Inhalation anesthesia Flashcards
1
Q
What is an inhalation anesthetic?
A
- Chemical compound that enters the body through the lungs and is carried by the blood to tissues (e.g. brain) to induce general anesthesia
- Provides unconsciousness, amnesia, antinociception, muscle relaxation, immobility
- Nitrous Oxide additionally provides analgesia (antagonism of the NMDA receptor)
2
Q
What are Volatile anesthetics?
A
- Isoflurane, Sevoflurane, Desflurane
- Liquids at room temperature
- Require the use of a special device (vaporizer to efficiently & precisely deliver anesthetic vapor to the patient
- Provide rapid induction to unconsciousness
- Allow rapid & precise changes in anesthetic depth
- Rapid recovery
- Minimal hepatic metabolism/renal clearance
- Short-lived residual effects
3
Q
What is general anesthesia?
A
controlled but reversible CNS depression
4
Q
What are the chemical characteristics of anesthetics?
A
- Mostly organic compounds
- Nitrous oxide inorganic compound
- Classification:
- Straight or branched hydrocarbons (e.g. halothane)
- Ethers (general ROR structurs)
- Halogenated
- Additional chlorine or bromine increases potency
- Addition of fluorine increases stability
5
Q
What is the difference between a Gas and a Vapor?
A
- Gas: substance that exists as a gas at ambient temperature and pressure (20C, 760 mmHg)
- Oxygen, Nitrous Oxide
- Anesthetic Vapor: gaseous phase of a liquid anesthetic with sufficient partial pressure at room temperature to produce general anesthesia
- Isoflurane, Sevoflurane
6
Q
What is Vapor Pressure?
A
- Molecules of inhalant move & collide with each other and the walls of the closed container creating pressure (force per unit area)
- Vapor Pressure = pressure exerted by vapor molecules when the liquid & vapor phases are in equilibrium
- measure the ability of an inhalant anesthetic to vaporize
*
- measure the ability of an inhalant anesthetic to vaporize
7
Q
What is vaporization
A
molecules in the liquid phase gain sufficient velocity to overcome attractive forces of neighboring molecules and enter the vapor phase
8
Q
What is Saturated Vapor pressure?
A
- The maximum concentration of molecules in the vapor state that can exist for a given inhalant anesthetic at a given temperature
- Depended only on temperature
- As temperature increases, number of molecules in the vapor phase increases
9
Q
Example: What is the saturated vapor concentration (in volume percent) of isoflurane at ambient conditions (20C, 760 mmHg)
A
- Determined by relating vapor pressure to the ambient pressure
- Vapor pressure of isoflurane:
- 240 mmHg at 20C
- (240 mmHg/760mmHg) x 100 = 32%
- Vapor pressure of isoflurane:
- A concentration of 32% of isoflurane vapor is possible under ambient conditions
- Isoflurane vaporizer dial setings range from 1 - 5 %, commonly set to 1.5 - 2.0% to maintain general anesthesia
10
Q
What is the function of a vaporizer?
A
- A vaporizer is required to dilute out the vapor generated from the liquid anesthetic to a concentration appropriate for anesthetic management of a patient
11
Q
What is the difference between Concentration and Partial Pressure?
A
- Concentration (volume %)
- relative ratio of gas molecules in a mixture
- clinically used when describing DOSE of inhalant
- Partial Pressure (mmHg)
- Absolute value
- individual pressure of each gas in a mixture of gases
- Air: 21% O2 78% Nitrogen
- Partial pressure of Oxygen: (0.21 x 760mmHg) = 159.6 mmHg
- Partial pressure of nitrogen (0.78 x 760 mmHg) = 592.8 mmHg
- Oxygen + Nitrogen = 752 mmHg
12
Q
How do inhalation anesthetics flow through the system/body
A
- Move down a series of partial pressure gradients
- Driving force - partial pressure differences between tissues
- Equilibrium reached when
- PA = Partery = Pbrain
- concentrations vary between tissue because of differences in solubility
13
Q
What is required to achieve general anesthesia?
A
- Adequate partial pressure of anesthetic in the brain to cause a desired level of CNS depression
- Inhalant anesthetics diffuse across a lipid membrane based on differences in partial pressures between tissues (NOT concentrations
14
Q
What is solubility of inhalant anesthesia?
A
- Major characteristic in the rate of uptake and distribution of an inhalant agent within the body
- Solubility in blood (& other body tissues) determines the quantity of anesthetic removed from blood
- Greater solubility in a tissue
- Longer amount of time to saturate tissue
- Longer induction/change in anesthetic depth/recovery
- Primary determinant of the speed of anesthetic induction, recovery, and change in anesthetic depth
15
Q
What is the partition Coefficient (PC)
A
- Measure of the solubility of an anesthetic inhalant in various tissues
*
16
Q
What are the pharmacokinetics of Inhalation Anesthetics?
A
- Inhalation anesthetics move down a series of partial pressure gradients until equilibrium is reached at the target tissues
- Alveolar partial pressure (PA) rapidly equilibrates with highly vascular tissues, known as the Vessel Rich Group (VRG)
- Anesthetic depth varies directly with partial pressure of anesthetic within the brain
- Changing the partial pressure of anesthetic within the alveoli ⇢ changes partial pressure of anesthetic within the brain
22
Q
What is the Minimum Alveolar Concentration?
A
- Minimum Alveolar Concentration of an inhaled anesthetic preventing gross purposeful movement in response to a noxious stimulus in 50% of individuals
- Values are equipotent
- 1 MAC iso produces the same anesthetic depth as 1 MAC sevo
- Quantitative assessment of anesthetic potency
- equivalent to the ED50 of an injectable drug
