Flashcards in Week 3 - Pharmacokinetics of Inhaled Anesthetics Deck (70)
Inhaled anesthetics pharmacokinetics involve what?
Absorption - uptake from alveoli to pulmonary blood
Distribution - to CNS, VRG (vessel rich group), skeletal muscle, fat
Metabolism - variable % of metabolism of volatile agents (sevo 3-5%)
Elimination - Lung primary site for elimination
Aging does what to inhaled pharmacokinetics?
Alters pharmacokinetics due to
1. Decreased lean body mass
2. Increased % body fat
3. Increased Volume of distribution (Vd)
4. Decreased Hepatic function
5. Decreased Pulmonary gas exchange
6. Decreased Cardiac output (the lower the CO, the more rapid the onset via inhalation) - per Lewis is counterintuitive
Partial pressure gradients propel inhaled anesthetics across what?
(NOT AN ATP REQUIRING PROCESS)
1. Anesthesia machine/circuit - absorbs minimal amounts of agent.
2. Alveoli - anesthesia is very concerned with this.
4. Cell membranes
All tissues (given enough time) will equilibrate with what?
Partial pressures of inhaled anesthetics. (not likely to see this in single case.) If you do "you've had a bad day"
PA = Pbr
Pa = arterial partial pressures
PA = Alveolar partial pressures (imagine this is a capital LITTLE a)
Pbr = brain partial pressures
Given enough time, Arterial blood partial pressures equilibrate with what?
alveolar partial pressures
Where is the first place that IA's go once it gets into arterial blood?
Equilibration between Pa & PA means what?
The partial pressures are the same in both phases.
PA and Pbr are determined by what?
- PI - inhaled partial pressures
- Alveolar ventilation
- Delivery system
- Anesthetic solubility in tissue - BGPC
- CO - cerebral blood flow
- Alveolar to venous partial pressure differences (A-vD)
Alveolar concentration = ??
BGPC is what?
Blood gas partition coefficient
Inspiratory concentrations of IA's are ALWAYs ___ than expiratory concentrations.
Inhaled partial pressures (PI) must be ___ during initial phase of anesthetic
- High initial partial pressures offset impact of uptake
- Accelerates induction by increased rate of rise of PA (Alveolar partial pressures)
- As uptake rate decreases, PI may be decreased
Concentration effect can be described as what?
The higher the PI (inhaled partial pressure), the more rapidly the PA (alveolar partial pressure) approaches PI.
IE - giving higher doses of inhaled anesthetic, the faster the equilibration.
PI offsets anesthetic uptake
PI also provides augmentation of tracheal gas flow - (which Lewis said was Phuey (sp?))
The second gas effect is what?
Reflects the ability of a high volume uptake of one gas (nitrous) to ACCELERATE the rate of increase of the PA of a concurrently administered second gas (volatile agent)
Increases tracheal gas flow
Concentration of volatile agent as uptake of nitrous occurs.
Alveolar ventilation is what?
Higher rates of alveolar ventilation promote the uptake of the anesthetic gas. (IE - the faster you breathe, the faster your PA gets to PI, so if you want your patient to get drugs faster, bag them.
- Increased alveolar ventilation = increased rate of increase in the PA toward the PI
- The increased alveolar ventilation to FRC ratio, the more rapid the rate of induction.
Neonate alveolar ventilation to FRC ratio is what?
5:1 (neonates induced quicker than adults)
Adult alveolar ventilation to FRC ratio is what?
Volatile agents are ventilatory ___?
Spontaneous ventilation provides a negative feedback protective mechanism here, in that when ventilation is reduced, the delivery of anesthesia is reduced. (IE - your patients RR drops, less volatile agent is delivered, which is why open drop ether was so safe.)
- If patient is on controlled ventilation, this mechanism is lost.
The impact of alveolar ventilation changes on rate of increase in PA toward PI depends ___ of anesthetic in the blood.
Changes in alveolar ventilation influence rate of increase of PA towards PI more with ___anesthetics than with ____ anesthetics
Inhaled agent solubility in blood and tissues is quantified by what?
partition coefficients - is a ratio describing how inhaled anesthetics distribute themselves between 2 phases at equilibrium
Anesthesia machine's characteristics influences the rate of increase of __ towards PI __.
-volume of breathing system, solubility of agent in circuit, fresh gas flow rates.
Have fresh gas flows equal to or greater than ___ to negate effects of volume of anesthetic breathing system.
(higher flows on induction and emergence will speed both processes along)
The rate of increase of PA towards PI is ___ proportional to solubility of anesthetic in the blood.
Blood is a pharmacologically __ __?
The size of the blood reservoir depends on the solubility of anesthetic in blood.
High BGP (Blood Gas partition coefficient)
- Large amount of anesthetic must dissolve in blood for Pa to equilibrate with PA.
(body has a LARGE sponge)
Low BGP (Blood Gas partition coefficient)
Small amount of anesthetic must dissolve in blood for Pa to equilibrate with PA.
(body has a SMALL sponge)
BGP of Iso, Nitrous, Des, and Sevo
Iso - 1.46
Nitrous - 0.46
Des - 0.42
Sevo - 0.69