Inhaled Anesthetics Flashcards
(42 cards)
Atomic Number
Protons + neutrons - both carry weight
Ionic v Covalent bonding
ionic = complete loss of electron (NaCl) Covalent = shared e- , can be polar (non-equal sharing, a relative charge on one side) or polar
Forms of Inhaled anesthetics
non-organic (no carbon) - NO, xenon
organic
alkane
ether R-O-R
Molecular structure of inhaled anesthetics
covalent bonds.
physical characteristics determined by molecular structure - solubility, boiling point, vapor pressure
Vapor Pressure
- the pressure exerted by a gas above its liquid phase at atmospheric pressure
- the max pressure it can exits as both a liquid and gas at standard conditions.
- determines the max concentration a gas can be under standard conditions
the effect of temperature on vapor pressure
increasing temp = increased vapor pressure
3 Gas Laws
Boyles: P1V1 = P2V2 (pressure & vol are inversely related at std temp)
Charles: V is directly related to T (constant P)
Gay-Lussacs: P directly to T (constant V)
Avogadros Law
the gram molecular weight of an ideal gas at standard temp is occupies 22.4L and contains 6 x 10^23 particles
Action of an Inhaled Anesthetic depends on what?
therapeutic tissue concentration in CNS
Induction Goal
reach equilibrium (the gas exerts the same partial pressure on either side of the membrane, there is no net movement) Fi/Fa = 1. does NOT mean equal number of molecules.
What factors effect reaching equilibrium?
solubility - less soluble quicker
CO - lower CO quicker
fresh gas flow
minute ventilation
Three Factors of getting the gas from vaporizer to alveoli
- partial pressure gradient
- effect of alveolar ventilation
- the time constant - if unopposed (no uptake) the rate is determined solely by this (capacity/flow L/L/min)
Time Constants & % Change
0.7 TC = 50%
1 TC = 63%
2 TC = 86%
3 TC = 98
Factors Affecting Inspiratory Concentration
FGF rate
Circuit volume/absorption
gas concentraion
3 Factors that affect uptake of an inhaled anesthetic
- solubility
- pulmonary blood flow (CO)
- partial pressure difference
How uptake effects the Fi:Fa ratio
uptake prevents alveolar concentration from rising. Fi/FA <1. means a slower onset - the partial pressure doesn’t build up (alveolar PP determines blood PP which determines brain PP)
Solubility of a Gas
lower solubility = less uptake = faster onset
expressed by a b/g coefficient.
a lower b/g coefficient means lower solubility.
B/G Coefficient
the ratio of the concentrations of the anesthetic gas in each of two phases at steady state - equal partial pressures in the two phases.
Ex: desfluranes b/g is 0.42 meaning 1ml of blood contains 0.42 as much desflurane as 1 ml of alveolar gas
B/G Coefficients of Common Inhaled Anesthetics
Des : 0.42 NO: 0.47 Sevo: 0.65 Iso: 1.4 Enflurane: 1.9 Halothane: 2.4 Methoxy: 15
Fat Solubility of a Vapor.
potency is directly related to the fat/blood coefficient. highest fat solubility = lowest MAC. All agents (except N2O) have significant fat solubility.
Partial Pressure effect on uptake
greater gradient = more uptake
Uptake into Tissue Groups
determined by solubility, blood flow, and partial pressure gradient.
1- vessel rich (most perfusion & CO)
2- muscle
3- fat - very high solubility = a capacity that takes days to fill
4- vessel poor
Ventilation Effect on Uptake
increased ventilation constantly replaces alveolar concentration to increase partial pressure quicker. effect more noticeable with highly soluble agents that have greater uptake. with insoluble has minimal effect - these agents already reach a FA/Fi ratio of 1 quickly
Elimination of Anesthetics
primarily via exhalation. minimal biotransformation and insignificant transcutaneous.
Recovery is usually faster than induction unless prolonged anesthesia. this is due to continued uptake by tissues that have not yet reached steady state which helps drop alveolar partial pressure
