Inhalant anesthetics Flashcards
use of nitrous oxide as anesthetic?
- Patients remain conscious (not a true general anesthetic) but alleviates anxiety and is an excellent analgesic
- Very safe; still used to alleviate anxiety & pain
drawbacks of early inhalant general anesthetics
- Explosive when administered with oxygen
- Produce toxic metabolites
ether plus oxygen creates what? how can we solve this problem?
-Ether is flammable, and can be explosive when mixed with O2
-Substituting halogens for hydrogens reduces the flammability of a hydrocarbon
To be both safe and effective, general anesthetics must:
inhibit cerebrocortical activity while maintaining brainstem function (cardiovascular & respiratory system regulation)
All areas of the brain are reversibly inhibited by general anesthetics in a dose-dependent manner
The cerebral cortex is most sensitive, and its function is virtually abolished at concentrations that inhibit brainstem function only partially
The safety of general anesthetics is therefore dependent upon the extent to which cardiovascular & respiratory functions are impaired at concentrations required to maintain general anesthesia
The other major safety issue relates to the toxicity of the inhalant’s metabolites
-In general, hepatic and renal toxicity are directly related to the extent to which an inhalant is metabolized
In general, hepatic and renal toxicity are directly related to: (regarding inhalant anesthetics)
the extent to which an inhalant is metabolized
difference in metabolism between halothane and isofluorane, and consequences?
Halothane: up to 40% metabolized > significant hepatotoxicity
Isofluorane: <0.2% metabolized > no organ toxicity
issues with halothane? how did this inform us about inhalant safety?
Halothane sensitizes the myocardium to epinephrine, promoting arrhythmias
Early testing of various halogenated hydrocarbons revealed that those with an ether linkage do not have this property
> the ether linkage C-O-C is therefore desirable for safety
The main inhalants used today in veterinary medicine are:
– Isoflurane
– Sevoflurane
Characteristics of an ideal inhalant anesthetic: (7)
- Produces unconsciousness while maintaining
brainstem function (respiration, circulation) - Negligible visceral toxicity
(i.e. no metabolism > 100% of drug exhaled intact) - Non-flammable
- Non-pungent odour
- Compatible with anesthetic machine materials (does not degrade rubber or metal parts or react with soda lime)
- Chemically stable without preservatives
- Potent
mechanism of action of inhalant anesthetics?
Uncertain:
“Volatile general anesthetics inhibit neurotransmitter release by a mechanism not fully understood.”
Facilitation of GABA is one known effect and
possibly the main mechanism
Pharmacokinetics of inhalant anesthetic
Anesthetic vapour delivered w/ oxygen
> absorbed from lungs into blood
> distributed to the tissues
Constant delivery of fresh gas with controlled % of anesthetic maintains anesthesia (like a constant rate IV infusion)
Eliminated mainly by exhalation; some metabolism
the depth of anesthesia is proportional to what property created by the anesthetic?
The depth of anesthesia is not, however, proportional to the concentration of the anesthetic in the brain, it is proportional to the partial pressure created by the anesthetic
ANESTHETICS THAT ARE HIGHLY SOLUBLE IN TISSUES PRODUCE a partial pressure that is higher or lower? why?
Anesthetics that are highly soluble in tissues interact in some manner with the tissue (e.g., form reversible bonds with molecular constituents of the tissue) and therefore move less rapidly, producing a lower partial pressure compared to anesthetics that are relatively insoluble in the tissue and do not interact as much with its molecules
rate of onset and recovery for an anesthetic depends on _____
The rate of onset and recovery also depends on partial pressure
If a conscious patient inhales an anesthetic, how will the tissue solubility affect how fast they are rendered unconscious?
If a conscious patient inhales an anesthetic, they will be rendered unconscious quickly if it is a drug with low tissue solubility because a small quantity of that drug will be enough to produce the necessary partial pressure.
-also will recover quickly when drug is removed
In comparison, if the patient were to inhale an anesthetic that is highly soluble in tissues, it would take longer to become unconscious because much more drug would have to be absorbed before it created a sufficient partial pressure.
