Inhaled Agents

Question 1

How can inhaled agents cause distress?

Answer 1

Properties of the agent (pungency, hypoxia, etc.) and conditions by which agent is administered (home or new cage)

Question 2

What are disadvantages of using SNS and HPA axis activation as indicators of stress?

Answer 2

Activated in response to both physical and psychological stressors and not necessarily associated with higher-order CNS processing and conscious experience. Can see activation with inhaled agents during the period between loss of consciousness and death, which doesn’t reflect emotional state experienced by the animal.

Question 3

Are inhaled agents aversive?

Answer 3

Yes, all have been identified as being aversive to some degree.

Question 4

What is time to unconsciousness with inhaled agents dependent on?

Answer 4

Displacement rate, container volume, and concentration

Question 5

Define the administration of direct application of products of combustion of sublimation.

Answer 5

Not acceptable due to unreliable or undesirable composition and/or displacement rate.

Question 6

Why might induction be delayed in sick or depressed animals to excited animals?

Answer 6

Due to the rise in alveolar gas concentration being delayed due to decreased ventilation or increased cardiac output.

Question 7

Which types of animals have a greater capacity for holding their breath and for anaerobic metabolism?

Answer 7

Reptiles, amphibians, and diving birds and mammals.

Question 8

What is the concern with rapid gas flows?

Answer 8

Can produce noise or cold drafts, leading to animal fright and escape behavior.

Question 9

What are the two physical processes that impact the change in gas concentration in an enclosed space?

Answer 9

1. Sash-in of new gas
2. Time constant required for that change to occur within the container for a known flow rate

Question 10

What type of form describes the rate of change of gas concentration within any enclosed space?

Answer 10

Exponential, whereas the quantity under consideration rises toward a limiting value at a rate that progressively decreases in proportion to the distance it still has to rise.

Question 11

What is the equation for the time constant? What is 1, 2, and 3 time constants equal to?

Answer 11

T = volume/flow rate.
1 time constant = Concentration of inflowing gas to rise to 63.2% of inflowing gas concentration, or for remaining gas concentration to fall to 36.8% of initial volume.
2 = Rise to 86.5%, or fall to 13.5%
3 = Rise to 95%, or fall to 5%
Infinity = Gas concentration in container to equal inflowing gas concentration, or remaining gas to fall to 0%

Question 12

What is unique regarding commercially supplied CO2 compared to N, Ar, and CO?

Answer 12

CO2 is supplied as a liquified gas under high pressure

Question 13

Describe euthanasia using halothane, enflurane, isoflurane, sevoflurane, desflurane, diethyl ether, and methoxyflurane.

Answer 13

Halothane - Induces anesthesia rapidly, effective for euth
Enflurane - Less soluble in blood than halothane, but because of its lower vapor pressure and lower potency, induction rates may be similar to halothane. Convulsions at deep anesthetic plane.
Isoflurane - Less soluble than halothane and induces anesthesia more rapidly. Pungent odor, so onset of unconsciousness may be delayed due to breath holding. Less potent than halothane, and thus requires more drug.
Sevoflurane - Less potent than Iso or Evo and has a lower vapor pressure. Can induce rapid anesthesia, but more drug needed. Less objectionable odor than Iso, some species still struggle. Also induces epileptiform electrocortical activity
Desflurane - Least soluble potent inhaled anesthetic, but vapor is pungent which may slow induction. So volatile it could displace O2 and induce hypoxemia during induction if supplemental O2 not provided.
Diethyl ether and methoxyflurane - Highly soluble, may be accompanied by agitation due to anesthetic induction being slow. DE irritating to eyes, noses, and resp airways, possesses risks due to flammability and explosiveness, and has been used to create a model for stress.

Question 14

Which inhaled agents are least aversive for mice and rats? Do rodents develop learned aversion?

Answer 14

Mice: Halothane and enflurane
Rats: Halothane
Yes, rodents are more likely to leave a test chamber on subsequent exposures to inhaled anesthetics.

Question 15

Which agent is the least potent of anesthetic agents? What is the median effective dose, and what impact does this have?

Answer 15

Nitrous oxide. Median effective dose in animals is 200%, thus it can bot be used alone at 1 atmosphere of pressure without producing hypoxia prior to resp or cardiac arrest.

Question 16

How can nitrous oxide be used in euthanasia?

Answer 16

Added to inhaled agents to reduce the time to loss of consciousness.

Question 17

What are the occupational limits for inhaled agents?

Answer 17

NIOSH Inhaled agents: Not to exceed 2ppm (1-hour ceiling) when used alone or 0.5ppm when combined with 25ppm N2O
8hr Time-Weighted average: 50ppm for N20, 50ppm for halothane, 75ppm for enflurane

Question 18

How were the safety standards for NO2, halothane, and enflurane established?

Answer 18

These concentrations were found to be attainable utilizing clinical scavenging techniques. No controlled studies proving exposure at these concentrations are safe

Question 19

What are the NIOSH standards for isoflurane, sevoflurane, and desflurane?

Answer 19

No established limits

Question 20

Inhaled anesthetics are particularly useful for euthanasia of what size of animals?

Answer 20

Smaller animals (<7 kg)

Question 21

Inhaled anesthetics are aversive for what species?

Answer 21

Rabbits and laboratory rodents.

Question 22

What inhaled anesthetic is irritating, flammable, and explosive?

Answer 22

Ether

Question 23

Induction with what inhaled anesthetic agent can be unacceptably slow?

Answer 23

Methoxyflurane

Question 24

What are disadvantages of using nitrous oxide alone for euthanasia?

Answer 24

Creates a hypoxic atmosphere and supports combustion at high concentrations

Question 25

Use of inhaled anesthetics is unacceptable for what group of animals?

Answer 25

Food-producing animals

Question 26

At which concentrations in CO explosive?

Answer 26

>12%

Question 27

How does CO cause death?

Answer 27

Produces hypoxemia - Readily combines with hemoglobin and blocks uptake of O2 by erythrocytes by forming carboxyhemoglobin.

Question 28

In the past, mass euthanasia was accomplished using what three methods for generating CO? What were the problems associated with these methods?

Answer 28

1. Chemical interaction of sodium formate and sulfuric acid 2. Exhaust fumes from gasoline internal combustion engines 3. Commcerically compressed CO in cylinders First two techniques associated with production of other gases, inadequate production, inadequate gas cooling, inability to quantify delivery rate, and maintenance of equipment

Question 29

How does the aversion of CO compare to CO2?

Answer 29

Averse in rats, but less so than CO2

Question 30

What are the advantages of using CO for euthanasia?

Answer 30

1. Induces loss of consciousness without pain and minimal discomfort, depending on species 2. Hypoxemia is insidious 3. Death occurs rapidly if 4-6% concentrations are used

Question 31

What are the disadvantages of CO euthanasia?

Answer 31

1. Aversive for rodents and may be the same for other species 2. Safeguards to monitor exposure of personnel 3. Exposed electrical equipment must be spark free and explosion proof

Question 32

How do nitrogen and argon cause death?

Answer 32

Displace air (O2) causing anoxia.

Question 33

Do rats prefer CO2 or Ar? How do they respond to increasing concentrations of Ar?

Answer 33

Ar, but are sensitive to small changes in the concentration of O2 and will stay only a few seconds in either gas. When exposed to gradually increasing concentrations, they always left the chamber before losing consciousness.

Question 34

What species demonstrate little to no aversion caused be hypoxia due to inert gases?

Answer 34

Turkeys and chickens

Question 35

What are the advantages of Ar or N2 use?

Answer 35

1. Not aversive to chickens or turkeys and resulting hypoxia appears to be non- or only mildly aversive. Similarly do not appear to be directly aversive to pigs and reduce behavioral responses to hypoxia. 2. Nonflammable, nonexplosive, and readily available as compressed gases 3. Hazards to personnel are minimal when used with properly designed equipment 4. Argon and N2-CO2 mixtures are heavier than air and can be contained within an apparatus into which animals and birds can be lowered or immersed

Question 36

What are the disadvantages of N2 and Ar?

Answer 36

1. Hypoxia is aversive to rats, mice, and mink 2. Gradual displacement may result in exposure to hypoxic conditions prior to loss of consciousness. May be preceded by open-mouth breathing and hypercapnea, which may be distressing for nonavian species. 3. Reestablishing low concentration of O2 (>6%) will allow immediate recovery 4. Exposure times >7min needed for pigs 5. As for CO2, rats demonstrate alveolar hemorrhage consistent with terminal asphyxiation 6. Ar costs 3x more than N2 7. Gases tend to cause more convulsive wing flapping in poultry than CO2 in air mixtures

Question 37

What does inhalation of CO2 cause?

Answer 37

Respiratory acidosis and a reversible anesthetic state by rapidly decreasing intracellular pH.

Question 38

What CO2 concentrations result in an increased pain threshold and anesthesia?

Answer 38

Pain threshold: 7.5% Deep anesthesia and death with prolonged exposure: 30%

Question 39

What are the three mechanisms by which CO2 has the potential to cause distress?

Answer 39

1. Pain due to formation of carbonic acid on respiratory and ocular membranes 2. Production of air-hunger and feeling of breathlessness 3. Direct stimulation of ion channels within the amygdala associated with the fear response

Question 40

At what CO2 concentrations do nociceptors respond to CO2? At what point do humans report discomfort?

Answer 40

Nociceptors: 40% Discomfort: 30-50%, intensifies to overt pain at higher concentrations

Question 41

Why might inhaled irritants induce reflex apnea and heart rate reduction?

Answer 41

Reduce transfer of harmful substances into the body.

Question 42

Why is gradual displacement of CO2 preferred for euthanasia?

Answer 42

Less likely to produce pain prior to unconsciousness where unconsciousness occurs before chamber concentration reaches levels associated with nociceptor activation. Bradycardia is reported to occur in rats prior to loss of consicousness.

Question 43

At what concentrations does CO2 cause air hunger? Does supplemental O2 help?

Answer 43

As low as 8%, severe at 15%. Adding O2 may or may not help, but will prolong time to death and may delay onset of unconsciousness.

Question 44

What species demonstrate less/no aversion to CO2?

Answer 44

Turkeys and chickens, up to 60%. Diary goat kids, up to 10-30%. Certain genetic lines of pigs (Yorkshire or Dutch Landrace).

Question 45

Why is exposure to CO2 innately fear-inducing in some species?

Answer 45

Presence of ASICs sensitive to elevated CO2.

Question 46

Generally, how do rabbits respond to CO2 exposure for euthanasia?

Answer 46

Requires prolonged exposure times.

Question 47

How does learned aversion in lab rodents differ between CO2 and inhaled anesthetics?

Answer 47

Always develop learned aversion to inhaled anesthetics, but increased aversion to CO2 does not increase with subsequent exposure.

Question 48

Why might CO2 not be effective for rodents and amphibians?

Answer 48

They may breathe too slowly for its use.

Question 49

At what concentration can CO2 euthanasia result in postmortem pulmonary and URT lesions?

Answer 49

When induction of loss of consciousness occurs at concentrations <80%.

Question 50

What must occur if animals are anesthetized with inhaled agents prior to completing the euthanasia process with CO2?

Answer 50

Sufficient time should be allowed to prevent rapid recovery during the wash-in of CO2 and wash-out of inhaled agent.