SCV 202: Lecture 4 Induction

Goals of induction

Take the patient from sedated to Stage III anesthesia smoothly and quickly, and place an ET tube to control and protect the airway

How induction is usually done

With an injectable drug (IV or IM), then intubation and connection to an anesthetic machine that delivers an inhalant

Induction protocol rules

Part of a balanced drug protocol; place an IV catheter if giving IV; give no more than the minimum dose needed for Stage III; titrate (give to effect)

Induction drug categories

• Dissociatives (ketamine, tiletamine), propofol, alfaxalone, and others (barbiturates, guaifenesin)

Dissociatives — name

• Ketamine hydrochloride

• tiletamine hydrochloride (in Telazol)

Dissociatives — family

Dissociative anesthetics (NMDA receptor antagonists)

Dissociatives — effects

Chemical restraint, unconsciousness, partial analgesia

Dissociatives — side effects

• increased heart rate and blood pressure

• Reflexes stay present

• sensitivity to light and sound

• eyes stay open (need lube)

• increased muscle tone

• possible seizures

• apneustic breathing (choppy breathing)

• hallucinations

• increased salivation and airway secretions

Dissociatives — mechanism

NMDA receptor, non-competitive, antagonist.

• Blocks the NMDA receptors responsible for pain "wind-up," and disrupts signals in some brain areas while stimulating others.

• This causes a trancelike (cataleptic) state: the animal looks awake but is unaware and immobile

Dissociatives — reversible?

No

Dissociatives — controlled?

Yes

Dissociatives — analgesic?

Partial, yes (NMDA antagonism).

• It resets pain receptors, so it is good for neuropathic pain

Dissociatives — routes, onset, duration

IV or IM; onset about 90 seconds IV and 2–10 minutes IM

• lasts 20–30 minutes

MLK CRI

Morphine + Lidocaine + Ketamine as a constant rate infusion; an example of ketamine's analgesic use

Ketamine + diazepam or midazolam

Good induction combo to Stage III: gives muscle relaxation (benzodiazepine - diazepam or midazolam), minimal cardiac depression (dissociative - ketamine), smooth recovery, and mild analgesia (ketamine)

Telazol

Tiletamine + zolazepam

• labeled for IM and IV use in dogs and IM in cats

• more respiratory depression (tiletamine is more potent than ketamine) and longer recoveries

"Kitty magic"

Ketamine + dexmedetomidine + butorphanol given IM

Ketamine combos for small mammals

Ketamine/xylazine and ketamine/medetomidine

Propofol — name

Propofol (PropoFlo 28 is more shelf stable)

Propofol — family

Ultrashort-acting, non-barbiturate injectable anesthetic

Propofol — effects

Creates Stage III unconsciousness and anticonvulsant (used for status epilepticus when other drugs fail)

Propofol — side effects

• Respiratory depression/apnea (esp high dose or given too fast IV)

• excitement and muscle tremors (especially given too slow IV)

Propofol — mechanism

Binds GABA receptors and keeps them open, which inhibits neurotransmission

Propofol — reversible?

No

Propofol — controlled?

No

Propofol — analgesic?

No

Propofol — metabolism

Fat soluble. Goes first to high blood-flow organs (brain, heart, kidneys, liver), then quickly redistributes to muscle and fat

Propofol — how it's given

• Bolus for induction or CRI (continuous rate infusion) for short procedures

• relatively safe in liver or kidney disease (quick redistribution and clearance)

• preoxygenate 3–5 minutes before induction

Propofol — induction technique

Titrate the bolus to effect over 1.5–2 minutes: give 1/4 of the calculated dose every 30 seconds, OR give 1/2 over the first minute and the rest over the next 30–40 seconds

Propofol — storage and handling

• Milky because it is an emulsion of egg lecithin, glycerin, and soybean oil

• not water soluble

• some preparations must be discarded 6 hrs after opening at room temp or 24 hours if refrigerated (PropoFlo 28 lasts longer at room temp)

Alfaxalone — name

Alfaxalone

Alfaxalone — family

Ultrashort-acting injectable anesthetic

Alfaxalone — effects

Used to induce general anesthesia

Alfaxalone — side effects

Mild cardiac depression and mild hypotension; may see respiratory depression or apnea

Alfaxalone — mechanism

Enhances the effects of GABA, which inhibits neurotransmission

Alfaxalone — reversible?

No

Alfaxalone — controlled?

Yes

Alfaxalone — analgesic?

No

Alfaxalone — administration

Faster than propofol. Titrate the bolus over 1 minute, OR give 1/4 of the calculated dose every 15 seconds to effect

Barbiturates — name and family

Ultrashort-acting: thiopental, methohexital (small animals and horses). Short-acting: pentobarbital. Intermediate/long-acting: phenobarbital

Barbiturates — controlled?

Yes, all of them

Pentobarbital — use

Status epilepticus and euthanasia solutions

Phenobarbital — use

Not used for anesthesia; used as an anticonvulsant for seizure management

Guaifenesin (GG) — name and family

Guaifenesin (GG); a muscle relaxant

Guaifenesin — effects

Given to large animals (horses and cattle) to improve muscle relaxation, make intubation easier, and ease induction

Guaifenesin — other info

Often combined with other agents (e.g., the equine "triple drip": GG + ketamine + an alpha-2 agonist); also an expectorant in human cough medicines

Functions of intubation

1) Efficient delivery of O2 and anesthetic gas (connects to machine or ambu bag, reduces staff exposure to waste gas, reduces anatomic dead space). 2) Protects the airway (prevents aspiration of vomit, blood, saliva; leave in until the swallow reflex returns).

3) Allows manual or mechanical ventilation

3 key elements for successful intubation

1) Patient at Stage III (unconscious, no voluntary movement, no reflexes including swallow, enough muscle relaxation). 2) Correct restraint by another tech or assistant. 3) Intubation equipment ready

Intubation equipment to lay out

• 12 cc insufflation syringe (inflates cuff)

• 3 ET tubes of slightly different diameters

• gauze squares to grasp the tongue

• water-soluble ET tube lube on 4x4 gauze

• gloves

• ET tube tie (IV tubing, rolled gauze, or tape)

• laryngoscope with the right blade size (has the light)

• lidocaine injectable, gel, or spray to control laryngospasm in cats (optional)

• eye lube

Parts of an ET tube

Pilot balloon valve, pilot balloon, machine end, connector, tube tie, ruler (length marks), internal size marking, cuff, patient end, Murphy eye

ET tube types

PVC: clear and stiffer.

Murphy silicone: pliable, strong, less irritating, resists collapse.

Red rubber: flexible and less traumatic, absorbent, may kink or collapse

Murphy tube — what's unique

Has a Murphy eye, a small side opening near the patient end that lets gas pass if the tip is blocked

Cole tube — what's unique

Uncuffed, with a narrowed (tapered) patient end

ET tube size

Measured by internal diameter (ID), from 1 mm to 30 mm; the length scale marks distance from the patient end in centimeters

ET tube diameter selection

Small enough not to injure the trachea, large enough to seal with the inflated cuff

ET tube length selection

Measure from the nose to the thoracic inlet; keep it short to reduce dead space

Other things to consider when choosing a tube

Species, conformation, and breed

ET tube cuff prep

Inflate, then deflate the cuff before intubating to make sure it holds air

Laryngoscope — function and parts

Increases visibility of the larynx while placing the ET tube. Handle (batteries), blade (depresses tongue and epiglottis), light source

Laryngoscope — sizes and blade types

Small animal blades 0 to 5 inches; large animal blades up to 18 inches. Blade types: Miller and McIntosh

Miller vs. McIntosh blade

Miller is a straight blade; McIntosh is a curved blade

Intubation steps: before the tube goes in

Preoxygenate; give IV induction drug(s) to effect; remove the syringe and flush the catheter; restrainer puts the animal in sternal recumbency, holds the head by the maxilla behind the upper canines, and extends it upward; tongue is held to drop the lower jaw; laryngoscope goes over the tongue to flip the epiglottis down; pass the tube smoothly through the glottis into the trachea

Intubation steps: after the tube is in

Tie the tube (behind the ears or over the muzzle) while sternal; move to lateral recumbency and check the tube has no bends or kinks; partially inflate the cuff with about 1 cc of air; with O2 on, connect to the machine; fully inflate the cuff using listen-and-squeeze; set the vaporizer to the calculated setting; lubricate the eyes; keep monitoring stage and plane

Inflating the ET tube cuff (listen and squeeze)

Connect the insufflation syringe to the pilot valve; put your ear to the patient's mouth; assistant closes the pop-off valve and gives a breath (squeezes the reservoir bag 1–2 times), then reopens the pop-off; listen for leaks; inflate until the leak stops

Confirming ET tube placement

See the tube in the larynx; watch the reservoir bag move with breathing; feel air at the connector on exhale; look for fogging in the tube; watch the unidirectional valves move; palpate the neck (feeling 1 tube = the ET tube is in the trachea; feeling 2 tubes = the ET tube is in the esophagus); a patient who can vocalize or coughs means the tube is misplaced

Laryngospasm

Reflex closure of the glottis when touched by an object or substance. Common in cats, swine, and small ruminants at light planes; makes intubation hard and the larynx easy to damage; may cause cyanosis or hypoxemia

Preventing laryngospasm

Make sure anesthesia is deep enough; wait for the glottis to open; don't force the tube; use 2% injectable lidocaine, gel, or spray in cats

Other intubation complications

Clogged tube (secretions, vomit, poor cleaning); swallowed or chewed tube if removed too late; tracheal or laryngeal irritation leading to postsurgical cough or tracheal tears (rough attempts, overinflated cuff); drops in heart rate (vagal stimulation); breed challenges (e.g., bulldogs)

Safety after intubation

Support the patient (especially the head) as it loses consciousness; always disconnect the breathing circuit before moving the animal; always deflate the cuff before repositioning the tube in the trachea

Extubation — signs the patient is ready

Palpebral and swallow reflexes return; you want to see 3 swallows

Extubation — steps

Deflate the cuff by drawing air out of the pilot balloon; untie the tube; keep the neck in a natural, slightly extended position

Extubation — cautions

Be careful with early extubation in brachycephalic breeds; watch for delayed extubation in cats

Supraglottic airway device (SAD)

A lesser-used alternative to intubation; helpful for hard-to-intubate patients such as cats and rabbits