Inhalation Agents Flashcards
3 most common inhalation anesthetics
- Isoflurane
- Desflurane
- Sevoflurane
(N2O, we give often… but not a true anesthetic)
Inhalation agents are direct…
Myocardial depressants
The primary objective of inhalation anesthesia is
To achieve a constant and optimal partial pressure of anesthetic administered in the brain
The target organ in inhalation anesthesia is the __
Brain
Equilibrium is referred to as ___
Steady state
At equilibrium (steady state)
Constant and optimal pressure is maintained
Partial pressure of a gas are…
Equal in both the blood and the gas phase (both sides of the partition)
Lambda coefficient is also called ___
Ostwald Coefficient/Partition Coefficient
Ostwald/partition coefficient refers to potentials of substances in (2 phases)
Blood and gas phases
The gas phase occurs in the ___
Alveoli
The blood phase occurs in the ___
Arterial/venous blood
At equilibrium…. the ___ = ___ = ____
Alveoli = arterial blood = brain
Partial alveolar is abbreviated as
PA
Partial arterial pressure is abbreviated as
Pa
Partial pressure is abbreviated as
P
Partial brain pressure is abbreviated as
Pbr
While partial pressures are equal… the anesthetic ___
Drug concentration is not equal
We determine equilibrium by
Input into alveoli - uptake of drug from alveoli into arterial blood
Input into alveoli, uptake of drug from alveoli into arterial blood occur during
Induction
Alveoli are the
“Windows to the brain”
Leaky bucket concept refers to
Drug absorption uptake
Drug concentration is (directly/inversely proportional) to partial pressure
Directly (but not equal)
Increased concentration =
Increased partial pressure
Decreased concentration =
Decreased partial pressure
Alveoli are window to the brain… we measure what is in alveoli by
End tidal CO2
At equilibrium, alveoli =
Blood partial pressure
Diseased lungs require
Increased concentration
Transfer from the machine to the alveoli is determined by
Vaporizer and flow meter settings, as well as type of equipment used
Never use oxygen <
30%
How to flush anesthesia machine for someone concerned with MH (option 1)
100% oxygen at 10L/min for at least 20 minutes
Replace breathing circuit and carbon dioxide canister
Inactivate/remove vaporizers
***newer anesthesia machines may require washout for 104 minutes
How to flush anesthesia machine for someone concerned with MH (option 2)
Adding commercially available activated charcoal filters to the circuit will remove anesthetic gases and remove need for purging system
***Anesthesia machine will still need to be flushed with high fresh gas flows >= 10L/min for 90 seconds
Higher fresh gas flow & lower absorption =
Faster induction
Pediatrics often only use…
Inhalation induction
Adults need IV induction as well, then you start inhalation. In theory you have ___
Two inductions
IV bypasses
Absorption
Inhalation must be ___
Absorbed
Alveoli to arterial blood (3 factors)
- B/Gλ (lipid solubility of agent)
a. Drug you choose/solubility - C.O. (pulmonary blood flow)
- Alveolar to venous partial pressure difference (diffusion gradient)
Goal in inhalation induction is to: (3)
- Decrease gas solubility coefficient
- Decrease/maintain normal C.O.
- Increase arterial/venous partial pressure
Drug, ventilation and cardiac factors are determined by (2 things) with alveoli to arterial blood
- Agent chosen
- Patient’s heart and lungs
Tissue factors: (3) with arterial blood to brain
- Brain/blood λ
- Cerebral blood flow
- Arterial to venous partial pressure difference (diffusion gradient)
Arterial blood to brain tissue factors are determined by 2 things:
- Agent chosen
- Patient’s heart and lungs
Goal of inhalation absorption to tissue: (3)
- Decreased blood gas solubility coefficient
- Normal cerebral blood flow
- Increased arterial/venous partial pressure
Barriers that must be overcome to produce an effective anesthetic:
- Delivered
- Inspired
- Alveolar
- Arterial
- Tissue
MAC stands for
Minimum alveolar concentration
MAC compares
Potency of all inhalation agents
We breathe in…
78% Nitrogen
21% Oxygen
0.04% CO2 and other mixed gases
Pre-oxygenation is reallu
De-nitrogenation
At equilibrium, partial pressures are ___ and are reflective of
Equal
Total pressure exerted in alveoli
At equilibrium, concentrations are ___, so you must ___
Never equal
Dial the concentrations in
Pharmacodynamic effects of inhaled anesthetics must be ___
Based on a dose
Pharmacodynamic effects are based on
MAC
Each drug has a MAC, each MAC is different, but at that #….
It has equal potency at each MAC
Factors that reduce MAC
Increased age
Hypothermia
Admin of sedatives, co-administration of other anesthetics
Alpha 2 agonists
Opioids
Acute ethanol consumption
Hypoxemia
Hyponatremia
Hypermagnesemia
Anemia
Hypotension
Pregnancy
Lithium
Factors with no influence on MAC
Duration of anesthesia
Gender
Hypocapnia/hypercapnia
Metabolic alkalosis
Hypertension
Hyperkalemia/hypokalemia
Factors that increase MAC
Young age
Hyperthermia
Hyperthyroidism
Hypernatremia
Acute admin of CNS stimulant drugs
Red hair in females
Chronic alcohol abuse
1 MAC assesses…
Pt movement (aka ED50)
1MAC =
ED50 (no recall, LOC)
1.3MAC =
ED95
MAC value for N20
105
MAC value for Isoflurane
1.15
**1 in elderly
MAC value for Desflurane
5.8
**5.17 in elderly
MAC value for Sevoflurane
2.0
**1.45 in elderly
Pts < 1 year of age…
Have increased MAC
MAC decreases what percentage per each decade of life?
6-7% each decade
Do you use 105 of N2O?
No, you use 0.6MAC (60%)
If MAC is increased, you must give a(n) ____ dose
Increased
If MAC is decreased, you must give a(n) ___ dose
Decreased
Smaller the dose…
The more potent it is
MAC is highest between ____ of age
6-12 months
MAC-awake refers to
Approximately 1/3 of MAC
50% will open eyes when asked
MAC for loss of recall and awareness are
0.4-0.5 MAC
1-MAC
Derived value for agent that assesses movement on incision
Surgical-MAC
1.2-1.3x MAC
Anesthetic requirements > MAC by 20-30%
**Surgical MAC is where we operate
MAC-BAR
1.6x MAC
**This is where you block adrenergic/stress response
What MAC do you block adrenergic/stress response?
MAC-BAR (1.6x MAC)
Blood/gas λ of N2O
0.47 (in blood) and 1 in gas
Blood/gas λ of Isoflurane
1.4 (in blood) and 1 in gas
Blood/gas λ of Desflurane
0.42 (in blood) and 1 in gas
Blood/gas λ of Sevoflurane
0.6 (in blood) and 1 in gas
Fat/blood λ of N2O
2.3:1
Fat/blood λ of Isoflurane
45:1
Fat/blood λ of Desflurane
27:1
Fat/blood λ of Sevoflurane
48:1
What is the preferred inhalation agent in bariatric patients?
Desflurane because fat/blood is lowest true anesthetic of 27
FA/FI = 1 = ___
Equilibrium
The speed of anesthetic induction (with inhalation) is determined by ___
The rate at which alveolar partial pressure approaches inspired partial pressure
FA/FI is
Alveolar: Inspired
The key is to look at what is in alveoli (lung) to ___
To determine how fast they will go to sleep
FA/FI (interchangeable with FA:FI) is the degree ___
To which alveolar concentration is becoming closer to inspired concentration of anesthetic
If alveolar concentration equals the inspired concentration….
The ratio will be 1/1 or 1:1 or simply 1
FA/FI = 1 is the same as saying ___
Rate at which alveolar concentration of anesthetic equals inspired concentration of anesthetic
**AKA equilibrium
3 factors determining uptake from lung (absorption) alveoli–arterial blood
- B/G λ solubility of the agent
- C.O.
- Alveolar to venous anesthetic partial pressure difference between lung and blood (diffusion gradient)
Blood is a pharmacologically ____
Inactive reservoir
Blood (inactive reservoir) size is determined by ___
Solubility of anesthetic agent in blood
If the B/G λ is high, a ___ amount of anesthetic must be dissolved in blood before the Pa (arterial) equilibrates with alveoli (PA)
LARGE
If the B/G λ is low, ___ of the anesthetic must be dissolved in blood before the Pa (arterial) equilibrates with PA (alveoli)
Less
An increase in anesthetic uptake (absorption) results in…
A slower induction— a SLOWING of anesthetic induction (or increase in induction time)
The B/G λ (Ostwald) is a distribution ratio describing ___
Itself between two phases: blood and gas
Equilibrium is equal to
Partial pressure on both sides
B/G λ =
Anesthetic blood concentration/anesthetic alveolar gas concentration
Isoflurane’s B/G λ is…
1.4
With isoflurane:
At equilibrium, the concentration in blood will be ___x the concentration in the alveoli but the partial pressures will be ___
1.4 and equal
1.4:1 or 1.4/1
In predicting MAC from O/G λ, what is the formula?
MAC = 150 / (O/G λ)
MAC = 150 / (O/G λ is good at determining
Anesthetic potency
The correlation between lipid solubility and potency follows which rule?
Meyer-Overton rule
What type of oil was used to determine oil/gas solubility?
Olive oil
Oil/gas partition coefficient of N20
1.4
Oil/gas partition coefficient of Isoflurane
99
Oil/gas partition coefficient of Desflurane
18.7
Oil/gas partition coefficient of Sevoflurane
50
If lipid solubility is increased…
So does anesthetic potency
If you know oil/gas…
You can figure out MAC
Meyer-Overton rule:
Anesthetic potency of inhalation agents correlates directly with lipid solubility
Meyer-Overton rule implies that ___
Anesthesia results from molecules dissolving at specific lipophilic sites
Problem with Meyer-Overton rule:
All lipid-soluble molecules are not anesthetic and some are convulsants (correlation between anesthetic potency & lipid solubility is only approximate)
O/G λ
How efficiently drug can access the sites of action (potency)
B/G λ
How fast drug is delivered to tissue (solubility)
Inhalation anesthetics are…
a. potentiaters
b. additive
c. syngergistic
d. antagonists
Additive
0.5 MAC of one agent + 0.5 MAC of 2nd agent =
1.0 MAC
What gas is the only gas you can add?
Nitrous oxide
General rule of thumb using N2O…
1% reduction in MAC for each 1% N2O
Uptake (absorption) is ___ proportional to C.O.
Directly
Increased C.O. =
Increased uptake (slower induction)A
An increased cardiac output means a slower/faster induction
Slower
Decreased C.O. = ___ uptake
Decreased uptake (more rapid induction)
A decreased cardiac output means a slower/faster induction
Faster
Increased C.O. = ___ lung perfusion
Increased
Increased lung perfusion =___
Increased drug absorption (increased uptake)
Increased drug absorption (uptake) =
Decreased rate of rise in alveolar drug (slower induction)
Drug absorption (uptake) is also known as the…
Leaky bucket
Decreased C.O. = ___ lung perfusion
Decreased
Decreased lung perfusion = ___ drug absorption (uptake)
Decreased
Decreased drug absorption (uptake) =
Increased rate of rise in alveolar drug (more rapid induction)
The leaky bucket (lung) theory correlates with
Drug absorption
The greater the difference in partial pressure between the agent in the alveoli and the ___ the greater the uptake
Will the induction be slower or faster
Venous blood
Slower
High pressure in lung is required to get __
Rise
If there is no uptake (absorption) of anesthetic gas from alveoli, FA would…
Rapidly equal the concentration of anesthetic gas in inspired gas FI
No uptake =
Faster response
FA (alveolar) lags behind FI because…
Pulmonary circuit takes up anesthetic gas and lowers concentration in alveoli
The greater the amount of uptake (absorption)…
The slower the FA/FI ratio rise toward 1
Uptake delays, but it happens to ___ people
ALL
Uptake is ____ related to rate of rise of the FA/FI ratio
Inversely
There are 3 phenomena influencing uptake & distribution of inhaled agents
- Concentration (overpressure)
- Second gas effect
- Diffusion hypoxia
Overpressurization is analogous to…
An upfront IV loading bolus of a drug
Overpressurization definition:
The admin of a higher partial pressure of anesthetic than the alveolar concentration actually desired for the patient
When you reach equilibrium with overpressurization… what should you do immediately after and why?
Turn gas down because it causes major cardiac depression
Concentration effect is d/t: (2)
- Concentrating effect
- Hyperventilation effect
Concentrating effect definition:
Total volume of lung is decreased by amount of uptake of inhalation agent by the blood (concentrates remaining gas in lung)
Hyperventilation Effect/Increased tracheal inflow…
When a large volume of gas is taken up by the blood a negative pressure is created and fresh gas inflow augments alveolar ventilation
Concentration effect’s biggest difference occurs in how many breaths?
2-3
Concentration effect speeds up induction by the concentrating effect and the ___
Hyperventilation effect
Net effect means
You are delivering a greater concentration of agent than is actually being administered
Second gas effect is the uptake of large volumes of a ___
1st primary gas (N2O) that accelerates alveolar rate of rise of a 2nd gas (isoflurane) administered at the same time
Second gas effect theory:
Rapid uptake of primary gas concentrates the second gas because of the decrease in total lung volume
To tell the difference between concentration vs. second gas… ask yourself…
Are you giving 1 or 2 gases?
Concentration and second gas effects do what to induction and emergence?
Hasten (speed up)
Concentration and second gas occur during ___ for approximately ___
InductionF
First 5-15 minutes (most dramatic and significant within first few breaths)
***Occurs during emergence for first 5 min
Diffusion hypoxia (anoxia) occurs during what phase… induction, maintenance, or emergence?
Emergence
Diffusion hypoxia ONLY occurs with the use of ___
N2O
**Postop first 5 min
To avoid diffusion hypoxia you should….
Wake the patient up with oxygen
Why does diffusion hypoxia occur?
Nitrogen comes in/out and crowds the lungs and causes hypoxia
SHUNTS:
Examples of right-to-left shunt (2)
Intra-cardiac shunt
Intra-pulmonary shunt (bronchial intubation)
SHUNTS:
Example of left-to-right shunt (1)
Tissue shunt
Which type of shunt can cause de-oxygenation?
Right-to-left (they are cyanotic)
Examples of tissue shunts (not significant in de-oxygenation)
AV fistula
Volatile anesthetic induced in increase in cutaneous blood flow)
Right-to-left shunts could slow the rate of induction d/t a ____
Dilutional effect
Right-to-left shunts have greater impact on:
poorly soluble agents like:
Desflurane
Sevoflurane
N2O
Right-to-left shunt compensation for deficits: (2)
Increased concentration of insoluble agents
Increased ventilation w/ soluble agents
Right-to-left shunt = decreased or increased PA
Decreased
Left-to-right shunts result in delivery to ___
Lungs of venous blood with a higher partial pressure of anesthetic than that present in blood (that has passed through tissue)
Left-to-right shunts increase or decrease PA tension
Increase
Is a left-to-right shunt alone detectable?
No
What type of shunt is detectable?
Right-to-left
In the presence of a right-to-left shunt, a left-to-right shunt will ____
Offset dilution effects (could prolong IV induction)
Three stages of anesthesia:
Induction
Maintenance
Recovery
Induction has three:
Excitement (see in children mostly)
Rigidity (set their jaw)
Relaxation
Maintenance has two:
Constant (set anesthesia machine)
Variable (we adjust w/ variable)
Recovery has two:
Crisis (very rapid wake up)
Lysis (smooth even w/ wake up)
We want patients in the ____ phase of recovery
Lysis
What gas is preferred in bariatric?
Desflurane
Three factor equation for uptake from lung:
UL = λB Q(PA - PV) / BP