Mod 1 - Inhalation Anesthetics 8/18

Question 1

what is an inhalant?

Answer 1

liquid that is reduced to vapor and picked up by fresh gas flow = anesthetic

Question 2

define vapor pressure

Answer 2

pressure exerted by a vapor at equilibrium with its liquid phase at a given temp in a closed system

Question 3

the higher the vapor pressure, the ?.

as temp inc., vapor pressure ?, and vice versa.

Answer 3

more volatile it is
(volatile = easily evaporated at normal temps)

inc.

Question 4

the more volatile a vapor is, the (more/less) controllable it is.

Answer 4

less

Question 5

volatile anesthetics have high vapor pressures. what are the vapor pressures of:
1. isoflurane
2. sevoflurane
3. desflurane

Answer 5

1. 250 mmHg
2. 160 mmHg
3. 664 mmHg

Question 6

why do desflurane vaporizers need to be heated (they are the only ones that do!)?

A. vapor pressure is so low that desflurane needs to be heated to inc. vapor pressure
B. heating inc. uptake into the body
C. vapor pressure is so high that the heat of vaporization actually cools down the liquid & makes desflurane delivery unpredictable
D. original desflurane vaporizers were designed to be used in Antarctica

Answer 6

C.

Question 7

a vaporizer regulates the amount of anesthetic gas put into the system. 1% of fresh gas flow bypasses the vaporizer while 2% of it enters the vaporizer.

if the vaporizer is for isoflurane, and the iso vapor pressure is 250 mmHg (at 20C), how much isoflurane is delivered to the patient? Recall atmospheric pressure is 760 mmHg.

Answer 7

1. 90%
2. 10%

(250 mmHg/760 mmHg) x 100 = 32.8% or 33%

33%/10% (of FGF entering vaporizer) = 3.3% iso delivered

Question 8

can sevoflurane be delivered in a vaporizer in circuit (VIC) configuration?

A. No! Always too dangerous.
B. Yes, if glass bottle vaporizer is used and the O2 flow is really low (100-200 mL/min)
C. Yes, if the patient is hyperventilating & won’t stay asleep
D. Yes, if the patient is <4m old

Answer 8

A. & B.

Question 9

what is MAC?

the lower the MAC, the ?.

Answer 9

minimum alveolar concentration of a vaporized anesthetic agent for which 50% of patients show no response to painful stimulus

more potent the anesthetic!

Question 10

which inhalant anesthetic is most potent?

A. isoflurane
B. sevoflurane
C. deseflurane

Answer 10

A.

Question 11

the rate of change of gas conc. within a breathing circuit can be predicted from the ?.

what is it?

Answer 11

time constant = circuit volume divided by the gas inflow rate
C = V/f

Question 12

one time constant = 1% of the desired conc. of inhalant anesthesia.

three time constants = 2%

four time constants = 3%

Answer 12

1. 63%
2. 95%
3. 98%

Question 13

increasing the flow rate (inc./dec.) the time constant, and vice versa.

Answer 13

dec.

Question 14

T/F - once the anesthesia circuit is equilibrated, the brain conc. of isoflurane reaches its peak.

Answer 14

False - doesn’t necessarily mean the brain has equilibrated too

Question 15

a small animal anesthesia machine = 5-8 L volume that must be filled with anesthetic gas.

how long would it take for the conc. of isoflurane to reach 95% at the end of the breathing circuit with a flow rate of 1 L/min, using a machine of 5 L?

Answer 15

5 L/1 L/min = 5 min (for one time constant)

5 x 3 = 15 min (for three time constants, or to reach 95% desired conc.)

Question 16

how can we reduce the time constant?

Answer 16

inc. flow rate

Question 17

the more soluble the drug is in the blood, the (faster/slower) the uptake is in the brain, and therefore, the (faster/slower) the removal, and vice versa.

Answer 17

slower
slower

Question 18

what are 2 factors that affect uptake and redistribution of anesthetic gas?

Answer 18

1. delivery to alveoli
   - inspired conc.
   - alveolar ventilation
2. removal from alveoli: uptake by blood
   - solubility
   - CO
   - alveolar to anesthetic partial pressure difference

Question 19

which inhalant would provide the fastest mask induction?

A. nitrous oxide
B. desflurane
C. sevoflurane
D. halothane

Answer 19

B.

Question 20

what is the main inhibitory NTM of the brain?

Answer 20

GABA

Question 21

binding of GABA to GABA-A Cl- channel causes what?

Answer 21

1. inc. Cl- ion conductance - inc. intracellular conc. of neg. ion
2. GABA (inhibitory) competes with glutamate (excitatory) to make it harder for excitation of neuron to occur
3. amnesia, unconsciousness

Question 22

isoflurane, desflurane, and sevoflurane will all cause mean arterial pressure (MAP) to (inc./dec.) because ?. This, in turn, will cause HR to (inc./dec.) (when using iso or des).

Answer 22

dec.
causes relative hypovolemia due to vasodilation of vessels (larger container with same amount of blood)
inc.

Question 23

explain the 4 steps of how the respiratory system is affected by anesthetic inhalants.

Answer 23

1. dec. tidal volume
2. compensatory inc. in resp. rate
3. BUT dec. minute ventilation
4. THEREFORE inc. PaCO2

inc. MAC = dec. TV = inc. PaCO2

Question 24

which inhalant would be safest to use in a patient with liver disease?

A. halothane
B. sevoflurane
C. isoflurane
D. desflurane

Answer 24

C. & D.
- both have very little metabolized in the liver