3. Inhalational Anesthetics Flashcards
apneic threshold
highest blood CO2 level at which a pt can remain apneic if holding their breath
IOW: how high their CO2 can climb before they have to take a breath
high apneic threshold
pt who can hold breath longer
have less drive to breathe
low apneic threshold
pt who cant hold breath as long
have more drive to breathe
what can raise the apneic threshold
volatile agents
narcotics
propofol
versed
how do anesthetics raise apneic threshold
suppress pts drive to breathe
it will take higher CO2 to stimulate breathing
what lowers the apneic threshold
pain
how does pain lower apneic threshold
stimulates respirations (faster RR)
difficult to hold breath while in pain
Steps to get pt to spontaneously ventilate prior to wakeup
- Reverese muscle paralysis (if necessary)
- allow pt CO2 to increase
- slow RR
- allow them to go completely apneic - pt begins to breath when apneic threshold is reached
- aka - when the CO2 is high enough
hypoxic drive
lowest oxygen level at which the pt can no longer remain apneic
*makes pts w/high CO2 (smokers) breathe
- if pt is holding breath, lowest level of O2 that a pt can tolerate before they have to take breath
- O2 level is stimulating pt breathing
hypoxic drive level (normal pt)
PaO2 ~ 60mmHg
SpO2 = 90%
- if CO2 didnt stimulate breathing, a normal pt could hold their breath until SpO2 dropped to 90%
do smokers have high or low CO2 levels?
high
decrease alveolar gas exchange
decrease exhalation
increase CO2
what stimulates breath in healthy pts
hypercarbia»hypoxia
high CO2»low O2
what stimulates breath in pts w/lung disease
hypoxia»hypercarbia
low O2»high CO2
*less responsive to CO2 increase
apneic threshold and hypoxic drive correlation
inverse
central chemoreceptors
responsible for apneic threshold
respond to change in [H+] in CSF which is determined by PaCO2
peripheral chemoreceptors
responsible for hypoxic drive
respond to changes in PaO2
Primary reasons volatile agents are given
- prevent movement (non-paralyzed pts)
- prevent anesthetic awareness (all pts)
is pt ready for surgical incision?
- is pt going to move?
- is pt going to remember?
is it easier to prevent movement or awareness during surgery?
awareness is easier than movement
*takes more VA to keep someone STILL than it does to keep them ASLEEP
if we give enough inhalational agent to keep someone STILL…
its impossible for them to have awareness
if we only need the agent to prevent awareness and not movement
we dont need to give as much agent
2 ways to prevent MOVEMENT
- give paralytics
- give high enough concentration of volatile agent
why would you give a paralytic, even if the surgeon doesnt require one?
to keep the pt still withouth having to keep them as deeply anesthetized
higher concentration of volatile agent can make __________ more likely
hypotension
Minimum Alveolar Concentration (MAC)
minimum alveolar concentration of an exhaled gas that will prevent movement in 50% of pts during incision
*a pt is 50% likely to move if they are exhaling the MAC percentage of agent
< 1 MAC means
more than 50% likely to move
> 1 MAC means
less than 50% likely to move
1 MAC Desflurane
6%
1 MAC Sevoflurane
2%
1 MAC Isoflurane
1.2%
1 Mac Nitrous Oxide
105%
most potent volatile agent
Isoflurane
lease potent volatile agent
Desflurane
what does MAC tell us clinically?
how much volatile agent we need to give in order to prevent someone from moving
inspiratory concentration
concentration of agent in the machine/inspiratory limb of the circuit
inspiratory concentration matches
concentration of the agent selected on the vaporizer dial
expiratory concentration
“end tidal”
concentration of agent that the pt is actually exhaling
which concentration do we care about for MAC values?
expiratory
(et)
expiratory concentration ____ inspiratory concentration
et less than in
MAC values reflect
potency
pts are less likely to move when breathing a ______percentage of agent
higher percentage (MAC value)
MAC Awake
0.4 MAC
MAC Awake is needed to
prevent awareness w/o surgical incision
prevent awareness during surgery w/narcotics/paralytics
0.8 MAC
prevent awareness during surgery w/o narcotics/paralytics
1 MAC
how much MAC will prevent movement in 95% of pts
1.3 MAC
0.4 MAC Sevo
0.8%
0.4 MAC Iso
0..48%
0.4 MAC Des
2.4%
0.8 MAC Sevo
1.6%
0.8 MAC Iso
0.96%
0.8 MAC Des
4.8%
1.3 MAC Sevo
2.6%
1.3 MAC Iso
1.56%
1.3 MAC Des
7.8%
when do you look at RR for guaging pt readiness for incisions?
spontaneously ventilating pts only
SV pt w/high RR
might not be anesthetized enough to tolerate incision w/o moving
need more narcotic/volatile agent
SV pt w/low RR
more likely to tolerate an incision w/o moving
DECREASE MAC requirement
we dont need to give as high of a concentration to achieve same effect
7 factors that DECREASE MAC requirement
Pregnancy
Blood (hypoxia, hypercarbia, anemia, hypotension)
IV anesthetics
Old age
Temperature (hypothermia and mild hyperthermia)
Acute alcohol intoxication
Signs of Infection (sepsis)
how much does MAC decline w/age?
4-6% each decade
decline starts before age 40
how does pregnancy decrease MAC req?
pregnancy is a high progesterone state
progesterone is sedative and decreases reqs by up to 40%
hypotension MAP that decreases MAC
<40mmHg
INCREASE MAC requirement
we have to give more of the agent to have same effect
4 factors that INCREASE MAC requirements
Young (peds pt)
Hot
Salty
Alcoholics
Young
sevo - highest MAC req is from 0-1 mo
others - highest MAC req is 6 mo
Hot
extreme hyperthermia (<42C)
burn injury
Salty
hypernatremic pts
Alcoholics
Chronic alcohol abuse
do thyroid conditions impact MAC values?
nope :)
