura mater, lowering the pressure in the arachnoid space, resulting in traction on the blood vessels. The post-spinal headache is different from any headache the patient has experienced before.
• It is worse on sitting up and relieved by lying down. It is aggravated by external stimuli, such as light or noise, and relieved by increasing intra-abdominal pressure. It is located in the occipital region and associated with pain down the neck relieved by increasing abdomnal pressure
• The incidence of post spinal headache is related to the size of the needle; the finer the spinal needle, the lesser the incidence.

Prevention

• Use a fine needle, avoid multiple punctures, nurse the patient flat for the first 6 hours post-operatively, and avoid coughing and straining post-operatively.

Treatment for a Headache

• Reassurance, frequent drinks, sedation and analgesia, I.V fluids, and epidural blood patch
• Administer a small pillow under the lumbar region and sedation.
• To prevent this, avoid traumatic punctures.

Other Potential Complications of Spinal Anesthesia

• Retetion of urine where nursing measure and standing the patient out of bed if possible aids it
• Catheterise If retention persists.
• Sepsis or infection that may take varios forms:. Extra dural abscess, Meningitis(inflammation of the meninges, Encephalitis(Inflammation of brain and spinal cord), and Radiculitis (inflammation of the nerve roots)

Meningism

• Irritation of the meninges without actual infection due to introduction of irritants into the sub?arachnoid space during the tap
• Presents like menigitis, but the CSF is normal.

Indications for Spinal Anesthesia

• Routine anesthesia for surgery below the umbilicus in a fit patient, cases in which it is safer in patients with full stomachs, cases in which who have a contraindication to general anesthesia, In cases where a situation in which oxygen is available but must be conserved, and situations where cost is an important factor.

Contraindications to Spinal Anesthesia

• Local infection over the site of injection, raised intracranial pressure, young children, unco-operative patients and patients with chronic headaches, very ill patients with reduced blood volume, deformed backs, disoders of blood clotting, and cases where enlarged prostate in which the surgery performed is not performed on the prostate.

Advantages of Spinal (and Epidural) Anesthesia

• Simple technique, safe, and minimal interference with physiological function (patient is conscious and breathing spontaneously.)
• The costs of drugs and equipment for spinal is much less than for general anaesthesia, and good operating conditions instance exist for where intra-abdominal surgery is done below the umbilicus

Advantages include:

• Reduced bleeding, and no explosion risk
• Fewer post-operative chest compliations,relaxed abdomen,Good analgesia.

Disadvantages

• Patient acceptability (not all patients accept being awake) and complications.