Local and regional anesthesia by dr. S. Bradulskis General surgery department , Kaunas

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Local and regional anesthesiaby dr. S.
Bradulskis General surgery department , Kaunas
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Local Anesthetic

• A local anesthetic is an agent that interrupts
  pain impulses in a specific region of the body
  without a loss of patient consciousness.
  Normally, the process is completely
  reversible--the agent does not produce any
  residual effect on the nerve fiber.

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Chemistry all local anesthetics are weak bases,
classified as tertiary amines.
 
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• Characteristics
• Poorly water soluble weak basic amins (pKa
  7,5-9)
• Molecular weight between 220 and 288
• Lipophilic aromatic ring tertiary hydrophilic
  amin
• Link Ester (-COO-) or Amid (-NHC-) chain

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Local Anesthetic Classification

• Aminoesters
• Cocaine
• Procaine
• Chloroprocaine
• Tetracaine
• Aminoamids
• Lidocaine
• Prilocaine
• Mepivacaine
• Etidocaine
• Bupivacaine
• Ropivacaine
• Levobupivacaine

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Local Anesthetic Metabolism

• Aminoesters (cocaine, procaine, tetracaine, and
  chloroprocaine )
• Quick degradation
• Hydrolysis, plasma and liver cholinesterase
• Produces para-aminobenzoic acid (PABA, allergy.)
• Aminoamids (lidocaine, mepivicaine, prilocaine,
  bupivacaine, and etidocaine )
• Slower degradation
• Liver microsomal enzymes
• Excretion (metabolites and lt5 unchanged drug)
  via kidneys
• Prilocain (very large doses)
• - Accumulation of metabolites risk of
  methemoglobinemia

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Mechanism of Action

• Local anesthetics work to block nerve conduction
  by reducing the influx of sodium ions into the
  nerve cytoplasm.
• Sodium ions cannot flow into the neuron, thus the
  potassium ions cannot flow out, thereby
  inhibiting the depolarization of the nerve. 
• If this process can be inhibited for just a few
  Nodes of Ranvier along the way, then nerve
  impulses generated downstream from the blocked
  nodes cannot propagate to the ganglion.

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Effect on Action

• Effect of protein binding - increased binding
  increases duration of action
• Effect of diffusibility - increased diffusibility
  decreased time of onset (pK)
• Effect of vasodilator activity - greater
  vasodilator activity decreased potency and
  decreased duration of action

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Vasoconstrictors

• Vasoconstrictors decrease the rate of vascular
  absorption which allows more anesthetic to reach
  the nerve membrane and improves the depth of
  anesthesia, it .
• There is variable response between LA and the
  location of injection as to whether
  vasoconstrictors increase duration of action.
  1200,000 epinephrine appears to be the best
  vasoconstrictor.

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Effect of lipophilicity ANESTHETIC POTENCY

• Lipid solubility appears to be the primary
  determinant of intrinsic anesthetic potency.
  Chemical compounds which are highly lipophilic
  tend to penetrate the nerve membrane more easily,
  such that less molecules are required for
  conduction blockade resulting in enhanced
  potency.
• more lipophilic agents are more potent as local
  anesthetics

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• Two forms exist simultaneously
• Ionised kation (BH)
• Non ionised base (B)
• Relation between the two forms
• depends on
• pKa of the local anesthetic drug
• Tissue (and solution) pH
• Both forms necessary for the action
• Neutral base
• - Penetrates the membrane
• of the nerve cell
• Kation Active form
• - Blocks Na-channels (intracellular)

The lower the difference between pKa and pH (less
basic LA), the more non ionised molecules
(base). More non ionised molecules quicker
onset of action
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Order of sensory function block

• 1. pain
• 2. cold
• 3. warmth
• 4. touch
• 5. deep pressure
• 6. motor

Recovery in reverse order
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Susceptibility to block by local anesthetics of
types of nerve fibers

• In general, small nerve fibers are more
  susceptible than large fibers, however
• the type of fiber
• degree of myelination
• fiber length and
• frequency- dependence are also important in
  determining susceptibility

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A FIBER SIZE AND FUNCTION

• a (dia 12-20um cond vel 70-120m/s) largest,
  afferent to and efferent from muscles and joints.
  Actions motor function, proprioception, reflex
  activity.
• ß (dia 5-12um 30-70m/s) large as A-alpha,
  afferent to and efferent from muscles and joints.
  Actions motor proprioception, touch, pressure,
  touch and pressure.
• ? (dia 3-6um 15-30m/s) muscle spindle tone.
• d (dia 2-5um 12-30m/s) thinnest, pain and
  temperature. Signal tissue damage.

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B FIBER SIZE AND FUNCTION

• B fibers (dia 2-5um) Myelinated preganglionic
  autonomic. Innervate vascular smooth muscle.
  Though myelinated, they are more readily blocked
  by LA than C fibers.

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C FIBER SIZE AND FUNCTION

• C fibers (dia 0.4-1.2 um) Nonmyelinated, very
  small nerves. Smallest nerve fibers, slow
  transmission. Transmit dull pain and temperature,
  post-ganglionic autonomic.
• Both A-d and C fibers transmit pain and are
  blocked by the same concentration of LA.

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TOXICITIES OF LA

• Essentially all systemic toxic reactions
  associated with local anesthetics are the result
  of over-dosage leading to high blood levels of
  the agent given. Therefore, to avoid a systemic
  toxic reaction to a local anesthetic, the
  smallest amount of the most dilute solution that
  effectively blocks pain should be administered.

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Toxicity of LA

• Signs of toxicity occur on a continuum. From
  early to late stages of toxicity, these signs
  are circum-oral and tongue numbness,
  lightheadedness, tinnitus, visual disturbances,
  muscular twitching, convulsions, unconsciousness,
  coma, respiratory arrest, then cardiovascular
  collapse.

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Toxicity of LA

• Hypersensitivity. Some patients are
  hypersensitive (allergic) to some local
  anesthetics. Although such allergies are very
  rare, a careful patient history should be taken
  in an attempt to identify the presence of an
  allergy. There are two basic types of local
  anesthetics (the amide type and the ester type).
  A patient who is allergic to one type may or may
  not be allergic to the other type.

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Toxicity of LA

• Central Nervous System Toxicities.
• Local anesthetics, if absorbed systematically in
  excessive amounts, can cause central nervous
  system (CNS) excitement or, if absorbed in even
  higher amounts, can cause CNS depression.

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Toxicity of LA to CNS

• Excitement. Tremors, shivering, and convulsions
  characterize the CNS excitement.
• Depression. The CNS depression is characterized
  by respiratory depression and, if enough drug is
  absorbed, respiratory arrest.

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Toxicity of LA to Cardiovascular system

• Cardiovascular Toxicities. Local anesthetics if
  absorbed systematically in excessive amounts can
  cause depression of the cardiovascular system.
• Peripheral vascular action arteriolar dilation
  (except cocaine which is vasoconstrictive
• Hypotension and a certain type of abnormal
  heartbeat (atrioventricular block) characterize
  such depression. These may ultimately result in
  both cardiac and respiratory arrest.

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Systemic toxicity prevention

• IV access secured before injection of the LA
• Chose least toxic drug suitable
• Consider block type and patient specific max.
  dose ranges
• Start with a typical dose
• Consider adding a vasoconstrictive adjuvant
  (epinephrine)
• Careful aspiration during injection
• Observe clinical reactions
• Talk to the patient and monitor ECG/blood
  pressure to realize
• early symptoms of central-nervous and
  cardiovascular toxicity
• Stop injection immediately when early symptoms
  are realized
• Consider the time course for development of
  toxic signs
• (5-10 min. after correct injection.)

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Systemic toxicity treatment

• Stop injection immediately
• Treat
• Give oxygen, (hyperventilate - mask or airway
  device)
• Stop cerebral excitation (Benzodiazepines,
  Barbiturates, Propofol (Midazolam 2-5
  mg,Thiopental 50-150 mg,Propofol 50-100 mg)
• Correct hypotension and arrhythmias
• (crystalloids, vasopressors, antiarrhythmic drugs
  (Ephedrin 5-10 mg, Epine-phrine 10-100 µg )
• Cardiopulmonary resuscitation for cardiac arrest
  / VF
• Avoid / treat aggravating factors Hypoxia and
  acidosis (respiratory and metabolic)

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Influencing factors patient-related

• Age
• Old age (gt 70 yr) elimination prolonged
• - 10-20 dose reduction for continuous
  applications
• Newborns (lt 4 months) elimination of amid LA
  prolonged
• - 15 dose reduction per kg
• Renal dysfunction
• Excretion reduced
• - 10-20 dose reductions relative to degree of
  dysfuncion
• Hepatic dysfunction
• Low liver blood flow or poor liver function
• - Higher blood levels of amid local anesthetics
• - 10-50 dose reduction for repeated or
  continuous applications

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Influencing factors patient-related

• Body size
• In very small adults, the dose for blocks
  requiring large doses (brachial plexus, IVRA)
  should be reduced.
• Pregnancy
• Hormonally increased sensitivity of the CNS to
  LA
• Reduced requirements
• Risk for toxicity ?
• Reduced protein binding of bupivacaine
• Increased cardiac output, perfusion ? and uptake
  ?
• Anatomic and physiologic changes - 10 dose
  reduction

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Influencing factors patient-related

• Infected tissue
• Low tissue pH
• More ionised kations
• Less uncharged base available for penetration
• Vasodilation
• Uptake into circulating blood ?
• Reduced effect of the injected local anesthetic

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Influencing factors not patient-related

• Alkalinization (pH ? with sodium bicarbonate)
• Uncharged base?, diffusion rate through nerve
  membrane ?
• - Time to onset ?
• And Injection is less painful ! (higher pH)
• But Duration of action ?
• Recipe
• 9 ml LA (lidocaine, mepivacaine) 1 ml NaBic 8.4
• Adjuvant (Epinephrine 1 200000 (5 µg/ml))
• Vasoconstriction Intravascular uptake ?
• - Duration of action 30-50?
• only in combination with short acting LA (Lido-,
  Prilo-, Mepivacaine!)
• - Toxicity ? (all LA)
• Recipe
• 20 ml LA 0.1 mg Epinephrine
• Contraindication for epinephrine
• Local anesthesia around
• end arteries (finger, ear, penis)!

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Types of Local Anesthesia

• Surface Anesthesia. This type of anesthesia is
  accomplished by the application of a local
  anesthetic to skin or mucous membranes. Surface
  anesthesia is used to relieve itching, burning,
  and surface pain (for example, as seen in minor
  sunburns).

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• Lidocaine
• 5 ointment, 2 gel, 4 solution, 10 aerosol,
  100 mg suppository
• Onset 3-5 min
• Bensocain
• 14-20 solution, gel,
• Onset 30 s

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Types of Local Anesthesia

• Local Infiltration Local infiltration occurs when
  the nerve endings in the skin and subcutaneous
  tissues are blocked by direct contact with a
  local anesthetic, which is injected into the
  tissue. Local infiltration is used primarily for
  surgical procedures involving a small area of
  tissue (for example, suturing a cut).

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Intravenous regional anesthesia- Bier anesthesia

• How does it work?
• Injection into a previously
• exsanguinated and occluded limb
• Retrograde spread of the distally
• injected local anesthetic agent
• Very rapid onset
• For arm/leg procedures lt 1hour
• Can be performed with
• Prilocaine, Chloroprocaine,
• Lidocaine (0.5 solution, 40-60 ml)
• Possible complications
• LA intoxication when tourniquet
• is insufficient or released less
• than 15 - 20 minutes after injection

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Types of Local Anesthesia

• Peripherial Nerve Block. In this type of
  anesthesia, a local anesthetic is injected around
  a nerve that leads to the operative site. Usually
  more concentrated forms of local anesthetic
  solutions are used for this type of anesthesia.
• Major nerve block - (plexus brachialis)
• Minor nerve block - (n. radialis)

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Peripheral nerve blocks

• Choice of agent
• Most local anesthetics can be used. Choice
  depends on intended duration of the block
• Dose / concentration
• Lidocaine, Mepivacaine, Prilocaine (1
  solutions)
• Bupivacaine (0.5 solution), Ropivacaine (0.75
  solution)
• (10 ) 20 40 ( 50) ml (depending on nerve or
  plexus type)
• Onset of action and duration
• Onset is rapid for Lido/Mepi/Prilocaine and slow
  for Bupi/Ropi. Time to onset and duration with
  considerable variations (depending on distance of
  LA deposit to nerves)
• Epinephrine prolongs duration of
  Lido/Mepi/Prilocaine,
• but is less effective with Bupivacaine

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Brachial plexus block

• Block possible at different sites
• Interscalene Supraclavicular, Infraclavicular
• Axillary
• Chosen site
• Depends on planned surgical procedure
• Injected volume 30 50 ml
• Time to onset lt 10 min (Lido, Mepi, Prilo) up to
  25 min (Bupi, Ropi)

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Spinal and epidural anesthesia Main indications

• Epidural
• Indications as for spinal,
• - Thoracic approach in combination with
• general anesthesia
• abdominal and thoracic surgery
• Obstetric analgesia
• Treatment of acute and chronic pain

• Spinal
• Surgery below the umbilicus
• lower abdomen
• lower extremities
• transurethral and vaginal procedures
• Cesarean section

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Spinal and epidural anesthesiaContraindications

• Sepsis
• (Severe) Coagulopathies
• Shock, severe hypovolemia
• Infection at/near puncture site
• Refusal
• Certain neurologic diseases
• Severe aortic valve stenosis
• Communication problems
• Elevated intracranial pressure
• Anatomical abnormalities

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Spinal and epidural anesthesia effects

• Main effects
• Anesthesia and analgesia
• Cardiovascular system
• Hypotension (related to extent of sympathetic
  block, volemia)
• Bradycardia (blocked sympathetic
  cardioaccelerator fibersYoung males more
  frequent)
• Both effects more pronounced with spinal than
  with epidural anesth.
• Respiratory system
• Reduced active exhalation with high block level
• - Caution in patients with severe COPD!

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Spinal and epidural anesthesia effects

• Gastrointestinal and urogenital
• Unopposed parasympathetic activity
• Nausea (associated with high block level)
• Increased secretions, relaxed sphincters, bowel
  constriction
• Long lasting block of sacral parasympathetic
  nerves
• Postoperative urinary retention possible
• Endocrine-metabolic
• Less perioperative stress-response
• Reduction of protein catabolism, hyperglycemia,
  sodium and
• water retention, fever, tachycardia, increased
  minute ventilation
• Coagulation
• Reduced hypercoagulability, reduced
  thromboembolic events

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• Spinal Anesthesia. In spinal anesthesia, the
  local anesthetic is injected into the
  subarachnoid space of the spinal cord

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Spinal anesthesia

• Only use drugs without preservatives!
• Commonly used 0.5 Bupivacaine (long action)
• hyperbaric (with 8 glucose) or plain (isobaric)
  solution
• No hyperbaric (heavy) lidocaine (transient
  neural symptoms)!
• Rapid onset (injection close to nerve roots)

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Epidural Anesthesia. This type of anesthesia is
accomplished by injecting a local anesthetic into
the epidural space

• Lido-, Mepiva-, Prilo-, Bupiva-, Ropivacaine
• Onset 5-15 min (Lido/Mepi/Prilo) to 20-30 min
  (Bupi/Ropi)
• Anesthesia
• high concentration 2 Lido/0.5 (L-)Bupi/0.75
  Ropi
• Analgesia postoperative without motor deficit
• 0.125-0.25 (L-)Bupi/0.2 Ropi

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Postoperative Analgesia

• Local anesthetics for postoperative analgesia
• Thoracic Epidural analgesia
• Continuous peripheral nerve blocks (Shoulder,
  Arm, Leg)
• Continuous wound infiltration (Shoulder,
  Tram-Flap, .)

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Postoperative epidural analgesia

• Advantages compared with systemic opioid-based
  analgesia
• Decreased postoperative pain
• Better analgesic effect
• Reduced opioid related side-effects
  (Nausea/Vomiting, Pruritus, Sedation)
• Also suitable for some outpatients (elastomere
  pumps)
• Less sedation, less postoperative fatigue
• Higher Health related quality of life
• Earlier mobilization
• Better respiratory function, better exercise
  capacity,better bowel function, earlier oral
  nutrition
• Earlier ready for discharge (not done, other
  factors.)
• No difference incidence of postoperative
  surgical complications
• But
• Method-specific side effects (similar to regional
  anesthesia

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