6. Special Techniques

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Local Anesthesia Assisted and Controlled Ventilation Neuromuscular Blocking Agents

Special Techniques

Chapter 6

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Local Anesthesia

Use of a chemical agent on sensory neurons to disrupt nerve impulse transmission leading to temporary loss of sensation

Indications: Tractable animal General anesthesia is undesirable or high risk Means to deliver general anesthesia are not

available

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Local Anesthesia (Cont’d)

Advantages Low cardiovascular toxicity Inexpensive Excellent pain control immediately postoperatively Minimum patient recovery time

Uses Ruminant obstetric and abdominal procedures Complement standing sedation in horses In conjunction with general anesthesia for pain

control

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Local Anesthetic Agents

Skin infiltration and mucous membrane application: Lidocaine

• Administer at 0.5% to 2%• Dilute with sterile saline if necessary

Bupivacaine• Administer at 0.25% or 0.5%• Slower onset of action; long duration

Mepivacaine Procaine

Ophthalmic use: tetracaine, proparacaine

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Characteristics of Local Anesthetics

Not general anesthetics Don’t affect the brain and have no sedative effect

Few cardiovascular or respiratory effects Better for high-risk patients

Exert action close to site of injection Not distributed throughout the body

Don’t normally cross the placenta Used for cesarean sections and obstetric

manipulations

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Local Anesthetic Mechanism of Action

Drugs that affect primarily sensory neurons Drug must be placed in proximity to the neuron Blocks sodium channels and prevents generation

of electrical impulses (stops depolarization) Reversal occurs as drug is absorbed into the local

circulation Metabolized in liver

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Local Anesthetic Mechanism of Action (Cont’d)

Drugs that affect primarily motor neurons Cause temporary local paresis or paralysis Seen in conjunction with sensory neuron loss of

sensation• e.g., Epidural block

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Local Anesthetic Mechanism of Action (Cont’d)

Drugs that affect the autonomic nervous system Not always desirable Affect sympathetic neurons between the brain and

blood vessels and internal organs Sympathetic blockade = temporary loss of function Sympathetic blockade may affect the heart if local

anesthetic diffuses into the thoracic spinal cord Peripheral effect: vasodilation leading to local

flushing and increased skin temperature Vasodilation may lead to hypotension

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Routes of Administration of Local Anesthetics: Topical

Applied directly to intact skin Drug molecules can penetrate the epidermis to

reach the dermis Less pain relief and shorter duration of effect than

if administered by infiltration Clinical applications

Lidocaine patches Ethyl chloride spray Eutectic mixture Splash block Bupivacaine instilled through a chest tube Mucous membrane application

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Routes of Administration of Local Anesthetics: Infiltration

Drug injected into tissues in proximity to the target nerve

Lidocaine with or without epinephrine is most common

Intradermal, subcutaneous, or intramuscular administration

Provides analgesia for surgery involving superficial tissues

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Routes of Administration of Local Anesthetics: Infiltration (Cont’d)

Procedure Surgical prep Small-gauge needle (20- or 25-gauge) Amount of drug used varies with species and

procedure location Onset of action (lidocaine) = 3-5 minutes Test effectiveness prior to surgery: gently prick

skin with a 22-gauge needle

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Infiltration of Local Anesthetics

Effectiveness of local anesthetics Deep tissues are not affected if injection is

superficial Scar tissue, fibrous tissue, fat, edema, and

hemorrhage impede diffusion of drug Inflammation or infection decrease effectiveness

Duration of effect depends on drug used and

rate of absorption by local blood vessels Epinephrine may be added to local anesthetic to

affect rate of absorption Use of lidocaine without epinephrine

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Infiltration Techniques: Nerve Blocks

Injecting local anesthetic in proximity to a nerve

Desensitizes a particular anatomic site Decreases amount of general anesthesia

needed Provides short-term analgesia postoperatively

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Infiltration Techniques: Nerve Blocks (Cont’d)

Most commonly used in large animals Lameness examinations in horses Cornual blocks for dehorning cattle Paravertebral blocks for abdominal or obstetric

procedures in cattle Dental blocks in dogs and cats Intercostal nerve blocks for chest surgery Limb amputations Cat declaws

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Infiltration Techniques: Line Blocks

A continuous line of local anesthetic placed between the target area and the spinal cord

Ring block: line of local anesthetic completely encircles an anatomic part

Used in food animal and equine surgery Teat surgery or wound repair

L-block: a line block used for laparotomy surgery in ruminants

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Infiltration Line Blocks (Cont’d)

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Other Infiltration Techniques

Intraarticular administration Local anesthetics injected directly into a joint

Regional anesthesia Local anesthetic injected into a nerve plexus or in

proximity to the spinal cord Affects a larger area such as an entire limb or

caudal portion of the body

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Other Infiltration Techniques (Cont’d)

Paravertebral anesthesia (ruminants only) For standing laparotomies as an alternative to an

L-block Local anesthetic blocks the dorsal and ventral

branches of spinal nerves T13-L2 Provides wide, uniform area of anesthesia May produce hind limb weakness or scoliosis

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Infiltration Techniques: Epidural Anesthesia

A regional anesthetic procedure used in small and large animals

Blocks sensation and motor control of the rear, abdomen, pelvis, tail, pelvic limbs, and perineum

Uses: Tail amputation Anal sac removalPerianal surgery UrethrostomiesObstetric manipulations Cesarean sectionsSome rear limb operations

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Infiltration Techniques: Epidural Anesthesia (Cont’d)

Drug choice determined by procedure Local anesthesia

• 2% lidocaine or 0.5% bupivacaine Postoperative pain control

• Opioid (morphine) Opioid and local anesthetic mixture

• Anesthesia and analgesia Opioid and alpha2-agonist mixture

• Epidural anesthesia in horses and cattle

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Epidural Anesthesia (Cont’d)

Location Drug deposited in epidural space between spinal

cord and vertebrae Spinal nerves pass through this space and are

affected by drugs placed here Dogs: between the last lumbar vertebra (L7) and

the sacrum

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Epidural Anesthesia (Cont’d)

Effects: Local anesthetics block sensory and motor

neurons Sensory block controls pain Motor block may affect tail and limb mobility Opioids have minimal effect on motor neurons

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Infiltration Techniques: Bier Block

Intravenous regional anesthesia Provides short-term local anesthesia to a limb Lidocaine only Tourniquet applied to proximal superficial vein Lidocaine injected into distal superficial vein Effect lasts less than 1 hour

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Systemic Constant Rate Infusion

Lidocaine administered to healthy anesthetized animals

Reduces dose of general anesthesia or analgesic required

Used in dogs, cats, and horses

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Adverse Effects of Local Anesthetics

Loss of motor neuron function Loss of nerve function with direct injection Tissue irritation Paresthesia during recovery Allergic reactions from rash or hives to

anaphylactic shock Systemic toxicity Trauma to spinal cord or cauda equina Serious toxicity and death with infiltration into the

cranial portion of the spinal cord Sympathetic nerve block with infiltration into the

cervical or thoracic spinal cord

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Assisted and Controlled Ventilation

Positive-pressure ventilation (PPV) Assisted ventilation

Anesthetist delivers an increased volume of air or oxygen/anesthetic gases to the patient

Patient initiates inspiration Controlled ventilation

Anesthetist delivers all air required by the patient No spontaneous respiratory effort by the patient Anesthetist controls respiratory rate and volume

and pressure of gas inhaled

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Normal Ventilation

Physical movement of air into and out of the lungs and upper respiratory passageways

Active phase Inhalation Initiated by increased PaCO2 detected by

respiratory center in the brain Thoracic muscles (diaphragm and intercostals)

movement causes thoracic cavity to expand Air is pulled into the breathing passages and goes

to the alveoli When lungs are “full” the respiratory center stops

inhalation

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Normal Ventilation (Cont’d)

Passive phase Exhalation No active muscle movement PaCO2 begins to rise until respiratory center starts

inhalation again Normally twice as long as inspiration Normal tidal volume = 10-15 mL/kg

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Ventilation in Anesthetized Animals

Differs from ventilation in awake animals Reduced amount of air entering and leaving

the lungs Tranquilizers and general anesthetics

Decrease responsiveness of breathing center to carbon dioxide levels so inhalation doesn’t occur as often

Relax intercostals muscles and diaphragm so the chest doesn’t fully expand (VT is reduced)

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Potential Problems

Hypercarbia Related to the breakdown of carbon dioxide to

bicarbonate ions and hydrogen ions, which can lead to respiratory acidosis

Hypoxemia Related to less oxygen entering the lungs to be

absorbed into the blood Atelectasis

Related to decreased VT so alveoli don’t fully expand on inhalation, which can lead to partial collapse of the alveoli in some sections of the lung

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Types of Controlled Ventilation (PPV)

Patient is intubated and connected to anesthetic machine

Manual ventilation or bagging Anesthetist bags patient every 2-5 minutes

Intermittent mandatory ventilation Patient requires bagging throughout the anesthetic

period Mechanical ventilation

Lungs are filled with oxygen by pressure of gas from a ventilator

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Manual Ventilation

Lungs are filled with oxygen by pressure of gas entering airways Anesthetist is squeezing the reservoir bag Pop-off valve is fully or partially closed Exhalation is passive and occurs when positive

pressure is discontinued and pop-off valve is fully opened, which allows the lungs to empty

Bag one to two breaths every 2-5 minutes (sighs) Bag is squeezed for 1-1.5 seconds (inhalation

time) Pressure manometer reading: <20 cm H2O (small

animals); <40 cm H2O (large animals)

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Intermittent Mandatory Manual Ventilation (Cont’d)

Periodic bagging doesn’t provide enough ventilation

Animals with preexisting heart or lung disease; diaphragmatic hernias Patients show shallow breaths and respiratory rate

<6 bpm May be used prior to placing animal on the

ventilator Bagging superimposes positive pressure over

patient’s spontaneous breathing efforts, which will stop in about 1 minute

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Intermittent Mandatory Manual Ventilation (Cont’d)

Assisted ventilation rate Initially 8-20 bpm depending on size of the patient 6-12 bpm after control of respiration has been

established Wean patient off assisted ventilation near the

end of the surgical procedure

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Mechanical Ventilation

Similar to intermittent mandatory manual ventilation

Breathing is controlled by the ventilator, which replaces the reservoir bag and becomes part of the breathing circuit Ventilator bellows automatically compresses and

forces oxygen and anesthetic into the patient’s airways

Gases delivered on inspiration: pressure cycle, volume cycle, or time cycle ventilators

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Mechanical Ventilation (Cont’d)

Normal beginning settings RR = 6-12 bpm Duration of inspiration = 1-1.5 seconds Duration of expiration = 2-6 seconds Inspiratory/expiratory ratio = 1:2 to 1:3

Ventilator settings vary with the needs of the patient

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Ventilators

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Risks of Controlled Ventilation

Ruptured alveoli Decreased cardiac output Respiratory alkalosis Ventilator delivers more inhalant anesthetic to

patient Anesthetist monitoring

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Neuromuscular Blocking Agents

Muscle-paralyzing agents Animals remain conscious and feel pain

Limited use in veterinary medicine Animals on mechanical ventilation Orthopedic surgery and ophthalmic surgery Cesarean sections Facilitate difficult intubation Balanced anesthesia techniques

Administered when animal is unconscious and respiration is controlled by intermittent mandatory mechanical ventilation

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Neuromuscular Blocking Agents: Depolarizing

Interrupt normal impulse transmission from motor neuron to muscle synapse

Succinylcholine Single surge of activity at neuromuscular

junction Followed by period when the muscle end

plate is refractory to further stimulation Fast onset; short duration of effect Useful for rapid intubation

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Neuromuscular Blocking Agents: Nondepolarizing

Interrupt normal impulse transmission from motor neuron to muscle synapse

Gallamine, pancuronium, atracurium besylate, cisatracurium

Block receptors at end plates No initial surge of activity

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Neuromuscular Blocking Agents

Concurrent use with other drugs may affect potency

Administered by slow IV injection Effect seen within 2 minutes and lasts

10-30 minutes Repeated doses or continuous infusion can

be used with some agents Makes assessment of anesthetic depth

difficult Only voluntary skeletal muscles are affected

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Neuromuscular Blocking Agents: Reversal Drugs

Nondepolarizing agents reversed with a anticholinesterase agent Edrophonium, neostigmine, pyridostigmine Maintain at light anesthesia until reversal is

complete Make sure spontaneous breathing has started or

support respiration Pretreat patient with atropine or glycopyrrolate to

prevent adverse effects Depolarizing agents have no effective

reversal drugs