Pharmacokinetics of Inhalation Anesthetics

Question 1

What are key pharmokinetic properties of Inhalation GAs?

Answer 1

Partial Pressure (tension)

Oil/gas Partition Coefficient (lipid solubility)

Minimal Alveolar Concentration (MAC)

Blood/gas Partition Coefficient

induction

Question 2

What is Partial Pressure (tension)

Answer 2

Key factor that determines the CONCENTRATION of a volatile general anesthetic in alveolus, blood and tissues (including the BRAIN).

Question 3

What is the oil gas partition coefficient (lipid solubility)?

Answer 3

LIPID SOLUBILITY and POTENCY

Higher the oil:gas co-efficient (lipid solubility) of an anesthetic –the more POTENT

Question 4

What is the minimal alveolar concentration (MAC)?

Answer 4

Alveolar concentration of an anesthetic at 1 atm that prevents movement in 50% of patients in response to a noxious stimulus (e.g., surgical incision)

MAC of a volatile GA is INVERSELY related to its LIPID SOLUBILITY (oil:gas coefficient)(Mac = 1.3/λ

MAC is higher in infants than elderly

Question 5

What is the blood/gas partition coefficient?

Answer 5

Determines the rate of induction and recovery of Inhalational anesthetics
Solubility of GA in blood vs. air = #molecules in blood/#molecules in gas at STEADY STATE

Lower the blood:gas coefficient – FASTER the induction and recovery

Higher the blood:gas coefficient – SLOWER induction and recovery

Question 6

How is CNS partial pressure maintained?

Answer 6

regulating the INSPIRED PARTIAL PRESSURE of GA.expressed in Torr

Question 7

What is the MAC response and Alevelor concentration will respond for dose 0.5*MAC?

Answer 7

Response- MAC AWAKE

Alveolar concentration at which - 50%of patients can be awakened

Question 8

What is the MAC response and Alevelor concentration will respond for dose 1*MAC?

Answer 8

Response- MAC

Alveolar concentration at which - 50%will not move at surgical incision

Question 9

What is the MAC response and Alevelor concentration will respond for dose 1.3*MAC?

Answer 9

Response- ED95

Alveolar concentration at which - 95% will not move at surgical incision

Question 10

What is the MAC response and Alevelor concentration will respond for dose 1.5-2.0*MAC?

Answer 10

Response - MAC-Bar

Alveolar concentration at which - 50% have blocked autonomic responses

Question 11

What are other factors that influence induction?

Answer 11

Inspired anesthetic concentration
Pulmonary ventilation
Pulmonary and cerebral blood flow
Arteriovenous concentration gradients

Question 12

What affect does increasing the inspired anesthetic concentration?

Answer 12

Rate of Induction by inhalation GAs can be increased by increasing the INSPIRED CONCENTRATION

Question 13

Wha is the affect of increasing the pulmonary Ventilation?

Answer 13

Blood levels of GAs also determined by minute ventilation of patient
↑ VENTILATION = ↑ INDUCTION

Question 14

What is the affect of increasing the Pulmonary blood flow?

Answer 14

↑ BLOOD FLOW = ↓ Aterial Anesthetic gas tension

GA has a greater volume of blood to saturate — partial pressure will increase more slowly

Induction will be more rapid in patients with LOW CARDIAC OUPUT

Question 15

What are arteriovenous concentration gradients?

Answer 15

Blood levels of GAs are also determined
by differences in the perfusion of tissues.
Highly perfused tissues have greatest effect (75% of cardiac output) on arteriovenous concentration.

Question 16

What is the uptake of a GA into tissues based on?

Answer 16

based on their PERFUSION and SOLUBILITY

Question 17

Which tissues are the first to encounter inhaled GAs?

Answer 17

HIGHLY PERFUSED TISSUES — vessel rich group (VRG) (brain, heart, liver, kidney and endocrine organs) are the
FIRST to encounter inhaled GAs > muscle group > fat
group.
–
VRG has moderate
Solubility and small
volume — Limits GA capacity