Shared Flashcard Set

Details

L6: Inhalation anaesthesia
Advantages and disadvantages, uptake, potency, elimination, desirable properties, volatile agents
30
Veterinary Medicine
05/23/2012

Additional Veterinary Medicine Flashcards

 


 

Cards

Term
What are volatile anaesthetics commonly used for?
Definition
-maintentance of anaesthesia
-less commonly for induction
Term
Advantages of volatile anaesthetics
Definition
-more economical than injectables, esp for long procedures
-only relies on lung function for elimination, no other organs, usually rapid
-rapid changes to depth of anaesthesia
- usually administered w. a high percentage of inspired oxygen
-airway protected against aspiration / FB
Term
Disadvantages of volatile anaesthetics
Definition
-expensive equipment required
-slow induction and may be rejected on induction i.e. breath-holding, struggling, more obvious stage I and II of anaesthesia
-knowledge of equiment essential
-hazards associated w. compressed gas
-environment pollution w. global and personal contacts
-often difficult to transport for 'field' anaesthetics
Term
Commonly used volatile agents
Definition
Isoflurane
Sevoflurane
Term
Gaseous anaesthetics in use
Definition
-only nitrous oxide (N2O) is in common use, not used at all by many smaller practices
Term
Uptake of volatile anaesthetics
Definition
Agents move along the partial pressure gradient:
breathing system --> lungs --> alveolus -->arterial blood --> brain tissue (target)
Term
Why is speed of induction slower for volatile anaes. c.f. injectable?
Definition
-a couple of conc. gradients to cross
-the alveolar conc. is esp important
Term
Factors influencing the rise of alveolar partial pressure of volatile agents
Definition
-inspired concentration
-alveolar ventilation
-blood-gas partition coefficient
-cardiac output / pulmonary blood flow
-ventilation perfusion relationships
Term
Does a soluble or insoluble volatile agent reach useful partial pressure more quickly?
Definition
an INsoluble volatile agent reaches useful partial pressure more quickly than a soluble agent
Term
How does inspired concentration of a volatile agent influence the rise of alveolar partial pressure?
Definition
higher conc. will incr. uptake (provided it does not result in breath holding)
Term
How does nitrous oxide effect the rise of alveolar partial pressure when administered w. a volatile agent?
Definition
-nitrous oxide w. a volatile agent = second gas effect
-causes an incr. in concentration of the volatile and so faster uptake
-due to rapid uptake of nitrous oxide to the blood
Term
How does alveolar ventilation influence the rise of alveolar partial pressure of a volatile agent?
Definition
if ventilation incr., the alveolar partial pressure of the volatile agent will incr. and so the rate of uptake incr.
Term
How does the blood: gas partition coeffecient influence the rise of alveolar partial pressure of a volatile agent?
Definition
-if an agent is very soluble, large amounts will cross into the blood and so decr. the rate of rise of alveloar partial pressure - so induction will be slow
i.e.decr. solubility of a volatile agent will decr. the rate of induction
Term
How does CO/pulmonary BF influence the rise of alveolar partial pressure of a volatile agent?
Definition
-the higher the BF through the pul. circulation, the more rapidly the agent will be removed from the lungs and so the slower the rise in alveolar partial pressure will be
i.e. incr. pul. BF and dec. rate of induction
Term
How do ventilation perfusion relationshipe influence the rise of alveolar partial pressure of a volatile agent?
Definition
if there are areas of the lungs which are well ventilated but poorly perfused and areas that are well-perfused but poorly ventilated, the uptake of volatile agents will be delayed
Term
From the blood, where are volatile agents taken up?
Definition
-first taken up into vessel rich tissues
e.g. brain, heart, liver, kidney
-then muscle
-then fat and poorly perfused tissues
Term
Why is fat significant in induction and recovery from volatile anaethetics?
Definition
-due to volatile anaesthetics fat solubility
-the fat acts as a "sponge" and soaks up the anaesthetic agent - so the agent can not reach the brain and so induction slows
-on recovery the release from fat is slow so slows elimination and recovery
Term
What causes anaesthetic agents to equilibriate faster with a tissue?
Definition
-if the blood supply to the tissue is good
-if the agent is relatively INsoluble in that tissue - this means fewer molecules have to enter the tissue to saturate it
Term
Minimum Alveolar Concentration (MAC)
Definition
-the concentration of anaesthetic which will prevent purposeful mvmt in response to stimulus in 50% of patients
-used as a tool for comparison between agents
Term
Why can MAC never be an absolute guide to how much anaesthetic an individual needs?
Definition
-there is considerable individual variation - MAC prevents response in only 50% of patients!
-different factors incr and decr MAC
Term
What factors will cause an increase in MAC?
Definition
-hyperthermia
-drugs e.g. sympathomimetics
Term
What factors will cause a decrease in MAC?
Definition
-hypothermia
-pregnancy
-old age (even if healthy)
-drugs, esp. inj. anaesthetics e.g. opiods, sedatives - contribules to balanced anaesthesia
Term
Elimination of volatile agents
Definition
-opposite of uptake
-when there is no inspired concentration, the blood:alveolar conc. grad is reversed, so agent diffuses into lungs and is exhaled
-volatile agents are NOT dependent on metabolism for elimination (although variable proportions of different agents is metabolism)
Term
Isoflurane and sevoflurane % metabolised in liver
Definition
Isoflurane ~ 0.2%
Sevoflurane ~ 3%
Term
What are the desirable properties of inhalation anaesthetics?
Definition
NB: no agents fulfil all these critea, atm sevoflurane is closest but v. expensive
-easily vaporised in useful conc.
-non-irritant to mucous membranes
-little effect on CVS and resp syst.
-non-toxic i.e. minimal side effects
-low blood:gas solubility so rapid induction
-good muscle relaxation
-good analgesia (doesn't occur atm)
-compatible w. soda lime
-stable on storage
-non-flammable and non-explosive
Term
Describe isoflurane
Definition
-volatile agent used to induce anaesthesia
-easily vaporised
-non flammable, preservatives not required
-low blood:gas solubility so rapid uptake and elimination
-sl. irritant to MM
-aversive smell
-fair muscle relaxation
-ossibly analgesia - can't rely on, must give another agent
-resp. depression more marked than halothane
-less myocardial depression than halothane but more vasodilation
-licensed for D+C and H, some preparations for small animals
Term
Describe sevoflurane
Definition
-of all inhalation anaesthetics, the closest to fitting all desirable critea but atm very expensive
-non flammable, preservatiofves not required
-very low blood:gas solubility so very rapid uptake and elimination and quite smooth c.f. isoflurane
-non irritant to MM
-'pleasant' smell
-possibly muscle relaxation
-possibly analgesia p- can't rely on, must give another agent
-less resp. depression than isoflurane
-little CV depression
-licensed for dogs only, but is used in other spp.
Term
Describe nitrous oxide
Definition
-gas compressed as liquid, but don't need vaporiser
- MAC>100% so can not use alone
- used to smooth anaesthesia - dec. the incidence of sudden changes in depth
- provides analgesia
-decr. inspired % O2 and dec. conc. needed for volatile agents
-rel. insoluble so rapid uptake and elimination
-may improve speed and quality of recovery
-care in animals w. pneuothorax / gas in thorax/abdomen -tends to diffuse into gas filled space
Term
What is the second gas effect?
Definition
-admin of N2O and volatile acid
-nitrous oxide moves rapidly into blood which effectively decr. the volume of gas in the lungs and so incr. the conc. of other substances i.e. the volatile agents
-so incr. volatile agent conc. grad between alveoli and blood and so faster induction
Term
What is diffusion hypoxia?
Definition
-the reverse of second gas effect at the end of anaesthesia, involving N2O
-as N20 diffuses rapidly into the alveolar space from the blood there is a decr. % O2 in the alveoli
-ok if breathing a 100% oxygen but if breathing 21% O2 i.e. room air, the drop in O2 conc. could result in a hypoxic drug mixture
-hence admin 100% O2 for min. 5mins after discontinuing N2O admin