Injectable anesthetic agent Flashcards
General anaesthesia
State of reversible unconsciousness produced by general anaesthetic agents
Central acting
Coma like state
Absence of pain sensation over the entire body; not aware of stimulus, amnesia
Varying degrees of muscle relaxation, immobility
Always see changes in CV and pulmonary function
Temporary, only so long as the drug is active
The 4 stages of general anesthesia
Stage 1- voluntary excitement/conscious fear response
Stage 2- involuntary (unconscious) excitement
Stage 3- unconsciousness (surgical anesthesia)
Plane 1- light unconsciousness (goal of induction)
Plane 2- moderate unconsciousness (surgical plane)
Plane 3- deep unconsciousness (early overdose)
Stage 4- anesthetic overdose
stage 1 of anesthesia causes
Voluntary excitement; conscious fear response
Still conscious, but losing consciousness
Fear, excitement; fight or flight
Disorientation, struggling
Increased HR, increased RR, increased BP
Urination, defecation, panting and/or breath-holding
Stage 2 of anesthesia causes
Involuntary (unconscious) excitement
Unconscious fight or flight
Loss of voluntary control
Twitching, paddling, moaning, tremors, rigid limbs; dilated pupils
Irregular breathing
Towards end of stage 2, muscular relax, decreased reflexes, slowing down HR/RR
What is the goal for anesthetic induction
Goal during induction is to get the patient through stages 1 and 2 as quickly as possible to minimize their stress
Stage 3 of anesthesia causes
Unconsciousness
Surgical plane of anesthesia
Decreased cardiopulmonary function and decreased response to stimulus
Decreased sympathetic functions
Breathing is regular
There are 3 planes of unconsciousness within stage 3
Stage 3 plane 1 of anesthesia is
Light surgical plane
Light unconsciousness
Decreased muscle tone; decreased reflexes; decreased sensation, but still react to painful stimulus (HR,RR< and BP will increase if painful stimulus applied)
Movement is possible
This is the goal of induction
Can intubate and prep patient
Inadequate for surgery
Stage 3 plane 2 of anesthesia is
Moderate unconsciousness
Surgical plane
Adequate, ideal degree of unconsciousness for surgery
Depressed CV function and respiration
Vitals are steady and stable
Minimal muscle tone, no reflexes, no reaction to most painful procedures
If pulling on viscera e.g. ovarian ligament may still see transient increase in HR/RR/BP
Stage 3 plane 3 of anesthesia is
Deep unconsciousness (aka early overdose)
Extensive CNS depression
Significant CV, resp depression
NO muscle tone, no reflexes; limp
Early warning is that the patient is no longer stable. WARNING: TOO MUCH ANESTHETIC!
May need to manually support ventilation; will need to support BP
STage 4 of anesthesia is
Anesthetic overdose
Brainstem paralysis
CV, pulmonary collapse (aka shock)
Initially will see fight or flight response (dilated pupils, increase HR/RR); rapidly followed by shutting down of cardiovascular and resp functions (rapid increased in HR,RR,BP, mm go white)
Death will ensure in 1-5 mins if you do nothing
STOP ANESTHETIC, STRAT CPR
3 parts of preforming GA
Induction
Maintenance
Recovery
Induction part of GA
The process where patients move through stages 1 and 2; enter stage 3
Patient moves from conscious to unconscious
Induction should always be rapid as stages 1 and 2 are NOT nice (for patient or handler)
Induction is typically with a general anesthetic
Injectable GA provide the fastest induction; can be so quick that stages 1 and 2 are never noticed
Stage 1 can also be achieved by overdosing on a sedative. Recall that patient is still conscious in stage 1
Maintenance for GA
Where the patient is consistent at stage 3
Consistent depth of unconsciousness; CNS depression
Consistent CV and resp function
If painful procedures are being performed; then the patient should be in plane 2
Most commonly, patients are maintained with an inhalant anesthetic
Occasions where maintenance is with an injectable anesthetic
Recovery of GA
After the anesthetic is turned off or no longer being administered, the patient will go through the stages in reverse order
Like induction, a slow recovery can be rough
Smooth recoveries are typically fast, but controlled
The patient returns to a state of consciousness
Typically, use return of vitals to pre-anesthetic norms and sternal recumbency as end of recovery period
Knowing drugs and monitoring
Anaesthetic accidents are devastating
Can result in permanent injury, death, loss of licence, lawsuit
Majority can be prevented by
Knowing your drugs
Proper dosing and administration
Appropriate monitoring (know norms)
Accurate communication b/w vet and tech
Using highest standards of care
Always keep meticulous records
Injectable GA
Drugs given by IV route
Centrally acting depressants (all of them)
Must reach the brain to work properly
Designed to cross the BBB; usually very lipophilic
Used as part of balanced anaesthesia, with or without an inhalant anesthetics
Require inactivation/elimination by the liver/kidneys
Why are Injectable anesthetic drugs the preferred method
Prefered for induction in all species (over gas induction)
Most rapid onset of stage III
Considered the standard in all medium to large dogs and LA
Exception is small exotics, cats and very small dogs when inducing with an inhalant anaesthetic may be considered acceptable in specific situations
May be single drug or combination of drugs
Always given IV to effect
Except in large animals where calculated dose should be given ( a LA that is not fully anaesthetised is dangerous)
Limitations in injectable anesthetics
Causes CNS, CV and resp depression
Always a risk of debilitating or fatal overdose
Must monitor HR, ventilation and BP
Do not provide (sufficient) analgesia
Must use with analgesics for painful procedures
Do not provide muscle relaxation
Combine with muscle relaxants, inhalant anesthetics
Very low TI; no error for mistakes
Must dose accurately for the individual patient
Cannot be reversed or removed
Supportive care only of overdose or adverse reaction
Have a longer recovery period than inhalants
Why do IV anesthetics cause a longer recovery time
Require liver metabolism and/or elimination
Longer recovery periods have more excitement and/or hallucinations. Therefore, not preferred for recovery (when an inhalant is available)
In most cases, injectable anesthetic is used for induction; inhalant anaesthetic is used for maintenance and recovery
Using injectable anaesthetics
Always dose for lean body weight
Always administer “to effect” IV (SA)
Do not administer too rapidly
Giving too fast can cause arrhythmias and induction apnea (breath holding)
Wait 15-90 sec after each increment
Always use a catheter
Can give to effect easier
Ensures venous access since drugs will cause some degree of hypotension
Can use alone for short procedures
Induction is followed by 5-20 min of GA
Check and double check doses
Low TI, cannot reverse
How to administer IV anesthetics to effect
Calculate and draw up the full dose for the patient
Using an IV catheter, only give ¼ - ½ of total dose→flush→watch for effect. If more drug is needed, give another ¼ of total dose→ flush→ watch for effect,…Repeat until animal is in stage 3, then stop
Purpose: only give as much drug is required; decreased risk of overdose
Total intravenous anaesthesia (TIVA)
Method where only injectable anesthetic drug(s) are used to maintain general anaesthesia for any duration of time but typically for procedures <60 minutes
Including induction and maintenance
I.e., no inhalant is used
Drug is by IV infusion (ex. CRI)
Currently used in equine field work (e.g. triple drip, more recently medetomidine+ketamine+midazolam) and occasionally in SA
Examples of Injectable anaesthetic drugs
Barbiturates- thiopental, pentobarbital
Dissociative anaesthetics- ketamine
Guaifenesin
Propofol
Alfaxalone
Others include
Synthetic opioids- fentanyl, sufentanil, etorphine (distinct as these provide analgesia and are reversible)- wildlife
Etomidate
Barbituates are
Barbituates were commonly used as general anaesthetics, but due to development of newer injectable agents and inhalants and the loss of availability of Thiopental, they are now only used for specific inductions
GABA agonists
