Volatiles

Question 1

Factors affecting Fi

Answer 1

1) Fresh gas flow –> higher flow = higher FiO2
2) Volume of system –> lower volume = actual FiO2
3) Absorption of circuit –> lower absorption = actual FiO2

Question 2

Factors affecting FA

Answer 2

1) Uptake of gases
2) Alveolar Ventilation
3) Concentration of gas

Question 3

Gas uptake

Answer 3

greater the uptake = slower rise in FA/Fi ratio -> slower induction

Question 4

Factors affecting uptake

Answer 4

1) Blood solubility
2) Alveolar blood flow
3) Partial pressure difference

Question 5

Blood solubility of gas

Answer 5

affinity of a gas to be taken up in the blood
- more soluble gas => greater uptake => slower induction

Blood Gas Coefficient = high value means more soluble

Question 6

Alveolar Blood Flow

Answer 6

should equal cardiac output in absence of shunting

- as CO increases = more anesthetic uptake = slower induction

Question 7

Partial pressure difference

Answer 7

depends on difference between gas and venous blood

• vessel rich organs reach steady state first
• fat group has a tremendous solubility

Question 8

Ventilation

Answer 8

alveolar ventilation is the most important factors affecting FA
- constantly replacing anesthetic gas with more gas due to high ventilation will result in a higher FA => faster induction and faster equilibrium

Question 9

Gas Concentration

Answer 9

increasing the concentration of inspired gas => quicker rise in alveolar concentration = faster induction

Question 10

Second Gas Effect

Answer 10

nitrous is more soluble in the blood than other gases, if it is used at same time as other gases, those gases will be relatively more insoluble and reach FA/Fi quicker => faster induction

Question 11

Factors affecting Fa

Answer 11

V/Q mismatch => increased difference between alveolar/arterial

Question 12

Factors affecting Elimination

Answer 12

1) Exhalation - most important
2) High gas flow
3) Low anesthetic volume
4) High cerebral blood flow
5) Increased ventilation

Question 13

Volatile Pharmacodynamics

Answer 13

no single MOA identified - appears to be related to GABA function

Question 14

Minimum Alveolar Concentration

Answer 14

MAC is the alveolar [ ] that prevents movement in 50% of patients when surgical stimulus applied
- additive among agents

*lower the value, the more potent the volatile

Question 15

MAC of anesthetic gases

Answer 15

Nitrous = 105
Halothane = 0.75
Isoflurane = 1.2
Desflurane = 6.0
Sevoflurane = 2.0

Question 16

Factors that increase MAC

Answer 16

Young Age
Chronic EtOH
Hypernatremia
Hyperthermia
MAOI, amphetamines, cocaine

Question 17

Factors that decrease MAC

Answer 17

Old Age 
Acute EtOH
Hypothermia
Anemia
Hyponatremia
Hypercarbia
Hypoxia
Pregnancy

Question 18

General characteristics of volatiles

Answer 18

CV: decrease SVR
Resp: increase RR but decrease TV, blunt response to hypoxia and hypercarbia
CNS: increase CBF, ICP and CMRO2
NMJ: malignant hyperthermia
Renal: decrease RBF and GFR

Question 19

Nitrous Oxide

Answer 19

colorless and odorless
*NMDA receptor antagonist
- gas at room temperature, liquid under pressure
CV: stimulates sympathetics
Contraindication: air containing cavities => expanding space from being more soluble

Inhibits Vit B12 synthesis - aplastic anemia

Question 20

Halothane

Answer 20

halogenated alkane
CV: dose dependent reduction in MAP from myocardial suppression
Contraindications: don’t use with intracranial lesions due to increasing CBF and ICP
*Liver Failure = centrilobular necrosis

Question 21

Isoflurane

Answer 21

nonflammable and pungent
CV: preserves the baroreceptors, reflexes still in tact, dilation of coronaries (STEAL)
*Metabolized to trifluoroacetate -> fluoride
*Contraindications: just MH

Question 22

Desflurane

Answer 22

substitution of fluorine makes it different -> RAPID induction and emergence
CV: rapid increase in DES = stimulates catecholamines
Resp: pungent - laryngo/bronchospasm
*minimal transformation in humans, desflurane is degraded by dessication in the CO2 absorbent

Question 23

Sevoflurane

Answer 23

halogenated fluorine, non-pungent and non-flammable
- rapid increase in alveolar [ ] = rapid induction
- blood-fat solubility is close to iso (long case = long time to come off)
CV: reduction in SVR, possible QTc prolongation
Biotransformation by CYP450