IV General anesthetics Flashcards
main effects of general anesthetics. Are they effective analgesics?
- Produce unconsciousness
- Do not inhibit pain signal generation
-Patients do not feel mildly painful surgical manipulations because they produce only small numbers of pain signals, which are transmitted to the Reticular Activating System (RAS) in the medulla, but are insufficient to awaken the patient in the presence of the anesthetic
-If surgical manipulations produce large numbers of pain signals, the RAS may be stimulated to the point where the anesthetized patient begins to awaken
Administer an analgesic beforehand in these cases to inhibit pain signal generation
how can premedication effect the dose of induction anesthetic?
Premedication (e.g., with an opioid or benzodiazepine) can reduce the dose of induction anesthetic, often by ~50%
> also reduces the incidence of agent-specific adverse effects
Why bother with the induction agent? Why not just give the inhalant?
Onset of effect is too slow
>Want to get through excitement phase so rapidly that it may not even be experienced
A patient passes through various stages or planes of anesthesia during induction:
I. Stage of analgesia
Analgesia, amnesia, euphoria, then semi-consciousness
II. Stage of excitement
Struggling, delirium, irregular breathing, +/- vomiting, urination, defecation, combative behaviour (semi-conscious)
III. Stage or plane of surgical anesthesia
Unconscious; regular breathing returns, movements cease
If the process goes too far:
IV. Stage of medullary depression
Breathing stops, heart stops > death
Intravenous General Anesthetics
1) Propofol
2) Ketamine
3) Alfaxalone
4) Barbiturates
>pentobarbital (for euthanisa)
what is propofol? main use?
Sedative & anesthetic (not an analgesic)
Main use: induction
mechanism of action of propofol
GABA facilitation
* GABA is the major inhibitory NT in the
brain
* The GABA-receptor is a type of Cl- channel
that opens when GABA binds > the flow
of chloride across a membrane will
hyperpolarize it, inhibiting APs
(the same thing happens if potassium channels open)
* Propofol slows the dissociation of GABA from its receptor > more chloride flows into cell > longer inhibition of APs
propofol metabolism
- Recovery occurs due to redistribution from brain to muscle & adipose, and rapid hepatic metabolism
- Cats metabolize it more slowly (poor at glucuronide conjugation)
- Significant extrahepatic metabolism occurs (lung, kidney), making this a relatively safe choice for animals with hepatic or renal disease
Physiological effects of propofol on CNS
CNS:
* Dose-dependent sedation > anesthesia
Patient can be titrated to effect without risk of excitation
* Smooth induction
* Rapid, smooth recovery
Physiological effects of propofol on CVS
- Depression of myocardial contractility, peripheral vasodilation
> These combine to produce a dose-dependent depression of BP
> The degree of hypotension is worse with rapid injection
Caution in: - Hypotensive patients
effects of propofol on HR, contractility, CO, BO, RR
all down, bp down more
resp effects of propofol
- Inhibits respiration in a dose-dependent manner
Transient cyanosis may be observed (esp. tongue)
Usually not a problem in stable animals - If the label dose is administered as a bolus, apnea occurs for 30-60 s, then deep, regular breathing resumes > do not give full dose all at once
- Premedicate patient first so less is needed
effects of propofol on the blood
- Damages Hb
> precipitation within RBCs
> called Heinz bodies - Occurs esp. in cats, especially those receiving
the drug more than once over 2 or 3 days - A consideration if blood work is anticipated
contamination risk of propofol - why, and how to avoid?
All products contain carriers that facilitate bacterial growth (soybean oil, egg lecithin, etc.)
label recommends use for 28 days if aseptic technique used
* Ethanol can take 10 to 60 seconds to kill bacteria
* Stopper is often not adequately sterilized
-some products contain benzyl alcohol (may be toxic to cats)
adverse effects of propofol
- Pain at injection site, esp. if injected rapidly
>Worse in smaller veins - Toxicity if used repeatedly in cats over short term
o Cats are poor at glucuronide conjugation
-anesthetic induction with propofol alone in horses can cause excitement
adverse effects of propofol
- Pain at injection site, esp. if injected rapidly
>Worse in smaller veins - Toxicity if used repeatedly in cats over short term
o Cats are poor at glucuronide conjugation
-anesthetic induction with propofol alone in horses can cause excitement
anesthetic effects of ketamine
Produces a dream-like state of pseudo-unconsciousness known as dissociative anesthesia:
* Eyes open
* Swallow reflex intact
* Can hear normally
* Intense muscle rigidity
* Hallucinogenic
* “Disconnected” from surroundings & pain
mechanism of action of ketamine, and CNS effects. Also reason for some CVS effects.
NMDA receptor blockade (the major excitatory NT receptor in brain)
Also blocks several other receptor types, including opioid receptors and GABA receptors > CNS overstimulation, possibly in central SNS pathways > CVS effects
CNS effects:
* Unconsciousness
* May move in response to pain
* No post-op analgesia
* Hypothermia
CVS effects of ketamine, and other unwanted effects
Increased HR & BP due to SNS stim.
Other effects:
* Can precipitate seizures
* If used alone, excitement is common in dogs
> unacceptably rough recovery due to pain and disorientation (never use ketamine alone)
ketamine effect on HR, contractility, CO, BP, RR
HR: up up
contractility: NC or up
CO: NC or up
BP: NC or up
RR: down and cluster
clinical applications of ketamine in small animals
- Ketamine (IM) plus an opioid, alpha2 agonist, or benzodiazepine for heavy sedation or general anesthesia of cats (depending on dosage)
- Ketamine + diazepam (IV) for induction
- Add analgesic if required (e.g., painful surgery)
clinical applications of ketamine in large animals
- Ketamine + xylazine (with caution)
- Ketamine is often used for induction
alfaxalone: what is it, what is its mechanism of action and use
A neuroactive steroid
Mechanism of action:
Facilitates effects of GABA at receptor > inhibits APs
Main use is induction
alfaxalone CNS effects
- As with any general anesthetic, it inhibits APs in brain, not in peripheral pain pathways
- Dose-dependent sedation > anesthesia
- Efficacy is greater when the drug is injected more
slowly - Smooth induction
- Rapid recovery, but can be very rough if used alone
alfaxalone cvs effects:
- Similar to propofol
- Dose-dependent peripheral vasodilation and inhibition of HR, CO, & BP, though at typical dosages the CVS effects may be somewhat less pronounced than with propofol, so that BP may fall only slightly due to vasodilation with little or no change in HR or contractility
- Rapid injection tends to cause a greater drop in BP, which may cause a reflex increase in HR
difference in cot vs dog HR change with alfaxalone
DOG: NC or up
CAT: NC or down
resp effects of alfaxalone
- Dose-dependent inhibition (similar to propofol)
- If administered as a bolus, initial period of apnea
is longer
contamination risk of alfaxalone
As for propofol, alfaxalone contains nutrients
> supports bacterial growth
Re-use of single-use vials is not recommended by the manufacturer
Mechanism of action of barbiturates, and general effects
Inhibit dissociation of GABA from its receptor > inhibits APs
* Similar to benzodiazepines & propofol, but more profound effect
> more intense depression, slower metabolism, low therapeutic index > more dangerous
metabolism of barbiturates
Hepatic metabolism by P450 enzymes is slow
clinical use of barbiturates
1) Euthanasia
barbiturates used for euthanasia? caution about what?
Pentobarbital sodium
CAUTION:
Excitement (may be severe) if less than ~half the calculated dose is given
Convulsions if accidentally injected into carotid artery
protocol for euthanasia
- Sedate animal (unless already moribund)
- Place IV catheter
- Flush with saline
- Induce (e.g., propofol)
- Administer 1.5 x dose of pentobarbital
- Examine animal to verify death
- Give owner a moment alone with animal
T-61 use
- Only used for euthanasia
- Used by humane societies since no licence required
